Astroparticle, Particle And Space Physics, Detectors And Medical Physics Applications (Proceedings of the 9th Italian Conference)

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l/cos(theta) Figure 5. ToTs vs l/cos0 and MC comparison and MC comparison As the hit strip multiplicity, the ToTs in the track layers increases linearly with the l/cos0 (proportional to the track length in the SSDs) as expected by the MC simulation (Fig.5).

5.

Conclusions

The preliminary results from analysis of cosmic ray data taking during LAT integration are shown in this paper. The study of tracker behavior shows a good agreement between data and MC simulation predictions. Cosmic rays test with the complete LAT configuration and beam test that will be performed during 2006 will be usefully to tune the Monte Carlo. After that, the GLAST Collaboration will be able to use the Monte Carlo simulations to predict the performance of the instrument.

References 1. 2. 3. 4. 5. 6.

http://glast.gsfc.nasa.gov/ http://www-glast.stanford.edu/pubfiles/proposals/bigprop/ http://proj-gaudi.web.cern.ch/proj-gaudi/welcome.html M.N Mazziotta et al., Nuc. Instrum. And Meth. A533:322-343, 2004 M. Brigida et al. (2002), LAT internal note, LAT-TD-0105 Eduardo do Couto e Silva and Lee Steele (2005), LAT-TD-04631-02

B A C K G R O U N D ANALYSIS OF CUORICINO I N V I E W OF THE FUTURE EXPERIMENT CUORE

S. CAPELLI, 0 . CREMONESI, M. PAVAN, S. PIRRO, E. PREVITALI Dipartimento di Fisica dell'Universitd. di Milano-Bicocca e Sezione di Milano dell'INFN, Milano I-20126,Italy The study performed on the measured CUORICINO background spectrum is reported. The extrapolation to CUORE background and the R&D begun in order to reduce it to a value compatible with the wanted sensitivity is also presented.

1. Introduction The discovery of neutrino oscillations has unequivocally demonstrated that neutrinos are massive and mixed particles. Experiments looking for the Neutrinoless Double Beta Decay (0/3(Ov) ) can probe the Majorana nature of the particle and the absolute value of the Majorana mass (mee) . The value of (mee) is unknown but can be partially inferred from oscillation data. The possible ranges of values at 99% C.L. for (m ee ) are (1.1 - 4.5) meV for normal hierarchy [1], (12 - 57) meV for inverted hierarchy [1] and (10 - 55) meV for quasi degenerate spectrum [2]. The sensitivity reached by the running CUORICINO experiment ranges between 0.1 and 1.1 eV [3], allowing to explore part of the quasi degenerate mass spectrum range. Next generation p/3(Qv) experiments will require at least a sensitivity of the order of 10 meV in order to explore the inverted hierarchy mass spectrum range. The f}j3(0v) is a lepton violating process, where a nucleus (A,Z) decays into (A,Z+2) with the emission of two electrons and no neutrino. The searched signature is a peak at the Q value of the transition in the sum energy spectrum of the two electrons. The decay rate of this process is directly proportional to (m ee ) squared and depends on the nuclear matrix elements for the decaying nucleus. The chosen isotope in the CUORICINO experiment is 130 Te , whose high transition energy (2528.8 ± 1.3 keV) and high isotopic abundance (33.8%) make it good DBD candidate. The detectors are Te02 crystals

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operated (at a temperature of ~10 mK) as bolometric detectors [4],

2. CUORICINO and C U O R E set-up The CUORICINO detector consists in 62 Te02 crystals, arranged in a tower like structure made by 11 planes of 4 crystals of 790 g and of 2 planes of 9 crystals of 330 g, for a total mass of ~40.9 kg. It is installed in the hall A of LNGS (3200 mwe), in a diluition refrigerator able to reach temperatures of about 10 mK. The tower is surrounded by copper shields, 1.5 cm of Roman lead shield, 20 cm of commercial lead shield and 10 cm of borated P E T shield. The whole set-up is kept in nitrogen overpressure and placed in a Faraday cage. Particular care was devoted to selection and cleaning of materials. Crystals and copper structure were subjected to a dedicated surface treatement. While being a self consistent experiment CUORICINO is also a feasibility test for the next generation experiment CUORE. CUORE [5] will be constituted by 988 Te02 790 g crystals, arranged in 19 towers made by 13 modules of 4 detector each, for a total mass of ~750 kg. The detector will be placed in a specially produced dilution refrigerator and provided with copper and Roman lead inner shields and externally shielded by 30 cm of commercial lead and 10 cm of borated PET. The detector set-up will be kept in nitrogen overpressure and placed in a Faraday cage.

3. Background sources for a D B D experiment Due to the rarity of the searched process the sensitivity of a /3/3(0v) experiment can be strongly limited by the background counting rate in the region of interest. Background understanding and reduction is therefore mandatory. The main background sources for a bolometric detector like CUORICINO can be divided in: • External sources: photons and neutrons from the Gran Sasso rock or muons and muon induced particles. • Internal sources: intrinsic radioactivity and cosmogenic activation of all the experimental materials and surface contaminations of crystals and materials facing them.

146

4. Background study No peak at 2528 keV appears in the anticoincidence background spectrum obtained with the statistics collected so far with CUORICINO (5.87 kg x year of 130 Te ). Using a Maximum Likelihood procedure to evaluate the maximum number of DBDOi/ events compatible with the measured background in the /?/?(0^) region (0.18 ± 0.02 c/keV/kg/y), a lower limit of 2.0 x 10 24 years at 90%C.L on the 130 Te half-life for the /?/?(0i/) was set. A dedicated study of the measured CUORICINO background was performed in view of the future CUORE experiment. An analysis method based on sophisticated procedures and with the support of Montecarlo (MC) simulations was developed. By analysing coincidences, counting rates in different energy regions, gamma and alpha peaks position, intensity, rates and shapes, the most probable sources contributing to the measured background are identified. MC spectra are then generated, reproducing the contribution that each of these sources would give in the detector. A dedicated code to find the linear combination of input MC spectra that gives the best agreement with the measured spectrum is then used in 3 steps: a peak region (E>4 MeV), a continuum (E>3 MeV) and 7 and /3 region. The Geant4 package has been used to write a C + + based code able to simulate the entire structure of CUORICINO and CUORE. Decay processes due to natural ( 232 Th , 238 U , 2 1 0 Pb , 4 0 K ) and cosmogenic isotopes in the bulk and on the surfaces of the various experimental parts have been simulated, as far as LNGS environmental radioactivity. 4.1. CUORICINO

background

analysis

From the analysis performed on the CUORICINO data the main sources responsible of the background in the P/3(0v) region have been identified, a decays occurring on the crystals surface have been clearly identified from their characteristic asimmetric peak at the alpha transition energy and the coincidence pattern, while a decays occurring in the crystals bulk show a gaussian peak at the Q value of the decay. The flat continuum measured between 3 and 4 MeV can be ascribed to deep (1-5 /xm depth) surface contaminations of materials facing the crystals (the biggest surface is the one of the copper structure) as well as to surface contamination of the detector themselves. The contribution to the /30(Ov) region from these sources has been evaluated to be of ~10±10% and ~50±20% respectively for the Te02 and Cu surfaces. The remaining background in the /?/?(0z/) region can be ascribed to 2 3 2 Th contaminations of experimental part distant from

147 the detector ( ~30±10%). A limit of ~10 _ 8 Bq/cm 2 has been evaluated for the surface contamination of copper and crystals in 2 3 2 Th , 238 U or 2 1 0 Pb . For the bulk contamination a limit of 1 pg/g and 0.1 pg/g for copper and TeC>2 has been set. 4.2. Extrapolation

to

CHORE

In order to reach the sensitivity required to investigate (mee) in the inverted hierarchy, it is necessary to reach in CUORE a background rate in the /3/?(0f) region of the order of 1 0 _ 2 - 1 0 ~ 3 c/keV/kg/y. The construction of the CUORE detector will require about five years; a dedicated R&D is on the way, aiming to decrease the background sources that could limit the sensitivity of the experiment. The background reachable with CUORE was evaluated on the basis of the background model developed for CUORICINO and on the basis of the radioactive contaminations measured so far for the various materials commercially at our disposal [5]. Negligible contributions are expected from thermal neutrons and environmental gammas, thanks to the use of specially designed neutron and lead shields. The evaluated contribution from high energy neutrons (both produced from rock radioactivity and from muon interactions in the rock and in the lead shields) is of ~ 4 x l 0 - 4 c/keV/kg/y. Copper and crystals bulk contamination expected rate in the flfi{Qv) region is less than 3xl0~ 3 c/keV/kg/y. The most worrisome background source is the surface contamination: for contamination values of ~10~ 8 Bq/cm 2 a rate of 2xlO~ 2 c/keV/kg/y and 5xlO~ 2 c/keV/kg/y could arise for crystals and copper surface contaminations respectively. 5. R & D for background reduction In order to fulfill the CUORE goal with respect to background reduction an intense R&D had begun with respect to surface cleaning and passivation, aiming at reaching at least reduction by factors 10 and 4 in copper and crystals surface contamination respectively. Dedicated measurements have been performed in a second and smaller cryostat, installed in the hall C of LNGS, provided with 5 cm of Copper shield and 15 cm of commercial lead. No neutron shield is present. In the first measurement (RAD1) 8 crystals of 760 g subject to surface etching and polishing with clean and measured powders have been mounted in CUORICINO like tower. The copper used for the tower structure was

148

also subject to a specially studied cleaning procedure: etching, electropolishing and passivation. The measured background, with a statistics of ~6750 h x detector, gave very encouraging results with respect to crystal surface 2 3 2 Th and 238 U contamination reduction. The rate of the a peaks due to these sources were in fact reduced by a factor of ~ 4 with respect to CUORICINO, fulfilling the first CUORE milestone. The continuum background between 3 and 4 MeV was compatible with the one measured with CUORICINO. Further testswere therefore necessaries in order to find out the source responsible for this contribution. In a second test (RAD2) with the same detector array the crystals were faced to a higher mass and surface of teflon (a 10x10x0.5 cm 3 sheet facing 4 crystals), gold wires (12.7 m facing 2 crystals) and silicon heaters (~30 cm 2 facing 2 crystals), thus increasing the sensitivity for these components contaminations with respect to CUORICINO. The measured background have proved that this source can't be responsible of the rate measured in CUORICINO and in RAD1 in the 3-4 MeV energy region. Another test (RAD3) is presently running in hall C in order to have a clear answer with respect to copper surface contamination. All the copper parts facing the detectors have been covered with 5 layers of ~12/im low contaminated polyethylene sheet, in order to stop a particles generated by decays occurring in the Cu surface. In a second run thermal neutrons will be tested by changing the shields: the 5 cm of copper will be sostituted by lead and 1 cm of CB4 and 6 cm of paraffin will be added. Preliminary results show a background reduction in the 3-4 MeV region of a factor ~ 2 . 6. Conclusion The analysis performed on CUORICINO data was fundamental in order to disentangle the main sources responsible of the background measured in the (3/3(Qv) energy region. And intense and promizing R&D has begun in order to reduce the identified contaminations in view of the next generation CUORE experiment. References 1. 2. 3. 4. 5.

A.Strumia and F.Vissani Nucl.Phys. B726, 294-316 2005 S.Pascoli and S.T.Petkov Nucl. Phys. B580, 280 2004 C.Amaboldi et al. (hep-ex/0501034) D. Twerenbold, Rep. Prog. Phys. 59 349 1996 A.Arnaboldi et al, CUORE proposal

APPLICATION OF NUCLEAR TRACK DETECTORS IN ASTROPARTICLE AND NUCLEAR PHYSICS S. CECCHINI IASF/INAF, 40129 Bologna, Italy INFN, Sez. Bologna, 40127Bologna, Italy S. BALESTRA, M. COZZI, G. GIACOMELLI, M. GIORGINI, S. MANZOOR, E. MEDINACELI, L. PATRIZII, V. POPA, G. SIRRI, M. SPURIO, V. TOGO Dipartimento di Fisica, Universita di Bologna, 40127Bologna, Italy INFN, Sez. Bologna, 40127 Bologna, Italy T. CHIARUSI

Dipartimento di Fisica, Universita la Sapienza, 00185 Roma, Italy After a brief presentation of the technical methods employed and the calibrations performed on different Nuclear Track Detectors (NTDs), the results obtained by their use in different fields of astroparticle and nuclear physics are presented. In particular the results of searches for magnetic monopoles, nuclearites and other exotica in the cosmic radiation carried out by the Bologna Group. Also an application of these detectors in an experiment for the determination of the charge spectrum of primary cosmic ray particles is described. Example of the use of the same detectors for the measurement of the total and partial charge changing fragmentation cross sections of relativistic ions in several targets are also reported.

1. Introduction The technique of Solid-state Nuclear Track Detectors (SSNTDs) was born almost 50 years ago. Since then the field has grown enormously; numerous Conferences have been dedicated to the topic and several books cover different aspects of this technology [1]. Starting from 1986 the Bologna group has devoted part of its activity to the use of plastic materials (CR39, Makrofol, Lexan) in various research fields and to the improvement of industrial production of CR39 expressly for science applications [2]. Here we briefly summarize the results obtained during the decennial experience acquired in Astroparticle and Nuclear physics experiments using this detection technique. 149

150

2. The Nuclear Track Detectors (NTDs) The working principle of plastic NTDs is qualitatively presented in Fig.l. When a particle crosses such a detector the polymeric chains along its path are affected giving origin to the so-called "latent track". In the case of a particle with an electric charge Z and velocity (3 (= v/c), the produced damage can be related to Z/p. The latent tracks become visible under an optical microscope after chemical etching, typically in aqueous solution of NaOH or KOH - for the materials considered here - when the etching velocity along the latent track (vx) is larger than the one for the bulk of the material (vB). The temperature and concentration of the etching bath are chosen according to the type of application. For a through going particle, in the initial stage of the etching two cones are formed on both sides of the detector. The size and shape of such cones depend on the energy loss of the incident particle; the measurement of the cone base area or heights allows, through an appropriate calibration, to determine the characteristics of the particle.

Figure 1. (left) The breaking of polymeric chains of plastic NTD by a crossing particle; (right) crossview of the formation of the track after etching for two different times [1].

The relation between the parameter p =vT /vB, the observable quantity, and the REL is obtained through calibrations. A typical set-up at an accelerator ion beam would consist in few sheets of detectors upstream of some target material (Cu, Pb, C, Al and so on) used for the ion fragmentation, and several NTD sheets placed downstream of the target in order to record the tracks of the nonfragmented ions as well as of the fragments generated in the target. Fig 2 shows the distribution of the etch cone base areas produced by 158A GeV In + ions and their fragments at the CERN SPS in CR39. By computing the REL values corresponding to the different relativistic ions, the NTD calibration curves (p vs. REL) can be obtained [3].

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Different calibrations for different materials and etching conditions appropriate for the specific experiment carried out have been obtained [4]. 200

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3. Astroparticle Physics The use of NTDs can play a key role for the detection of particles incident (or hypothetically incident) on the Earth atmosphere. In many cases the event flux is expected to be small and NTDs for their characteristic of being passive, easy to use, low cost and adaptable at the scope represent a good solution.

3.1. Primary Cosmic Rays Abundances The nuclear composition of primary CR is a rich probe for understanding their origin. Precise measurements of the nuclei abundances beyond Z > 30 in fact could allow to discriminate between two competing models [5] and [6] proposed for the composition at their sources. Especially the ratios among heaviest elements could be the best discriminator for most current theories regardless of the propagation history of these nuclei. In order to be able to measure these

152 ratios, high charge resolution and high statistics are needed since the flux of heavy nuclei is very low (< 1 part/m2h). Until now, the largest statistics above the Fe threshold came from experiments of the HEAO-C3 [7] and Ariel-6 [8] missions, which unfortunately suffered from low charge resolution. The use of NTDs onboard space vehicles to study the radiation environment is well know [9, 10]; they were also used for measurements of nuclei with Z > 60. The major advantage of NTDs is the possibility of building experiments with large geometrical factors, balloon flights of 20-40 days appear feasible from Antarctica or in north circumpolar routes. Another possibility can be offered by the ISS for exposure of large area (tens of m2) for few months. The competition with active detectors is favoured for what concerns weights, cost and power consumption.

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The large area exposed requires automatic scanning and track recognition, as it is unthinkable to use operators at optical microscope for such large areas. For this purpose we have developed a prototype experiment CAKE that has been flown on a stratospheric balloon that reached a plafond altitude of 38-40 km for 18 hours during a trans-mediterranean flight. First results of this tests have been reported [11] (Fig. 3) and suggest that it is possible to reach the sensitivity, charge resolution and statistics needed for discriminating between different scenarios of the origin of CR with reasonable dimensions and exposure times of the detectors.

153 3.2. Exotica searches Different exotic particles have been suggested to be part of the cosmic radiation flux such as Magnetic Monopoles (MMs), Nuclearites, Q-balls [12]. All of them can be constituents of the dark matter component of the Universe, and could have very wide mass ranges (from multi-TeV to 1017 GeV and beyond). A search for GUT MMs was one of the primary goals of the MACRO experiment at LNGS (3600 m w. e. depth). Different sub-detector systems were used to have redundancy for the detection of MMs with different charges and velocities; among them 1280 m2 of NTDs were used for a total acceptance of ~ 10000 m2sr. From a 10-years exposure it has been possible to establish a 90% CL flux upper limit that which at present is still the lowest for direct searches of such particles in the 4. 10"5 < p < 0.8 [13]. Also multiply charged MMs with masses from 107 to 1013 GeV could be present in the cosmic radiation and accelerated to high velocities [14]. In this case one has to look for MMs with p > 10"2. Detector on the Earth surface could detect IMMs coming from above if they have masses > 105 GeV [15]; lower mass MMs may be searched for with detectors located at high mountain altitude, balloons and satellites.

jS*wc

Figure 4. 90% CL. flux upper limits for different charged MMs obtained with the analysis of 239 m2 and average exposure time of 3.7 years with SLIM experiment

154 The SLIM experiment [16], based on 440 m2 of NTDs exposed for 4 years at the Chacaltaya (Bolivia) High Altitude Laboratory for CR (5290 m a.s.l.) is sensitive to IMMs with masses > 104 GeV for g = gD magnetic charge [17]. NTDs have been used also to search for Dirac MMs produced at high-energy accelerators [18]. Similarly interesting are other particles such as Nuclearites andQ-balls[12].

4. Other applications of NTDs We recollect here examples of measurements we performed with NTDs connected to the fields of particle physics and of the natural environmental radiation. 4.1. Fractional charged nuclei The charge resolution of NTDs is the best feature for such kind of technique. Charge resolution as low as 0.05e in the range 5 < Z < 14 has been obtained for CR39 by tracking the ions through a stack of 12 detector sheets (Fig. 5). Using 200

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these measurements a search for the production of fractionally charged nuclei was made. A limit at the level of 1/104 was established [19].

155 4.2 Fragmentation cross-section measurements Experimental results on the fragmentation properties of high-energy nuclei are relevant for nuclear physics, cosmic ray physics and astrophysics. We measured the fragmentation cross-sections in high-energy nucleus-nucleus collisions using relativistic Sil4+, S16+ and Pb82+ ion beams on different targets by means of CR39 NTDs which yielded an easy charge identification [20]. 4.3 Radon Indoor Radon concentration in Bologna and in the LNGS tunnel was monitored using CR39 based detectors [19]. With this type of technique it is possible to measure radon concentrations integrating over time periods of the order of months. After exposure the tracks left by alpha particles from radon decay are observed and counted under an optical microscope with 40x objective and lOx eyepiece, Fig .6. It has been also established a calibration constant 1 track/cm2 for 1 month exposure = 0.41 Bq/m3 ~ 0.011 pCi/1

G

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as Figure 6. Microphotograph of the alpha particle tracks in CR39 visible at a microscope after exposure in environment where Radon was present.

5. Conclusions Plastic NTDs are easy to use detectors. They offer very good charge resolution (probably the best) and have wide applicability as we have tried to illustrate above.

156 Acknowledgments R. Giacomelli, A. Kumar, G. Mandrioli, M. Errico, E. Bottazzi, L. Degli Esposti, C. Valieri all these colleagues are duly acknowledged for their collaborations References 1.

R.I. Fleischer et al., Nuclear Ttracks in Solids, Univ. California Press, Berkeley, 1975 S. A. Durrani and R. Ilic, Measurements by Etched Track Detectors: Applications in Radiation Protection, Earth Sciences and the Environment, World Scientific, Singapore, 1997 D. Nikezic and K.N. Yu, Material Sci. Eng R 46, 51 (2004)

2.

S.P. Ahlen et al., Nucl. Tracks Radiat. Meas. 19, 641 (1991)

3.

S. Cecchini et al., Nucl. Tracks Radiat. Meas. 22, 555 (1993); II Nuovo Cim. 109A, 1119 (1996); Rad. Meas. 34, 55 (2001); G. Giacomelli et al., Nucl. Instr. & Meth., A411,41 (1998)

4.

S. Cecchini et al., hep-ex/0502034

5.

M. Casse and P. Goret, Astrophys. J. 346, 997 (1978)

6.

C.J. Cesarsky and J.P. Birbing, IAU Symposium, Bologna (1981)

7.

W.R. Binns et al, Astrophys. J., 346, 997 (1989)

8.

B. Byrnak et al., 18th ICRC (Bangalore, 1983) Conf. Papers, 2, 29

9.

J. Donnelly et al., Proc. 26th ICRC (Salt Lake City, 1999) Conf. Papers, 3,113

10.

B.A Weaver and A.J. Westphal, Proc. 27th ICRC (Hamburg, 2001) Conf. Papers, 5, 1720

11.

S. Cecchini et al., 16th ESA-PAC Symp., SP-530 (S. Gallen, 2003) 529; Proc. 29th ICRC (Pune, 2005) Conf. Papers , astro-ph/0510717

12.

D. Bakari et al., hep-ex/0004019; Astrop Phys. 15, 137 (2001); M. Ambrosio et al., Eur. Phys. J. C13,452 (2000)

13.

M. Ambrosio et al., Phys. Lett. B406, 249 (1997) and Eur. Phys. J. C 25, 511 (2002)

14.

P. Bhattacherjee et al, Phys. Rept. 327, 109 (2000) and reference therein

15. J. Derkaoui et al., Astrop. Phys. 9, 173 and 339, 1998 16.

S. Cecchini et a l , 28th ICRC (Tsukuba, 2003) Conf. Papers, 3,1657

157 17.

S. Balestra et al., 29th ICRC (Pune, 2005) Conf Papers, hep-ex/0508043

18.

M. Bertani et al., Europhys. Lett. 12, 613 (1990); K. Kinoshita et al., Phys. Rev. D46,R881(1992)

19.

S. Cecchini et al., Astrop. Phys. 1, 369 (1993); H. Dekhissi et al., Nucl. Phys. A662, 207 (2000); S. Cecchini et al., Nucl. Phys. A707, 513 (2002)

20.

G. Giacomelli et al., II Nuovo Saggiatore, 4, 5 (1990); M. Beozzo et al., Nucl. Tracks Rad. Meas. 19,297 (1991); Arpesella et al., Health Phys. 72, 629 (1997)

RECENT DAMA RESULTS

R. B E R N A B E I , P. B E L L I , F . M O N T E C C H I A A N D F . N O Z Z O L I Dipartimento di Fisica, Universita and INFN, Sezione di Roma2,

di Roma "Tor 1-00133 Rome,

Vergata" Raly

F . C A P P E L L A , A. D ' A N G E L C ^ A . I N C I C C H I T T I A N D D . P R O S P E R I Dipartimento di Fisica, and INFN, Sezione

Universita di Roma "La di Roma, 1-00185 Rome,

Sapienza" Raly

R. C E R U L L I INFN

- Laboratori

Nazionali

del Gran Sasso,

1-67010 Assergi

(Aq),

Italy

C.J. DAI, H.L. H E , H.H. K U A N G , J . M . M A a n d Z.P. Y E f IHEP,

Chinese

Academy,

P.O. Box 918/3,

Beijing

100039,

China

DAMA is an observatory for rare processes and it is operative deep underground at the Gran Sasso National Laboratory of the I.N.F.N.. The experiment developes and uses low background scintillator for rare processes investigation. In this paper, after a short presentation of the main DAMA set-ups, the DAMA/Nal apparatus (~ 100 kg highly radiopure Nal(Tl)) and its main results in the Dark Matter field will be addressed. This experiment, in particular, has effectively investigated the presence of a Dark Matter particle component in the galactic halo by exploiting the modelindependent annual modulation signature over seven annual cycles (total exposure of 107731 kg x day), obtaining a 6.3 a C.L. model-independent evidence for such a presence. In addition, some corollary model-dependent quests to investigate the nature of a candidate particle will be recalled. The new additional analysis for a pseudoscalar and for a scalar bosonic candidate (whose detection only involves electrons and photons/X-rays) will be addressed as well. Some perspectives of the second generation DAMA/LIBRA set-up (~ 250 kg highly radiopure Nal(Tl)), presently in measurement deep underground, will be mentioned.

* also: Scuola di Specializzazione in Fisica Sanitaria, Universita di Roma "Tor Vergata", 1-00133 Rome, Italy talso: University of Jing Gangshan, Jiangxi, China

158

159

1. Introduction DAMA is an observatory for rare processes based on the development and use of various kinds of radiopure scintillators. Several low background setups have been realized, in particular: i) DAMA/Nal (~ 100 kg of highly radiopure Nal(Tl)), which took data underground over seven annual cycles and was put out of operation in July 2002 1.2.3.4.5.6.7; ii) DAMA/LXe (~ 6.5 kg liquid Xenon) 8 ; iii) DAMA/R&D, which is devoted to tests on prototypes and small scale experiments 9 ; iv) the new second generation DAMA/LIBRA set-up (~ 250 kg highly radiopure Nal(Tl)) in operation since March 2003. Moreover, in the framework of devoted R&D for radiopure detectors and PMTs, sample measurements are carried out by means of the low background DAMA/Ge detector, installed deep underground since more than 10 years and, in some cases, by means of Ispra facilities. By profiting of the radiopurity of these apparata, several rare processes have been investigated obtaining also some competitive results 8 ' 9 . In the following, we will address, in particular, the results obtained by the DAMA/Nal experiment exploiting the annual modulation signature to point out with a model-independent approach the presence of a particle Dark Matter component in the galactic halo. This signature (originally suggested in 10 ) is very distincive; in fact, the expected signal must simultaneously satisfy of all the following requirements: 1) the rate must contain a component modulated according to a cosine function; 2) with one year period, T; 3) phase, to, around ~ 2nd June; 4) this modulation must only be found in a well-defined low energy range, where Dark Matter particle can induce signals; 5) it must apply to those events in which just one detector in a multi-detector set-up "fires" (single-hit events), since the Dark Matter particle multi-scattering probability is negligible; 6) the modulation amplitude in the region of maximal sensitivity is expected to be <7% for usually adopted halo distributions, but it can be larger in case of some possible scenarios such as e.g. those in ref. n > 1 2 . To mimic such a signature either spurious effects or side reactions should be able not only to account for the whole observed modulation amplitude but also to contemporaneously satisfy all the requirements; none has been found (see ref. 4 ' 5 and references therein). 2. The model-independent result of D A M A / N a l The experimental residual rate of the single-hit events in the (2-6) keV energy interval is reported in Fig. 1. The data offers a 6.3 a C.L. evidence for

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h/ \ \\A -0.1

1

$l\

^ ,,,,

500

1000

1500

i , ,

2000

,

11

2500 Time (day)

Figure 1. Experimental residual rate for single-hit events in the cumulative (2-6) keV energy interval as a function of the time over 7 annual cycles (total exposure 107731 kg X day). The experimental points show the errors as vertical bars and the associated time bin width as horizontal bars. The superimposed curve represents the cosinusoidal function behaviour expected for a Dark Matter particle signal with a period equal to 1 year and phase exactly at 2nd June; the modulation amplitude has been obtained by best fit. 4 - 6

the presence of an annual modulation of the measured rate of the single-hit events in the lowest energy region. In fact, fitting the experimental points with modulated cosine-like function (A- cosu(t — to)), a modulation amplitude equal to (0.0200 ± 0.0032) cpd/kg/keV, t0 = (140 ± 22) days and T = — = (1.00 ± 0.01) year, are obtained. The period and phase agree with those expected in case of an effect induced by Dark Matter particles in the galactic halo (T = 1 year and to roughly at ~ 152.5*'1 day of the year). The hypothesis of unmodulated behaviour can be excluded being the X2/d.o.f. = 71/37 on the (2-6) keV residual rate, which corresponds to a probability of 7 • 10~ 4 . Other model-independent analyses on the data have also been pursued; they give similar results and support the annual modulation effect observed at the same statistical significance. For a complete discussion see ref. 4 ' 5 . A careful investigation of all the known possible sources of systematic and side reactions has been regularly carried out and published at time of each data release and quantitative discussions can be found in ref. 3 ' 4 ' 5 . No systematic effect or side reaction able to account for the observed modulation amplitude and to satisfy all the requirements of the signature has been found. A further relevant investigation has been performed on the multiplehits events collected during the DAMA/NaI-6 and 7 running periods (see Fig. 2), thanks to an improvement in the electronics performed before

161

500 600 Time (day) Figure 2. Experimental residual rates over seven annual cycles for single-hit events (open circles) - class of events to which Dark Matter particle events belong - and over the last two annual cycles for multiple-hits events (filled triangles) - class of events to which Dark Matter particle events do not belong - in the (2-6) keV cumulative energy interval. The initial time is taken on August 7 t h . 6

those cycles. The multiple-hits event class - on the contrary of the singlehit one - does not include events induced by Dark Matter particles since the probability that a Dark Matter particle would interact in more than one detector is negligible. The fitted modulation amplitudes are: A = (0.0195 ± 0.0031) cpd/kg/keV and A = -(3.9 ± 7.9) • 10~ 4 cpd/kg/keV for single-hit and multiple-hits residual rates, respectively. Thus, evidence of annual modulation is present in the single-hit residuals (event class induced by Dark Matter particle), while it is absent in the multiple-hits residual rate (event class due only to background events). Since the same identical hardware and the same identical software procedures have been used for the two classes of events, the obtained result offers an additional strong support for the presence of Dark Matter particles in the galactic halo further excluding any side effect either from hardware or from software procedures or from background. In conclusion, the data of the seven annual cycles support at 6.3 a C.L. the presence of an annual modulation in the residual rate of the single-hit events in the lowest energy interval (2 - 6) keV, satisfying all the features expected for a Dark Matter particle component in the galactic halo. No systematic effect or side reaction able to account for the observed effect has been found. No other experiment, whose result can be directly compared with this one in a model independent way, is available so far in the field of Dark Matter investigation.

162

3. Some corollary model-dependent quests for a candidate On the basis of the obtained 6.3 u model-independent result, corollary investigations can also be pursued on the nature of the Dark Matter particle candidate. This latter investigation is instead model-dependent and - considering the large uncertainties which exist on the astrophysical, nuclear and particle physics assumptions and on the parameters needed in the calculations - has no general meaning (as it is also the case of exclusion plots and of the Dark Matter particle parameters evaluated in indirect detection experiments). Thus, it should be handled in the most general way as we have pointed out with time passing 2>4-5>6. Some wide discussion on this topics and on the real validity of any claimed model-dependent comparison in the field can be found in ref. 4 ' 5 ' 6 and in the references therein. The corollary quests for a Dark Matter candidate particle will be discussed considering some candidates belonging to the WIMP class particles (i.e. particle that interact via scattering off to target nuclei) and considering light bosonic candidates (i.e. particles that interact via the conversion of the particle mass in electromagnetic radiation). For simplicity, the results of these analysis are presented in terms of allowed volumes/regions obtained as superposition of the configurations at given C.L.. Details on the followed procedure can be found e.g. in ref.4'5,6. 3.1. Corollary

quests for WIMP

class

candidates

For this class of Dark Matter particles, it has been considered so far low (of order of few GeV) and high mass (up to hudereds of GeV) candidates interacting with ordinary matter via: i) mixed Spin-Independent & SpinDependent coupling; ii) dominant Spin-Independent coupling; iii) dominant Spin-Dependent coupling; iv) preferred Spin-Independent inelastic scattering. A detailed discussion on the allowed volumes/regions for these candidates can be found e.g. in ref. 4 ' 5 . As an example, here we have only reported in Fig. 3-left the results obtained in the general case of particle belonging to the WIMP class with mixed Spin-Independent & Spin-Dependent. The coulored regions are example of slices (of the 4-dimensional allowed volume) in the plane £asi vs £,
163

defined in the [0,7r) interval). In Fig. 3-right, instead, obtained allowed region for the case of a particle belonging to the WIMP class with dominant Spin-Independent interaction for some model frameworks is reported 4 ' 5 . Obviously, larger sensitivities than those reported in these figures would e=o

e=itf4

e = wi

e=z.43S

Figure 3. On the left: example of slices (of the 4-dimensional allowed volume) in the plane £asi v s £
be reached when including the effect of other existing uncertainties on the astrophysical, nuclear and particle Physics assumptions and related parameters; similarly, the set of the best fit values would also be enlarged as well. It is worth to note that the DAMA allowed region of Fig. 3-right is also compatible with theoretical expectations in the particular case of a neutralino candidate in MSSM with gaugino mass unification at GUT scale released and with purely Spin-Independent coupling 13 ' 14 . The corollary study of other candidates is given in ref. 4 ' 5 . 3.2. New corollary quest for light bosonic pseudoscalar scalar candidates

and

Recently, DAMA collaboration has also pursued a corollary quest for the candidate particle considering a light (~ keV mass) bosonic candidate, either with pseudoscalar or with scalar couplings, as Dark Matter component

164

in the galactic halo 6 . For these candidates, the direct detection process is based on the total conversion in Nal(Tl) crystal of the mass of the absorbed bosonic particle into electromagnetic radiation. Thus, in these processes the target nuclei recoil is negligible and is not involved in the detection process; therefore, signals from these light bosonic Dark Matter candidates are lost in experiments based on rejection procedures of the electromagnetic contribution to the counting rate. For detailed discussion on this corollary quest for a bosonic candidate, see ref. 6 . The considered Dark Matter light bosonic candidates are non-relativistic since they should be trapped in the galactic halo and they can interact via "Compton - like" effect, "Axioelectric" (photoelectric-like) effect, or Primakoff effect.

Figure 4. Case of a pseudoscalar boson candidate. Left: region allowed at 3cr C.L. in the plane gase vs ma by the DAMA/Nal annual modulation data in the considered model framework. Right: allowed region in the plane gary vs ma (crossed hatched region). All the configurations in this region can be allowed depending on the values of all the gajjIn the plot examples of the expectation for some models have been reported. The solid line corresponds to a particle with lifetime equal to the age of the Universe; at least all the ga-,-,'s below this line are of cosmological interest. 6

In all these processes the total (including the secondary processes: Xrays and Auger electrons) energy release, Erei, in the detector (providing that its detection efficiency is ~ 1 for low-energy electrons and low-energy photons) matches the total energy of the a particle, Ea ~ ma since the a velocity is of the order of 10 _ 3 c. In the case of a pseudoscalar boson, the results can be presented in terms of gaSe, coupling constant of the particle to the electrons, and m Q , the particle mass (see Fig. 4-left). This allowed region is almost independent on

165 the adopted gaUu and gajd coupling constants since the axioelectric effect largely dominates on the other detection processes 6 . The pseudoscalar boson particle can decay into two photons but its lifetime can be also of cosmological interest as shown in Fig. A-right, where the allowed region in the plane gayi vs ma is reported. z10 \~z.

-2

-7

10

-8

10

io~ -10 10

0.5

1

2

3

4 5 6 78910

m h (keV) Figure 5. Region allowed at 3a C.L. in the plane ghfjN vs m/, for a scalar light boson Dark Matter candidate coupled only to the hadronic matter in the given frameworks 6 . Several configurations correspond to particles of cosmological interest (see ref. 6 ) .

Furthermore, in the case of a scalar boson, only a coupling to hadronic matter is considered in order to account for the DAMA observations 6 and the result is an allowed multi-dimensional volume in the space denned by m/j (scalar boson mass) and by all the ghjf coupling constants to quarks. For semplicity here we report only a slice obtained considering the ghNN coupling constants to nucleons (see Fig. 5). This region does not allow a direct information about the effective coupling constant to photons, gh-n, and, therefore, about h lifetime; however, the large number of free coupling constants allows the existence of a large number of h configurations of cosmological interest that can be calculated in some model frameworks 6 . 4. The new D A M A / L I B R A set-up and beyond In 1996 DAMA proposed to realize a ton set-up 15 and a new R&D project for highly radiopure Nal(Tl) detectors was funded at that time and carried out for several years in order to realize as an intermediate step the second generation DAMA/LIBRA experiment (successor of DAMA/Nal) with an exposed mass of about 250 kg. Thus, as a consequence of the results of

166 this second generation R&D, the new experimental set-up DAM A/LIBRA (Large sodium Iodide Bulk for RAre processes), ~250 kg highly radiopure Nal(Tl) crystal scintillators (matrix of twenty-five ~ 9.70 kg Nal(Tl) crystals), was funded at end 1999 and realised. In fact, after the completion of the DAMA/Nal data taking in July 2002, the dismounting of DAMA/Nal occurred and the installation of DAMA/LIBRA started. The experimental site as well as many components of the installation itself have also been implemented (environment, shield of the photomultipliers, wiring, HighPurity Nitrogen system, cooling water of air conditioner, electronics and data acquisition system, etc.). DAMA/LIBRA is taking data since March 2003 and the first data release will, most probably, occur when an exposure larger than that of DAMA/Nal will have been collected and analysed in all the aspects. The highly radiopure DAMA/LIBRA set-up is a powerful tool for further investigation on the Dark Matter particle component in the galactic halo 5 having many intrinsic merits and a larger exposed mass, an higher overall radiopurity and improved performances and increased sensitivity with the respect to DAMA/Nal. At present a third generation R&D effort towards a possible Nal(Tl) ton set-up has been funded and related works have already been started. 5. Conclusion DAMA/Nal has pointed out at 6.3 a C.L. the presence of a Dark Matter particle in the galactic halo by investigating the model independent annual modulation signature in seven annual cycles. No systematic effect or side reaction able to account for the observed effect has been found. Corollary quests for the investigation on the nature of the Dark Matter candidates have also been pursued considering the present poor knowledge on the many astrophysical, nuclear and particle physics related aspects. Here, the new corollary quest for light bosonic candidate has shortly been summarized. At present after a devoted R&D effort, the second generation DAMA/LIBRA (a ~250 kg more radiopure Nal(Tl) set-up) has been realised and put in operation since March 2003 and a third generation R&D toward a possible ton Nal(Tl) set-up, we proposed in 1996 15 , is also in progress. References 1. R. Bernabei et al., Nuovo Cimento A 112, (1999) 545; R. Bernabei et al., Phys. Lett. B 389, (1996) 757; R. Bernabei et al., Nuovo Cimento A 112, (1999) 1541.

167 2. R. Bernabei et al., Phys. Lett. B 424, (1998) 195; R. Bernabei et al., Phys. Lett. B 450, (1999) 448; P. Belli et al., Phys. Rev. D 61, (2000) 023512; R. Bernabei et al., Phys. Lett. B 480, (2000) 23; R. Bernabei et a l , Phys. Lett. B 509, (2001) 197; R. Bernabei el al., Eur. Phys. J. C 23, (2002) 61; P. Belli et al., Phys. Rev. D 66 , (2002) 043503. 3. R. Bernabei et a l , Eur. Phys. J. C 18, (2000) 283; 4. R. Bernabei et a l , La Rivista del Nuovo Cimento 26, (2003) 1-73. 5. R. Bernabei et a l , Int. J. Mod. Phys. D 13, (2004) 2127. 6. R. Bernabei et al., ROM2F/2005/19, to appear on Int. J. Mod. Phys. A. 7. R. Bernabei et al., Phys. Lett. B 408, (1997) 439; P. Belli et al., Phys. Lett. B 460, (1999) 236; R. Bernabei et a l , Phys. Rev. Lett. 83, (1999) 4918; P. Belli et al., Phys. Rev. C 60, (1999) 065501; R. Bernabei et a l , Phys. Lett. B 515, (2001) 6; F. Cappella et al., Eur. Phys. J. direct C 14, (2002) 1; R. Bernabei et al., Eur. Phys. J. A 23, (2005) 7; R. Bernabei et al., Eur. Phys. J. A 24, (2005) 51. 8. R. Bernabei et al., Nuovo Cimento A 103, (1990) 767; Nuovo Cimento C 19, (1996) 537; Astrop. Phys. 5, (1996) 217; Phys. Lett. B 387, (1996) 222 and Phys. Lett. B 389, (1996) 783 (err.); Phys. Lett. B 436, (1998) 379; Phys. Lett. B 465, (1999) 315; Phys. Lett. B 493, (2000) 12; New Journal of Physics 2, (2000) 15.1; Phys. Rev. D 61, (2000) 117301; Eur. Phys. J. direct C 11, (2001) 1; Nucl. Instrum. Methods A 482, (2002) 728; Phys. Lett. B 527, (2002) 182; Phys. Lett. B 546, (2002) 23; in the volume Beyond the Desert 03 (Springer, 2004) 541. R. Bernabei et al., in publication on the Proceed, of the NPA Int. Conf., Debrecen (Hungary), May 2005 9. R. Bernabei et a l , Astrop. Phys. 7, (1997) 73; R. Bernabei et al., Nuovo Cimento A 110, (1997) 189; P. Belli et a l , Astrop. Phys. 10, (1999) 115; P. Belli et a l , Nucl. Phys. B 563, (1999) 97; R. Bernabei et al., Nucl. Phys. A 705, (2002) 29; P. Belli et al., Nucl. Instrum. Methods A 498, (2003) 352; R. Cerulli et al., Nucl. Instrum. Methods A 525, (2004) 535. R. Bernabei et al., ROM2F/2005/18 in publication on Nuclear Inst, and Meth. A 10. K.A. Drukier et al., Phys. Rev. D 33, (1986) 3495; K. Freese et al., Phys. Rev. D 37, (1988) 3388. 11. D. Smith and N. Weiner, Phys. Rev. D 64, (2001) 043502. 12. K. Freese et al., Phys. Rev. Lett. 92, (2004) 11301. 13. A. Bottino et al., Phys. Rev. D 67, (2003) 063519; A. Bottino et al, hepph/0304080; D. Hooper and T. Plehn, MADPH-02-1308, CERN-TH/200229, hep-ph/0212226; G. Belanger, F. Boudjema, A. Pukhov and S. RosierLees, hep-ph/0212227. 14. A. Bottino et al., Phys. Rev. D 69, (2004) 037302. 15. R. Bernabei et a l , Astrop. Phys. 4, (1995) 45; R. Bernabei, "Competitiveness of a very low radioactive ton scintillator for particle Dark Matter search", in the volume The identification of Dark Matter (World Sc. pub.,1997) 574.

D O U B L E BETA D E C A Y M E A S U R E M E N T W I T H C O B R A

J. V. DAWSON ON BEHALF OF THE COBRA COLLABORATION* Department of Physics and Astronomy, University of Sussex, Brighton. BN1 9QH UK E-mail: [email protected]

The COBRA double beta decay experiment is described. Preliminary new limits from a total exposure of 3.82 kg days are presented.

1. Introduction The COBRA concept is to use a large array of Cadmium Zinc Telluride (CZT) semiconductors, to search for neutrinoless double beta decays 1 in which:

(Z,A)^(Z + 2,A) + 2e~

(Ov00 - decay). 3

(1)

Currently, the COBRA arrays are formed using 1cm CZT crystals, each with an active mass of 6.53g. The cube size ensures that for any double beta decay event, the emitted electrons are unlikely to escape the crystal. The COBRA design is modular, allowing easy upgrade and offering the ability to discriminate background events by rejecting events where more than one crystal is triggered. With this ability, a large proportion of high energy gamma ray events can be rejected, as Compton scattering in more than one crystal is likely. Additionally, events that produce a gamma ray cascade such as thermal neutron capture can be noted. Semiconductors, in general, offer good energy resolution and are produced in a clean environment. This is vital for a double beta decay experiment like COBRA. CZT semiconductors operate at room temperature, allowing an experiment which does not suffer the complication of cooling. For an experiment housed underground in a remote laboratory this characteristic is extremely valuable. * http://cobra.physik.uni-dortmund.de

168

169 There is much industrial and scientific interest in the development of CZT. Pixellated devices are used for hard X-ray/7-ray astronomy, medical imaging applications and standard devices sold as general gamma ray detectors. Pixellated devices are of interest for future COBRA detectors, potentially these devices could be used for tracking, with the detector operating as a solid-state TPC. The best energy resolution for double beta events can be achieved if the double beta isotopes are contained within the detector. This is easily obtained with CZT without any doping or isotopic enrichment.

2. Double Beta Isotopes For a double beta decay experiment, CZT offers great potential. Of the 35 known isotopes able to undergo double beta decay, CZT contains 5 of them. Table 1 shows the percentage natural abundance in the crystals, the Q value of the decay and the decay mode. Four of the isotopes can also undergo double positron decay, hence emitting positrons instead of electrons ((3+{3+). Although, due to energy constraints only 106 Cd can emit two positrons, whilst the other isotopes decay via single electron capture of a K-shell electron (/3 + /EC) and double electron capture {EC/EC) modes. In addition, the atoms can be left in excited states after the decay, subsequently emitting gamma rays to return the atom to the ground state. Whilst these events are less likely, the signals produced in the COBRA array are unique. Both electrons would be contained within one cystal, depositing a characteristic energy, and the emitted gamma ray(s) could be detected in a different crystal(s), again with a characteristic energy deposition. The likelihood of a background event mimicking these signature events is small, but further investigation is required to determine how unlikely this is. Decays with high Q-values are desirable in a double beta decay experiment, as the decay rate increases with increasing energy. Also, the background rate generally decreases with increasing energy and the line resolution improves. Three of the isotopes have Q-values greater than 2.5 MeV. One of these isotopes, 116 Cd, has a Q-value greater than the highest energy gamma ray emitted in the decay of natural thorium (2.614 MeV), and so this signal should be unaffected by the natural gamma ray background. 130 Te has the lowest Q-value of these 3 isotopes at 2.529 MeV but the highest natural abundance at 33.8%.

170 Table 1. Isotope

Double Beta Isotopes present in CZT

Nat. Abundance (%)

Q (keV)

0.62

1001

28.7

534

Decay Mode

Cd

7.5

2805

128Te

31.7

868

130Te

33.8

2529

0-00-00-00-00-0-

64

48.6

1096

0+/EC

™Zn u4

Cd

116

Zn

106

Cd

1.21

2771

108

Cd

0.9

231

i20Te

0.1

1722

0+0+ EC/EC 13+/EC

3. Technology The semiconductors are cut out of large boules. The material shows defects. Whilst the material quality has improved with better production techniques, further improvement is likely. The electron mobility is superior to that of the holes, such that the holes are trapped before reaching the cathode, so the signal is formed from the electrons only using the CoPlanar Grid technique (CPG) 2 . The CPG structure forms a virtual Frisch grid below the anode, and the resulting signal is produced by electrons travelling past this virtual grid to the anode. In this way there is almost no position dependence on the signal size. 4. Backgrounds The background events that could be detrimental to the COBRA experiment could be caused by: • High Energy Gamma Rays - of less concern with high Q-valued isotopes, and vetoed by multiple crystal triggers. • Muons - rate lowered by rock shielding and muon scintillator vetoes. • Alpha and Beta Particles - only events close to the crystal surfaces will be able to penetrate and be detected. • Cosmogenics - the time crystals spend on the Earth's surface should be minimised. • Neutrons - MCNP and GEANT 4 simulations resulted in shielding designed to reduce the ambient thermal neutron flux by 8 orders of magnitude. • 2v{3f3 - this is the irreducible background. To reduce contamination

171 of the neutrinoless peak region by the 2i//3/3 continuum, a good resolution is required. The contamination fraction is 3 F = —-^— me

(2)

5. Experimental Status Currently 4 CZT cubes are installed in the laboratory at Gran Sasso, with a cumulative exposure of 3.82 kg days. Preliminary limits for some of the double beta decay modes possible are shown in Table 2. These and limits on 0^/? + /3 + decay modes will be published soon. Previously published results can be found in 4 . Table 2. Isotope

90% C.L.s on Oi//3_/3~ half-lives derived from the fits.

Daughter State

TTC

Cd Cd 116 Cd 116 Cd 116 Cd 116

i3oTe

G.S. 1294 1757 2027 2112 G.S.

Energy (keV) 2805 1511 1048

778 693

Te

536

2529 1993

l28Te

G.S. G.S.

1001

130

70

Zn

868

Events Fitted

T ! / 2 limit (10 1 9 years)

14.79 ± 17.61 75.87 ± 27.7 -7.68 ± 34.04 -46.69 ± 43.8 -12.14 ± 57.98 10.90 ± 17.25 11.82 ± 20.94 31.67 ± 41.55 -37.13 ± 38.15

1.32 0.478 0.977 2.07 0.725 6.34 5.18 3.48 0.0253

6. T h e 64 Array The next stage of COBRA development is the 64 crystal array, shown in Figure 1. This will bring the COBRA mass to 418g. The 64-array will be the first COBRA detector able to search for the characteristic signatures of double beta decays to excited states. Greater background reduction is also anticipated with the ability to reject multiple site events. At present the crystals are being tested, and installation into Gran Sasso should begin in early 2006. 7. Conclusion Whilst the mass of the COBRA experiment is low, the project is in a research and development phase, competitive limits for some neutrinoless

172

Figure 1.

Artist's impression of the COBRA 64 crystal array, courtesy of D. Dobos.

double beta decay modes have been made. The 64 crystal array is being constructed, and is expected to improve the sensitivity to neutrinoless double beta decays by at least an order of magnitude. The 64-array is intended to test the COBRA principles, paving the way towards a large scale experiment with a total mass of 500 kg. In addition to increasing the mass of the experiment, isotopic enrichment of 116 Cd is possible. GEANT 4 simulations are already underway to explore the potential characteristics of a large (40 x 40 x 40) array. References 1. 2. 3. 4.

K. Zuber, Physics Letters B 519, 1 (2001). P. Luke, IEEE Trans. Nucl. Sci. 42, 4 (1995). S. Elliot, P. Vogel, Ann. Rev. Nucl. Part. Sci. , (2002). H. Kiel et al, Nuclear Physics A 723, 499 (2003).

ULTRAVIOLET DIAMOND PHOTODETECTOR

A.DE SIO Dep. of Astronomy and Space Science, University ofFirenze, Largo Enrico Fermi, Firenze 50125 Italy F. MONTI, F. MALVEZZI, E. BURATTINI Dep.of Information Science, University of Verona, Strada le Grazie 15, 37134 Italy A. MARCELLI National Laboratories ofFrascati, National Institute of Nuclear Physics, 00044Frascati, Roma, Italy E. PACE Dep. of Astronomy and Space Science, University ofFirenze, Largo Enrico Fermi, Firenze 50125 Italy Diamond is a very promising material for UV and X-ray sensing applications. Due to its wide bandgap (of 5.5 eV) diamond present low thermal noise, low dark current levels, high visible-XUV signal discrimination and high sensitivity to radiation with energy greater than the bang gap one. Furthermore diamond radiation hardness is very appealing for applications in extreme environments or as monitor of high fluency radiation beams. In order to analyse the performances of diamond based photoconductors under synchrotron UV light a series of measure at the DA
1. Introduction Diamond physical properties offers

unique advantages for

electronic

devices realization. In fact, due to wide bandgap, electronic and physical properties,

and

radiation hardness

diamond

based

devices

should

have

intrinsically high performances and long term stability even if used in extreme and harsh environments as space, waste solution or under high fluencies radiation.

Moreover

diamond

based

detectors

show

high

sensitivity

at

wavelength with energy over band gap (225nm) and, at lower energies signal rejection that can be up to 9 order of magnitude (solar or visible blindness). This

173

174

last characteristic is very appealing in all applications where the suppression of visible signal is needed. Thus diamond is one of the most appealing material for UV and soft X-ray detection [1]. In the last years the growth of single crystal samples was become possible with the improvement of the High Pressure High Temperature (HPHT) and omoepitaxial CVD growth technique. The usage of such kinds of diamond samples has improved the detector sensitivity and reliability [2]. In this work the use of single crystal diamond based photodetectors in synchrotron radiation monitoring is discussed. For such purpose some preliminary measurements session at the UV DAONE-Light synchrotron beam line have been carried on to verify single crystal diamond sensors detectivity, reliability and linearity versus the impinging radiation intensity.

2. Experimental The DAONE-Light DXR-2 beam line can provide radiation in the 800nm 2nm spectral range (corresponding to 1.55 eV to 1 keV). These limits are respectively imposed by the small acceptance angle and to maximum energy reflected at 2.2° by the gold coated mirror. Actually the spectral interval is further on limited, at higher energies, by the sapphire window at the end of the vacuum channel and by the many in air reflections before monochromator. The monochromator is a Jobin-Yvon (mod. HR460MST2-2XM) in Zernic Turner configuration f/5.3 with a focal length of 460mm.The equipped 2400 line/mm holographic grating, optimised for 250nm, allow to work in the 190-600nm spectral region with a resolution of 0.1-0.3%. The radiation intensity at each wavelength has been calibrated with a silicon photodiode. The current signal was amplified and converted into a voltage signal by a Keithley 427 Current Amplifier (with a maximum gain of 109 V/A) and then recorded by a digital multimeter. Different spectra of the beamline light source are reported in Fig 1. The two spectra in square dots are related to two different, non consecutive, measurements performed in order to verify the stability of the photons beam and the reliability of our apparatus. The measurement, in circle dots, is related to the same spectral range while the glass long wave pass filter, that we developed to suppress the second order of the monchormator in the 380nm 500nm spectral range, is applied. II the picture insert the measurements of the transmittance of the filter is reported. The diamond device was based on a commercially available High Pressure High Temperature (HPHT) lb diamond crystal, produced by Sumitomo. It was cut and mechanically polished to {100} within 4°, with final 4 x 4 mm2 size and thickness of 350 um. The sample appears yellowish, due to the 100 ppm nitrogen concentration, but it is optically transparent. A couple of 2 x 2 mm2

175 interdigitated coplanar gold contacts were deposited on one of its surfaces. Contact pitch and width are 20 um. The diamond sensor current signal was measured with a Keithley electrometer 6517A making use, for the device bias, of its intrinsic voltage source up to 1000 Volts.

150

10

-

:__.

150

200

250

300

OOOUX^ I

,

200

350

4Q0

450

500

550

,

l

600

Wavelength (nm) I

250

i

I

i

300

I

350

i

I

400

,

I

450

i

l

500

" ,

550

:

600

Wavelength (nm)

Fig 1: Spectra of the synchrotron light source (in squares). In circle dots is shown the same spectra applying a glass philter which transmittance is reported in the insert.

3. Results and discussion Using a simple photo-generation and collection model for photons fully absorbed in the material [3], the measured current, that is the sum of the dark current and the photoconductive signal (Iph, defined as the difference between the current measure under illumination and in the dark), is given by: Ime S ured=Idark+Iph= Idark +
(1)

where q is the electric charge, F0 is the number of incident photons per second, 0 < 77 < 1 is the quantum efficiency, //ris the mobility-lifetime product, E is the applied electric field and d is the electrode spacing. Defining the photoconductive gain G as the ratio between the drift collection distance L = JUTE and d, it is possible to define the External Quantum Efficiency (EQE) as the ratio EQE = I p h /qF 0 = nL/d = TiG

(2)

176

Therefore EQE can have values higher than unity if L > d. This can be achieved by either using very small electrode gaps or if the UT value is very high as is expected in high quality material. The EQE spectra are estimated by measuring the photocurrent at each wavelength.

150

200

250

300

350

400

450

500

550

600

Wavelength (nm)

Fig2: Sensitivity of the diamond device.

The response of the photodetector to UV continuous monochromatic illumination was then measured in the 200nm 500nm spectral range. The results of these measurements are reported in Fig 2. The sensor EQE is far from the unity but even in this case the detectivity at wavelength above the band gap is good. The rejection to wavelength greater than the band gap is three order of magnitude in 50 nm and is actually limited by the sensitivity of the experimentl apparatus. Therefore the linearity of the detector response as a function of the impinging flux has been measured at 200nm (adopting the second order of the monochromator at 400nm) and 21 Onm. The results obtained are shown in Fig. 3. Both response are linear in the whole flux interval. From the linear fit parameters, combined with the expression (1), is possible to calculate, in an independent way, the EQE at each wavelength. The comparison between the measured EQE values and the calculated one is reported in Tab 1. Table 1. Comparison between the EQE directly measured (EQEm) and calculated from the linear fit (EQEc). The fourth column shows the difference between the two data. EQEc

Diff.

200

EQEm 0.0129

0.0135

4.5%

210

0.00863

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6%

177

-40

< Q.

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-60

8.0x10'° 1.0x10" 1.2x10" 1.4x10" 1.6x10" 1.8x10" 2.0x10" 2.2x10" Photon Flux (Ph/s)

Fig3: Linearity of the photoresponse as function of the impinging flux.

4. Conclusion A photoconductive device based on HPHT single crystal diamond has been tested under UV synchrotron light irradiation. Good detectivity, reproducibility and visible solar blindness have been observed. Linear response versus the impinging flux has been recorded at two different wavelengths (200nm and 210nm). Al measurements are corroborating synthetic diamond as one of the most promising material for UV synchrotron light monitoring.

References 1. E. Pace, A. Cidronali, S. Libertino, M. Uslenghi, S. Coffa, M. Focardi, G. Collodi, M. Fiorini, A. Pini, A. De Sio,, SPIE Proc. 4498, 121-130 (2001). 2. A.De Sio, A De Sio, J Achard, A. Tallaire , R S Sussmann, A T Collins, F Silva, and E Pace, App. Phys. Lett. 86, 213504 (2005). 3. S.M.Sze, Physics of semiconductor Devices, 2nd Ed. Wiley, New York, Chapt. 5,7,13 (1981)

STATUS OF THE KATRIN EXPERIMENT OTOKAR DRAGOUN* for the KATRIN Collaboration Nuclear Physics Institute, Academy of Sciences of the Czech CZ 250 68, Rez near Prague, Czech Republic

Republic,

The KATRIN experiment will search for the neutrino mass in a model independent way by the precise measurement of the tritium P-spectrum. A windowless source of gaseous tritium with 95% isotopic purity and column density of 5 1 0 " T2 molecules/cm2 will operate at temperature of 27 K stabilized to 0.1 % by a boiling liquid neon cryostat. The P-particles will be adiabatically guided from the source through two electrostatic retardation spectrometers up to the position-sensitive semiconductor detector by means of about 30 superconducting magnets producing fields of 3-6 T. The main spectrometer (diameter of 10 m, length of 23.3 m, vacuum better than 1 0 " mbar) will operate with resolution of 0.9 eV at 18.6 keV. The retarding high voltage will be monitored to ppm level by precise voltage measurement and independently by a third retardation spectrometer with radioactive sources of monoenergetic electrons. The pre-spectrometer will reduce incoming flux of P-particles from lO'V1 to 1 0 V in the main spectrometer. KATRTN sensitivity on the neutrino mass should reach 0.2 eV after 1000 days measurement, which is an order of magnitude improvement in comparison with the best present limits.

1. Introduction According to the present knowledge the neutrinos oscillate and thus are massive. However, the absolute scale of the neutrino mass eigenstates is still unknown. The most sensitive model-independent method for the neutrino mass determination is a precise measurement of p-spectrum in the endpoint region. The most stringent direct upper limits on m(ve) were achieved using a new type of an electron spectrometer, so called MAC-E-Filter (Magnetic Adiabatic Collimation combined with an Electrostatic Filter). Such instruments were independently developed and extensively utilized in Mainz (Germany) and Troitsk (Russia). These two spectrometers exceeded their predecessors in luminosity of tens percent of 4rc even at the resolution of several eV in measurements of the endpoint region of the tritium p-spectrum at 18.6 keV. The Mainz group reported recently their final result m(ve) < 2.3 eV at 95 % This work was supported by the Grant Agency of the Czech Republic under contract No. 202/03/0889 and by the IRP AV0Z1048050

178

179 confidence level [1]. The Troitsk group, after applying a phenomenological correction for yet unexplained small anomaly in their tritium P-spectrum, reported the upper limit /n(ve) < 2.05 eV [2]. 2. The requirements for a new direct neutrino-mass experiment As for the radioactive source, the tritium P-decay, 3H —>3He+ + e~ + ve, is the best possible choice: 1. The low endpoint energy, £"0 = 18.6 keV, maximizes the fraction of P-decays in the v-mass sensitive region close to E0. This fraction is proportional to (1/ E0)3 and tritium has the second lowest endpoint energy of all Punstable isotopes. 2. The tritium decay is superallowed ('/2+ —*V-i transition) and therefore the shape of its p-spectrum is independent of the nuclear structure. 3. In present experiments, it is not possible to distinguish P-decay branches leading to various excited states of the daughter 3He ion. Thus the recorded P-spectrum is the sum of partial spectra with different endpoint energies and different relative intensities. For simple molecules like T2 and TH, the excitation energies and the population probabilities can be reliably calculated [3] although these calculations could not yet be verified experimentally. The first electronic excited state of (T3He)+ is at 27 eV. This is advantageous for investigation of narrow region just under the p-spectrum endpoint. Nevertheless, there remain the rotational-vibrational excitations of the (T3He)+ ground state caused by the recoil of the 3He nucleus. These excitations with average energy of 1.6 eV and widths of 0.4 eV have to be included into the P-spectrum analysis. Of course, a dreamed source of atomic tritium would be free of such low-energy excitations. 4. Tritium can be investigated in gaseous form and the energy losses of P-particles within the gaseous source can be determined more precisely than in the solid sources. In fact, the last 12 eV region of P-spectrum is free of electrons scattered inelastically within the gaseous source due to scattering threshold for T2 molecules. 5. The tritium half-life of 12.3 years is still acceptable from the point of view of the source specific activity. The electron spectrometer of the MAC-E-Filter type is obviously the most suitable instrument for precise measurement of the endpoint region of tritium Pspectrum thanking to its excellent combination of large luminosity even at high resolution. Since the p-ray intensity falls drastically with increasing energy, the integral character of the measurement does not represent any difficulty.

180

3. The next generation v-mass experiment with sub-eV sensitivity In June 2001, researchers from Germany, Russia, USA and Czech Republic founded the KATRIN Collaboration (Karlsruhe Tritium Neutrino Experiment, [4]). Its aim is to reach the m(ve) sensitivity of 0.2 eV which is one order of magnitude better than that of the best current experiments. The experiment will be located at the Forschungszentrum Karlsruhe (FZK) since there is the only scientific laboratory equipped with a closed tritium cycle and licensed to handle the required amount of tritium. Detailed description of the components and measurement strategy can be found in the KATRIN Design Report 2004 [5]. The KATRIN set-up will consist of three main functional units: (i) the windowless gaseous tritium source with the differential pumping and cryopumping sections, (ii) system of two electrostatic retardation spectrometers, (iii) the position-sensitive electron detector system. 3.1. The windowless gaseous tritium source (WGTS) The molecular WGTS will be a standard source for long-term measurements of P-spectra since it offers the highest possible luminosity and small systematic uncertainties. Such a source represents a technological challenge since the traces of tritium decaying within the spectrometer would increase the detector background. The WGTS activity should be monitored with the highest achievable precision due to single channel mode of the P-spectram taking. A stainless-steel source tube of 10 m length and 90 mm inner diameter will be continuously filled in the middle with molecular tritium of isotopic purity >95% at pressure of 3.4T0"3 mbar. The tritium injection rate will be 1.7-1011 Bq/s and the transport time of T2 molecules through the WGTS will be of the order of 1 s. The column density will be fixed to 5T017 T2 molecules/cm2. The source tube will be cooled to 27 K to decrease the Doppler broadening. The temperature stability of ±30 mK will be achieved by means of boiling liquid neon. Longitudinal magnetic field of Bs= 3.6 T will adiabatically guide the Pparticles to both ends of the tube. A modular differential pumping system and cryo-pumping system will be installed to keep the flow of T2 and TH molecules into the pre-spectrometer smaller than 10"14 mbar 1/s. 3.2. Electrostatic spectrometers KATRIN will utilize three spectrometers of the MAC-E-Filter type: the prespectrometer and the main spectrometer in the beam line of P-particles to analyze their energy and a monitor spectrometer to check the stability of the

181

retarding high voltage by measuring monoenergetic electrons from a suitable radioactive source.

The pre-spectrometer (Fig.l) will work at a fixed retarding energy of about 300 eV below the (3-spectrum endpoint thus reducing the incoming flux of Pparticles from 1010 s"1 to 104 s"1 in the main spectrometer. In order to analyze |3particles with resolution of 0.93 eV, the main spectrometer must have diameter of 10 m and length of 23 m. The unprecedented instrument size together with the required pressure of <10' n mbar in the volume of 1400 m3 represents a challenge since XHV vessels of this size have never been built before. The pre-spectrometer as well as the main spectrometer will be pumped with a combination of turbomolecular pumps (TMP) and non-evaporable getter (NEG) pumps. In order to reach working pressure of 10"u mbar pumping speed of about 5T0 5 1 s"1 is necessary requiring about 3 to 6 km of NEG strips. The effective pumping speed of the TMP has to be of the order of 10000 Is"1. There will be a heating/cooling system that will allow reaching temperature of 350°C needed to activate the NEG pumps.

182 In the previous MAC-E-Filters, the vacuum vessels were grounded and the retarding high voltage was supplied to a system of cylindrical electrodes inside the spectrometer. The KATRTN will utilize a novel approach in which the accurate HV will be put on the wire system of inner electrodes while the vacuum tank put on less precise HV will serve as a guard electrode. The electrodes will be put on slightly more negative potential than the tank itself in order to electrically screen low energy electrons released from the spectrometer wall by cosmic muons and the ambient or intrinsic radioactivity. The inner electrodes should have very small geometrical coverage to prevent the electrodes themselves to become a significant source of secondary electrons. The Mainz MAC-E-filter verified recently the idea of background suppression by means of "massless" inner wire electrodes. Together with the cylindrical-mirror analyzer at Rez [6], it is currently utilized to develop radioactive calibration and monitoring sources. In future, it will serve as the KATRTN high voltage monitor independent of direct voltage measurements. 3.3. Position-sensitive electron detector The KATRTN focal plane detector will be position sensitive with about 400 pixels. Using monoenergetic electrons it will allow mapping the inhomogeneities of the electric retarding potential in the analyzing plane and performing necessary off-line correction of measured p-spectra. The detector requirements are summarized in Tab. 1. The intrinsic detector background < 1 mHz will be achieved by low level of radioactivity in construction materials (especially ceramics), a passive shielding by pure copper and low-activity lead and by active veto against contribution from cosmic rays. Table 1. Requirements for the KATRIN position-sensitive electron detector Property Performance goal Electron energies E 5-100 keV Detection threshold 5 keV Sensitive area 78 cm2 Insensitive areas <5% Detection efficiency e > 0.9 < 600 eV Energy resolution A£DCI at 18.6 keV Position resolution (5x, 5y) 0.3 mm < 5x, 5y < 3.5 mm Time resolution At < 0.5 us Detector thickness d = 200 -300 urn Dead layer entrance window X < 50 nm (Si) Dark currents < 2 nA/cm2 Operation in magnetic fields up to 6 T Operating temperature 100-250 K Use of low-level radioactive tested materials Use of outgassing approved materials

183

A post-accelerating the P-particles up to 25 keV after they have passed through the main spectrometer is considered since it would rise the signal above much of the low-energy backgrounds thus improving the signal to noise ratio by a factor of 5-7. A "dart board" geometry with segmentations along the radial direction and in polar direction is the preferred pixel configuration. Segmented PIN-diodes in a monolithic array having uniform entrance window with minimum insensitive area seem to meet the requirements. 4. Monitoring and calibration of the KATRIN energy scale Precise knowledge of the actual retarded energy EKt for each recorded event is one of crucial conditions for a reliable single-channel measurement of the tritium p-spectrum. A numerical study on the energy scale imperfections [7] showed e.g. that an unrecognized shift of EKi by 0.05 eV between partial Pspectra taken at different times would result in the systematic error of the fitted neutrino mass as large as 0.04 eV. Two methods will be utilized to monitor Elet: 1. The direct measurement of the retarding voltage up to 35 kV using a highprecision voltage divider. With support of the PTB in Braunschweig [8] we aim to develop a HV divider with long-term stability and precision on 1 ppm level. 2. The measurement of energetically well-defined and sharp conversion electron and photoelectron lines of energy not far from the tritium endpoint energy by means of the monitoring spectrometer connected to the same retarding high-voltage as the main spectrometer. The first suitable candidate is the K-conversion line of the 32 keV transition in 83 Kr (energy of 17.8 keV, natural width of 2.7 eV). The 83mKr can be applied in gaseous form for an energy calibration as well as for investigation of potential distribution within the WGTS. Recently, we have determined the binding energy of the K-shell electrons in gaseous krypton to be 14327.26(4) eV and the nuclear transition energy in 83Kr to be 32151.7(5) eV. The sub-monolayer calibration sources of quenched-condensed 83mKr films were employed successfully in the Mainz neutrino experiment [1]. We try to improve their reproducibility by annealing a graphite substrate with laser radiation and applying pre-plated films of neon at temperatures below 10 K. Film thicknesses are measured by laser ellipsometry. Since the 83mRr half-life is only of 1.8 h, we try to prepare for monitoring purposes vacuum evaporated sources of 83Rb/83mKr with more suitable half-life of 86 d. We are also developing an 241Am/Co calibration and monitoring source where the photoelectrons ejected by 26 keV y-rays from the K-shell of metallic cobalt have kinetic energy very close (~ 60 eV) and above the tritium end-point.

184 The natural width of the atomic K-shell in cobalt amounts to favorable 1.3 eV. A feasibility tests were recently completed with the electron spectrometers at Rez and Mainz. 5. Expected uncertainties and sensitivity of the KATRIN experiment A great attention is paid to investigation of systematic uncertainties [5].The physical quantity examined is m\ve). No systematic effect contributing more than Amv2 = 0.0075 eV2 was specified. A quadratic adding of the systematic uncertainties quantified so far yields an estimate of osysttot ~ 0.01 eV2. Since a complete analysis of all systematic effects cannot be performed in this early stage of the experiment we anticipate osysttot < 0.017 eV2 for the m\ve). This corresponds to 5 individual systematic uncertainties of maximal value of Amv2 = 0.0075 eV2 added quadratically. For the analysis interval (Eo-30eV, is0+5eV), background rate of 10 mHz and the net exposure time of 1000 days, the statistical uncertainty amounts to o5tat = 0.018 eV2. Thus otot =0.025 eV2 and m(ve) <0.2 eV at 90 % C.L. with no finite neutrino mass being observed. The true neutrino mass of 0.35 eV (0.30 eV) should be seen as the 5o (3a) effect. The manufacture of the largest components, the WGTS and the main spectrometer vessel started in 2004, the construction of new buildings at the site of the Forschungszentrum Karlsruhe began recently. With more than 100 collaborators from 14 institutions the KATRIN Collaboration endeavors to start tritium measurements in 2008. Aimed m(ve) sensitivity of 0.2 eV should be reached within 5 calendar years. References 1.

2. 3. 4. 5. 6. 7. 8.

Ch. Kraus, B. Bornschein, L. Bornschein, J. Bonn, B. Flatt, A. Kovalik, B. Ostrick, E. E. Often, J. P. Shall, Th. Thummler and Ch. Weinheimer, Eur. Phys. J. C40, 447 (2005) V. M. Lobashev, Proc. 17. Int. Conf. Nuclear Physics in Astrophysics, Debrecen/Hungary, 2002, Nucl. Phys. A719,153c (2003) A. Saenz, S. Jonsell and P. Froelich, Phys. Rev. Lett. 84, 242 (2000) http: //www-ik. fzk.de/katrin/ http://www-ik.fzk.de/katrin/publications/index.html O. Dragoun, M. Rysavy, N. Dragounova, V. Brabec and M. Fiser, Nucl. Instr. Meth. A365, 385 (1995) J. Kaspar, M. Rysavy, A. Spalek and O. Dragoun, Nucl. Instr. Meth. A527, 423 (2004) R. Marx, IEEE Trans. Instr.Meas. 50, 426 (2001)

THE RPC SYSTEM FOR THE OPERA SPECTROMETERS

S. DUSINI* INFN Sezione di Padova, via Marzolo 8, 35131 Padova, Italy

OPERA is one of the two detectors foreseen in the CNGS (CERN Neutrino to Gran Sasso) project, devoted to the detection of i/^ <-» vT oscillations in the parameter region suggested by SuperKamiokande. The tracking system inside the iron yoke of the muon spectrometer makes use of Resistive Plate Chambers (RPC) with bakelite electrodes in a large scale application. We present here full design of the project. Four R P C planes were instrumented and the first tests were performed confirming a good behavior of the installed RPCs in terms of intrinsic noise and operating currents and efficiency. In the paper we present also some results on R P C aging studies.

1. Introduction OPERA is part of the CNGS (Cern Neutrino to Gran Sasso) project and is an experiment dedicated to the observation of the v^ <-> vT oscillation through direct observation of vT charged current interaction. The detector designa is based on a massive target of lead/nuclear emulsions sandwiches complemented by electronic detector (scintillator bars) that allow the location of the event and drive the scanning of the emulsions. The instrumented target is followed by magnetic spectrometer which measures charge and momentum of penetrating tracks. The target section and the relative spectrometer constitutes a super-module. OPERA is made of two super-module for a total target mass of 1.8 Kton. Each spectrometer of OPERA consist of a dipolar magnet made of two iron walls of rectangular cross section (8.75 x 8.2 m 2 ), interleaved by pairs of high resolution trackers. Each wall is made of 12 iron plates 5 cm thick separated by 2 cm of air gap where RPC are installed. The iron is magnetized by a current of about 1200 A circulating in the top and bottom copper coils. The magnetic flux density in the tracking region is 1.55 T with vertical field lines of opposite "On behalf of the RPC-OPERA group (Bologna, LNGS, LNF, Napoli, Padova, Zagreb). For a detailed description of the detector together with its physics potential see [1].

a

185

186 directions in the two magnet walls. The high resolution trackers consist of vertical drift tube planes with an intrinsic resolution of 0.5 mm in the bending direction. The two tracker planes housed between the two magnet walls provide an angular measurement of the track with a 100 cm lever arm. The lever arm for the external trackers is 50 cm. This design leads to a momentum resolution better than 30% in the relevant kinematical domain. Left-right ambiguities are resolved by the two-dimensional measurements of the spectrometer RPCs and by two additional RPC planes with inclined strips (±43.6° with respect to the vertical axis) located just after the two most upper stream drift tubes stations. 2. The O P E R A R P C system The RPC technology used for OPERA is the same as that developed for LHC experiments, BaBar and Argo. Given the very low rate capability required, high resistivity bakelite electrodes are employed (p > 5 x 10 n ft cm at T = 20°). The RPCs planes are composed of 21 RPC of 2.9 x 1.1 m 2 arranged in 7 rows and 3 columns. In order to fit the large bolts that hold together the magnet, 18 RPCs have grooves of semi-circular shape with radius R = 4.3 cm along one of the two long side of the RPC. With this particular shape the RPCs plane acceptance is 97%. The double coordinate read-out is performed by means of 2.6 cm pitch vertical strips, which measure the coordinate in the bending plane, and 3.5 cm pitch horizontal strips. To guarantee the strip adherence to the RPCs, polyester fiber are used to fill the residual of the 2 cm gap space between the iron slab. The RPCs are operated with a gas mixture of argon, tetrafluoreothane and isobutane in volume ratio of 76/20/4 with the addition of 0.5% of SF6Each RPC row is flushed separately at 5 refills per day, using an open flow gas system. Copper and stainless steel pipes are used to avoid excessive humidification of the gas mixture. To control the temperature of the RPC 10 thermistor are placed on each RPC plane between the external side of the read-out strips and the filling material. 3. O P E R A R P C read-out electronics The RPC are operated in streamer mode which provide large amplitude signals (~ 100 mV). The signals from the strips are collected by means of ~ 15 meter long twisted pairs cables and carried to the top of the magnet where the read-out electronics2 is located. The input stage of the RPC Front-End Board (FEB) make use LVDS receivers as comparators. Those

187 devices, because of their differential inputs and high common mode rejection ratios allow a good matching of the signals transmitted through twisted cables. Figure (1) left shows a single LVDS-discriminator channel with the resistive network that provide the threshold bias and the capacitor to reject the DC components. The variable threshold bias is obtained with a DAC with ±300 mV voltage spam. Figure (1) right show the RPC efficiency at the working point as a function of the LVDS threshold voltage.

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Figure 1. (Left) Input stage of the R P C Front End Board. (Right) R P C efficiency versus LVDS-discriminator threshold voltage for positive and negative signals.

4. Underground test After the installation of the spectrometers the last 4 plane of the first spectrometer were tested (~ 20% of all installed RPCs) in order to check the RPC performance in underground conditions. The measurements were performed with no magnetic field and using self-triggering FEBs of MACRO experiment. Muon tracks were selected requiring the coincidence of signals in three planes (for additional forth plane efficiency study as a function of HV). In the sample collected in about 62 hours, 1307 single-muon, 24 di-muons, 3 four-muons and 1 six-muons events have been found after visual scanning. The number of single muon correspond to the rate of 21.0 ± 0.6 fj,/h. By requiring a very clean track topology (only one strip cluster per view) 973 muons and a rate of 15.6±0.5 fi/h has been measured, in good agreement with the Monte Carlo b prediction of 15.2 ± 0 . 3 n / h . Figure (2) show the angular distribution of the single muon events. The shape of the distributions reflect the detector acceptance and the non-uniformity b T h e underground muon flux simulation is based on an empirical formula for vertical muon intensity modified to account for the Gran Sasso rock structure 3 .

188

of the rock distribution around the laboratory. The average counting rate of the installed RPCs at the operating voltage of 5.6 kV is 17 Hz/m 2 . e
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5. Results on R P C aging studies As the RPCs inside the magnet will not be accessible during the entire OPERA lifetime, a big emphasis has been placed on the long term operation and on quality control tests. The good quality of installed RPCs is ensured by a full chain of quality control tests 4 performed in dedicated facilities at the Gran Sasso external laboratory. The quality of the RPCs is determined on the basis of mechanical test (gluing of spacers and gas tightness), electrical test (dark and working currents, resistivity) and response to cosmic ray (RPC efficiency) and noise distribution inside the chamber (noise map). This last test turns out to be the most selective one with a rejection fraction of 14%. The test is performed by locating on the detector the hits acquired in self-triggering mode. The counting in pixel of 3.5 x 3.5 cm 2 are converted into local rate by scaling it to the global RPC counting rate. An RPC is rejected if the maximum of the local rate at the operating voltage exceed 5 Hz/pixel. We call this a hot-spot. Six rejected RPCs have been operated at cosmic ray fluxes for 240 days. All RPC show an increase of the hot-spot rate with the temperature which rise during the summer from 20°C to 27°C. When the temperature return to 20°C the hot-spot rate of the high resistivity RPCs (> 1012flcm) return almost to the initial value. Contrary for the RPC with the lowest resistivity ((0.36 ± 0.01) • 1012ficm) the hot-spot rate increase by a factor 2, while the average pixel rate outside the hot-spot remain stable. This is a clear indication of a local damage of the RPC surface. The damaging of the RPC electrodes has been observed

189 on an RPC (RPCl in [5]) which was operated at the cosmic ray fluxes for 460 days. The RPC showed 10% loss of efficiency in an area corresponding to the noisiest pixels (~ 350 Hz/pixel). The RPC has been opened and the electrode surface visually checked. The surface, in correspondence of the noisiest pixels had change color turning to yellow. In that zone the anode was covered by small, 0.5mm of diameter, white spots. In correspondence on the cathode were visible small whiskers. Figure (3) left show a magnification of the white spots with an optical microscope. The same structure, see figure (3) right, has been observed on the anode of a new RPC rejected by the quality control test in correspondence of the pixel with the highest rate. This show that the noise map test is able to identify defect of the RPC electrode that can, with time, degrade the RPC performance.

LJ Figure 3. Magnification, with an optical microscope, of (left) a white spots on the anode of R P C l of [5] and of (right) the anode electrode, in correspondence of an hot-spot, of a new R P C rejected by the quality control test.

6. Conclusion The OPERA spectrometers make use of RPC in a large scale application. The R P C installation ended in May 2005 and preliminary test on 4 installed RPC planes (~ 20% of all installed RPC) confirm the good behavior of the chambers in terms of intrinsic noise and operating currents and efficiency. The first OPERA spectrometer will be commissioned in spring 2006. References 1. M. Guler et al., CERN/SPSC 2000-028. M. Guler et al, CERN/SPSC 2001-025. 2. M. Ambrosio et al, Nucl. Instrum. Meth. A 533 (2004) 173. 3. M. Ambrosio et al., Phys. Rev. D52 (1995) 3793. 4. A. Bergnoli et al., Nucl. Instrum. Meth. A 533 (2004) 203. 5. G . Barichello et al. Nucl. I n s t r u m . M e t h . A 5 3 3 (2004) 42.

ELECTROMAGNETIC SHOWER RECONSTRUCTION IN EMULSION CLOUD CHAMBER

L. S. E S P O S I T O * Laboratori Nazionali del Gran Sasso, S.S. 17 BIS km. 18.910 67010 Assergi LAquila

Atmospheric neutrino d a t a from the MACRO, Soudan II and Super-Kamiokande experiments are consistent with the hypothesis of i/M —• vT oscillations. The OPERA experiment aims to prove definitively this hypothesis with the direct observation of vT neutrinos in the v^ beam produced at CERN (CNGS). The apparatus, in construction at the Gran Sasso Underground Laboratory, is equipped with electronic detectors and a sensitive target. The target is highly segmented in units, bricks, composed of alternate nuclear emulsion plates and lead sheets. An algorithm to reconstruct electromagnetic showers in a brick was developed. The algorithm was optimized using experimental data from 1, 3 and 6 GeV electron exposures and cross-checked with detailed Monte Carlo simulations. Finally, a neural network was used as electron/pion separator.

1. I n t r o d u c t i o n Neutrino physics is one of the most challenging topics in particle physics. There is convincing evidence for neutrino oscillations, hence neutrino masses and mixing, in particular the combined analysis of atmospheric neutrino experiments favors the hypothesis of a purely v^ —• vT oscillation. The final proof for neutrino oscillations should be the detection of a vT in a terrestrial artificial (almost) pure v^ beam (appearance experiment). The OPERA experiment 1 , in preparation at the Gran Sasso Underground Laboratory, aims at the direct observation of u^ —» vT oscillations in the CNGS (CERN Neutrinos to Gran Sasso) neutrino beam produced at CERN. Since the ve contamination in the CNGS beam is low, OPERA will also be able to study the sub-dominant v^ —» ve oscillation channel. OPERA is a large scale hybrid apparatus divided in two supermodules, each equipped with electronic detectors, an iron spectrometer and a *On behalf of the OPERA Collaboration.

190

191 highly segmented 0.9 kton target section made of "Emulsion Cloud Chamber" units. Each emulsion-lead unit, called brick, is composed of alternate nuclear emulsion films and lead sheets. The use of the ECC satisfies the need of a large mass, required in long-baseline neutrino experiments, and a high precision tracking capability to detect r decays. The target trackers and the spectrometers provide the trigger and a "prediction" of the individual brick where the neutrino interaction happened. The candidate brick is then extracted and disassembled. The emulsion films are photographically processed and then scanned with automatic microscopes for an event-byevent topological classification (event location, search for decays) and further measurements (particle identification and momentum measurement). Very fast automatic scanning systems are needed to cope with the analysis of the large number of emulsion plates associated to neutrino interactions 2 . Tau leptons can decay into a hadron, a muon or an electron. The electron channel has a characteristic signature due to the electromagnetic shower produced by the electron. It is important to have a good particle identification, and, at the same time, to reduce as much as possible the mis-identification, e.g. a low energy pion can undergo a charge exchange process n~p —> w°n and produce an electromagnetic shower. Good electron-pion separation can be achieved by studying the different behavior of these particles in passing through an OPERA brick. At the energies of our interest (1 4- 10 GeV), an electron generates an electromagnetic shower, while a charged pion loses energy essentially through ionization. An experimental study of electron identification using ECC was performed with an electron-enriched ir~ beam 3 . This analysis was based on multiple Coulomb scattering. An experimental study has been performed at DESY where few bricks were exposed to a pure electron beam. A dedicated shower reconstruction algorithm and an electron-pion separator based on a Neural Network have been implemented 4 . To complement this approach, a multiple Coulomb scattering analysis with a pure pion beam has been performed5.

2. Shower reconstruction It is worth to better define the terminology used in the following. An emulsion film is one of the two 43 /im thick emulsion layers put on one side of the plastic base. The emulsion plate is the whole of the plastic base

192 and the two emulsion films. A micro-track is a chain of aligned clusters in one emulsion film. A base-track is built by connecting the two micro-tracks that cross the plastic base. A volume-track is constructed from two or more base-tracks, each one in a different emulsion plate. To reconstruct a shower, all base-tracks inside a cone opened around the primary base-track can be collected. These crude strategy provides a high efficiency in collecting the base-tracks belonging to the shower but can not avoid casual matches with background base-tracks. The bricks analyzed in this work integrated many cosmic ray tracks before and after the exposure, so it was necessary to develop a dedicated algorithm that starts the propagation of the shower from the primary basetrack inside the bricks. Another issue concerns the overlapping between adjacent showers. Since the density of the electrons impinging on the brick is ~100 electrons/cm 2 , the average distance of two primary electron is ~ 1 mm, that is of same order of shower lateral spread. In this "critical" experimental situation, the cone is used as a fiducial volume. All the base-tracks that are outside the cone are not considered during the propagation. Moreover, also base-tracks inside the cone are eliminated if they belong to a volume-track that starts outside the cone itself, i.e. they are considered as "passing-through" and interpreted as cosmic rays. To be efficient in the shower reconstruction, the angular and position tolerances must be increased with respect to a standard propagation of a mip. The Moliere angle and displacement for 100 MeV particles are of the order of 100 mrad and 100 fim, respectively. It is expected that, with this "large" spatial tolerances, background contamination will be reduced at the level of ~0.2/base-tracks/cm 2 by simple computation based on phase space reduction.

3. E x p e r i m e n t a l test Some OPERA bricks were exposed to a pure electron beam at the DESY electron synchrotron. The experimental set-up is made of a pair of scintillation counters used as trigger system, a Multiwire Proportional Chamber to measure beam profile and a lead glass that provides an energy measurement of the electrons. The bricks used in this work were exposed to different energies (1,3 and 6 GeV) with a density of 50-rl00 particle/cm 2 .

193 To reconstruct base-tracks, we used FEDRA 6 , an object-oriented tool based on C + + language developed in ROOT framework. 4. Monte Carlo simulation The Monte Carlo simulation takes into account the brick geometry and materials. All the generated primary and secondary tracks are traced through a brick considering all the relevant physical processes. Secondaries are propagated down to 1 MeV kinetic energy. Two different Monte Carlo codes were used to cross-check their reliability: FLUKA 7 and GEANT3 8 . The results are compatible. For each emulsion film, Monte Carlo provides a pair of hits, one at the entrance of the emulsion, the other at the exit. These pair of hits are then connected to form a micro-track. Micro-tracks are then properly treated by a dedicated software9. A Gaussian smearing was applied both to the micro-track angle and base intercepts. Micro-track reconstruction efficiency was simulated using the mean value of the base-track efficiency as measured with the microscope in other reference plates. The number of clusters for each micro-track was sampled from a Gaussian distribution, whose parameters were obtained from experimental data a . No local effects (e.g. distortions) were simulated. They are strongly dependent on the emulsion development phase and its simulation is a very hard task. In our case the emulsion thickness is small, so the distortions should play a minor role. No smearing of the micro-track z position was applied. The emulsion-base surface, as measured by the automatic system, changes within few microns inside the scanned area. For the simulated data, an "ideal" planar surface was considered. Once smearing and efficiency corrections are applied to the micro-tracks, base-tracks are formed. Each top micro-track is linked with the corresponding bottom one using the tolerances estimated from real data. The same area was measured in all the plates of a brick. This defines the scanned volume. To simulate the scanned volume in the Monte Carlo data, the region that includes all the primary electrons in the first plate of the simulated brick was projected into the downstream plates. All base-tracks that intercept the downstream plates outside this region were discarded. a I n the real data the distribution of the number of clusters of micro-tracks follows a poissonian distribution: however, a poissonian distribution with a mean value ~12-Hl3 is well approximated by a gaussian distribution.

194 From now on, both experimental and simulated data follow the same analysis flow, with the exception of the alignment procedure performed only for real data. The shower reconstruction algorithm has been developed using Monte Carlo simulation and an iterative comparison with experimental data. A reconstructed shower of a 6 GeV electron is shown in Fig. 1.

i Beam direction

Figure 1. A reconstructed shower generated by a 6 GeV electron. The entrance point of the primary electron is the top left angle of the figure according to the beam direction.

5. Results An electron-pion separator based on a Neural Network was implemented and applied to experimental electron data. Monte Carlo simulations were used in order to correctly take into account background tracks in the signal region. The results for electron data and Monte Carlo simulations are given in Table 1. where ee-+e is the efficiency to correctly identify an electron, %_+„• is the probability of electron mis-identification and SNC is the fraction of "not classified" particles. A particle is "not classified" if it has only 1 or 2 base-tracks besides the primary: for these particle the available information is too small.

195 Table 1. Electron identification efficiency for experimental data. Monte Carlo E (GeV) 1 3 6

£e—*e

*/e-7T

0.88 0.91 0.96

0.05 0.05 0.04

Data

SNC

0.08 0.04 0.01

£e—+e

Ve-nr

0.86 0.92 0.95

0.07 0.06 0.04

SNC

0.08 0.02 0.01

6. C o n c l u s i o n s a n d o u t l o o k s A shower reconstruction algorithm was developed and the Neural Network separator implemented. T h e y were applied to t h e electrons d a t a of 1, 3 and 6 GeV bricks exposed at DESY. Electron identification efficiencies are 86%, 92% and 95%, respectively, in good agreement with Monte Carlo results. In t h e future the realization of a hybrid algorithm based on the shower reconstruction and Multiple Coulomb scattering analysis will provide better performances.

Acknowledgments I acknowledge t h e cooperation of the members of the O P E R A Collaboration for the realizations of the experimental test and t h e d a t a sharing and I t h a n k my colleagues of University of Bologna for discussions and suggestions, in particular M a x Sioli and Michela Cozzi. References 1. OPERA Collaboration, OPERA: An appearance experiment to search for uM <-> vT oscillations in the CNGS beam, Experimental proposal, CERN-SPSC-2000-028 (2000); http://operaweb.web.cern.ch/operaweb/documents/index.shtml. 2. G. Rosa et al., Nucl. Instr. Meth. A394, 357 (1997). N. D'Ambrosio [OPERA Collaboration], Nucl. Instrum. Meth. A525, 193 (2004). 3. K. Kodama et cd., Rev. Sci. Instrum. 74, 53 (2003). 4. L. S. Esposito, Study of electron identification in the Emulsion Cloud Chambers of the OPERA experiment, Ph.D. Thesis, University of Bologna (2005). 5. M. Cozzi, Study of pion identification in the Emulsion Cloud Chambers of the OPERA experiment, Ph.D. Thesis, University of Bologna (2005). 6. http://ntslab01.na.infn.it/fedra. 7. A. Fasso, A. Ferrari and P. Sala, SLAC-REPRINT-2000-116. 8. GEANT 3.21, CERN Program Library Long Writeup W5013. 9. http://people.na.infn.it/~marotta/ORFEO.

A LOW B A C K G R O U N D M I C R O M E G A S D E T E C T O R FOR T H E CAST E X P E R I M E N T

P. ABBON, S. ANDRIAMONJE, S. AUNE, D. BESIN, S. CAZAUX, P. CONTREPOIS, N. DUPORTAIL, E. FERRER RIBAS, M. GROS, I. G. IRASTORZA, A. GIGANON, I. GIOMATARIS, M. RIALLOT AND G. ZAFFANELA DAPNIA, Centre d'etudes de Saclay, 91191 Gif sur Yvette CEDEX, Prance G. FANOURAKIS, T. GERALIS, K. KOUSOURIS AND K. ZACHARIADOU Institute of Nuclear Physics, NCSR Demokritos, Aghia Paraskevi 15310, Athens, Greece T. DAFNI* Institut fur Kernphysik,

TU-Darmstadt, Schlossgartenstr. Darmstadt, Germany

9, D-64289

T. DECKER, R. HILL, M. PIVOVAROFF AND R. SOUFLI Lawrence Livermore National Laboratory, 7000 East Avenue, CA94550, USA

Livermore

J. MORALES Instituto de Fisica Nuclear y Altas Energias, Universidad de Zaragoza, Zaragoza 50009, Spain

A low background Micromegas detector has been operating on the CAST experiment at CERN for the search of solar axions during the first phase of the experiment (2002-2004). The detector operated efficiently and achieved a very low level of background rejection (5 x 1 0 - 5 counts keV _ 1 cm _ 2 s - 1 ) thanks to its good spatial and energy resolution as well as the low radioactivity materials used in the construction of the detector. For the second phase of the experiment (2005-2007), the detector will be upgraded by adding a shielding and including focusing optics. These improvements should allow for a background rejection better than two orders of magnitude.

"now at DAPNIA, Centre d'etudes de Saclay, 91191 Gif sur Yvette CEDEX, France

196

197 1. Introduction The CAST (Cern Axion Solar Telescope) collaboration is using a decommissioned LHC dipole magnet to convert solar axions into detectable x-ray photons. Axions are light pseudoscalar particles that arise in the context of the Peccei-Quinn1 solution to the strong CP problem and can be Dark Matter candidates 2 . Stars could produce axions via the Primakoff conversion of the plasma photons. The CAST experiment aims to track the Sun in order to detect solar axions. The detection principle is based on the coupling of an incoming axion to a virtual photon provided by the transverse field of an intense dipole magnet, being transformed into a real, detectable photon that carries the energy and the momentum of the original axion. The axion to photon conversion probability is proportional to the square of the transverse field of the magnet and to the active length of the magnet. Using an LHC magnet (9 T and 9.26 m long) improves the sensitivity by a factor 100 compared to previous experiments. A more detailed description of the principle of the experiment can be found in Zioutas et al.3. For the first phase of the experiment, 2002-2004, three types of detectors have been developed to detect the x-rays originated by the conversion of the axions inside the vacuum of the magnet: a time projection chamber (TPC), a CCD and a Micromegas detector. The CCD detector has been working in conjunction with a mirror system to focus x-rays coming out of the magnet bores improving its signal to background ratio. The analysis of the data collected during 2003 shows no excess of signal over background and has resulted in the most restrictive experimental limit on the coupling constant of axions to photons 4 . The CAST second phase, 2005-2007, will allow us to scan axion masses greater than 0.02 eV/c 2 . This is made possible by filling the magnet bore with a buffer gas (4He or 3 He) allowing the photon to acquire an effective mass. During 2005 a He gas system has been designed and constructed, allowing systematic changes of the He pressure. Cold polypropylene windows have been installed in the magnet bore to minimize the thermal coupling between the cold bore and the outside of the magnet. 2. Performance of the Micromegas detector during phase I 2.1.

Description

The Micromegas detector is a double gap chamber. It consists of a conversion gap separated from an amplification gap by a gauze-light electroformed conducting micromesh. A full description of the detection principle can be found in Giomataris et al.5. For CAST, the conversion gap is 20 (30) mm

198

thick and the amplification gap used was of 50 (100) /txm in the 2003 (2004) data taking. The gas mixture is 95% Argon and 5% Isobutane. The field applied to the amplification gap is about 40 times higher than the conversion field. The charge is collected on a two dimensional readout by means of X and Y strips of 350 /xm pitch. The mechanical pieces of the detector are of Plexiglas because of its low natural radioactivity to limit as much as possible the inherent background of the detector. The Micromegas detector, which operates at atmospheric pressure, is interfaced with the LHC magnet by two thin vacuum windows. These two windows, made of polypropylene, act as a buffer for the pressure gradient and at the same time they need to maximize the x-ray transmission in the energy range of interest for the axion search. The integrated x-ray efficiency of the detector in the solar axion energy spectrum (1-8 keV) is of 73% taking into account the loss in transmission due to the thin windows as well as the conversion efficiency of the gas.

2.2.

Results

The detector has run successfully during the first phase of the experiment. The Micromegas detector records tracking data at sunrise, and during the rest of the day background data is taken. The detector is calibrated daily using a 55 Fe source. The energy resolution at 6 keV is 20% (FWHM). Tracking and background data have been analysed. Signal events (photons with a mean energy of 2-8 keV) have a well defined signature giving a typical cluster in the read out strips and a typical pulse in the micromesh. Background events, coming from cosmic rays and natural radioactivity, give out a bigger cluster in the strips, and the pulse shape in the micromesh is very different favouring an efficient rejection based on the micromesh pulse shape and on the cluster topology. Figure 1 shows the energy spectra for background events for 2003 and 2004 data. In these plots, a flat background is observed with a superimposed peak at 8 keV. This peak has been identified as the copper fluorescence peak coming from the detector materials (micromesh and readout strips). Thanks to the upgrading of the detector in 2004 and the elimination of residual cross-talk between strips, the background rejection was improved of a factor 3 with respect to 2003, achieving a level of 5 x 1 0 - 5 counts k e V - 1 c m - 2 s - 1 . The combined analysis of the three CAST detectors for the 2003 data has resulted in a limit on the coupling constant of photon to axions 4 . This limit, 1.16 x l O - 1 0 G e V - 1 for ma < 0.02 eV, improves

199 [WJ

;

-4-

!00

: ; : ; -*: '•*• ; Figure 1.

i

,

] -+

10

|MV]

Background spectra for the 2003 and 2004 data.

the existing experimental limit by a factor 6. The 2004 combined analysis should allow us to extract a limit close to the theoretical bound coming from astrophysical considerations.

3. The new Micromegas line for phase II During the transformation of the magnet for phase II, the Micromegas group has been designing an upgraded Micromegas detector surrounded by a shielding and coupled to an x-ray optic, as shown in Figure 2. This new line should be installed at the CAST experiment during spring 2006. These upgrades will improve significantly the performance of the detector. First, the x-ray optic, a concentrator with a 1.3 m focal length and 47 mm diameter, will allow us to increase the signal to noise ratio by a factor ~ 100 by focusing the photon flux in a 2 mm spot. This concentrator will consist of 14 nested polycarbonate shells, each 125 mm long and coated with iridium. The optic will transmit ~ 36% of the 0.5-10 keV flux emerging from the magnet bore. Second, the shielding, composed of copper, lead, cadmium, nitrogen and polyethylene, is expected to reduce the background by a factor of 4. Third, by changing the gas of the chamber from Argon to Xenon, the photon conversion probability can be improved by at least ~ 10%. The new detector will be running with the same electronics and acquisition developed for phase I. First tests of a prototype of the detector have started. The complete line, with the integrated optics, is expected to be operational for characterisation tests and calibration at the PANTER x-ray test facility in Munich beginning of 2006. Recent developments on the integration of Micromegas with pixel CMOS readout 6 , may lead to an ultimate upgrade of the detector for the 2007 data taking to profit of the spatial and energy resolution of pixel sensors.

200

Figure 2. The new Micromegas line consisting of a new detector with integrated x-rays optics and shielding.

4. Conclusions A low background Micromegas detector has been operating at the CAST experiment achieving a remarkable background rejection (5 x 1 0 - 5 counts k e V _ 1 c m - 2 s _ 1 ) . For the CAST second phase, an upgraded Micromegas detector with integrated x-ray optics and shielding will be installed at the experiment during spring 2006. These improvements should lead to a reduction of the background level by at least two orders of magnitude. Acknowledgments This work has been performed within the CAST collaboration. our colleagues for their support. Work at LLNL was performed auspices of the U.S. Department of Energy by the University of Lawrence Livermore National Laboratory under Contract No. ENG-48.

We thank under the California W-7405-

References 1. R. D. Peccei and H. R. Quinn, Phys. Rev. Lett. 38, 1440 (1977). 2. P. Sikivie, Int. J. Mod. Phys. D3 S, 1 (1994) and Phys. Rev. Lett. 51, 1415 (1983). 3. K. Zioutas et al., Nucl. Instrul. Meth. A 425, 480 (1999). 4. CAST Coll., "First results from the CERN Axion Solar Telescope (CAST)", Phys. Rev Lett. 94,121301 (2005), hep-ex/0411033. 5. Y. Giomataris, P. Rebourgeard, J. P. Robert and G. Charpak, Nucl. Instrum. Meth. A 376, 29 (1996). 6. P. Colas et al., Nucl .Instrum .Meth. A 535, 506 (2004).

IMAGING OPTICAL ADAPTERS FOR MULTI-ANODE PHOTO MULTIPLIERS DETECTORS * LISA GAMBICORTI + Dep. Of Astronomy and Space Science, Florence University, L.go E. Fermi 2, 50125 Florence, Italy PIERO MAZZINGHI CNR-INOA, L.go E. Fermi 6, 50125 Florence,

Italy

EMANUELE PACE Dep. Of Astronomy and Space Science, Florence University, L.go E. Fermi 2, 50125 Florence, Italy

In several experiments of Nuclear and Neutrino Physics and Cosmic-rays detection, Multi-Anode Photo-Multipliers (MAPMTs) detectors from Hamamatsu corp. are used. Aim of this work is to describe the ancillary optical system, projected to improve the photons collection efficiency of PMT detectors focusing all photons inside the sensitive area. This system optimized with ray-tracing simulations allows to maintain imaging and filtering capability, improving the signal-to-noise ratio by limiting the wave band to the region of interest. The filter glass employing and the multi-layer coating optimization allows to avoid background photons from nearby spectral regions and to minimize the wavelength shifting when the incident angle increases. The transmissivity spectrum has been realized with an high transparency in wavelength range of interest and with a sharp cut-off outside. First prototypes have been realized.

* This work is supported by INFN (Florence section), CNR-INOA and Dep. Of Astronomy and Space Science of Florence University. t Lisa Gambicorti, PhD Student, Dep. of Astronomy and Space Science of Florence University, L.go E. Fermi 2, 50125 Florence, Italy. Tel & Fax +39-055-2307750. E-mail: lisag(aarcetri.astro.it

201

202

1. Introduction The detectors developments involve several and wide areas of applied physics. The employment in experiment in which a high photons collection efficiency is required determines the motion to a research and a develop devoted to an efficiency improvement. Several up to date experiments, in particular in Nuclear and Neutrino physics fields, High Energy and Cosmic-rays detection, primary optical systems with large collection area are used, detecting the photon signal generated by the interaction of such high energy particles crossing a medium, that works as a scintillator. To collect information on the wide angle and to reconstruct the particles trajectories, large pixelated focal planes are required. So, such kind large area focal surface, have to be completely covered with detectors to collect the larger amount of photons [1,2]. Advanced typologies of Multi-Anode Photo Multipliers (MAPMTs) detectors, developed by Hamamatsu Corp., represent an appealing solution to cover large collection area. In particular these detectors will be used for EUSO (Extreme Universe Space Observatory) experiment, a telescope that will observe the Earth's atmosphere with a 60° of FoV (Field of View) from 400 km altitude on board of the ISS (International Space Station). The large volume of atmosphere observed will be necessary to detect a high number of EAS (Extensive Air Shower) events at energies above 1020eV, allowing to obtain a good statistic. A UV filter is required to select the wavelength of collected photons and to improve the signalto-noise ratio in range 300+400 nm. Therefore a possible item to improve collection efficiency is to study an ancillary system to constrain the photons impinging on total area to be focalized only on sensitive area [3,4]. Aim of this work is to describe the project and the optimization of the adapters system designed for three series of detector, which have a sensitive-total area ratio ranging from 45% to 89%. In particular the efficiency reach for the R8900 PMT series, with a sensitive-total area ratio of 79% will be shown. Part of this work deals with the study of materials to obtain an adapter with high transmissivity in wavelength range of interest. 2. The optical adapters A specific ancillary optical system is studied to improve collection efficiency for each R7600/R8900/R7400 PMTs series. An optical adapter lens with hemispherical shape has been projected according to sensitive-total area MAPMTs ratio, allows maintaining the imaging capability that, in several experiments, is a requirement necessary to obtain a well defined image on detector. The adapter materials should have high transmittance in wavelength

203

range of use. The employment in EUSO of the R7600/R8900 MAPMT series is limited by their sensitivity between 300nm and 650 nm, peaked at 420nm, a filter solution must be take in consideration to avoid the 400nm 650nm unwilled signal [5]. Therefore the BG3 Schott glass filter has been selected as EUSO baseline filter[6]. This filter presents a good transmittance in the 300-500nm spectral range. The adapters performances analysis have to be done considering the optical properties of used materials. Main parameters are the bulk transmittance and the refractive index of materials, depending on the wavelength. In particular, the internal transmittance as a function of thickness allows calculating the absorption coefficient (extinction) of material for a given wavelength, as shows the Lambert-Bouguer absorption law [7]. Data on optical and physical properties of materials and filters have been used in ray- tracing simulations. In particular considering the R8900 MAPMT, baseline detectors for EUSO experiment, the projected adapter is an hemispherical lens with a squared section, the same lateral side of each MAPMT series considered to cover the full area of detector. Lateral sides of lens forming a reflecting empty truncated pyramid with four mirrors, in order to collect all the photons coming out of the lens and to fit the lens size with the sensitive area of the detector side. The lens dome is made of PMMA {Poly-Methyl-Metha-Acyilate) with a curved BG3 glass filter glued on top side respectively. To further on reduce the pass band of the BG3 a multi layer coating has been studied and optimized in wavelength range of interest and with a sharp cut-off outside. In case of space experiments, as EUSO, the use of PMMA allows reducing the adapters' weight [8]. 3. The optical performances To evaluate the performance of the filter lens adapter, a set of simulations have been performed with an extended uniform source, emitting at atmospheric fluorescence wavelengths of EUSO. The adapters performances have been obtained, calculating two parameters that together determine the collection system efficiency: the geometric^riG) and the radiometric efficiency*(r|R). Fig. 1 for example shows as look like the top view of R8900 PMT coupled to the optical adapter after the ray-tracing simulation of 105 rays at 357nm. The resulting geometric efficiency increase from 79% to 99% because only a small fraction of rays falls in dead area of detector. Even if the absorption coefficients 'Geometric efficiency r|G: Ratio between rays collected on sensitive area with respect to the number of rays startedfromsource. "Radiometric efficiency nR: Ratio of photons on sensitive area with respect to photons startedfromsource, considering the absorption coefficient of adapter materials.

204

of materials are taken in account the radiometric efficiency reaches the 95%. A simplified prototypes in Fused silica have been fabricated in order to test the optical performances directly.

<4

•

27.0 mm Figure 1. Top view of R8900 PMT coupled to the optical adapter after the rat-tracing simulation of 10s rays at 357nm.

In general applications where are not weight limits, an hemispherical lens adapter optimized for the R8900 MAPMT has a BG3 glass filter, glued on back side of squared hemispherical lens dome of Fused Silica [9,10], A similar study have been realized also for Flat Panel. 4. Conclusion In Nuclear and Neutrino Physics, High Energy and Cosmic-rays detection, the employment of the R7600/R8900/H8500 series MAPMT detectors is limited by their small sensitive-total area ratio. In this paper an optical adapters system is studied to improve the collection efficiency including filtering capabilities, implementing a glass filter to improve the signal-to-noise ratio and reducing the MAPMT wavelengths pass band to the UV range. An optical adapter system,

capable to improve the collection efficiency of each PMTs series detector, has been projected. A filtering adapter solution for the 8900 MAPMT series has been proposed and studied to match also the detector wavelength pass band to the EUSO experiment requests. The simulations show that, using these optical adapters, the detector efficiency improves from 79% to 95%.

205

Due to its general characteristics, the hemispherical lens optical parameters can be modified with different filter solutions, studied and optimized depending by the spectral range of work. References 1. R. Pestotnik et al., Lens-based collection system for a proximity focusing RICH, Nucl. Inst. andMeth A 504 (2003) 237-239 2. A. Petrolini, A comparative study of light collector systems for use with multi-anode photo-multipliers, Nucl. Inst. andMeth A 497 (2003) 314-330 3. V. Bratina, P.Mazzinghi, L.Gambicorti, Overview of possible optical adapter for EUSO, Proc. Spie Vol. 5164, UV/EUV and Visible Space Instrumentation for Astronomy II; Oswald H. Siegmund; Ed.(2003) 100-112 4. P. Mazzinghi, V. Bratina, Concentration of radiation on EUSO focal plane detectors: theoretical limitations and practical implementation, EUSO Tech. Rep., EUSO-Florence collaboration, (2000) 5. Multi-anode Photo-Multiplier tube R7600-M16/M64 series, R8900M25/M36 series, R7400-M256 series, Hamamatsu Photonics K. K., Electron Tube Center, 314-5, Shimokanzo, Toyooka-village, Iwata-gun, Shizuokaken, 438-0193, Japan; http://www.hamamatsu.com 6. Schott Glass BG3 filter glass and Fused Silica datasheet, Schott Glass software code "FILTER'99"http://www.schott.com/ 7. Bouguer-Lambert law. Middleton, W. E. K., 1961: Pierre Bouguer's Optical Treatise on the Gradation of Light, Translation Pfeiffer, H. G., and H. A. Liebhafsky, 1951: J. Chem. Educ, 28, 123-125. Malinin, D. R., and J. H. Yoe, 1961: J. Chem. Educ, 38, 129-131. 8. Gambicorti L., P. Mazzinghi, E. Pace: 2003, UV filters and optical adapters for the EUSO focal surface detectors, 9th Pisa Meeting on Advanced Detectors Frontier Detectors for Frontier Physics, La Biodola, Isola d'Elba (Italy) May 25-31. 9. L. Gambicorti, V. Bratina, A. Gherardi, G. Corti, R. Ciaranfi, P. Mazzinghi, E. Pace, Improving the collection efficiency of EUSO focal surface detectors, Frascati Physics Series Vol. XXXVII (2004) pp. 477- 484. 10. L.Gambicorti, P. Mazzinghi, E. Pace, EUSO Optical adapters: Comparison between UV filtering optical adapters optimized for EUSO focal surface, EUSO- FS Tech. Rep. 012, 20 July 2004 11. L. Gambicorti, P. Mazzinghi, E. Pace, Optical adapters for MAPMT detectors, Nucl. Inst. andMeth A, (Submitted)

A 2D STOCHATSIC M O N T E C A R L O FOR T H E SOLAR M O D U L A T I O N OF GCR: A P R O C E D U R E TO FIT I N T E R P L A N E T A R Y P A R A M E T E R S C O M P A R I N G TO T H E E X P E R I M E N T A L DATA

P. B O B I K 1 , M . J . B O S C H I N I 2 r M . G E R V A S I 2 - 3 , D . G R A N D I 2 A N D P.G. RANCOITA2 INFN

Institute of Experimental Physics, Kosice, Slovak Republic Istituto Nazionale di Fisica Nucleare sezione di Milano, Milano and University of Milano Bicocca, Italy E-mail: [email protected]

Italy

We realized a 2 dimensional (radius and latitude) stochastic MonteCarlo for modelling the Galactic Cosmic Ray (GCR) propagation in the Heliosphere. The model solves numerically the transport equation of particles in the heliosphere, including major processes affecting the heliospheric particle transport: diffusion, convection, adiabatic energy losses and drift of particles. We estimated the cosmic rays spectrum at 1AU using this model formalism: our method allows the study of modulation behaviour in dependence on energy of interstellar particles and heliosphere parameters as Solar wind speed and the tilt angle a of the neutral sheet. We compared our simulations with the Primary CR proton spectra measured by AMS and we realized a fine tuning of our parameters in order to match real data. This work, done for the A > 0 period, can be done also for A < 0, where the best value found for the diffusion coefficient is different.

1. Introduction Models of heliospheric propagation of galactic cosmic rays (GCR) need to take into account all the features of the solar cavity in order to reproduce the observed flux, we can mention for the example the structure of heliospheric magnetic field [1], after Ulysses measurements. We used a Parker field model for the heliosphere [2], that also became time dependent due to the measurements connected to drift effects [3]. First we have produced a ID heliospheric model using a stochastic simulation approach [4,5], useful for testing the method and successful in reproducing the main feature of the *also CILBA, Segrate Milano

206

207

solar cavity. Then we have implemented a two dimensional (radius and latitude) drift model of GCR propagation in the heliosphere [6], depending on the charge sign of particles due to curvature, gradient and current sheet drifts, and time dependent too due to the variation of the measured values of the solar wind velocity in the ecliptic plane (V) and the tilt angle (a). 2. Monte Carlo 2D model with drift effects A 2D stochastic simulation is based on the equation for GCR transport in the heliosphere (without drift terms) [7,8] Coordinates variation in a small time step At for a test particle results:

AT = l d ( r 5 r r ) + VAt + Rg^2K^At H

d

AHi=

— dfi

X_

dr

2*9

At+ RAI 2(1-^)^-At

(1)

Where r is radial distance, \i = cos0 where 0 is the heliolatitude, Ar is the radial variation, A/j, = AcosO is the latitudinal variation of the particle, V is solar wind velocity, Rg is the Gaussian distributed random number with unit variance, At is the time step of calculation. Radial diffusion coefficient is Krr = Kucos2ip + K_isin2ip, where ip is the angle between radial and magnetic field directions [7]. The latitudinal coefficient is Kgg = K±. Parallel and perpendicular coefficients are

K\\ =

K0pKp(9t)^ Kx = (tfj.)o#||

(2)

where KQ — 2 x 10 2 2 cm 2 s _ 1 [9] /? is the particle velocity, Kpffi) take into accounts the dependence on rigidity (in GV), (K±)o is the ratio among parallel and perpendicular diffusion coefficient, BQ ~ 5nT is the value of heliospheric magnetic field at the Earth orbit, and B is the Parker field [10]. In our model the solar wind speed V(9) is a function of the heliolatitude 0 [11]: V = V0(l + sin0), for 0° < 0 < 60° and V = 750 km s~x, for 60° < 0 < 90°. VQ is the velocity of solar wind in ecliptic plane. Drift effects are included through analytical effective drift velocities [12]. The average drift velocity is Vd = V x ( f § ) . 5ft is the CR particle's rigidity. The Parker model allows analytical solution for drift velocities and better

208

evaluation of diffusion tensor, in this spiral field we added to the previous formulas 1 gradient, curvature and drift along the neutral sheet to calculate the position of a test particle during a time step At: Ard = Ar + {vg + v%s Afid = cos arccos(A/x) + arctan

fv$At\

(3)

\ r / Arj is the radial variation with drift effect, Afid is the latitudinal variation of the particle due to drift, vg is the velocity of gradient drift, v™ is the velocity of neutral sheet drift and v$ is the velocity of curvature drift. We use Burger's model [13] as Local Interstellar Spectrum of protons (LIS). Our model has been optimized by fine tuning the parameters Kp($l) and (-ft'x)o of the diffusion tensor, performing long time consuming simulations and comparing results with experimental data (flux spectrum at 1AU). 3. Results 3.1. Comparison

with

data

For A > 0 we selected June 1998 as the best period for testing the model results, because AMS-01 [16] experiment up to now has measured one of the most precise proton spectra, and we obtain as best value (Kj_)0 = 0.025, the same as reported in [14], but the rigidity dependence we have found is different: best spectra is obtained for Kp($l) ~ 9ft0,78 = 5R2-9, where q is the slope of the LIS at high energy. Simulation based on the same set of parameters do not fit the data for A < 0. For negative solar field polarity we obtained, comparing with IMP8 as best values the following set of parameters: (-Kjjo = 0.05 and Kp($t) ~ 3?. For both periods (A > 0 but especially A < 0) we also used Creme96 [15] model for comparison. For positive periods we have used solar wind speed in the range Vo = 100 — 1000km 5 _ 1 and tilt angle in the range a = 10 — 50°. Same set of values has been investigated for A < 0, and for both periods we evaluated the effect of different drift components (gradient, curvature and neutral sheet) turning off each one of them separately and finally all three together. 3.2. Dependence

on a and Vsw

We used the tilt angle a as main parameter for the level of the solar activity: the higher the value of a the lower the expected GCR flux, for both solar

209 V „ 400 km/s - A < 0

Va, = 400 km/s - A>0

!

block donates

o - 15*

block squo'es

a-SC

\

, Sp*CU»m ot 1 AU

Figure 1.

Different values of tilt angle a, A > 0 left and A < 0 right.

field polarity. Besides for the same value of a a higher flux of protons is expected for A > 0. In Fig. 1 we present results of our model for several value of a for A > 0 (left panel) and for A < 0 (right panel). Best values are consistent with measured ones, a ~ 25° for A > 0 and a ~ 15° for A < 0 (from Wilcox Solar Laboratory [17]).

a = 30° - A>0

a = 30° - A < 0 <#dlUP8 fc'u* US s w c t i u m

Speclrum Ot I AU

Figure 2.

Speclfum o l I Ay

Different values of solar wind velocity V, A > 0 left and A < 0 right.

We also evalueted the effect of ecliptic solar wind velocity VQ in both polarities. A higher value of V means higher solar activity and In Fig. 2 we present results of our model for several value of V for A > 0 (left panel)

210

and for A < 0 (right panel). Also for solar wind results are consistent with measured ones: V ~ 350 - 400 km/s for A > 0 and V ~ 450 - 500 km/s for A < 0 (from OMNIWEB [18])

3.3. Dependence

on the drift

effect

To study the effect of the drift velocity terms we considered a period when a = 30° and V = 400 km/s both for A > 0 and A < 0. After computing the flux including all the terms we have excluded the drift from the transport equation. Results are shown in Figure 3 for A > 0 and A < 0. The flux computed with no drift is similar for positive and negative periods, but the drift effect is more evident in positive periods. Minor differences are due to the different parameters used for the two periods.

v* 400 km/s —rt

1

I

I ••

\ / Lp2 U r i o igl s Di i . sq u ire i §t f

Vw 400 km/s - A < 0

= 30° A>0

|\

P

1

Clrun

-i

. •• '^ .

id .Irion les o squa

:

Spectrum at 1 AU

Figure 3.

\

•

t 5

I ^

( Ct'U"

\\

K

N

Specl'um ot 1 AU

Drift effect: turning off single components, A > 0 left and A < 0 right.

We can distinguish three terms involving drift velocities in the heliosphere: gradient, curvature and neutral sheet drifts. We have studied as each one of the drift terms affects the modulated spectrum. As you can see drift effect seems to be dominated by the neutral sheet one (for both solar polarities but stronger for A > 0), we may suppose that drifts play a more important role than convection. We can also see the several drift terms have different effect both in terms of intensity and shape, and that their global effect is not obtained by a simple superposition (neutral sheet drift alone is stronger than all three together).

211 4.

Conclusions

We built a 2D stochastic model of particles propagation across t h e heliosphere. Our model takes into account drift effects and show quantitatively good agreement with measured values. P r o t o n spectra predicted by model are decreasing with increasing tilt angles and flux ratio during positive and negative solar periods is in agreement with measurements. It is possible t o reach a better agreement with measured values by fine t u n i n g of parameters, solar wind velocity and tilt angle, as we did for A > 0 comparing results with AMS-01 data, and for A < 0, using I M P 8 d a t a , obtaining good results especially for A > 0. Agreements with d a t a is b e t t e r for periods with A > 0 t h a n for A < 0, t o investigate this we need accurate d a t a for this phase of the solar activity, t h a t will be available with AMS-02 measurements for C R flux starting at t h e end of 2008 and lasting for 3 t o 5 years.

References 1. L.A. Fisk, J. Geophys. Res. 101, 15547-15553 (1996). 2. R. A. Burger and M. Hitge, American Geophysical Union SH71A-04, Fall Meeting 2002. 3. G. Wibberenz, S. E. S. Ferreira, M. S. Potgieter, H. V. Cane, Space Science Reviews 97, 373 (2001). 4. M. Gervasi et al., Nuclear Phys. B (proc. suppl.) 78, 26 (1999). 5. M. Gervasi et a l , Proceedings of the 26th ICRC SH 3.1.18, 69 (1999). 6. P. Bobik et al., Proceedings of ICSC 2003ESA SP-533, (2003). 7. M. S. Potgieter, J. A. Le Roux, L. F. Burlaga, F. B. McDonald, Astrophysical Journal, Part 1 (ISSN 0004-637X) 403, no. 2, 760 (1993). 8. L. A. Fisk, J. Geophys. Res. 81, 4646 (1976). 9. M. S. Potgieter, J. A. Le Roux, Astrophysical Journal 423, 817 (1994). 10. R.A. Burger and M.S. Potgieter, The Astrophys. J. 339, 501 (1989). 11. J. P. L. Reinecke, C. D. Steenberg, H. Moraal, F. B. McDonald, Advances in Space Research 19, Issue 6, 901 (1997). 12. M. Hatting and R.A. Burger, Adv. Space. Res. 16, No. 9, 213 (1995). 13. R. A. Burger, M. S. Potgieter, B. Heber, J. Geophys. Res. 105, Issue A12, 27447 (2000). 14. J. Giacalone, J. R. Jokipii, Astrophysical Journal 520, Issue 1, 204 (1999). 15. A. J. Tylka et a l , IEEE Trans. Nuclear Sci. 44, No. 6, 2150 (1997). 16. J. Alcaraz et.al., Physics Letters B 490, 27 (2000). 17. http://quake.stanford.edu/ wso/wso.html 18. http://omniweb.gsfc.nasa.gov/form/dxl.html

SPATIAL D I S T R I B U T I O N OF E N E R G E T I C H E A V Y IONS N E A R THE EARTH

M. HAREYAMA, N. HASEBE, K. SAKURAI, S. KODAIRA, N. KAJIWARA, M. TAKANO, T. DOKE, M. FUJII Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan T. GOKA, H. KOSHIISHI, H. MATSUMOTO Institute of Space Technology and Aeronautics, Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan T. YANAGIMACHI Department,

of Physics, Rikkyo University., 3-4-14 Nishiikebukuro, Tokyo 171-8501, Japan

Toshimaku,

The observations have been made of energetic heavy ions in various places from 1 to 220 Re by means of the satellites as MDS-1 and GEOTAIL. The elemental abundances from C to Fe were observed by MDS-1 at 1 - 10 Re, GEOTAIL at 10 220 Re in the magnetosphere and beyond. Comparing these abundances with that of galactic cosmic rays (GCRs) measured by ACE/CRIS at 230 Re at LI point beyond the magnetosphere, we have found some differences between the GCR composition and our results. Referring to the results obtained thus far, we show the relative abundances and the spatial distribution obtained by these satellites. We also discuss the relation between the heavy ion composition in GCRs and the particle composition within the magnetosphere.

1. Introduction The observations of galactic cosmic rays (GCRs) and energetic particles in geomagnetic field have been done precisely by various satellites at various positions. However, it is not clear whether there are some differences of the abundances between particles in the magnetosphere and GCRs. This report examines the particle modulation effect in magnetosphere by comparing the relative abundances among three satellite data, Heavy Ion Telescope Array onboard MDS-1 called TSUBASA satellite in the near earth, GEOTAIL in distant geomagnetic tail and CRIS onboard ACE be-

212

213

charge histogram oblakd by HIT/MDS-1

£ 150

2 .-.

il - t , I , t . 0

2

4

6

8

10 12 14 16 18 20 22 24 26 28 charge Z

Figure 1. Charge histogram observed by HIT onboard MDS-1.

Figure 2. Charge Histogram observed by HEP/HI onboard GEOTAIL.

yond the magnetosphere. Then we consider the relation between the GCR composition and heavy ion composition within the magnetosphere. 2. Observations 2.1. HIT onboard

MDS-1

The Mission Demonstration Test Satellite-1 (MDS-1) was in operation from February, 2002 to September, 2003 just after the solar maximum period. It was in the geostationary transfer orbit with a perigee of 500 km and an apogee of 36000 km at the inclination of 28.5° which corresponds to i

a 10 -

2

i

r-1" I

I

i

i

i "i' i

§

i

1

i •^ |

I

i

i

i

f• l

l

• CRIS/ACE (GCRs) o HEP/GEOTAIL A HIT/MDS-1


a 03

t3

S3 3

*

•O

I ° i t

« *

1

N

O

Ne Mg element

Si

Fe

Figure 3. The relative abundances of heavy ions at 100 MeV/n obtained by HIT/MDSl(triangle), HEP/GEOTAIL (circle) and CRIS/ACE(square). These d a t a are normalized at iron.

214 L w 1 ~ 10. Its orbital period was of 10 hours and 35 min and its spin rate was of 5 spins/min. The Heavy Ion Telescope (HIT) onboard the MDS1 was designed to measure energetic ions in space by 2 layers of position sensitive Si detectors and 16 PIN typed Si detectors 1 . The HIT measured heavy ions from He to iron with energy between 20 and 180 MeV/n. This instruments separate heavy ions with 0.06 charge unit for C and O. The charge histogram obtained by HIT is shown in Fig.l. 2.2. HEP onboard

GEOTAIL

The GEOTAIL satellite was launched in July, 1992. It observed energetic particles between 10 Re to 220 Re in a wide range of the geomagnetic tail region. In the first two years, the apogee was from 80 Re to 220 Re in the distant tail in magnetosphere. Later, the apogee was 30 Re to 50 Re in the near-Earth tail region. The inclination has been set at -7 degree. In this work, we use the data set observed in the first three years during the decreasing phase in the solar activity. The instrument called HEP/HI (High Energy Particle) measured heavy ions between boron and iron with 50 ~ 200 MeV/n 2 . The charge histogram obtained by HEP is shown in Fig.2. 2.3. ORIS onboard

ACE

The ACE satellite has been observing energetic particles at 230 Re of Llpoint beyond the magnetosphere. This work used the data set obtained by CRIS instrument between February 2002 and September 2003 as overlapping the HIT data. The CRIS observed from Li to Zn with energy range between 40 and 500 MeV/n. The data set analyzed in this work were taken from CRIS level-2 data of "The ACE Science Center (ASC)". Hereafter, we treat these data as GCRs. 3. Results and Discussion The relative abundance of heavy ions between C and Fe obtained by three satellites is shown in Fig.3. The abundances are normalized at Fe = 1. The GEOTAIL data show comparable to the values from GCRs(CRIS) data as a whole, though there is a small deference at Ne and Si. The GEOTAIL observed heavy ions mostly at the distant tail region of magnetosphere far from the earth. Geomagnetic field in the distant tail is much weaker in the strength than that in the neighborhood of earth's surface

215

e[km]

800

1

" \

f :

1

•00

03

U)

1 H N

\

X

\

X

\

2-

Ar

|

j

x\l

h VL P- > 4

' \

^ \ -.

600

\ v

400

1 i-

(b)

^ \ \

1

1a

x

,

800

"

i-|km|

^\

1

N

J" \

l\ \

i

o o,

\ >

X

Af

A

]

kw

' \V \ \ \

-

^

\

^\

\

300

o' ,o'°

to' 2

io 1 4

>o16

number density [m" ]

IO'8

1010

. 1012

.

. 10'4

,

.

,

.

,018

. to18

number density [m" ]

Figure 4. The variation of atmospheric neutrals with altitude during (a):solar minimum period and (b):solar maximum period reported by U.S. Standard Atmosphere (1976).

and it is almost comparable to interplanetary magnetic field at LI point. Therefore, there is no or just a small modulation effect of geomagnetic field in the distant tail region. The HIT data seem to be enhanced more in the lighter nuclei than iron when compared with GCRs (CRIS) and GEOTAIL a . We consider one of the possible reasons that the heavier ions collide relatively easy with residual atmosphere while traversing to the deeper magnetosphere and then destroy themselves. So, the lighter ions will be enhanced near the earth. The variation of atmospheric neutral composition with altitude is shown in Fig.4 3 . The number density of neutral He and O during the solar maximum period is much larger than that of proton at more than 500 km (maybe up to about 10000 km), whereas proton is dominant component in the region higher than 500 km during the solar minimum period. If our assumption of energetic particles traversing in the magnetosphere while interacting with residual atmosphere is acceptable, their different abundance in the solar minimum phase may be observed when compared that in the solar maximum phase with our result. Then, we must consider this interaction effect to estimate precise GCRs composition even if satellite obsera

However, it is noted that no ions heavier than carbon was observed in L < 2 region, because of cut off rigidity itself.

216 vations in the neighborhood of the earth aim to detect heavy components particularly. 4. Conclusion We showed the relative abundances of heavy ions near the solar maximum period, which were obtained by three satellite, MDS-1 observed at 1 10 Re, GEOTAIL observed at 10 - 220 Re in the magnetosphere and ACE observed 230 Re beyond the magnetosphere. Comparing these data, we have found the results shown in the following : • The relative abundance observed by HEP/GEOTAIL in the distant tail far from the earth is similar to the abundance of GCRs observed by CRIS/ACE beyond the magnetosphere. • The abundance observed by HIT/MDS-1 in the magnetosphere near the earth is different from that in the distant tail and beyond the magnetosphere as GCRs. As one of possible reasons, we need to consider the cause of the observed abundances of nuclei presented here due to the interaction with residual atmospheric neutrals. Acknowledgements We thank the ACE CRIS instrument team and the ACE Science Center for providing the ACE data. Two of authors, M.H. and S.K, were partially supported by a Grant-in-Aid for The 21st Century COE Program (Physics of Self-Organization Systems) at Waseda University from MEXT and one of author, M.H., also supported by a Grant-in-Aid of Scientific Research of JSPS (Grant No. 17740153). References 1. H. Matsumoto et al, Jpn. J. Appl. Phys. , 44, 9A 6870 (2005) 2. T. Doke et al, J. Geomagn. Geodector., 46 713 (1994) 3. U.S. Standard Atmosphere, NOAA, NASA, USAF, Washington, (1976)

A P H O T O N TAG CALIBRATION B E A M FOR T H E AGILE SATELLITE

S. HASAN, M. PREST Universita degli Studi dell 'Insubria e INFN sezione di Milano, Italy L. FOGGETTA, C. PONTONI CIFS, Consorzio Interuniversitario

per la Fisica Spaziale, Torino, Italy

A. MOZZANICA Universita degli Studi di Milano e INFN sezione di Pavia, Italy G. BARBIELLINI, M. BASSET, F. LIELLO, F. LONGO, E. VALLAZZA Universita degli Studi di Trieste e INFN sezione di Trieste, Italy G. BUONOMO, G. MAZZITELLI, L. QUINTIERI INFN, Laboratori Nazionali di Frascati, Italy P. VALENTE INFN, sezione di Romal,

Italy

F. BOFFELLI, P. CATTANEO, F. MAURI INFN, sezione di Pavia, Italy

The AGILE satellite will be launched in 2006 for the study of gamma rays in the energy range 30 MeV-50 GeV. The satellite has to be calibrated using gamma rays of known energy. The calibration facility is being developed at the Beam Test Facility (BTF) at the INFN Laboratories in Frascati. The photons are produced by bremsstrahlung of electrons with a maximum momentum of 750 MeV/c. The electrons are tagged using a dipole magnet whose internal walls are covered by microstrip silicon detectors: depending on the energy loss, they impinge on a different strip once the dipole current has been set to a given value. The correlation between the direction of the electron measured by a pair of x-y silicon chambers and the impinging position on the tagging module inside the magnet allows the tagging of the photon. The paper describes the calibration layout and tests and the results, compared with the Montecarlo simulation, in terms of production rate and energy resolution.

217

218

1. Introduction The AGILE mission x will provide a powerful Observatory for 7-ray astrophysics in the energy range 30 MeV-50 GeV, during the years 2006-2008. AGILE will be launched by the Indian launcher PSLV on a equatorial orbit at 550 km with Malindi as ground base. The scientific instrument is light (wlOO kg) and will be able to detect and monitor 7-ray sources within a large field of view (wl/4 of the whole sky). Given that its launch is foreseen for the beginning of 2006, the calibration of the scientific payload has to be performed before the end of this year. The calibration facility is being developed at the Beam Test Facility (BTF) of the INFN Laboratories in Frascati. Its working principle and the experimental setup are described in Section 2, while Section 3 presents the results of the commissioning of the beamline and the comparison with the simulation.

E> >mi i i M P i h i i

Figure 1.

Working principle of the photon tag calibration beamline.

219 2. T h e calibration facility The DAFNE Beam Test Facility (BTF) 2 is a beam transfer line for bunches of electrons and positrons in the energy range tens of MeV - 750 MeV in a wide range of multiplicities down to the single electron mode. The pulse duration is 10 ns and the pulse rate is 50 Hz. Fig. 1 shows the basic idea of the photon tag beamline. Photons are produced in the bremsstrahlung process by the impinging electron beam. The last dipole magnet in the line is used as a "tagger" of the electrons that have undergone bremsstrahlung in the silicon beam chambers positioned before the magnet itself. These electrons have lost part of their energy in the photon and thus they curve towards the inner walls of the magnet itself, which is completely covered by silicon microstrip detectors.

(a)

(b)

Figure 2. (a) One of the silicon detectors of the beam chambers, (b) One of the tagging modules located inside the bending magnet; the frontend PCB and the silicon detector are clearly visible. The maximum clearance for the modules is 4 cm.

The silicon beam chambers (Fig. 2(a)) before the magnet are the medium for the bremsstrahlung process and they have to reconstruct the direction of the primary beam. They consist in two pairs of 8.9x8.9 cm 2 380 /um thick strip detectors (Micron Semiconductor ltd) with a pitch of 228 /xm. Each tile is readout by 3 TAA1 ASICs (IDEAS, Norway).The two tiles of each pair are positioned in a x-y configuration and the distance between the two pairs is 15 mm. Given the pitch and the analog readout, the spatial resolution is w40 /im while the angular one is w 2 mrad.

220

Fig. 2(b) presents one of the tagging modules, which is a 12 cm long 300 /jm thick silicon detector with a strip pitch of 100 /im and a readout pitch of 300 (im in the central region and 600 ,um in the outer 3 mm. The tagging system consists in 12 tagging modules positioned in the 4.1 cm gap of the bending magnet. Given this value, a very compact design has been chosen: the frontend electronics (3 TAAls per module) is located above the silicon detector and is connected to a pitch adapter which in turn is bonded to the silicon strips. The 12 modules are organized in 2 boxes.

10O

150

200

250

300

350

400

450 500 Energy MeV

Figure 3. Bremsstrahlung spectra for a primary beam energy of 424 MeV. The results from the Geant 3.21 simulation (triangles) are in good agreement with the data measured by the Nal calorimeter (full line).

3. Results In the commissioning phase, the bremsstrahlung spectrum and the tagging system response have been verified using a 15 Xo Nal(Tl) calorimeter. Fig. 3 shows the bremsstrahlung spectrum measured by the calorimeter when one of the tagging modules has detected an electron. The data are compared with the simulation obtained from the code written with Geant 3.21. The mimimum threshold for the calorimeter is of the order of 50 MeV. Fig. 4(a) presents the relationship between the value of the photon energy as measured by the Nal(Tl) calorimeter and the value given by the tagging

221

0

50

100

150

2M

2S0

300

3S0

400

450

-300

-200

-100

0

100

E t p w t w t m r g y M*V

(a)

200

300

400

(Expactwt • mtaturad) • " • ' O Y

500 * V

(b)

Figure 4. (a) Relationship between the value of the photon energy as computed using the tagging modules (X axis) and the one measured by the Nal calorimeter (Y axis). (b) Difference between the expected (tagging modules) and measured (Nal)photon energy.

system. Fig. 4(b) shows the residuals of the same distribution. The system energy resolution is limited at the moment by the calorimeter one, which is of the order of 6% from 424 MeV down to 100 MeV and then increases up to 18%. The system allows to compute the photon energy both when there is a single track in the beam chambers and for multi track events (the maximum allowed multiplicity is around 5 electrons per bunch). In this case, the entrance angle of the electron in the magnet is computed with a weighted average of the track positions in the chambers.

4. Conclusions A photon tagged beam facility has been commissioned at the BTF (LNF, Frascati). Photons are produced with a bremsstrahlung spectrum from 750 MeV electrons and their energy is measured tagging the interacting electron with a silicon spectrometer inside the last bending magnet. The energy resolution is limited at the moment by the calorimeter used in the system calibration phase. The photon production rate strongly depends on beam multiplicity and is of the order of 1 Hz.

222

References 1. G. Barbiellini et al, Nucl. Instr. and Methods A 490, 146-158, 2002. 2. G. Mazzitelli et al., Nucl. Instr. and Meth. in Phys. Res. A 515, 524-542 (2003).

OBSERVATION PROGRAM OF ISOTOPE COMPOSITION IN THE ULTRA HEAVY COSMIC RAYS

N. HASEBEA, M. HAREYAMAA, S. KODAIRAA, K. SAKURAIA, N. YAMASHITAA, T. MIYACHIA, O. OKUDAIRAA, M. TAKANOA, S. TORIIA, T. DOKEA, K. OGURAB, N. YASUDAC, Y. UCHIHORIc, H. TAWARAD, S. NAKAMURAE, T. SHIBATA", T. YANAGIMACHI0 AND S. WANAJO" A) Advanced Research Institute for Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan B) National Institute of Radiological Sciences, Inage, Chiba 263-8555, Japan C) College of Industrial Technology, Nihon University, Narashino, Chiba, 275-8576Japan D) High Energy Accelerator Organization, Tsukuba, Ibaraki 305-0801, Japan E) Department of Physics, Yokohama National University, Yokohama, Kanagawa 2408501, Japan F) Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 229-8558, Japan G) Department of Physics, Rikkyo University, Toshima-ku, Tokyo 171-8501, Japan H) Research Center for the Early Universe, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

The observational study of isotopic composition of ultra-heavy cosmic rays (UH-CRs) is the next important step after the ACE measurement of light isotopes, since no data has been available for both elemental and isotopic abundances in these cosmic rays. These abundances provide the clue for neutron capture nucleosynthesis, which is an essential for understanding the origin of galactic cosmic rays (GCRs). Long-duration-superpressure balloon over Antarctica capable of carrying large scientific payloads for a long period is under planning. A Large Isotope Spectrometer Array (LISA) made of solid-state track detectors (SSTD) will make the first measurement of the elemental and isotopic composition of GCRs for Z > 30 in the energy above 100 MeV/n. 1.

Introduction

The observations of nuclear components consisting of stable and radioactive isotopes with various half-lives in ultra-heavy cosmic rays (UH-CRs) give crucial constraints on a source of galactic cosmic rays (GCRs), the stellar nucleosynthesis, the chemical history of Galactic material, and offer new possibilities for the study of the acceleration and propagation mechanisms of

223

224

charged particles in space [1]. Although the elemental composition of UH-CRs has been observed by several previous experiments [2-4] and recent TIGER experiment [5], the isotopic one of UH-CRs has not been observed yet because of the flux in GCRs is very low. In this paper, we present the observation program of isotopic and elemental compositions of UH-CRs for Z > 30 with the energy above 100 MeV/n using a Large Isotope Spectrometer Array (LISA) made of solid-state track detectors (SSTD) onboard long-duration-super-pressure balloon over Antarctica. 2. Scientific significance for the observation of UH-CR isotopes The element Ne in GCR abundance is enhanced relatively as compared with the solar-system abundance from the previous observations of isotopic compositions for Z < 30 based on most recent experiment ACE/CRIS [6]. This is thought as due to the contribution from Wolf-Rayet stars which usually exist in superbubble [6,7]. The measurement of the abundances of 59Ni and 59Co gives the average delay time greater than about 105 years between nucleosynthesis and acceleration, and suggests that the supernovae (SN) do not accelerate their own ejecta [8]. Furthermore, from the consideration using the condensation temperature of each nuclide, it is pointed out that the abundances of UH-CRs with high condensation temperature are enhanced in GCRs [9]. Therefore, those nuclei which are relatively overabundant in the source composition seem to have been accelerated by multiple SN shocks in the OB-association [7,10]. Moreover, the trans-iron nuclei are produced by the process different from one for below iron-group. Although the elemental composition of UH-CRs was measured by some experiments [2-5], the measured one is modified by the elemental fractionation effect. Pure s, r, /^-process isotopes such as 92Mo, 96Mo, 100Mo will thus be useful for the origin of GCRs. In particular, the separation of pure r and /^-process isotopes for Z > 30 are important since the mass number of adjacent isotopes are separated by two atomic mass units. Moreover, the comparison of the abundance in a source of GCRs (mean life time: about 107 year) with those in primitive metal-poor star (about 1010 yr ago) or solar-system (4.5xl0 9 yr ago) will be useful for understanding of the chemical evolution of the Galaxy. Using the radioactive isotopes in UH-CRs, we will be able to determine the GCRs life time with 127I (TI / 2 =1.7X10 7 yr) or 135Cs (t 1/2 =2.1xl0 6 yr), the delay time between nucleosynthesis and acceleration with 81Kr (TI/ 2 =2.3X10 5 yr), and search for the nearby source due to SN explosion with 79Se (T 1 / 2 =6.5X10 5 yr) or 93Zr (TI/2=1.5xl06yr)-

225

Thus, the observation of isotopes for Z > 30 will enable a great progress in the studies of the origin of GCRs, the stellar nucleosynthesis, the chemical evolution and GCR acceleration and propagation mechanisms. 3. Detector concept and measurement technique The SSTDs such as CR-39 plastic detector or BP-1 glass one are very promising for the large-scale observation of these nuclei in space. This is due to the fact that it is easy to realize a low-cost large detector array. The LISA consists of many SSTD stack modules of 25 cm x 25 cm x 3 cm' and total detector area is 2 m x 2 m (8 x 8 modules). Its geometric factor and weight are ~ 24 m2 sr and ~ 400 kg, respectively. The LISA has the hodoscopes at the top and bottom of instrument, which select only trans-iron events both with the energy below 800 MeV/n at the top of instrument and with the relativistic energy and determine their trajectories by searching for their through-holes. The temperature in the instrument is kept within ± 3 K. We have verified the capability of a CR-39 plastic detector for isotope identification using heavy ion beam from NIRS-HIMAC. In the case of the SSTD, we use the relation of I values and R values to identify the mass, where L denotes the cone length of the etch-pit which is a function of dE/dx, and R is the residual range of an ion. The mass identification of the ion can be achieved using the so-called "L-R pairs" [11] by tracing the trajectory (see Fig. 1). Although the preliminary mass resolution for iron isotopes in CR-39 is 0.28 amu in rms (Fig. 2), by eliminating these errors as much as possible, the mass resolution is expected to be improved to ~ 0.20 amu [12]. The BP-1 glass detector has been used to measure the elemental composition of Pt-Pb group in GCRs as TREK experiment onboard the Russian space station Mir [4]. However, the mass resolution in BP-1 has not been investigated yet. Therefore, a good SSTD with high mass resolution for ultra-heavy isotopes is under development and we will employ the most suit-able detector for the observation of UH-CR isotopes. The handling of solid-state track detector historically has been required a large number of year x person to scan and analyze the etch-pit produced on the detector. Although LISA has very large area of 4 m2 and its handling is difficult, we developed the automated digital imaging optical microscope (HSP-1000, Fig. 3) to scan and analyze the etch-pit produced on the detector, whose image acquisition speed is 50-100 times faster than conventional microscope system [13]. This scanning speed of etch-pit image produced on the detector allows quickly to analyze the hodoscopes and detector stacks. However, the development of new technique to measure the cone length of etch-pit with high accuracy as well as the diameter of etch-pit mouth is required to separate more

226

clearly the adjacent isotopes. Therefore, we have been now developing new three-dimensional scanning system for measuring the cone length with high accuracy.

Fig. 1 Cone length (Lj) and residual Fig. 2 Mass distribution for Fig. 3. High speed auto range (R,) for a particle passing iron nuclei in CR-39. scaning microscope of through detector sheets to the HSP-1000. stopping point. 4. Expected number of particles over Antarctica Since the flux of UH-CRs is very low, the detector with a large collecting power is required to observe such rare events. Definitive measurements are almost avail-able for long duration flight using super-pressure balloon over Antarctica capable of carrying very large scientific payloads for a long extended period, whose advantages are a long duration flight of ~ a few month or more, a relatively small vari-ation of air thickenss and a stability of high altitude without ballast. This super-pressure balloon is currently under development. 10'

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227 HEAO-3/C2 [14]. The number of particles was estimated in a 60-day of a 4 m2 detector area for two cases, the observation of isotopes with the energy below 1000 MeV/n at the top of atmosphere and the observation of nuclear charges with relativistic energy above 1000 MeV/n at the top of atmosphere, respectively. The relative abundance of UH-CRs is taken from HEAO-3/C3 [3] as data. The result of assuming expected number of particles for UH-CRs is shown in Fig. 4.

5. Summary The study of isotopic composition of UH-CRs is the next important step after the ACE measurement of light isotopes (Z < 30), since no data of isotopic abundances in the UH-CR regions nor decisive data of elemental abundance in the region are available. Isotopic abundance is less biased as for the chemical fractionation, which leads us to bring a preferred probe to study the origin. LISA made of SSTDs onboard long-duration-super-pressure balloon over Antarctica will be the first measurement of the elemental and isotopic composition of UHCRs with sufficient statistics for Z > 30 with the energy above 100 MeV/n.

Acknowledgments This work was partially supported by the Grant-in-Aid for The 21st Century COE Program (Physics of Self-Organization Systems) at Waseda University from MEXT. This work was performed as a part of the accelerator experiments of the Research Project (15P146) at NIRS-HIMAC. We would like to express our thanks to the staff of NIRS-HIMAC for their supports.

References 1. N. Hasebe and M. Kobayashi: Symposium on the Origin of Matter and Evolution of Galaxies, 2000, eds. T. Kajino et al, p.94. 2. P. H. Fowler et al, Astrophys. J., 314 (1987) 739. 3. W. R. Binns et al, Astrophys. J., 346 (1989) 997. 4. A. J. Westphal et al, Nature, 396 (1998) 50. 5. S.Geietetal.,Th 29th Int. Cosmic Ray Conf., (2005) (OG1.1). 6. W. R. Binns et al, Astrophys. J., (2005) (in press). 7. J. C. Higdon and R. E. Lingenfelter, Astrophys. J., 590 (2003) 822. 8. M. E. Wiedenbeck et al, Astrophys. J., 523 (1999) L61. 9. N. Hasebe et al., Nucl. Phys., A758 (2005) 292c. 10. R. E. Lingenfelter et al, Astrophys. J., 591 (2003) 228.

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11. 12. 13. 14.

G. F. Siegmon et al, Nucl. Instrum. Metk, 128 (1975) 461. S. Kodaira et al, Jpn. J. Appl. Phys., 43 (2004) 6358. N. Yasuda et al, Radiat. Meas., 40 (2005) 311. J. J. Engelmann et al., Astron. Astrophys., 233 (1990) 96.

HIGH-ENERGY COSMIC RAYS INVESTIGATED BY AIR-SHOWER MEASUREMENTS WITH KASCADE, KASCADE-GRANDE, AND LOPES A. HAUNGS^4, W.D. APEI/ 4 , P. BADEA A , L. B A H R E N 5 , K. BEKK A , A. BERCUCI C , M. BERTAINA D , P.L. BIERMANN 5 , J. BLUMER A ' F , H. B O Z D O C 4 , I.M. BRANCUS G , M. BRUGGEMANN G , P. BUCHHOLZ G , S. B U I T I N K 5 , H. B U T C H E R 6 , A. CHIAVASSA D , K. DAUMILLER A , A.G. DE BRUYN B , C M . DE V O S B , F. DI P I E R R O D , P. D O L l / , R. ENGEL A , J. ENGLER A , H. F A L C K E 3 " 6 - 5 , H. GEMMEKE 7 , P.L. GHIA J , H.-J. GILS 71 , R. GLASSTETTER*", C. GRUPEN G , D. HECK A , J.R.HORANDEL F , A. H O R N E F F E R 7 ^ , T. H U E G E A ' F , K . - H . K A M P E R T K , G.W. K A N T B , H.O. KLAGES A , U. K L E I N L , Y. K O L O T A E V G ,

Y. KOOPMAN B , O. KROMER 7 , J. KUIJPERS 7 7 , S. LAFEBRE 7 7 , G. MAIER A , H.J. MATHES A , H.J. MAYER A , J. MILKE A , B. MITRICA C , C. M 0 R E L L 0 7 , M. MULLER A , G. NAVARRA 75 , S. NEHLS 71 , A. NIGL 77 , R. OBENLAND 71 , J. OEHLSCHLAGER A , S. OSTAPCHENKO 71 , S. OVER G , H.J. P E P P I N G 5 , M. PETCU* 7 , J. PETROVIC 7 7 , T. PIEROG 71 , S. PLEWNIA 71 , H. REBEL 71 , A. RISSE M , M. R O T H F , H. SCHIELER 71 , G. SCHOONDERBEEK B , O. SIMA C , M. STUMPERT F , G. T O M A c , G.C. TRINCHERO J , H. ULRICH 71 , J.VAN BUREN 71 , W.VAN CAPELLEN 5 , W. WALKOWIAK G , A. WEINDL 71 , S. WIJNHOLDS 5 , J. WOCHELE 71 , J. ZABIEROWSKI M , J.A. ZENSUS 5 , D. ZIMMERMANN G Institut fur Kernphysik, Forschungszentrum Karlsruhe, Germany B ASTRON Dwingeloo, The Netherlands c NIPNE Bucharest, Romania D Dpt di Fisica Generale dell'Universita Torino, Italy E Max-Planck-Institut fur Radioastronomie, Bonn, Germany Institut fur Experimentelle Kernphysik, Uni Karlsruhe, Germany, Fachbereich Physik, Universitat Siegen, Germany Dpt of Astrophysics, Radboud Uni Nijmegen, The Netherlands 1 IPE, Forschungszentrum Karlsruhe, Germany 1st di Fisica dello Spazio Interplanetario INAF, Torino, Italy K Fachbereich Physik, Uni Wuppertal, Germany Radioastronomisches Institut der Uni Bonn, Germany Soltan Institute for Nuclear Studies, Lodz, Poland

229

230 Recent results from the multi-detector set-up KASCADE on measurements of cosmic rays in the energy range of the so called knee (at ss 3 PeV) are presented. The multidimensional analysis of the air shower data indicates a distinct knee in the energy spectra of light primary cosmic rays and an increasing dominance of heavy ones towards higher energies. This provides, together with the results of large scale anisotropy studies, implications for discriminating astrophysical models of the origin of the knee. To improve the reconstruction quality and statistics at higher energies, where the knee of the heavy primaries is expected at around 100 PeV, KASCADE has recently been extended by a factor 10 in area to the new experiment KASCADE-Grande. LOPES is set up at the location of the KASCADE-Grande experiment and measures radio pulses from extensive air showers. LOPES is designed as digital radio interferometer using high bandwidths and fast data processing and profits from the reconstructed air shower observables of KASCADE-Grande.

1. Introduction The all-particle energy spectrum of cosmic rays shows a distinctive discontinuity at few PeV, known as the knee, where the spectral index changes from —2.7 to approximately —3.1 (Fig. 1). At that energy direct measurements are presently hardly possible due to the low flux, but indirect measurements observing extensive air showers (EAS) are performed. Astrophysical scenarios like the change of the acceleration mechanisms at the cosmic ray sources (supernova remnants, pulsars, etc.) or effects of the transport mechanisms inside the Galaxy (diffusion with escape probabilities) are conceivable for the origin of the knee as well as particle physics reasons like a new kind of hadronic interaction inside the atmosphere or during the transport through the interstellar medium. It is obvious that only detailed measurements over the whole energy range of the knee from 10 14 eV to 10 18 eV and analyses of the primary energy spectra for the different incoming particle types can validate or disprove some of these models 1 . The multi-detector system KASCADE-Grande (KArlsruhe Shower Core and Array DEtector and Grande array) 2 , approaches this challenge by measuring as much as possible independent information from each single air-shower event. Recently, the original KASCADE experiment 3 was extended in area by a factor 10 to the new experiment KASCADE-Grande. KASCADE-Grande allows now a full coverage of the energy range around the knee, including the possible second knee at energies just below 10 18 eV. With its capabilities KASCADE-Grande is also the ideal testbed for the development and calibration of new air-shower detection techniques like the measurement of EAS radio emission, which is performed in the frame of the LOPES project 4 .

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2. The KASCADE-Grande and LOPES Experiments These experiments are located at the Forschungszentrum Karlsruhe, Germany, (49.1°n, 8.4°e, llOma.s.l.) and measure extensive air showers in a primary energy range from 100 TeV to 1 EeV. Their combination provides multi-parameter measurements on a large number of observables concerning electrons, muons at 4 energy thresholds, hadrons, and the radio emission. The main detector components are the KASCADE array, the Grande array, and the LOPES antenna array. The KASCADE array measures the total electron and muon numbers (Ei,,kin > 230 MeV) of the shower separately using an array of 252 detector stations containing shielded and unshielded detectors at the same place in a grid of 200 x 200 m 2 . The excellent time resolution of these detectors allows also decent investigations of the arrival directions of the showers in searching large scale anisotropics and, if existent, cosmic ray point sources. The KASCADE array is optimized to measure EAS in the energy range of 100 TeV to 80 PeV. A muon tracking detector measures the incidence angles of muons relative to the shower arrival direction. These measurements provide a sensitivity to the longitudinal development of the showers. The hadronic core of

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the shower is measured by a 300 m 2 iron sampling calorimeter installed at the KASCADE central detector: Three other components offer additional valuable information on the penetrating muonic component at different energy thresholds. The complementary information of the showers measured by the central and the muon tracking detectors is predominantly being used for tests and improvements of the hadronic interaction models underlying the analyses 5 . The multi-detector concept of the KASCADE experiment (Fig. 2), which is operating since 1996 has been translated to higher primary energies through KASCADE-Grande 6 . The 37 stations of the Grande Array (Fig. 3) extend the cosmic ray measurements up to primary energies of 1 EeV. The Grande stations, 10 m 2 of plastic scintillator detectors each, are spaced at approximative by 130 m covering a total area of ~ 0.5 km 2 . For the calibration of the radio signal emitted by the air shower in the atmosphere an array of first 10 and meanwhile 30 dipole antennas (LOPES) is set up on site of the KASCADE-Grande experiment 7 . The antennas operate in the frequency range of 40-80 MHz. The radio data is collected when a 'large event' trigger is received from KASCADE which translates to primary energies above 10 16 eV.

233

3. K A S C A D E Results The KASCADE data analyses aims to reconstruct the energy spectra of individual mass groups taking into account not only different shower observables, but also their correlation on an event-by-event basis. The content of each cell of the two-dimensional spectrum of reconstructed electron number vs. muon number is the sum of contributions from the individual primary elements. Hence the inverse problem g{y) = / K{y,x)p(x)dx with y = (Ne,NJ?) (see Fig. 4) and x = (E,A) has to be solved. This problem results in a system of coupled Fredholm integral equations of the form

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the probability PA is obtained by Monte Carlo simulations on basis of two different hadronic interaction models (QGSJET01 8 , SIBYLL2.1 9 ) as options embedded in CORSIKA 10 . By applying these procedures (with the assumption of five primary mass groups) to the experimental data energy spectra are obtained as displayed in Fig. 1, where the resulting spectra for primary oxygen, silicon, and iron are summed up for a better visibility. A knee like feature is clearly visible in the all particle spectrum, which is the sum of the unfolded single mass group spectra, as well as in the spectra of primary proton and helium. This demonstrates that the elemental composition of cosmic rays is dominated by the light components below the knee and by a heavy component above the knee feature. Thus, the knee feature originates from a decreasing flux of the light primary particles 11 . Comparing the unfolding results based on the two different hadronic interaction models, the model dependence when interpreting the data is obvious. Modeling the hadronic interactions underlies assumptions from particle physics theory and extrapolations resulting in large uncertainties, which are reflected by the discrepancies of the results presented here. In Fig. 4 the predictions of the Ne and AT'r correlation for the two models are overlayed to the measured distribution in case of proton and iron primaries. It is remarkable that all four lines have a more or less parallel slope which is different from the data distribution. There, the knee is visible as kink to a flatter Ne-Nff dependence above lg Nff « 4.2. Comparing the residuals of the unfolded two dimensional distributions for the different models with the initial data set we conclude that at lower energies the SIBYLL model and at higher energies the QGSJET model are able to describe the correlation consistently, but none of the present models gives a contenting description of the whole data set. These findings are confirmed by detailed investigations of further shower observables measured by KASCADE 5 .

234

Figure 4. Two dimensional electron (Ne) vs. muon (JV£r = number of muons in 40200m core distance) number spectrum measured by the KASCADE array. T h e lines display the most probable values for proton and iron primaries obtained by CORSIKA simulations employing different hadronic interaction models.

Investigations of anisotropics in the arrival directions of the cosmic rays give additional information on the cosmic ray origin and of their propagation. Depending on the model of the origin of the knee and on the assumed structure of the galactic magnetic field one expects large-scale anisotropics on a scale of 1 0 - 4 to 1 0 - 2 in the energy region of the knee. The limits of large-scale anisotropy analyzing the KASCADE data are determined to be between 1 0 - 3 at 0.7 PeV primary energy and 1 0 - 2 at 6 PeV 12 . These limits were obtained by investigations of the Rayleigh amplitudes and phases of the first harmonics. Taking into account possible nearby sources of galactic cosmic rays like the Vela Supernova remnant the limits of KASCADE already exclude particular model predictions. The interest for looking to point sources in the KASCADE data sample arises from the possibility of unknown near-by sources, where the deflection of the charged cosmic rays would be small or by sources emitting neutral particles like high-energy gammas or neutrons. The scenario for neutrons is very interesting for KASCADE-Grande, since the neutron decay length at these energies is in the order of the distance to the Galactic center. In KASCADE case no significant excess was found 13 . 4. Status of KASCADE-Grande Fig. 5 shows, for a single shower, the lateral distribution of electrons and muons reconstructed with KASCADE and the charge particle densities

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measured by the Grande stations. This example illustrates the capabilities of KASCADE-Grande and the high quality of the data. The KASCADEGrande reconstruction procedure follows iterative steps: shower core position, angle-of-incidence, and total number of charged particles are estimated from Grande Array data; the muon densities and the reconstruction of the total muon number are provided by the KASCADE muon detectors 14 . At the KASCADE experiment, the two-dimensional distribution shower size - truncated number of muons played the fundamental role in reconstruction of energy spectra of single mass groups. Hence, Figure 6 illustrates the capability of KASCADE-Grande to perform an unfolding procedure like in KASCADE. A hundred percent efficiency for KASCADE-Grande is reached for all primary particle types for energies above 2 • 1016 eV, providing still a large overlap with the KASCADE energy range. KASCADE-Grande started end of 2003 with combined measurements of all detector components. 5. LOPES Data Analysis The LOPES-10 data set is subject of various analyses using different selections: With an event sample obtained by stringent cuts the proof of principle to detect air showers in the radio frequency range was given 15 . With showers fallen inside KASCADE the basic correlations with shower parameters are shown 16 . Further interesting features are investigated with a sample of very inclined showers 17 and with a sample of events measured

236

during thunderstorms 18 . Additionally, an analysis is performed correlating the radio signals measured by LOPES-10 with EAS events reconstructed by KASCADE-Grande with distant cores included 19 . Grande is taking data in coincidence with KASCADE and LOPES and enables to reconstruct showers with primary energies up to 10 18 eV and with distances between shower core and the LOPES-10 antennas up to 700 m. A crucial element of the analysis method is the digital beamforming which allows to place a narrow antenna beam in the direction of the cosmic ray event. This is possible because the phase information of the radio waves is preserved by the digital receiver and the cosmic ray produces a coherent pulse. This method is also very effective in suppressing interference from the particle detectors which radiate incoherently. The procedure of time shifting of the radio signals in the antennas is relatively safe when based on the values provided by the reconstruction for shower core and shower axis using the data of the original KASCADE field array. Due to the high granularity of the detector stations the accuracy of core and direction reconstruction is high enough to obtain a good coherence of the radio signals. The shower reconstruction using information from the Grande array is re-

Figure 7. Radio signals in the 10 antennas after first beamforming Cupper panel) and after optimized beamforming (lower panel).

Figure 8. Correlation of primary energy reconstructed with KASCADE-Grande with the reconstructed height of the radio pulse signal,

237

quired for shower cores outside KASCADE. The Grande stations, 10 m 2 of plastic scintillator detectors each, are spaced at ~130 m and cannot assure an accuracy comparable with the original KASCADE array. So, a so-called 'optimized' beamforming is performed, which searches for a maximum coherence by varying the core and the direction around the values provided by the Grande reconstruction. Fig. 7 shows the raw radio signal of an example event before and after the optimized beamforming. The event has a mean core distance to the antennas of « 100 m, a primary energy of 5 • 10 17 eV, and a zenith angle of 40°. An noticeable increase in coherence is seen after the optimized beamforming. Fig. 8 shows the final result of this analysis, where the radio pulse amplitude per unit bandwidth (e„) is plotted versus the primary energy reconstructed by KASCADE-Grande. A clear correlation between radio pulse height and primary particle energy is visible. One has to recognize that at LOPES 10 the antennas are not absolute calibrated, i.e. ev is a relative value, only, just as well LOPES 10 measures only one (east-west) polarization direction of the signal.

6. Conclusions and Outlook From KASCADE measurements we do know that at a few times 10 15 eV the knee is due to light elements, that the knee positions depend on the kind of the incoming particle, and that cosmic rays around the knee arrive our Earth isotropically. Additionally KASCADE could show that no current hadronic interaction model which are unavoidably needed for the interpretation of air shower data, describes very well cosmic ray measurements in the energy range of the knee and above. These model uncertainties are due to the lack of accelerator data at these energies and especially for the forward direction of collisions. The extension of KASCADE to the KASCADE-Grande experiment, accessing higher primary energies, is expected to solve the question of the existence of a knee-like structure corresponding to heavy elements. KASCADE-Grande keeps the multi-detector concept for tuning different interaction models at primary energies up to 10 18 eV. To improve further the data quality a self-triggering, dead-time free FADC-based DAQ system will be implemented in order to record the full time evolution of energy deposits in the Grande stations 20 . At present, LOPES operates 30 dipole radio antennas (LOPES-30) positioned inside or nearby KASCADE 21 . The longer baseline of the set

238 u p will allow a more detailed investigation of t h e lateral extension of the air shower radio signal. Additionally these antennas will be absolute calibrated, so t h a t the obtained pulse amplitude can be directly compared with theoretical expectations for the radio field strength 2 2 .

References 1. A. Haungs, H. Rebel, M. Roth Rep. Prog. Phys. 66 1145 (2003). 2. G. Navarra et al. - KASCADE-Grande coll. Nucl. Instr. Meth. A 518 207 (2004). 3. T. Antoni et al. - KASCADE coll. Nucl. Instr. Meth. A 513 429 (2003). 4. A. Horneffer et al. - LOPES coll. Proc. SPIE 5500 129 (2004). 5. A. Haungs et al. - KASCADE coll. Proc. XIIIISVHECRI 2004, Pylos, Nucl. Phys. B (Proc. Suppl.) in press (2004), astro-ph/0412610. 6. A.F. Badea et al. - KASCADE-Grande coll. Nucl. Phys. B (Proc. Suppl.) 136 384 (2004). 7. A. Haungs et al. - LOPES coll. Proc. 22nd Texas Symposium, Stanford, CA, Dec. 2004 eConf C041213 2413 (2004). 8. N.N. Kalmykov, S.S. Ostapchenko Phys. Atom. Nucl. 56 346 (1993). 9. R. Engel et al. 26th ICRC, Salt Lake City, Utah p.415 (1999). 10. D. Heck et al. Report FZKA 6019, Forschungszentrum Karlsruhe (1998). 11. T. Antoni et al. - KASCADE coll. Astrop. Phys. 24 1 (2005). 12. T. Antoni et al. - KASCADE coll. Astrophys. J. 604 687 (2004). 13. T. Antoni et al. - KASCADE coll. Astrophys. J. 608 865 (2004). 14. R. Glasstetter et al. - KASCADE-Grande coll., Proc. of 29th ICRC, Pune, India (2005). 15. H. Falcke et al. - LOPES coll., Nature 435 313 (2005). 16. A. Horneffer et al. - LOPES coll. Proc. of 29th ICRC, Pune, India (2005). 17. J. Petrovic et al. - LOPES coll. Proc. of 29th ICRC, Pune, India (2005). 18. S. Buitink et al. - LOPES coll. Proc. of 29*h ICRC, Pune, India (2005); 19. A.F. Badea et al. LOPES coll. Proc. of 29th ICRC, Pune, India (2005); A.F. Badea et al. LOPES coll. Proc. of 29th ICRC, Pune, India (2005). 20. W. Walkowiak et al. - KASCADE-Grande coll. Proc. of IEEE Nuclear Science Symposium, Rome (2004). 21. S. Nehls et al. - LOPES collab., Proc. of 29th ICRC, Pune, India (2005). 22. T. Huege and H. Falcke, Astrop. Phys. 25 116 (2005).

SCINTILLATING OPTICAL FIBERS FOR ASTROPARTICLE PHYSICS W. R. BINNS, M. H. ISRAEL, K. RIELAGE1 Dept. of Physics and McDonnell Center for the Space Sciences, Washington University St. Louis, MO 63130, USA D. H. KAPLAN Department of Physics Southern Illinois University Edwardsville, IL 62026, USA Polystyrene-based scintillating optical fibers are manufactured at Washington University for use in astroparticle physics instruments. A hodoscope of 0.2-mm square-crosssection fibers has been operating on the Cosmic Ray Isotope Spectrometer on the ACE spacecraft at LI since 1997. A hodoscope of 1-mm fibers operated in a large-area cosmic-ray detector for a total of 50 days on stratospheric balloons over Antarctica. Ribbons of 1-mm fibers are part of the anti-coincidence shield on the Gamma-ray Large Area Space Telescope (GLAST). Laboratory tests of these fibers have demonstrated insensitivity to moderate radiation doses, well understood variation of response with temperature, and attenuation lengths in excess of a meter.

1. Production of Scintillating Fibers At Washington University we fabricate scintillating optical fibers of square cross-section for use in astroparticle physics instruments on stratospheric balloons and spacecraft [1]. The scintillating core is polystyrene (index of refraction, n, 1.62) with butyl-PBD primary dye and dimethyl-POPOP secondary dye, and the single cladding is acrylic (n=1.50). We have also fabricated double-clad fibers in which the second cladding is fluorinated acrylic (n=1.43). Various fiber dimensions have been used from 0.2 x 0.2 mm2 with 26-cm detector length to 1.0 x 1.0 mm2 with 1.2-m detector length. The dyes are dissolved in the styrene monomer and a boule is created with a 3.8 x 3.8 cm2 cross-section and 40 cm length. The cladding thickness ranges from 2% to 5% of the boule dimension, depending to which we are drawing the final fiber. With controlled heating a long thin fiber is pulled off the boule and

Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA

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wrapped on a large (1.8-m diameter) drum. The final dimensions of the fibers are controlled by controlling the speed at which the drum rotates and the speed at which the boule is lowered past the heating element. The cross-section of the fibers preserves the square shape of the original boule, and the relative dimensions of the core and clad are similarly preserved. 2. Fiber hodoscope for the CRIS instrument on the ACE spacecraft The Cosmic Ray Isotope Spectrometer (CRIS) [2] on the Advanced Composition Explorer (ACE) spacecraft measures the isotopic composition of the cosmic rays, using the Energy-loss/Total-energy technique with nuclei stopping in a stack of totally depleted lithium-drifted silicon detectors. Precise measurement of energy loss (dE/dx) requires knowledge of the angle the incident particle makes with the normal to silicon detector stack (the secO correction). Also, it requires knowledge of the location of the incident particle in the detector because the detector thicknesses are not perfectly uniform. The CRIS instrument is composed of four stacks of 10-cm-diameter Si(Li) detectors next to each other, with a 26 cm x 26 cm hodoscope above. The hodoscope is composed of three xy pairs of scintillating fibers. The fibers each have 200um square cross-section including lOum cladding all around. The cladding is coated with black ink to prevent optical coupling between fibers. The light output of the fibers is detected with an image-intensified CCD camera. CRIS achieved excellent mass resolution (a < 0.25 amu). Among the many astrophysical results that depend on the excellent resolution of CRIS are demonstration of the time delay between nucleosynthesis and acceleration of heavy cosmic rays [3] and the indication that the composition of the cosmic rays reflect acceleration in the superbubbles surrounding OB associations [4]. The ACE spacecraft was launched in August 1997 and has been in space near the LI Lagrange point on the Earth-Sun line about 1.5 x 106 km sunward of Earth for over eight years. During this time the entire CRIS instrument has been operating flawlessly. We estimate that the topmost layer of fibers has been exposed during these eight years to approximately 2 krad of total radiation dose. Figure I shows the variation with time of the mean integrated intensity of the spot of light in the topmost (XI) layer and the bottommost (Y3) layer from iron nuclei stopping in the instrument. We note approximately 9% degradation of the signal amplitude in XI over nearly eight years., and less in Y3. This degradation could be a combination of the effects of radiation damage on the scintillators themselves and degradation of the microchannel plate of the image intensifier, which has seen more total charge transported in the region imaging XI than in

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the region imaging Y3. However, the laboratory results described in section 5 below, suggest that this degradation is not due to radiation damage to the fibers. 3. Fiber hodoscope for the TIGER stratospheric balloon instrument The Washington University group has been building scintillating fiber detector systems for balloon-flight cosmic-ray instruments for many years. Early balloon flights with these fibers demonstrated their capability prior to their selection for the CRIS instrument. Our most recent balloon-borne instrument using scintillating fibers is the Trans-Iron Galactic Element Recorder (TIGER), an instrument with 1.2 m x 1.2 m area designed to measure the elemental composition of the rare cosmic rays with atomic number (Z) greater than 30. [5] The instrument measures Z for particles penetrating a stack of plastic scintillators and two Cherenkov detectors with index of refraction 1.5 and 1.04. The particle trajectories are determined with xy fiber layers above and below the detector stack. In this detector system the fibers have 1-mm square crosssection, including um cladding. The fibers are grouped in tabs of six or seven fibers and read out using 2.5cm photomultipliers. Each of the four hodoscope layers of 196 tabs is read out with just 28 photomultipliers, using a coding system. At one end (the course end) fourteen consecutive tabs are bundled and coupled to one photomultiplier. At the other end (the fine end) every fourteenth tab is bundled and coupled to one photomultiplier. The combination of signals in one course and one fine photomultiplier defines which of the 196 tabs was penetrated by the particle.

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4. Fiber anticoincidence strips for the GLAST spacecraft The Large-Area Telescope of the Gamma-ray Large Area Space Telescope (GLAST) measures high-energy gamma rays pair-producing in a tower of silicon-strip detectors. A key element of this instrument is its anticoincidence detector (ACD), which is required to reject nearly all of the charged particles, which are the background for any gamma-ray experiment. The requirement for the ACD is that it must have 0.9997 efficiency and hermeticity for singly charged particles [6]. The ACD is composed of individual tiles of scintillators, but to close off the inevitable cracks between tiles, our Washington University group has manufactured strips of scintillating fibers that are laid over the cracks. Calculations by the GLAST scientists (Moiseev, private communication) show that with the fiber ribbons the required ACD efficiency is achieved, while without the ribbon an efficiency of only about 0.9965 is achieved. 5. Laboratory tests of the effects of radiation on the fibers To test the effect on our scintillating fibers of moderate doses of radiation, as might be experienced on a space-flight instrument, we exposed a set of our 0.75-mm square cross-section fibers to beams of 42 MeV protons at the Indiana University Cyclotron Facility, accumulating a dose of 4 krad. (Because radiation damage can repair in the presence of oxygen, and that oxygen would not be available in a space exposure, the fibers were kept in a pure nitrogen environment during and after the exposure.) Figure 2 shows that within the accuracy and repeatability of the test set-up (-+/- 5%) there was no difference

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between the performance of these fibers before and after the radiation. In each case, these fibers displayed a 1/e attenuation length of approximately 160 cm, and the pulse-height spectra of light detected at one end from electrons penetrating at various distances from that end after the irradiation was indistinguishable from that recorded prior to the irradiation. 6. Laboratory tests of thermal effects on fibers Kaplan et al [8] have measured the light output of a fiber as a function of temperature over the range -30°C to +50°C. (Figure 3) These data are consistent with a model of self-absorption by the secondary dye, which is described in reference [8].

Relative Intensity

Figure 3 Signal intensity vs wavelength at various temperatures.

Acknowledgments This work was supported in part by NASA grants NNG05WC04G and NAG512929. The authors acknowledge the important work of our CRIS collaborators at Caltech, JPL, and NASA/GSFC, of our collaborators at Caltech and NASA/GSFC on TIGER, and of Alexander Moseeiv our GLAST contact at NASA/GSFC.

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References 1. A. J. Davis, et al., Nucl. Inst. & Meth. A276, 347 (1989). 2. E. C. Stone, et al., Space Sci. Rev. 86, 285 (1998). 3. M. E. Wiedenbeck, et al., Astrophys. J. 523, L61 (1999). 4. W. R. Binns, et al., Astrophys. J. in press (2005). 5. J. T. Link, et al., 28th Intl. Cosmic Ray Conf. (Tsukuba) 4, 1781 (2003). 6. A. Moiseev, et al., 26'h Intl. Cosmic Ray Conf. (Salt Lake) 5, 160 (1999). 7. K. Rielage, Ph.D. Thesis, Washington University (2002). 8. D. H. Kaplan, et al., Nucl. Inst. & Meth. in press (2005).

T H E T2K E X P E R I M E N T : STATUS A N D I N S T R U M E N T A T I O N OF T H E 280M N E A R D E T E C T O R

YU. G. KUDENKO* Institute for Nuclear Research of the Russian Academy of Sciences October Revolution Pr. 7A, 117312 Moscow, Russia E-mail: [email protected]

The Tokai-to-Kamioka (T2K) experiment is a second generation long baseline neutrino oscillation experiment which aims at a sensitive search for the ve appearance. The T2K near neutrino detectors at 280 m from the target will measure the neutrino beam properties prior to the oscillation and interaction cross sections. The main design features of these detectors and recent studies of new scintillator/WLS fiber detectors readout by multipixel APD's are presented.

1. Introduction One of the major achievements of the particle physics in the past decade is the discovery of the neutrino masses and mixing based on the experimental studies of the neutrino oscillation1. These results show the necessity for the extension of the Standard Model. Three generation neutrino oscillations depend on the three mixing parameters sin20i2,sin2f?23,sin2013 and the two mass-squared differences Am 2 ol = Am| x = m% — m\ and Am 2 t m = Arrijj = m\ — m\. Current oscillation parameters (3a interval) from the global three-neutrino analysis2 look as follows: A m ^ = (7.1 - 8.9) x 1 0 - 5 , Ara^, = (1.4 - 3.3) x 10" 3 , sin 2 0 12 = 0.23 - 0.37, sin2023 = 0.34 - 0.66, and sin2f?i3 < 0.051. The main road of neutrino physics will be the high precision measurements of these parameters, improve sensitivity and possibly measure #13 and CP-violation in lepton sector, distinguish between the normal and invert mass hierarchies, and to identify the neutrinos as being Dirac or Majorana. The next few years the new results are expecting from long baseline neutrino experiments at Fermilab MINOS 3 and OPERA 4 at CERN. These experiments will be able to test #13 with sensitivity a few 'Presenting the T2K Collaboration

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times better than CHOOZ experiment, but at least the improvement by more than order of magnitude is needed. 2. The T2K long baseline experiment The T2K project 5 is a second generation long baseline neutrino oscillation experiment which will use a high intensity narrow band neutrino beam produced by JPARC 50 GeV (initially 40 GeV) proton beam. The first phase of the T2K experiment is aimed at two main goals: the sensitive measurement of #13 and the more accurate determination of the parameters sin2#23 and Am| 3 . The design intensity of the proton beam is 3.3 x 1014 protons/pulse at a repetition rate of about 0.3 Hz. The fast extracted beam width is 5.6 /zsec and there are 8 (or 15) bunches in a spill. The width of each bunch is 58 ns. Three electromagnetic horns will be used to focus charged pions generated in a graphite target installed inside the first horn. The focused pions decay in a 130 m decay tunnel which follows the horns. The off-axis beam configuration will allow to change the offaxis angle between 2° and 3° that provides the almost pure v^ beam with mean neutrino energies from 0.5 to 0.9 GeV, respectively, and a fraction of ve of about 0.5% at the peak energy. The sensitivity of better than (5(sin2013) = 0.01 at 90% c.l. will be achieved for 5 years of running. 3. The near neutrino detectors To achieve the physics goals, it is important to provide precise measurements of the neutrino beam properties near the target, neutrino flux, spectrum and interaction cross sections. For these purposes, the near detector complex (ND280) will be built at 280 m from the target along the line between the average pion decay point and the Super-Kamiokande. The physics requirements for ND280 can be briefly summarized as follows. The energy scale of the neutrino spectrum must be understood at 2% level, and the neutrino flux should be monitored with better than 5% accuracy. The momentum resolution of muons from the charged current quasi-elastic interactions should be less than 10%, and the threshold for the detection of the recoil protons is required to be about 200 MeV/c. The ve fraction is to be measured with an uncertainty of better than 10%. The measurement of the neutrino beam direction with precision much better than 1 mrad is expected to be provided by the on-axis detector (neutrino monitor). The off-axis ND280, shown in Fig. 1, consists of the UA1 magnet operated with magnetic field of 0.2 T, a Pi-Zero detector (POD), a tracking detector which

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SMRD Figure 1.

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The cutaway view of the T2K ND280 near detector.

has time projection chambers (TPC's) and fine grained scintillator detectors (FGD's), an electromagnetic calorimeter (Ecal), and a side muon range detector (SMRD). POD Detector. The POD has been designed to be similar to the K2K SciBar 6 and MINERi/A 7 detectors. It is installed in the upstream end of the magnet and optimized for measurement of the inclusive n° production by v^ on oxygen. In addition, the POD will measure the intensity and spectrum of the beam-associated electron neutrino flux. The two measurements will allow to control two major sources of backgrounds in the electron appearance search. The POD consists of 76 tracking planes composed of polystyrene triangular scintillating bars (about 2 x 104) alternating with thin (0.6 mm) lead foils. Each bar has a 3 cm base, 1.5 cm height and a central hole for a WLS fiber. The measurement of neutrino interaction with oxygen is achieved by interleaving water target planes between the thirty upstream scintillating planes. The water cells consist of semiflexible pillow bladders, nominal dimensions 3 cm x 1.8 m x 2.1 m, holding about 100 kg of water each. The schematic view of the POD target is shown in Fig. 2. The total mass of the POD will be about 12 tons with a fiducial mass of 1.7 tons of water, 3.6 tons of plastic scintillators, and 0.8 tons of lead. The expected energy resolution (without photostatistics) for events

248 sjMiw

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fully contained in the active target is &E — 10% + 3.5%/\/E(GeV). The efficiency of the photon (ir°) detection and reconstruction will be about 80-90% (55%). It is expected about 1.7 x 104 neutral current single 7r° events in the water target for 10 21 protons on target (one year of running). Tracking detector. Three TPC's will measure the 3-momenta of muons produced by charged current interactions in the detector and will provide the most accurate measurements of the neutrino energy spectrum. In addition, dE/dx measurements will also determine the sign of charged particles and identify muons, pions, and electrons. The outer dimensions of the three TPC modules are assumed to be approximately 2.5 m x 2.5 m in the plane perpendicular neutrino beam direction, and about 0.9 m along the beam direction. A candidate for the gas mixture is Ar:C0 2 (90:10). At drift field of 200 V/cm, the transverse diffusion is expected to be 220 nm/y/cm and the drift velocity is 14 mm/fis. The proposed baseline design uses ether GEM's or Micromegas for the gas amplification. Fig. 3 shows the TPC module. The desired spatial resolution (300 ^m) determines a small size of about 8 mm x 8 mm for pads, leading to about 105 channels for 3 TPC modules. The FGD consisting of layers of segmented scintillator bars readout by WLS fibers allows tracking of all charged particles arising from neutrino interactions with the target nuclei. The ND280 will contain two FGD's, each

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Figure 3.

The layout of a T P C module.

with dimensions 2 m x 2 m x 30 cm resulting in the total mass of 1.2 tons. The first FGD will be an active scintillator detector, one layer of which will consist of 200 scintillator bars, and thirty layers will be arranged in alternating vertical and horizontal layers perpendicular to the beam direction. The tracking threshold is expected to be about 4 cm, which corresponds to 350 MeV/c for a proton. The second FGD will consists of x — y layers scintillator bars alternating with 3 cm thick layers of passive water. The water layer will constructed from sheets of corrugated polypropylene, of dimension 1.0 m x 2.0 m and an outer thickness 1.0 cm. An active water scintillator FGD is considered as a detector upgrade. Ecal. The main purpose of the Ecal is to measure photon energy from the decays of 7r°'s produced by neutrino interactions in the POD, FGD's and TPC's. The Ecal surrounds the POD and tracking region and consists of 15 layers, each of which has one sheet of 3-mm Pb alloy and one layer of 1 cm thick x 5 cm wide plastic scintillator bars (with signal readout by WLS fibers). Interior this section is the preradiator section, where each of the 3 layers consists of a lead alloy sheet backed by 3 layers of scintillator bars. SMRD. Muons which escape at large angles with respect to the neutrino beam can not be measured by TPC's. However, they will intersect the iron yoke and therefore a muon's momentum can be obtained from its range by instrumenting the iron at various depths. About 40% of muons from

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charged current quasi-elastic reactions and about 15% of muons from charge current non-quasi-elastic reactions are expected to intersect the SMRD. The UA1 iron yoke consists of 16 C-shaped elements made of sixteen 5 cm thick iron plates, with 1.7 cm air gap between the plates and is segmented in 12 azimuthal sections. The active component of the SMRD will consist of 1 cm thick scintillator slabs sandwiched between the iron plates of the magnet yokes. The readout will be provided by wavelength shifting fibers (WLS) to transport the light into photosensors. The scintillators can be coated by a chemical reflector to increase the light yield. The Monte Carlo study indicated that a muon energy resolution of less than 10% can be achieved. The SMRD is required to identify MIP's with very good efficiency. It is also desirable to have good inter-layer timing resolution to distinguish inwardfrom outward-going particles.

4. P h o t o s e n s o r s The Nd280 detector will widely use the WLS fiber readout with individual detection of the light from fibers by photosensors which have to operate in magnetic field and limited space inside the magnet. There are several candidates for photosensor that can satisfy these conditions: MicroChannel-Plate Multi-Anode PMT's, Avalanche Photodiodes, Fine-Mesh Multi-Anode PMT's and multipixel avalance photodiodes, which are considered as photosensors for FGD and SMRD and extensively studied for such an application. Detailed description of the multipixel avalanche photodiodes with metalresistor-semiconductor layer-like structure operating in the Geiger mode technique 8 can be found in 9 ' 1 0 . This detector (hereinafter SiPM) is a multi-pixel semiconductor photodiode which consists of pixels on a common substrate. Each pixel operates as an independent Geiger micro-counter with a gain of the same order as a vacuum photomultiplier. Geiger discharge is initiated by a photoelectron, thermally or by high electric field. A single pixel signal does not depend on a triggered number of carriers in a pixel. In such a way, the SiPM signal is a sum of fired pixels. Each pixel operates as a binary device, but SiPM on the whole is an analogue detector with the dynamic range limited by a finite number of pixels. The pixel size can be of 15 to 70 /im, and the total number of pixels is 100-4000 per mm 2 . Potential problem of such devices is a high dark rate. For example, at the bias voltage sufficient to provide the photon detection efficiency for green light of 10-12%, the dark rate is about 1 MHz at a threshold of 0.5

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photoelectron (p.e.) SiPM's also have rather high temperature gradient of 3-4% per degree for the signal amplitude. SiPM's of the sensitive area diameter 1.1 mm and 556 pixels were produced by the CPTA Company (Moscow). The remarkable feature of these devices is a low crosstalk that provides fast reduction of the dark rate from about 1 MHz at the threshold of 0.5 p.e. to 50 (3-5) kHz for thresholds of 1.5 (2.5) p.e., respectively. The first tests show that such devices can be used in several ND280 systems. The result of the measurement with cosmic muons of the SMRD scintillator slab with embedded a S-shape double-clad Kuraray Y l l WLS fiber and both-end SiPM readout is shown in Fig. 4. A light yield of about 17 p.e. (sum of both ends) provides high efficiency

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(more than 99%) for a minimum ionizing particle (MIP) detection. Moreover, the good timing resolution ~ 1 ns allows suppressing the dark rate in such detector using coincidence between both SiPM's and results in a space resolution of 6-8 cm for a MIP along such scintillator slab.

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Conclusion

T h e main goal of the T2K experiment is a sensitive search for the v^ -> ue transition using off-axis muon neutrino beam. T h e energy spectrum, flavor content, and interaction rates of the neutrino b e a m will be measured by a set of detectors (TV° detector, tracker, electromagnetic calorimeter, side muon range detector) located at 280 m from the neutrino production target and contained inside the UA1 magnet. T h e current design and instrumentation of the near detectors seem to be adequate the physics goals. This work was supported in p a r t by the P r o g r a m of the Russian Academy of Sciences for Basic Research "Neutrino Physics". References 1. Y. Ashie et al., Super-Kamiokande Collaboration, Phys. Rev. D 7 1 , 112005 (2005) and references therein; E. Aliu et al., K2K Collaboration, Phys. Rev. Lett. 94, 081802 (2005); Q.R. Ahmad et al, SNO Collaboration, Phys. Rev. Lett. 89 011301 (2002); K. Eguchi et al., KamLAND Collaboration, Phys. Rev. Lett. 90, 021802 (2003); M. Appolinio et al., CHOOZ Collaboration, Phys. Lett. B466, 415 (1999). 2. M. Maltoni et al., New J. Phys. 6, 122 (2004) [hep-ph/0405172]. 3. htpp://www-numi.fnal.gov/ 4. The OPERA experiment, H. Pessard, hep-ex/0504033. 5. Y. Ytow et al.„ hep-ex/0106019. 6. K. Nitta et al., Nucl. Instr. Meth. A535, 147 (2004). 7. D. Drakoulakos et al., hep-ex/0405002. 8. A.G. Gasanov et al, Lett. J. Techn. Phys. 16, 14 (1990) (in Russian); G. Bondarenko, V. Golovin, M. Tarasov, Patent for invention in Russia No. 2142175, 1999. 9. A. Akindinov et al., Nucl. Instr. Meth. A494, 474 (2002). 10. G. Bondarenko et al, Nucl. Instr. Meth. A442, 187 (2000).

RESULTS AND STATUS OF THE PICASSO EXPERIMENT F. AUBIN, M. BARNABE-HEIDER, M. DI MARCO, P. DOANE, M.-H. GENEST, R. GORNEA, R. GUENETTE, C. LEROY*, L. LESSARD, J.- P. MARTIN, N. STARINSKY, U. WICHOSKI AND V. ZACEK Departement de physique, Universite de Montreal Montreal, Quebec, H3C 3J7, Canada E. BEHNKE, W. FEIGHERY§, I. LEVINE AND C. MUTHUSI§ Department of physics & astronomy, ^Department of Chemistry, Indiana University South Bend, South Bend, Indiana, 46634, USA K. CLARK, C.B. KRAUSS AND A.J. NOBLE Department of physics, Queens University Kingston, K7L 3NG, Canada S. KANAGALINGAM AND R. NOULTY Bubble Technology Industries Chalk River, Ontario, KOJ1J0, Canada S. POSPISIL, J. SODOMKA AND I. STEKL Institute of Experimental and Applied Physics ofCTU Prague, CZ-128 00 Prague2, Czech Republic The PICASSO (Project In Canada to Search for SuperSymetric Object) experiment is searching for cold dark matter (CDM) through the direct detection of weakly interacting massive particles (WIMPs), in particular neutralinos (x) via their spin-dependent interactions with nuclei. The experiment is taking data in the Sudbury Neutrino Observatory Laboratory at a depth of 2070 m (6000 mwe). PICASSO uses a liquid fluorocarbon, C4F10 as the active material and searches for WIMP interactions on 19F. Measured limits on the spin-dependent x-proton and x-neuh"011 CT0SS sections are presented and compared with results from other experiments. Excluded regions in the plane of the effective x-proton and x-neutron coupling strengths ap and a„ are also reported. The next phases of PICASSO are presented. They will involve detectors of larger volume and increased active mass with higher sensitivity in the spin-dependent sector.

Corresponding author: [email protected]

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1. Introduction There is strong evidence that a large fraction of the matter in the Universe is non-luminous and non-baryonic. The most recent evidence is based on the measurement of the cosmic radiation background (CMB) by WMAP [1]. The amount of cold dark matter is predicted by WMAP as QCDM = 0.188-0.256. From the WMAP results one derive a cosmological matter density of £}m=0.27±0.04 that may be compared with the value of the baryon density of £}b=0.044±0.004 as indicated by nucleosynthesis and the height of the first acoustic peaks in the CMB data. The lightest neutralino, %, predicted in supersymmetric (SUSY) extensions of the Standard Model (SM) [2] emerges as the most promising candidate for dark matter. This stable spin Vi particle has no electrical charge and is a weakly interacting massive particle (WIMP) with a mass in the range from 10 GeVc"2 to 1 TeVc"2. These neutralinos are assumed to be concentrated in a halo around our galaxy [3]. In the Milky-Way galaxy, the velocity, v, of these particles is assumed to follow an isotropic Maxwellian distribution of the form /(v) = vV (v+V£)2//v ° 2 , where v0 = 230 kms"1 is the velocity dispersion of the dark matter halo and vE = 244 kms"1 is the velocity of the earth relative to the dark matter distribution (the effect of a ± 2 0 kms"1 annual variation is neglected here) [3]. The local neutralino mass density at the position of the solar system is assumed to be 0.3 GeVc' cm" and the velocity distribution is truncated at the escape velocity of matter in the Milky Way, vesc = 600 kms"1. In a direct dark matter search experiment, the detection of neutralinos is through the measurement of the energy deposited by the recoiling nucleus in the WIMP elastic scattering off nuclei in the detector. For all detector materials, this energy is expected to be less than 100 keV. The rate of interaction of neutralinos with ordinary matter is of the order of a fraction of an event per day per kilogram. Therefore, it is necessary to build detectors with large active mass and extremely low intrinsic background and, in practice, to operate the experiment in a deep underground site. 2. The experimental technique PICASSO detectors [4] consist of containers filled with droplets of diameters between 10 to 100 um of a superheated liquid at room temperature (presently a fluorinated halocarbon C4F10). These droplets are dispersed and trapped in a polymerized water based gel. Liquid-to-vapour phase transitions can be induced by nuclear recoils following the neutralino interaction with a nucleus (19F or 12C) in the droplet.

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These detectors are threshold detectors, since an explosive droplet-tobubble transition is only triggered if the energy deposited through the ionization process in the liquid by the recoiling nucleus exceeds a temperature-dependent energy threshold and within a critical length. The resulting shock waves are detected by piezoelectric transducers mounted on the container's exterior wall. The measurements of the detector response to monoenergetic neutrons as a function of operating temperature allow the extraction of threshold temperatures, T,h, for a given neutron energy and in turn the determination of the recoil threshold energy, ERi>h,, as a function of temperature for various pressures of operation. For a practical range of temperatures of operation (20° to 47°C), ERth follows an exponential temperature dependence: ER*{T)

=EbeK(T-Tb>

(1)

where K is a constant to be determined experimentally and Et is the threshold energy at the boiling temperature Tb. From purely kinematical considerations, nuclear recoil thresholds in droplet detectors fall into the same operating temperature range for neutrons of low energy (from 10 keV up to a few MeV) and massive neutralinos (from 10 GeVc"2 up to 1 TeVc"2) at velocities which are typical for dark matter particles in our galactic halo. In this operating range the detectors are insensitive to x-rays, y-rays and irunimum ionizing particles. It is found that the efficiency to detect nuclear recoils at a given temperature is not a step function, but rises gradually from threshold to full efficiency. The probability, P(E'R, E'Ri,/,(T)), that a recoil nucleus i at an energy E'R near threshold EiRjh will generate an explosive droplet-to-bubble transition is zero for E'R < E'Rlh and will increase gradually up to 1 for ElR > E'Rth. This probability is expressed as [4]: P(El R,El

Kth(T))

= l-exp

-(l.0±0.l)(ElR-E'Rjh(T)) E'R,th(T)

(2)

Combining Eqs. (1) and (2) (in the case of 19F struck by neutrons) allows the determination of the detector efficiency as a function of the recoil energy of I9F at a given temperature. In the temperature range from 20°C to 47°C, in which the PICASSO detectors are operated, 19F recoils from 500 to 6 keV can be detected with an efficiency of 50%. With this efficiency, the neutralino induced recoils become detectable above 30°C, the expected recoil energies being smaller than 100 keV. Knowing the neutralino induced recoil energy spectrum [3] and the detector efficiency for a l9F recoil energy at a given operating temperature, the

256

observable neutralino count rate, R<,bSi as a function of temperature, neutralino mass and cross section can be found to be [4] Robs(M,,
403 ATMXJ

"SD

Pb

Px 03GeVc~zcm-3

<"*>

230kms

e{Mr,T). (3)

where AT is the atomic mass of the target atoms, px is the mass density of the neutralino of mass Mx, (?SD is the spin-dependent neutralino-nucleus cross section, is the relative average neutralino velocity and e is the neutralino detection efficiency. The neutralino interaction with nuclei of ordinary matter is of electro-weak strength and has the general form given by •4Gl

MXMA MX+MAJ

CA

(4)

where GF is the Fermi constant, Mx and MA are the mass of the neutralino and the detector nucleus, respectively [5,6]. CA is an enhancement factor, which depends on the type of the neutralino interaction. Spin independent (SI) or scalar interaction proceeds via Higgs, or sfermion exchange with CA given by

CsA'=-~[zfp+(A-Z)fn}

(5)

where fnp are the neutralino couplings to the nucleons. For equal couplings to neutrons and protons the cross section is proportional to A2 (coherent interaction). Due to their large 19F content (19F is a spin-l/2+ isotope), PICASSO detectors benefit from a very favorable spin-dependent cross section and are particularly suitable to search for spin-dependent neutralino interactions [5]. The general spin-dependent cross section for WIMP scattering on 19F has the form [5] ^SD

=4G2F/J2FCF,

(6)

where GF is the Fermi coupling constant and fiF is the neutralino-19F reduced mass. cF = ($lx)(*p(sp) + a„(s„)) (J+l)/J is a spin-dependent enhancement factor, where ap, a„ are the effective proton and neutron coupling strengths, J = 1/2 is the total nuclear spin, (sp) = 0.441 and (s„) = - 0.109 are the expectation values of the proton and neutron spins within ,9 F [7]. In order to compare our results with other experiments and predictions of supersymmetric models, the

257

experimental cross section limits on 19F, oxF, are converted into limits on the neutralino-proton (o^,) and neutralino-neutron (a^) cross sections. Following the method developed in [3], we assume that all events are either due to neutralinoproton or neutralino-neutron elastic scatterings in the nucleus, i.e. a„= 0 or ap= 0, respectively, and obtain: ( a

a

xi =

2 A

£*-.

Mi

#

(7)

.A) r

where i = proton or neutron, /J; is the x-nucleon reduced mass (the mass difference between neutron and proton is neglected) and Cp(F) and C„(F) are the proton and neutron contributions to the total enhancement factor of 19F. They are related to the ap and a„ couplings by C,(f) = (8/w)af (S1,)2 (J+l)/J . cp and C„ are the enhancement factors for scattering on individual protons and neutrons. The values for the ratios Cp(F/ Cp = 0.778 and Cn(F/ C„ = 0.0475 are taken from [5,7]. As shown in [5], the allowed values of ap and a„ for a given neutralino mass are constrained by the inequality: ( a„

24G>,

vV°V

(8)

The sign in the parentheses is that of (S p) / (S n) . The allowed values of ap and a„ fall within a band defined in our case by two straight lines with a slope given by -<Sn /Sp> = 0.247:

<-^±a

a

'

(Sp)

± "

K

-

-\|24G2^%

a-

(9)

^

For experiments with two active nuclei with different < S p) I (S „ > ratios, the combination of the two bands lead to a closed elliptical contour. 3. The search for dark matter The PICASSO experiment has taken data with three detectors of 1 litre volume representing a total active mass of 19.4 ± 1.0 g of 19F [8]. The detectors were all calibrated in the same known neutron field of an Ac(Be) source before being installed in the water purification gallery of the Sudbury Neutrino Observatory Laboratory at a depth of 2070 m (6000 mwe). The detectors were placed in an acoustically and thermally insulated container in which the temperature was regulated and controlled with a precision of 0.1 °C. The data taking proceeded

258

continuously between April 2004 and October 2004 under remote control via Internet from Montreal, the detectors being operated at an ambient pressure of 1.23 bar and at temperatures varied in a random sequence from 20° to 47°C. Each detector was treated independently by the data analysis. All the events were first filtered in order to reject non-droplet like background events, such as acoustic or electromagnetic noises, for which the signal frequency, obtained by a fast Fourier transform, is usually below the one obtained for particle induced events. The acceptance of this filter was carefully studied [4] and found to be 6(=0.85±0.05 over the temperature range from 25 to 45°C, decreasing slightly for lower temperatures. In order to include only data taken when the detectors were in a stable operating configuration, time cuts were applied, described in detail in [8]. After time cuts, the effective exposure for the ensemble of three detectors was 1.98 ± 0.19 kg day of 19F. The error on the exposure comes from the uncertainty on the determination of the active mass (8%) and the statistical error on the determination of the filter efficiency (5%). There are several sources of external and internal background. The external background consists of neutrons, the detectors being insensitive to y and P radiation in the temperature operating range. Neutrons at a deep underground laboratory, like SNO, are mainly produced by spontaneous fission of 238U and following (a,n) reactions and more rarely by cosmic-ray muon interactions (photonuclear interactions of muons, muon capture). Water cubes surrounding the acoustically and thermally insulated box containing the detectors slow down these external neutrons and bring them below detection threshold. The main background is the internal background due to the alpha particles [4] produced in the decay chains of uranium and thorium nuclei which are present as contaminants in the constituents of the detectors. Purification of the detector ingredients was done by chemical and mechanical means and is described in details in [4,9]. The purification achieved so far was assayed at the level of 10"10 gU/g. The entire detector setup is designed to be radon tight. The 222Rn emanation rate for PICASSO containers was measured to be 15 ± 9 atoms/day [4]. The fit of the data to the alpha background response can be seen in the left part of Figure 1. 4. Current limits and future developments A combined fit of the alpha background and the neutralino response to the data allowed us to rule out any positive evidence for neutralino induced nuclear recoils, translating into a 90% C.L. upper limit of 1.31 pb on cr^, and 21.5 pb on o™ for a neutralino mass of Mx = 29 GeVc"2 [8]. The left part of Fi gure 1 shows

259 how the data points compare with the combined alpha background and expected response for a neutralino of mass 50 GeVc"2 with different cross section values (2, 5 and 15 pb). One can see from this figure that the data points are compatible with a response induced by alpha background only. The right part of Figure 1 and the left part of Figure 2 show our limits, compared with the limits obtained by other experiments, on the spin-dependent neutralino-proton cross section (o^) and neutralino-neutron cross section (o^) at the 90% C.L. as a function of the neutralino mass. Our comparison includes neutralino-proton and neutralinoneutron results obtained from the application of the method developed in [5] to the neutralino-nucleus cross section obtained in spin-independent experiments [10], [11]. The right part of Figure 2 shows the excluded regions in the plane of the effective coupling strengths ap , a„ for protons and neutrons for a neutralino mass of 50 GeVc"2 as obtained from Eq. (9) as well as the excluded regions obtained by other experiments. The sensitivity of PICASSO is presently limited by alpha emitting contaminants and its small active mass. The next step of the experiment (Phase lb, right part of Figure 1) is in preparation, with 2 kg of active mass, detector modules of 4.5 litre volume (Figure 3) with an active mass of 60 g, an order of magnitude higher than the loading achieved for the 1 litre volume detectors used up-to-now. Each module is equipped with 9 piezo-electric sensors mounted on the exterior wall for event localization. These detector modules will benefit from hydraulic recompression, improved purification and fabrication techniques. Phase lb will be followed by phases II and III (right part of Figure 1) with 25 kg and 100 kg of active mass, respectively. With Phase II one expects to achieve a sensitivity of lxlO"3 pb or better (reach tip of MSSM predictions) in the spin-dependent sector. With Phase III the expected sensitivity is between lxlO"4 pb and lxlO"5 pb (reach the core of the MSSM predictions). These two phases will utilize larger 30 liter modules.

260

/ /

SOS

/

f

w"

1 oo«

.

/

I |

•&

/

A 0 03

0 02

T I

LJ—4—

rU^-fr i

001

-

7

30

:5

30 35 Tempetatuie r*C(

«

4S

50

M,(GcV/c')

Figure 1. Left: Measured count rate for one of the three detectors compared to the count rate expected from an alpha background only (red curve) and with an alpha background plus a signal due to a neutralino of 50 GeVc'2 mass having a cross section of 2 pb (green dashed line), 5 pb (blue dotted line) and 15 pb (dashed black line). Right: PICASSO limit on the spin-dependent neutralinoproton cross section (o-jp) at 90% C.L. as a function of the neutralino mass (blue squares) compared with other experiments: NAIAD (green circles), DAMA/Nal 3a-allowed modulation signal (pink shaded region), EDELWEISS (red dot line), CRESST I (grey 'x') and CDMS (black solid line for Ge and black dash-dot line for Si), see [10],[11] and references therein. The SIMPLE experiment [12] reports results similar to PICASSO.

1 10-

10-

.

H

^ ^ ^

\

s ^ -

__CRESST _

n

—"~>^C

o ° ° '

rTC

'^•"""firASso

^_x>< -

—.

NAIADN

PICASSO^1

10»

\

^

^ 1

i n '

11) IOM.KicVVl

III

-20

-10

0

K

Figure 2. Left: PICASSO limit on the spin-dependent neutralino-neutron cross section (c^) at 90% C.L. as a function of the neutralino mass (blue squares) compared with other experiments: DAMA/Nal 3o-allowed modulation signal (pink shaded region), EDELWEISS (red dot line), CRESST I (grey V ) , ZEPLIN I (red triangles) and CDMS (black solid line for Ge and black dashdot line for Si), see [10],[l l] and references therein. Right: Excluded regions (outside the bands) in the plane of the effective coupling strengths ap, a„ for protons and neutrons for a neutralino mass of 50 GeVc"2 as obtained from Eq. (9) for PICASSO, compared with some other experiments, see for example [10],[11] and references therein.

261

Figure 3: Left: a module of the new 4.5 litre detectors with an active mass of 60g. The container is entirely fabricated from acrylic. The stainless steel cage serves as reinforcement during recompression. Each module is equipped with 9 piezo-electric sensors for event localization. Right: "artist's view" of the arrangement of 32 modules in two layers for the 2 kg detector ensemble of Phase lb. In reality 4 detectors each are installed in a temperature and pressure control unit.

References 1. C.L. Bennett et al., Astrophys. J. Suppl. 148, 1-27 (2003); D.N. Spergel et al., Astrophys. J. Suppl. 148, 175-194 (2003). 2. G. Jungman, M. Kamionkowski and K. Griest, Phys. Rept. 267, 195-373 (1996). 3. J.D. Lewin and P.F. Smith, Astro. Phys. 6, 87-112 (1996). 4. M. Barnabe-Heider et al., PICASSO Collaboration, physics/0508098, to appear in NIM A. 5. D. R. Tovey et al, Phys. Lett. B 488, 17-26 (2000). 6. VA. Bednyakov, hep-ph/0310041, October 3, 2003. 7. A.F. Pacheco and D.D. Strottman, Phys. Rev. D 40, 2131 (1989). 8. M. Barnabe-Heider et al., PICASSO Collaboration, Phys. Lett. B 624, 186194(2005). 9. M. Di Marco, "Reduction du bruit de fond en vue de la detection de la matiere sombre avec le projet PICASSO", Ph.D. Thesis, Universite de Montreal, November 2004. 10. D.S. Akerib et al., astro-ph/0509269vl, September 10, 2005. 11. A. Benoit et al., Phys. Lett. B 616, 25-30 (2005). 12. T.A. Girard et al, Phys. Lett. B 621, 233-238 (2005).

DATA ACQUISITION SYSTEM AND TRIGGER ELECTRONICS FOR CACTUS J. LIZARAZO, P. AFONSO, M. CHERTOK, P. MARLEAU, S. MARUYAMA, J. STILLEY and S. M. TRIPATHI Physics Department, University of California One Shields Ave., Davis, CA 95616, USA CACTUS is a ground-based Air Cherenkov Telescope located at the Solar 2 facility in California, and operated by UC, Davis. It uses an array of 168 heliostats and a camera with 80 photomultiplier tubes to detect Cherenkov radiation produced by air showers. CACTUS incorporates novel techniques of time projection imaging and pattern triggering, implemented in FPGAs, thus improving upon the first generation sampling ACTs. Here we describe the telescope and its readout and triggering electronics.

1. Solar Two Field and CACTUS Secondary Optics CACTUS (Converted Atmospheric Cherenkov Telescope Using Solar2) is a ground-based air-shower detector located near Barstow, California [1,2]. The Solar Two site consists of about 1,800 heliostats (42m2 mirrors) that can be individually steered in elevation and azimuth. Motion controls are performed by remote-controlled driver electronics located at the base of each heliostat. Communications with a central computer are performed over a doublyredundant system using RS-232 lines. The cumulative tracking and pointing error for each individual heliostat is about 0.1 arc-degree. CACTUS utilizes 168 of these heliostats that fall within the field-of-view of a secondary mirror mounted 60 m above ground level on a tower. These 168 are spread over -20,000 m2, thus providing a 35% coverage with their mirror area. The spherical secondary mirror is a composite mirror made out of 13 hexagonal facets, each lm across. With an aperture of 4.4mx2.6m and a radius of curvature of 6m, it further focuses the 3m spot-size from the heliostats down to entrance apertures (~10 cm) of light-collecting parabolic Winston cones located at the focal plane of the camera. The camera consists of 80 photomultiplier tubes (PMTs) positioned according to the spot sizes of heliostats on the focal plane. The upper 35 PMTs are positioned to align with individual heliostats, while the lower 45 PMTs form a close-packed matrix which allows images from multiple heliostats to be captured in a single channel. We have exploited this feature by implementing a scheme in which we later (offline) sort out the signals from heliostats by measuring the relative delays in the times of arrival. In order to accomplish this, we have built high bandwidth electronics that can provide twopulse separation at the level of ~5ns. This is described below. 2. Readout and Trigger Electronics There are four main components in the CACTUS data acquisition system shown in Figure 1: a) front-end amplifiers and discriminators, b) trigger system, c) time to digital converters (TDCs) and readout system and d) anode current recording.

262

263 control bus data bus

TDCs

4-

ECLRVDS

translator^" PC

80 discriminator channels 80 channels

ECULVDS

Readout FPGA

translator

trigger out

Trigger FPGA

2^

': 80

PMT

amplifier t ^ parallel port interface

--| parallel port interface

USB

w:

'80 i current j monitor

Figure 1: A box-diagram of the CACTUS amplification and readout system. 2.1 Analog Front-end The front-end consists of inline (BNC-BNC) preamplifiers, mounted on the PMT bases, which drive 50Q co-axial cables to post-amplifiers located in a counting house. Both the preamps and the post-amps use monolithic broadband circuits (Mini-circuits ERA-3) [3] providing an overall voltage gain of ~30x and a bandwidth of ~1.6 GHz. The preamps are powered by a dc current carried on the same cable that is used for amplified pulses. A separate circuit senses this current and provides a voltage output that can be related to the anode current of the PMTs. The amplified signals are fed into CAMAC discriminators (LRS 4416) that provide dual outputs on ECL busses. The minimum pulse-width at the discriminator output is ~3.5 ns. Time-over-threshold measurements made above this minimum are directly proportional to pulse-heights. One set of ECL buses is connected to TDCs and the second one drives a trigger system. 2.2 Trigger System The task of the trigger system, based around a Xilinx XCV1600 FPGA, is to determine the presence of an air-shower in a given 10 ns window. The decision time must be less than 1 ixs because the storage depth in the TDCs is 2 lis. The ECL outputs from the discriminators are translated into LVDS levels before being fed into the FPGA. Figure 2 shows a schematic of the firmware loaded into the FPGA. It is divided into three main sections: a) delay lines, b) trigger logic and c) communications. ™\Majority The pipelines are used ^vLogic dynamically in real time to delay the Threshold signals from each of the 80 input Adjustable channels to compensate for flight Delay Line path differences among photons arriving from different parts of the Parallel Port field. Figure 3 shows a schematic of ' Communication a single channel pipeline. The building blocks consist of a flipflop, a multiplexer and a decoder. Figure 2: Schematic of the trigger system. These blocks are connected in a pipelined fashion such that an input signal gets introduced into the chain at a

264 point specified by the delay setting, and thence proceeds from one flip-flop to Delay DQ D9 Input Pulse

Line

4-

Delay Code •—

4 r l _ y Rip-Fiop r .

m.

-4— Multiplexer

J]

4—

4—

m.

[£]

—•—

Output

N-1

Figure 3: A schematic of the delay pipeline used in the trigger system. the next with one step advanced every rising edge of the 100 MHz global clock. Once signals arrive at the end of the pipeline, each delayed by the specified amount, an air-shower is represented by pulses synchronized in time, within ~5 ns for gamma-ray showers and -5-20 ns for cosmic showers. Light from secondary heliostats clusters in distinct patterns in time and can be included in more complex triggering schemes to further improve efficiency. The trigger logic is designed to detect timing patterns among the 80 channels. Various versions of the firmware can be downloaded into the FPGA. The simplest logic demands occupancy higher than some minimum number within a 10 ns window. Another version includes pulses from secondary heliostats and reconstructs the expected timing pattern for each channel. The "Majority Logic" module, which counts the number of channels with a hit in a 10 ns window, is pipelined in such a way that it uses only one logic cell per summing stage and is therefore capable of working at the required 100MHz clock. The summed value is truncated at six bits, thereby saving FPGA resources and making the electronics faster and more compact. This restricts the maximum allowed threshold to be 63. However, typical values used at CACTUS are in the 7-15 range. The "Threshold" module is a digital comparator which issues the final trigger when the sum value exceeds a user-specified threshold. The third part of the design is responsible for communicating with the host computer via its parallel port. A software package running on the host computer provides updates to path delays in real time, based on the tracked source's position in the sky. Delay data, in 8-bit format, are written to the data register of the parallel port and the FPGA loads them into an internal shift register that controls the delay pipelines. The coincidence level is also downloaded via the parallel port using its address register field. 2.3. Readout System Data acquisition in CACTUS consists of recording time of arrival (TOA) and time over threshold (TOT) of all pulses that arrive up to 2 us prior to the trigger time. This is accomplished by recording the rising and falling edges of discriminated signals in two different TDC channels, thus avoiding the minimum pulse-width constraint (8 ns) of the LRS 337 TDCs used in CACTUS. The TDCs register each edge independently with a 0.5 ns resolution. The TDCs are read out using a Xilinx XCV1000E FPGA as shown in Figure 4. The firmware used to program the FPGA is divided into three sections: a) TDC interface, b) 4-event buffer memory and c) data transfer, as described below.

265

ECL/LVDS

translator Jf8Q channels from discriminators (ECL)

FPGA

DPFIAM_4K

XCV1000E

PC

r

DPRAM_4K

DPRAM 4K HOST_XFER

parallel

I " J_-=±]b0rt

^~

DPRAM 4K

Figure 4: A schematic of the FPGA used in the TDC readout system. The TDC_CTL module in figure 4 is in an interface to the bank of TDCs via their front panel control bus. It implements a handshake protocol using 6 control lines: COM, WAK, REN are outputs and WST, BSY, PASS are inputs [4]. The TDCs are programmed in the "Common Stop" mode whereby they continuously acquire and store data in a 2 us buffer until a trigger is issued on the COM line. At this time the TDC stop recording and enter a readout mode. A handshake per word transfer (10 MHz) takes place between the TDCCTL module and the TDCs. The BSY line stays high during valid data transfer and the PASS line is asserted at the end. The PASS line is daisy chained to the REN line of the next TDC, thus allowing for a sequential readout of several TDCs. The readout time for typical events at CACTUS is about 2-4 ms. In order to minimize the system's dead time, the data from the TDCs are stored in buffer memory modules, DPRAM_4K, which are dual port 4K RAMs. The RAMSTORE module controls buffer sequencing and data are stored in a cyclical fashion. The buffers themselves keep track of data record length and of their own status. In case an event is longer than 4K, it utilizes contiguous buffers until the entire event is stored or all the buffers are full. The read-back of the buffers when transferring data to the host computer is also done cyclically, such that the oldest data are read out first. The RAM_SLECT module gathers the MEMORYSTATE outputs from each of the four buffers and instructs the RAMREAD module on read out sequencing. In this scheme there is no deadtime due to transfer to host computer until the trigger rate gets high enough such that the average time between triggers is equal to the average data transfer time. CACTUS operates at trigger rates of ~20 Hz with less than 10% deadtime. The HOST_XFER module implements an extended parallel port (EPP) protocol such that data can be transferred to the host computer's parallel port, regardless of the state of the TDC readout module. There are six control lines involved in an EPP transfer: nDATASTB, nADDRSTB and nWRITE are generated by the host computer and nWAIT is asserted by the FPGA, while nRESET and nlNTR are not used in this implementation. In addition, there is a bidirectional 8-bit bus that is used to transfer data and addresses. A C++

266

program, running on the host computer, continuously reads the address port for the presence of HxOC which indicates that an event is ready to be read out from the FPGA buffer memory. The host reads a few header words followed by the event record which is transferred at the maximum allowed EPP rate of ~1 MHz. 2.4 Anode Current Readout A voltage proportional to the PMT anode current is provided by the preamps and recorded using a commercial USB pod from National Instruments. The sampling rate can be varied in the 1-20 KHz range. The readout of these digitized voltages is asynchronous from the TDC readout and the data are correlated using time-stamps. The primary purpose of this data stream is for photometry which monitors the night sky background. 3. Results and Summary — All Heliostats

Figure 5 shows a distribution of Central ' Heliostats TOA from a sample of about 1,000 showers recorded by CACTUS. The lower curve is obtained by using only the primary heliostats (one per channel) while the upper curve includes pulses from secondary heliostats. The TOAs are with - 2 0 2 4 Rise Times (ns) respect to a trigger time for that Figure 5: A histogram of recorded TOA. event and display a leading edge coherence of about 1 ns. The trigger system provides a low threshold with an effective area of~10 4 m 2 at70 GeV rising to ~6xl0 4 m2 at 500 GeV. In summary, CACTUS is operational and the electronics systems have met specifications. The telescope has measured the differential flux of the Crab Nebula [5] that is in agreement with the world dataset above 300 GeV and provides new data in the regime below 100 GeV. Acknowledgements We are grateful to the University of California, especially Vice Chancellor for Research, Barry Klein, Dean Winston Ko, Senate Committee on Research, Senior Electronics Engineer, Britt Holbrook and the Office of the President. Prof Allen Zych, UCR, provided us with 50 PMTs. Mr. Judd Kilminik of Southern California Edison continues to support our activities at the Solar Two site. References [1] Tripathi, M., et al, "The Keck Solar Two Gamma-Ray Observatory", AIP Conference Proceedings, Vol. 587., 2001., p.93. [2] The CACTUS website http://ucdcms.ucdavis.edu/solar2/ [3] For a datasheet, see http://www.minicircuits.com/dg03-168.pdf [4] For a manual, see http://www.lecroy.com/lrs/dsheets/3377.htm [5] Marleau, P., et al, "Intermediate Energy Observations with CACTUS", Proceedings of CHERENKOV05, Paris, April 2005, in press.

T H E C E N T R A L PIXEL OF T H E M A G I C TELESCOPE FOR OPTICAL OBSERVATIONS *

F. LUCARELLI Dip. di Fisica, Universita degli Studi di Roma "La Sapienza" Pie. Aldo Moro 2, 00185 Rome, Italy E-mail: [email protected]

P. ANTORANZ, M. ASENSIO*, J.A. BARRIO, M. CAMARA J.L. CONTRERAS, R. DE LOS REYES, M.V. FONSECA M. LOPEZ, J.M. MIRANDA, I. OYA Dpto. Fisica Atomica, Facultad de Ciencias Fisicas, Dept. Infra., I. Sistemas Aeroespaciales y Aerop., 28040 Madrid, Spain

Universidad Universidad

Complutense Politecnica,

The MAGIC telescope has been designed for observation of the Cerenkov light generated in Extensive Air Showers. However, its 17 m. diameter and optical design makes it suitable for optical observations of varying astronomical objects. We report here on the installation of a dedicated photo-multiplier (PMT) for optical observations placed at the center of the MAGIC camera (the so-called central pixel). An electro-optical system has been developed in order to transmit through optical fiber the output signal to the counting room, where it is digitalized and stored for off-line analysis. The system was tested by observing the optical pulsation from the Crab pulsar. The results of the tests and the possibilities of the system will be presented here.

1. Introduction The MAGIC telescope l (Fig. 1) is an innovative detector aimed to detect very high-energy 7-rays from astrophysical sources using the Imaging Atmospheric Cerenkov technique. The telescope collects the very short flashes of atmospheric Cerenkov radiation (5-20 nsec in duration) emitted during the development of the Extended Atmospheric Showers (EAS) produced in the interaction of the cosmic 7-rays with the atmospheric nuclei. "This work was supported by the Spanish CICYT (project FPA2003-9543-c02-01)

267

268

Figure 1. The MAGIC telescope.

The main characteristics of the telescope are its 17m. tessellated mirror, a system of analog signal transmission based on optical fiber and signal digital sampling with 300 MHz Flash ADCs. The main detector, or camera, located at the focus of the telescope, is composed by a matrix of 576 fastresponse PMTs. Besides the main 7-ray observations, the large collection area of the MAGIC telescope can also be used to perform optical observations of slow varying astronomical objects. That can be done by measuring the variations in the DC-current output of a PMT installed at the telescope focus. This technique was already applied by other Cerenkov telescopes (HEGRA CT1 2 , Whipple 3 , Celeste 4 , HESS 5 ) which detected the optical pulsed emission from the Crab pulsar and estimated the photon content of the nebula surrounding the pulsar itself 2 . At commissioning time, the center of the MAGIC camera was deliberately left empty in order to host a dedicated PMT for optical observations. Such modified PMT, the so-called central pixel, was installed at the end of March '05 and tested successfully with the detection of the optical pulsed emission from the Crab pulsar. 2. The central pixel The PMT installed at the center of the MAGIC camera is a standard 1" MAGIC PMT, ET9116 6 , especially designed for fast pulsed-light detection. The DC branch of the pre-amplifier placed at the PMT base has been modified in order to have an integration constant r = 0.5msec. The overall tension was set to 1.08kV, corresponding to a gain of around 20k and an average current of 0.1-0.5^A under normal Night Sky Background (NSB) illumination. The transmission of the DC output signal from the PMT base to the

269

counting room for its digitalization and storing is made through optical fiber. The electro-optical transceiver (transmitter+receiver) was designed in order to transmit Crab-like signals, that is, analog pulsed signals with 10-100 Hz periods and widths of the order of milliseconds 8 . For the optical transmitter, installed inside the camera, a wide dynamic range LED was used (Honeywell HFE 4050-014 9 ) to perform the electro-optical conversion while at the receiver (placed after 170m of optical fiber, inside the counting room), a pin diode (Honeywell HFD3038-002 10 ) with an analog pre-amplifier implements the re-conversion. The LED was set to an operation point of 40mA, thus providing linear operation over a wide dynamic range (up to 200 mV). The bandwidth of the transceiver goes from 1Hz to 4kHz, thus allowing transmission of Crab-like signals and rejecting high and low frequency noise (mainly due to the night sky background). The transceiver allows to measure DC variations at the level of 0.2%. Both the central PMT and the optical transmitter are powered through independent and isolated power supplies installed on the side of the MAGIC camera. The ON-OFF switching of the central pixel is software-controlled through a Programmable Logic Controller (PLC).

3. Crab observations The whole system has been installed at the end of March '05 and tested by observing the Crab pulsar. The DC current from the central pixel, converted to voltage, was digitized at a sampling frequency rate of 2 kHz by means of a 16-bits National Instruments PCI-6034E ADC card u and stored for off-line analysis. The ADC card was set to a resolution of 0.16/xvolts. As the Crab pulses are completely embedded in noise (generated by the night sky background and the electronics), in order to observe its optical pulsation at the expected frequency, a time-correlated analisys of the data has to performed. A time stamp reflecting the sampling frequency was associated at each event and transformed to an inertial reference frame, which is assumed as the solar barycentric system. For this transformation, the TEMPO software 13 has been used. The corrected times then are folded with the radio ephemerides corresponding to the epoch of observation provided by the Jodrell Bank observatory 14 . A phaseogram was produced around the expected Crab rotational frequency for each independent test frequency defined by the Independent Fourier Spacing IFS=1/T 0 (, S . The folded intensities were then tested

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Figure 2. Upper plot: Optical light-curve of the Crab pulsar obtained with the central pixel of the MAGIC telescope. Lower plot: Zj^ statistical test. A clear peak appeared at the frequency expected for the observation epoch.

against a uniform distribution by performing a Z^ statistical test 15 . Figure 2 (lower plot) shows the Z\Q test scanning different frequencies around the expected Crab frequency. A clear peak appeared at the frequency expected for the observation epoch. Figure 2 (upper plot) shows the wellknown double-peaked lightcurve calculated at the maximum Z\0 value and after 20 min. of observation. 4. Conclusions In this work, we have reported about the installation in the center of the MAGIC camera of a photo-multiplier, the central pixel, dedicated to optical observations. The central pixel has been tested successfully by the detection of the Crab optical pulsations. The minimum time requested for a 5cr detection was lower than 1 minute. The expected detection time was 30 sec 2 . However, due to extreme weather conditions preceding the weeks immediately before the observations, the optical conditions of the telescope were not the optimal ones. The pointing error of the telescope was estimated offline in about 0.1°, while the Point Spread Function (PSF) was

271 around 0.1° ( F W H M ) . This limited the amount of collected light over t h e Central Pixel t o about 7-9% of the t o t a l light emitted by the pulsar. Recent alignments of the mirrors have reduced the P S F to less t h a n 0.05°. T h u s , we expect t o improve the detection time by at least a factor 2. T h e applications of the central pixel will be mainly focused in t h e simultaneous observations of the C r a b pulsar b o t h in the optical and in the 7 regimes, in order t o have real-time ephemerides for periodicity search in 7-rays. Before t h a t , the central pixel will be also used to test the whole timing system of the M A G I C telescope 1 2 . Optical observations of quasiperiodic flaring AGNs (Mrk 421, 501, ..) and X-ray binary systems (AE Aquarii) are also contemplated.

Acknowledgements T h e authors wish t o t h a n k the financial support given by the C I C Y T (project FPA2003-9543-C02-01) to make this work and the IAC for providing excellent working conditions in La Palma. T h e M A G I C telescope is mainly supported by B M B F (Germany), C I C Y T (Spain), I N F N and M U R S T (Italy).

References 1. Fonseca, M.V., Acta Physica Polonica B, vol. 30, 2331 (1999). 2. Ona-Wilhelmi, E., et a l , Astropart. Phys. 22, 95 (2004). 3. Srinivasan, R., et al., Proc. Towards a Major Atmospheric Cerenkov Detector V, Durban (1997) p.51. 4. De Naurois, M., et al., ApJ 566, 343 (2002). 5. Franzen, A., et al., Proc. 28th ICRC, Tsukuba (2003). 6. Ostankov, A., et al., NIM A, vol. 442, Issue 1-3, 117 (2000). 7. http://www.magic.iac.es/subsystems/centralpixel/ 8. Asensio, M., Lucarelli, F., Antoranz, P., et al., "Design, modelling and testing of the electro-optical transmitters for the central pixel of the MAGIC telescope", Proc. SPIE Vol. 5840, p.627-637 (2005) 9. content.honeywell.com/sensing/prodinfo/infrared/catalog/0331eng.pdf 10. content.honeywell.com/sensing/prodinfo/fiberoptic/application/on7eng.pdf 11. http://sine.ni.eom/nips/cds/view/p/lang/en/nid/11916 12. Lucarelli, F., et al., "The timing system of the MAGIC telescope", Proc. of the 19th ECRS, Florence (Italy), 2004 (to be appeared on IW) http://www.magic.iac. es/subsystems/timing, html. 13. Manchester, R.N., et a l , MNRAS 328, 17 (2001). 14. http://www.jb.man.ac.uk/ pulsar/crab 15. De Jager, O.C., ApJ, 436, 239 (1994).

STATUS R E P O R T OF A R D M P R O J E C T : A N E W D I R E C T D E T E C T I O N E X P E R I M E N T , B A S E D ON LIQUID A R G O N , FOR T H E SEARCH OF D A R K MATTER*

M. M E S S I N A A N D A. R U B B I A ETH Zurich, CH-8093, SWITZERLAND E-mail: marcello. messina@cern. ch

The goal of the ArDM project is to develop and operate a detector to search for direct evidence of Weakly Interacting Massive Particle (WIMP) as Dark Matter in the Universe. The experimental approach aims at detecting recoils of Argon nuclei induced by the collisions of WIMPs. Our immediate plan is to fully design and build a 1 ton prototype. This will involve a HV system, charge amplification + readout and light readout, as described in this paper. Our first milestone is a proof of principle on gamma-rays and beta electron vs nuclear recoils discrimination.

1. Introduction The understanding of the nature of Dark Matter is one of the most exciting problems of the particle physics. A reasonable hypothesis is that Dark Matter is composed of a new kind of non-baryonic, massive particle which could possess weak interactions with ordinary matter (WIMP). The detection of these WIMPs is based on the capability of measuring the recoils of target nuclei with kinetic energy in the range of 10 —100 keV. The signal is therefore quite elusive and it is a rare event given the weak coupling. Furthermore the rate is not easily predicted, since it depends on many poorly denned variables, even in the context of well defined extensions of the SM like e.g. SUSY. The ArDM project 1 aims running a high mass detector, almost 1 ton of effective mass to potentially reach a WIMP-nucleon cross-section sensitivity of a « 1 0 - 6 pb. The project is a natural evolution of the liquid Argon technology developed within the ICARUS 2 ' 3 programme, applied to the search of DM. The novelty of our design is to detect ionization and * On behalf of ArDM collaboration

272

273

scintillation signals independently such as to fully retain the features of the LAr TPC but with a threshold capable of detecting WIMP recoils.

2. Principle of detection and detector layout When a charged particle travels in the medium, scintillation light and ionization charge are produced. The amount of charge Q and light L depends on the energy release E and the applied electric field e according to the laws 4 : Q(e,E) = Q0(E)-^\n(l L(e,E) = L0(E)~Q(e)

+ ^)

(1) (2)

where C is a constant strongly dependent on the ionization power of the particle 5 ' 6 . C is large for high stopping power, e.g. like for nuclear recoils. Hence by comparing the charge and scintillation yields, one should be able to distinguish the nuclear recoils from e/7 signatures with high discrimination power. In addition the time decay of the scintillation light is given by two components, fast and slow. The amount of the slow signal depends on the stopping power of the particle, so with a pulse shape analysis one can further discriminate the nuclear recoils from the e/7 events. The detector is contained in a cylindrical vessel (Fig 1) where the liquid and vapour of the Argon are in equilibrium. The active volume is embedded in a high strength electric field (1 — 5 kV/cm). The high electric field is considered to reduce the recombination of the ionization electrons, even in the case of highly quenched slow nuclear recoils. At the bottom of the drift region a light transparent cathode is set at a potential (100-500 kV). A series of electrodes are installed and biased along the full drift path to keep the field uniform at a level of few %. The ionization electrons are drifted to the liquid-vapor interface and are extracted into the gas phase. In the vapour a LEM 7 is installed to provide the electron amplification by means of a high field generated in small (cylindrically shaped) holes; more details about LEM will be given in section 4. Lastly a series of photosensors are foreseen to be installed outside of the drift region below the cathode. The vessel wall is covered with foils of Mylar coated with a thin layer of Al to reflect the scintillation light.

274

Figure 1.

Schematic picture of the ArDM detector experiment.

3. High Voltage system The drift voltage is provided by an immersed cascade of rectifier cells (Greinacher/Crockroft-Walton circuit) which takes as input an alternate voltage of amplitude VQ and gives an output almost 2 • Vo times the number of stages. Such a solution has been chosen to overcome the task of feeding 500kV into the vessel. The immersed circuit exploits the high insulation capability of liquid Argon.

275

We envisage two possible configurations for the HV circuit, the first with a low number of stages. In this case capacitors capable to withstand up to 20 kV are needed. For this purpose we tested Mica capacitors 8 . The second solution is to use a larger number of stages, up to 250 with low voltage capacitors. A test with a circuit of 80 stages was realized, producing 120 kV in stable conditions. The test was done by using polypropylene capacitors with a maximum recommended voltage of 600 V and high voltage 2 fcVdiodes (Phillips BY505). 4. Large electron amplification device In order to address the task of multiplying the number of primary ionization electrons we started to develop a Large Electron Multiplier (LEM) 9 based on the GEM concept 10 . The former consists of perforated vetronite, with a thickness of ~ 1.5 mm with metallisation on both sides. The diameter of the cylindrical holes is 500 \xm and the distance between the centers of the 2 neighboring holes is 800 fim. An electron, following the electric drift lines will enter a hole of the LEM, where it will generate an avalanche given the high electric field. This avalanche produces a detectable signal on the LEM itself which is stripped to provide position coordinate determination. Single stage LEMs have been run in pure Argon, at room temperature, with different values of voltage applied on their faces(2.2 — 5.5 kV), at different values of Argon pressure (1—3.5 bar), and also with different values of the LEM thickness. In such conditions we got amplification factors in the range of 800 — 1200, particularly interesting is the result obtained at 3.54 bar which is the pressure at room temperature where the Argon gas has the same density as the vapour in equilibrium with liquid at T = 90°K. In this condition we got an amplification factor of 800 in stable operation. 5. Light detector and collector device For the purpose of reflecting the scintillation light we considered a thin layer of Al deposited on Mylar foil coated with a layer of MgFi to prevent the Al oxidation. The reflectivity of such mirrors at wavelength of 128 nm has been measured to be 91 ± 5 % u . As a baseline option we consider HAMAMATSU photomultipliers (PMT), R6235-01, modified to work in cold. They have their maximal sensitivity at a wavelength of ~ 420 nm and an overall gain of 3 • 105. In order to match the PMT to the scintillation light at A ~ 128nm we coat the PMT surface with a film of polystyrene doped with a wavelength-shifter,

276

Tetra Phenyl Butadiene (TPB). The polystyrene prevents the crystallization of the TPB. We also studied the features of a solid state light detector, the windowless Large Avalanche Photo Diodes (LAAPD) of Advanced Photonics. Such a device has shown a quantum efficiency of > 40 % at A ~ 128 nm12 . This result is quite promising even if the maximal gain of 103 of the APD, at room temperature, make them not very performing to detect single photons. 6. Conclusion We have presented our design of a 1 ton prototype whose goal is to demonstrate its validity for application to DM search.This requires a successful implementation and operation of a high drift field device, a LEM based charge readoutand a highly efficient Argon scintillation light detection system. A prototype is being setup at CERN. Our first milestone is a proof of principle on 7-rays and electron rejection vs nuclear recoils. Following its successful operation, we will consider a deep underground operation. A cknow

ledgments

We thank Mr R. Chandrasekaran, Dr. M. Laffranchi, G. Natterer, Ms P. Otiougova, Dr. C. Regenfus, for the precious contribution to this paper. This work was supported by ETH Zurich and the Swiss National Science Foundation. References 1. A. Rubbia, Talk given at IXth international conference on Topics in Astroparticle and Underground Physics (TAUP05), Zaragoza, (Spain) Sep. 2005. 2. A. Benetti et al, Nucl.Instrum.Meth. A327, 205 (1993). 3. F. Arneodo et al, Nucl.Instrum.Meth. A449, 147 (2000). 4. J. Thomas and D. A. Imel, Phys. Rev. A36-2, 614 (1987). 5. T. Doke et al, Nucl.Instrum.Meth. 134, 353 (1976). 6. A. Badertscher, M. Laffranchi and A. Rubbia, hep-ph/0412166. 7. P. Jeanneret et al, Nucl.Instrum.Meth. A500, 133 (2003). 8. http://www.reynoldsindustries.com. 9. P. Otiougova et al, Paper in progress. 10. F. Sauli, Nucl.Instrum.Meth. A386, 531 (1997). 11. A. Knecht, Diploma Thesis ETHZ-IPP Internal Report 2005-09. 12. R. Chandrasekaran, M. Messina and A. Rubbia, Nucl.Instrum.Meth. A546, 426 (2005).

DEVELOPMENT OF MULTIPLE PHOTON COUNTING COINCIDENCE (MPCC) TECHNIQUE FOR CHARACTERISATION OF SCINTILLATORS FOR CRYOGENIC APPLICATIONS* HANS KRAUS, VITALIIMIKHAILIK AND DAVID WAHL Department of Physics, University of Oxford, Keble Road Oxford, 0X1 3RH, United Kingdom A new method for measurements of the scintillation characteristics of materials has been developed. This method, called multiple photon counting coincidence (MPCC) technique, is based on the recording of a sequence of individual photon pulses resulting from a scintillation event. The distribution of the arrival times of these individual photon pulses provides information about the decay characteristic of the scintillation process and the number of photons recorded per scintillation event is proportional to the scintillation light yield. The ability to reject spurious events through off-line analysis is an important advantage of the MPCC method since it allows cleaning of the data set from pile-up events. It is shown that the MPCC technique is particularly well suited for the analysis of slow scintillation processes in the investigation of temperature-dependant scintillator properties. It is now used extensively by our group for the identification and optimisation of scintillating targets for cryogenic low-background rare event searches, such as Dark Matter and 0-v double beta decay experiments.

1. Introduction The search for rare events is one of the most active research areas of the astro-particle physics community. Finding neutrinoless double beta decay and the detection of weakly interacting massive particles require detectors capable of discriminating the rare signal in the presence of a dominating background caused by natural radioactivity and cosmic rays. Low-temperature detectors equipped with scintillating absorber achieve this discrimination via the simultaneous measurement of phonon and scintillation signals [1]. This ability to reject spurious background, together with other advantages of cryogenic phonon detectors, namely greatly enhanced energy resolution and low threshold, elevated these "hybrid" cryogenic phonon-scintillation detectors (CPSD) into the category of especially promising instrumentation for the next generation of experiments

* This work is supported by PPARC grant PPA/I/S/2001/00646.

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searching for WIMP Dark Matter [2, 3], 0-v double beta decay [4] and radioactive decay of very long-living isotopes [5, 6]. Inorganic scintillators are key elements of CPSD and the identification and characterisation of promising scintillating materials is becoming an important objective for future progress in this field. In this paper we introduce the multiple photon counting coincidence (MPCC) technique for measuring the light yield and decay time constants of scintillators over a wide temperature range. The development of this technique is part of a research initiative addressing the identification and optimisation of scintillating materials for targets in cryogenic searches for rare events. 2. Problem A large number of research projects on characterisation of scintillators has focused on measurements close to or at room temperature. There are quite a few challenges to overcome when expanding characterisation to a wider temperature range. The characterisation of scintillation detectors has so far relied almost exclusively on high-gain photomultipliers (PMT) or avalanche photodiodes (APD). However it is technically very difficult to carry out measurements of light yield using these conventional techniques when the scintillator is optically coupled to the detector since the responsivity of PMT and APD varies significantly with its operating temperature. In studies of decay time constants as function of temperature additional problems arise from some decay time constants increasing significantly as the scintillator sample is cooled. The delayed coincidence single-photon counting (DC-SPC) technique has excellent timing resolution (ca. 0.5 ns) and it is commonly used for measuring decay characteristics of traditional fast scintillators with decay time constants in the range of nano-seconds to micro-seconds [7]. For decay time constants in the region of several micro-seconds, pulse shape analysis (PSA) can be implemented. There is however a source of error inherent to both measurement techniques. It is not practically possible to fully control the rate at which ionising particles or photons interact with the scintillator. Hence there are always events recorded in which a second (or more) scintillation event has occurred during the measurement period of the first, so-called multiple excitation events. These events contribute to the total signal, resulting in a roughly even distribution of time differences throughout the decay time spectrum, hence creating a noticeable tailing effect. Thus, when analysing such data one might easily misinterpret this background as an additional decay component with a long time constant.

279 3. Description of the Technique To solve these problems we turned our attention to the idea of recording the time structure of multiple photons, all belonging to the same individual scintillation events. In practice, a sequence of single-photon pulses produced by a PMT is recorded for a duration which is long compared to the duration of an individual scintillation event. Each pulse in the sequence corresponds to an individual photon impinging on the photocathode of the PMT. The distribution of arrival times of these pulses provides information on the decay characteristics of the scintillation process. The number of single photon pulses recorded per event is proportional to the intrinsic light yield of the scintillator. By recording a large number of scintillation events (103 — 104) one can obtain the decay time characteristics and the light yield in a single measurement. Due to the large number of photons detected in each event, sufficient accuracy can be achieved already with far fewer scintillation events than needed for the DC-SPC technique, which only uses one time difference between arriving photons per scintillation event. A helium flow cryostat equipped with optical windows to allow the detection of scintillation light is the centre piece of our experimental setup used for MPCC measurements. The scintillator sample and the 241Am a-source are placed inside the cryostat and two PMTs see the sample at right angle to each other. The signals produced by each PMT and their associated pre-amplifiers are passed to integrating amplifiers. Only one PMT is connected to the input of a transient recorder while the trigger is based on coincidence in both PMTs in order to minimise the recording of spurious events. Event selection for triggering is based on the slow integrated and amplified signals. The transient recorder has sampling intervals ranging from 5 to 80 ns and is able to record up to 16384 samples per record. All scintillation events recorded by the transient recorder are stored in a binary data file and are then subjected to data analysis using tailor-made software. The algorithm for the elimination of double events is based on the ratio of likelihoods of the decay time constant X calculated for each event and the most likely time constant Xm taken from the distribution of X -values for all records. A detailed description of the experimental setup and data analysis procedure can be found elsewhere [8]. 4. Demonstration of the method An illustration of how well the data analysis works is given in Fig. 1, showing the decay time spectrum of CaW0 4 as it was measured (a) and after elimination of spurious events (b). It is clearly visible that when multiple events are rejected, the decay spectrum shows virtually no background. The example presented here demonstrates that the procedures implemented give a much

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281

clearer decay time spectrum of the scintillation process, which in turn permits the decay time parameters to be determined with better accuracy. The MPCC technique was used in our studies of scintillation properties of several tungstate and molybdate crystals. Figs. 2a and 2b show the results of these measurements for CaW0 4 . Both, the high and low temperature decay constants and the variation of the scintillator's light yield with temperature were found to be consistent with published data [9-11]. It was shown that the MPCC technique with its associated data analysis is most beneficial for characterising "slow" scintillators and it is definitely needed for characterisation of these scintillators at low temperatures where their response becomes even slower. 5. Conclusions It is concluded that the MPCC technique can be used successfully for investigations of scintillation characteristics (decay time and light yield) over a wide temperature range. It is particularly useful for studies of slow cryogenic scintillators used in searches for rare events. An additional benefit is that it is a costeffective experimental method which can easily merge with the conventional DC-SPC technique used routinely for decay time measurements. References 1.

P. Meunier, M. Bravin, B. Bruckmayer et al, Appl. Phys. Lett, 75, 1335 (1999). 2. S. Cebrian, N. Coron, G. Dambier, et al, Astropart. Phys. 21, 23 (2004). 3. G. Angloher, C. Bucci, P. Christ et al, Astropart. Phys. 23, 325 (2005). 4. A. Alessandrello, V. Bashkirov, C. Brofferio et al, Phys. Lett. B420, 109 (1998). 5. P. de Marcillac , N. Coron, G. Dambier et al, Nature 422, 876 (2003). 6. C. Cozzini, G. Angloher, C. Bucci et al, Phys. Rev.CIO, 064606 (2004). 7. M. Moszynski, B. Bengtson, Nucl. Instr. Meth. A 142 417 (1977). 8. H.Kraus, V.B.Mikhailik and D.Wahl, Nucl. Instr. Meth. (2005) in press. 9. G. B. Beard, W. H. Kelly and M. L. J. Mallory, J. Appl. Phys. 33, 144 (1962). 10. V. B. Mikhailik, H. Kraus, G. Miller et al, J. Appl.Phys. 97, 083523 (2005). 11. Yu. G. Zdesenko, F. T. Avignone III, V. B. Brudanin et al, Nucl. Instr. Meth. A538, 657 (2005).

RESULTS F R O M CUORICINO E X P E R I M E N T A N D P R O S P E C T S FOR C U O R E

M. Pedretti 9 , R. Ardito 1 ' 2 , C. Arnaboldi 1 , D. R. Artusa 3 , F. T. Avignone III 3 , M. Balata 4 , I. Bandac 3 , M. Barucci 5 , J.W. Beeman 6 , F. Bellini 14 , C. Brofferio1, C. Bucci 4 , S. Capelli 1 , F. Capozzi 1 , L. Carbone 1 , S. Cebrian 7 , M. Clemenza1, C. Cosmelli14, O. Cremonesi 1 , R. J. Creswick 3 ,1. Dafinei14, A. de Waard 8 , M. Diemoz 14 , M. Dolinski 6 ' 11 , H. A. Farach 3 , F. Ferroni 14 , E. Fiorini 1 , C. Gargiulo 14 , E. Guardincerri 10 , A. Giuliani 9 , P. Gorla 7 , T.D. Gutierrez 6 , E. E. Haller 6 ' 11 , I. G. Irastorza 7 , E. Longo 14 , G. Maier 2 , R. Maruyama 6 ' 11 , S. Morganti 14 , S. Nisi4, C. Nones 1 , E. B. Norman 13 , A. Nucciotti 1 , E. Olivieri5, P. Ottonello 10 , M. Pallavicini 10 , V. Palmieri 12 , M. Pavan 1 , G. Pessina 1 , S. Pirro 1 , E. Previtali 1 , B. Quiter 6,11 , L. Risegari 5 , C. Rosenfeld3, S. Sangiorgio9, M. Sisti 1 , A. R. Smith 6 , L. Torres 1 , G. Ventura 5 , N. Xu 6 , and L. Zanotti 1 (1) Dipartimento di Fisica dell'Universita di Milano-Bicocca and Sezione di Milano dell'INFN, Milano 1-20126, Italy (2) Dipartimento di Ingegneria Strutturale del Politecnico di Milano, Milano 1-20133, Italy (3) Dept.of Physics and Astronomy, University of South Carolina, Columbia, South Carolina, USA 29208 (4) Laboratori Nazionali del Gran Sasso, 1-67010, Assergi (L'Aquila), Italy (5) Dipartimento di Fisica dell'Universita di Firenze e Sezione di Firenze dell'INFN, Firenze 1-50125, Italy (6) Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA (7) Laboratorio de Fisica Nuclear y Altas Energias, Universidad de Zaragoza, 50009 Zaragoza, Spain (8) Kamerling Onnes Laboratory, Leiden University, 2300 RAQ, Leiden, The Netherlands (9) Dipartimento di Fisica e Matematica dell'Universita dell'Insubria e Sezione di Milano dell'INFN, Como 1-22100, Italy (10) Dipartimento di Fisica dell'Universita di Genova e Sezione di Gen282

283

(11) (12) (13) (14)

ova dell'INFN, Genova 1-16146, Italy University of California, Berkeley, California 94720, USA Laboratori Nazionali di Legnaro, 1-35020 Legnaro ( Padova ), Italy Lawrence Livermore National Laboratory, Livermore, California, 94550, USA Dipartimento di Fisica dell'Universita di Roma and Sezione di Roma 1 dell'INFN, Roma 1-16146, Italy

Cuoricino is a taking d a t a bolometric experiment searching for neutrinoless double beta decay (OfDBD) of 1 3 0 Te. The detector consists of an array of large cubic Te02 crystal bolometers. Cuoricino works at about 10 mK in the Gran Sasso Underground Laboratory. Good energy resolutions were obtained (2.1 keV at 911 keV and 3.9 keV at 2615 keV at best). The counting rate in the region of OfDBD is 0.18±0.02 c/keV/kg/y. The limit for the Oi^DBD half lifetime is 2.0 x 10 2 4 years at the 90% of C.L. This results correspond to a limit for the effective neutrino mass between 0.2 and 1.0 eV, depending on the nuclear matrix elements used. A large international collaboration is working on CUORE project, a future experiment with a mass of 741 kg of TeG-2 crystal bolometers. The experiment aims to probe the neutrino absolute mass down to 50 meV and to understand if the inverted hierarchy holds in the neutrino mass pattern.

1. Introduction The recent positive results coming from neutrino oscillation experiments have increased the attention on neutrino physics. Indeed these experiments indicate that the neutrino flavor and the neutrino mass eigenstates are different and that neutrinos have non zero masses. These results are important in view of new theoretical models beyond Standard Model. However many questions regarding the properties of this particle are still open. In particular, which is the neutrino mass hierarchy? Up to now three mass scenarios are possible: the normal hierarchy, where mi « inj « ni3, the inverted hierarchy with m 3 « mi < m^ and the quasi-degenerate one with mi ~ m 2 ~ 1113. Moreover the neutrino oscillations experiments indicate only the differences among the neutrino mass eigenvalues but not their absolute values, so what are the masses, M», of the neutrino mass eigenstates fi? Another open question regards the neutrino nature: are the neutrino and the antineutrino the same particle? Possible answers could come from Neutrinoless Double Beta Decay (Oi^DBD) experiments. 1.1. The Double Beta

Decay

The Double Beta Decay (DBD) is a rare spontaneous nuclear transition *,2 where a nucleus (A,Z) becomes a (A,Z±2) nucleus. This type of decay is

284

not favored with respect to the single beta decay and so it is possible to observe it only for those nuclei whose single j3 decay is either energetically forbidden or suppressed by a large change of the nuclear spin-parity state. Mainly two different channels of DBD are discussed: the first one, the 2z^DBD, is described by the following reaction 2*vDBD:

(A,Z) -> (A, Z + 2) + 2e~ + 2ue,

(1)

where two neutrinos are emitted and the lepton number is conserved. This type of decay is allowed by the SM and presently it has been observed for ~ 10 nuclei 3 . The second possible DBD channel is the Neutrinoless Double Beta Decay described by the reaction Oi/DBD :

(A, Z) -^(A,Z

+ 2) + 2e~

(2)

Here the emission and the re-absorption of a virtual neutrino mediates the decay. This last channel requires that neutrino and antineutrino are the same particle or, as said, a Majorana particle. Moreover the decay is allowed only if the neutrino has a non-zero mass. In Oz^DBD, the two electrons retain all the available kinetic energy. For this reason, the expected spectrum is just a spike at the transition energy. On the contrary, the 2^DBD is a four body decay and so it is expected a continuum spectra with a maximum value around one third of the Q value. The total expected energy spectrum is shown in fig. 1 (left). The DBD is characterized by a very long lifetime, for example those for the 2 J / D B D is in the range of ~ 10 18 - 10 22 years. The Oz/DBD probability is connected to the effective neutrino mass, (m„), by Fermi's golden rule: J

1

l/2

= G°" | M 0 l f (m„) 2

(3)

where G0v is the phase space integral that can be estimated exactly and |M°"| is the decay matrix element. The effective neutrino mass is a linear combination of the neutrino mass eigenvalues: (m,)=^^Mi|C/ei|2

(. = 1,2,3)

(4)

i

with \Uei\ the neutrino mass matrix elements. The <> /& phases that appear in last equation are the intrinsic neutrino CP parities. Their presence implies that cancellations are possible. From eq (4) it is clear that O^DBD could help to solve questions on the absolute value of neutrino masses and to probe the hierarchy scheme as shown in fig. 1 (right) where the neutrino oscillation results have been considered.

285

10"* Fraction of d r a y energy

10'

10 !

10'

| i g h l c s , ncmrino mass in cV

Figure 1. (Left) Expected spectrum of the sum of the energy of the electrons for DDB2f (dashed line) and DDBOi' (solid line). In the inset the contribution of the 2v decay to the Of DDB background is underlined. (Right) Plot of the effective mass (m„) as a function of the lightest neutrino mass (on a log-log scale). The dark regions are the possible neutrino mass range. Here neutrino oscillation experiment results have been considered with negligible errors.

From eq. (3), it is also clear that the evaluation of (m„) from an experimental measure of T®J2 will require the exact knowledge of the nuclear matrix elements. This could be the main limitation when using OfDBD results for implications on neutrino physics. 1.2. Experimental

approaches

Most of the recent experiments use direct measurement approaches to study the Of DBD that allow to determine the electron energy and its distribution, and sometimes permit also event reconstruction. In order to perform a sensitive experiment searching for Of DBD it is necessary to take into account the difficulties due to the very long half time of the DBD. Indeed, even if the signature of the Of DBD is in principle very clear, to experimentally recognize a peak over the continuum background could be very hard. In order to increase the sensitivity of the experiment usually it is necessary: • to work with very large source masses, of order of one ton or larger in next generation experiments. • to use good energy resolution and high efficiency detectors; good energy resolution is mandatory to improve the signal to background in the peak search; moreover poor resolution means that the 2f DBD spectra would be a source of dangerous background for the Of DBD

286

peak. • to perform the experiment in very radiopure conditions; this implies that detectors must be shielded from the environment and its associated radioactivity. Cosmic rays contributions are suppressed by performing experiments in underground laboratories. • to have long data taking periods. 2. The bolometric technique Large mass thermal detectors have been suggested since 1984 for search on rare decays 4 . Over the last few years, low temperature detectors have provided better performance as regard energy resolution, low energy thresholds and wide material choice than conventional detectors. A complete overview on these detectors and the bolometric technique is offered by proceedings of specific low temperature detectors conferences 5 . Bolometers are low temperature detectors sensitive to single particle interactions. The basic idea of bolometers is to measure the energy lost in particle interactions as a temperature variation of the detector. The two main components of a bolometric detector are an energy absorber, where particles lose all or part of their energy and produce elementary excitations, and a sensor which collects these excitations generating an electrical signal. As a first approximation, the height of the thermal pulse is given by the ratio of the deposited energy to the heat capacity C of the bolometer, whereas the pulse time constant is equal to the ratio between C and the thermal conductance of the detector to the heat sink. Consequently, in order to achieve big and fast signals, it is important to make bolometers with small heat capacity. This is obtained by using dielectric diamagnetic materials kept at very low temperatures (~10 mK in our case). Their heat capacity in fact scales as the cube of the ratio between the operating temperature T and the Debye temperature QD (Debye law). The mechanism that limits the energy resolution in these conditions is due to thermodynamic fluctuations. It can be shown that the intrinsic energy uncertainty is in this case 6

AErms

~ (kT^C)1'2

(5)

where k is the Boltzmann constant. The Milano group started some years ago a program to develop both large mass bolometers to search for Oz^DBD of various isotopes and microcalorimeters for X-ray and ^-spectroscopy. A series of bolometric experiments have been carried out by the Milano group since 1989 in the

287

Gran Sasso Laboratory, searching for the double beta decay of 130 Te. Prom the experience of this group and with an international collaboration, the Cuoricino experiment started at the beginning of 2003. In October 2003, an enlarged collaboration has proposed a new generation large mass experiment named CUORE (Cryogenic Underground Observatory for Rare

Events) that has the search for O^DBD of 130Te with the bolometric technique as a main goal.

3. The Cuoricino experiment: detector and results Cuoricino is the biggest running O K D B D experiment. The source of O J / D B D is 130 Te that, thanks to its high natural abundance, permits to work with large active masses also without isotopic enrichment. Moreover the transition energy value of the O^DBD of 130 Te is in a favorable position of the energy spectrum with respect to the natural radioactive background, indeed it is outside the 2 3 8 [ / background and in a window of low natural radioactivity: between the full energy and the Compton edge of 208Tl gamma peak (2615 keV). As absorbers of the bolometer, Cuoricino uses Tellurium Oxide (Te02) crystals, that of course contain 130 Te. In fact it is possible to grow large single Te02 crystals with excellent properties. Furthermore, the Debye temperature of TeOi is higher than pure Te and so lower heat capacity and higher pulses are obtainable. In addition, Te dominates in mass the compound and these crystals have a high radiopurity (< lpg/g in 232Th and 23SU). As sensors, Cuoricino uses germanium Neutron Transmutation Doped (NTD) thermistors, whose resistivity at low temperature depends steeply on temperature. More details on these devices could be found in 7 . Cuoricino detector is a tower-like structure that uses 40 kg of Te02- The array is composed by 44 crystals of 5x5x5 cm3 size and a 760 g weight and 18 crystals of 3x3x6 cm3 size and a 340 g weight placed in 13 elementary modules. A picture of Cuoricino detector is shown in fig. 2 (left). Cuoricino detector has reached good energy resolution in the DBD energy region (3.9 keV at the best, and 7 keV as average at 2615 keV with the big size crystals) and is characterized by good efficiency (86%). An example of energy spectrum obtained during a calibration measurement is shown in fig. 2 (right). The current background in Oi/DBD region is 0.18±0.02 c/keV/kg/y. The present best interpretation of this continuum background, coming from

288

1500 2000 Energy [keV]

3000

Figure 2. (Left) Picture of Cuoricino detector. It contains 62 TeC>2 crystals arranged in 13 planes. (Right) Example of an energy spectrum obtained during a calibration measurement. This spectrum is the sum of all the spectra obtained by the 44 5x5x5 cm 3 detectors.

Montecarlo simulation of the whole detector, is that it is mainly due to degraded alphas coming from passive materials that face the crystal absorbers, as the copper structures holding the detectors. This shows that Cuoricino experiment is not only a powerful experiment that is giving important results on DBD but it is also a good instrument to obtain information in view of CUORE Project. Cuoricino is presently giving a limit of 2.0 x 10 24 years at the 90 % of C.L. on T®Y2, corresponding to a bound on (mv) between 0.2 and 1.0 eV. The double decay region of the energy spectrum is shown in fig. 3. Cuoricino can reach a sensitivity on neutrino mass between 0.13 and 0.31 eV in a few years 8 . Unfortunately, due to the uncertainties on nuclear matrix elements, it will not be able to exclude totally the range indicated by part of the H-M collaboration for effective neutrino mass 10 , but has the potential to verify this claim. 4. The C U O R E experiment CUORE (Cryogenic Underground Observatory for Rare Events), that has been proposed and partially funded 9 , will be placed in hall A of the] Gran Sasso Underground Laboratory. The CUORE detector will be constituted by 19 Cuoricino-like towers packed in a cylindrical structure for a total of 988 Te02 detectors and a total mass of ~741 kg and corresponding to ~200 kg of 130 Te . Like Cuoricino detector, CUORE will work at about 10 mK in a new big dilution refrigerator built with low radioactive materials. CUORE

289

2<8Q

2520 iinergy

Figure 3. Energy spectrum in the Neutrinoless Double Beta Decay region.

is planned to start in 2010. In a conservative hypothesis about background (0.01 c/keV/kg/y) and with 10 keV of energy resolution CUORE should obtain in 10 years of data taking a sensitivity on half life of O K D B D of 2.1 1026 years corresponding to a neutrino mass limit ranging between 19 and 167 meV. In a optimistic, but realistic, hypothesis regarding the background (0.001 c/keV/kg/y), with 5 keV of energy resolution and 5 years of data taking, CUORE is foreseen to have a sensitivity on half life of 0 J / D B D of 6.5 10 26 years and a neutrino mass limit of 11-57 meV. 5. C U O R E R & D and the Surface Sensitive Bolometers Big efforts on CUORE R&D are performed in order to obtain a significant reduction of radioactive background in the O J / D B D region. Tests on a second dilution refrigerator in hall C of LNGS are in progress and the CUORE collaboration is working on both passive and active background reduction techniques. Last tests on the use of very radiopure powders to lap the crystals and on the use of chemical etching to polish their surfaces showed a reduction of about a factor 4 in the peaks of 238 U and 2 3 2 Th alpha decays. This reduction could be attributed to contaminations in the crystal surface. Study of cleaning procedures for the copper surface that surrounds the detector are in progress, in particular the collaboration is working on a plasma cleaning technique that is giving positive preliminary results. The CUORE collaboration is working on new detectors to make a discrimination of background events. These devices are called Surface Sensitive Bolometers (SSB) and are able to actively discriminate events orig-

290

inated near the crystals surface n . They are composite bolometers able to identify degraded alphas and beta particles coming from outside and reaching the crystal absorber. For this purpose thin bolometers (secondary bolometers) are glued around the main large TeC>2 bolometers and used as active shields. Tests with secondary bolometers of different materials were performed. As secondary bolometers and the TeC>2 absorbers are thermally coupled, pulses with different shapes are obtained depending on the particle impact point. If particles interact into the T e 0 2 crystal absorber, then both the sensors on shield absorbers and on the TeC>2 bolometer will develop pulses with the usual shape. Otherwise, in case that the interacting point of the particle is in a secondary bolometer, the sensor on this shield will read out fast and high pulses, whereas usual shape pulses will be read out by sensors on the other bolometers. The comparison between the pulses from two sensors could be carried out by means of a scatter plot, i.e. plotting the secondary bolometer pulse amplitudes versus the corresponding main bolometer pulse amplitudes. Two clearly separated bands distinguish events generated in the shield from events generated in the main crystal. Another possibility is to distinguish the origin of events by means of pulse shape discrimination.

(a)

Pulse amplitude (Ge)[mV] Surface events *r

J

400

o

it

(b)

/ Bulk / C _

100 200 300 Pulse amplitude (TeO z ) fmV]

6 8 10 Risetime (Ge thermistor) [ras] Figure 4. (Left) Scatter plot showing the relationship between the amplitudes of the pulses acquired in coincidence from the main absorber (X-axis) and from the active layer (Y-axis). In the inset, the energy spectrum obtained by selecting surface events is shown. (Right) (a) Rise Time distribution for pulses developed by sensor on the secondary bolometer, (b) Scatter plot where the different colors evidence the contribution due to fast and slow signal.

In fig. 4 (left) and 4 (right) it is possible to observe an experimental

291

Pufe*AnvUud«OT
Figure 5. Differences between the decay time achieved with a normal CUORE-like detector (left) and a detector with SSB (right). In the second detector, events coming from outside interact with the secondary bolometer and cause in the sensor on the main Te02 crystal pulses with different decay times with respect to those to events that interact directly with the main absorber. scatter plot obtained at Insubria University with a SSB composed by a 2 x 2 x 2 cm 3 TeC>2 bolometer and by a 1.5x1.5x0.05 cm 3 Ge secondary bolometer and working at about 20 m K . Tests of this new technique are in progress at the G r a n Sasso Underground Laboratory on a CUORE-like single module. Also, even if t h e results are really preliminary, they show a large discrimination capability and powerful rejection. Moreover they indicate the possibility t o discriminate the event origin by using not only pulse amplitude and rise time of pulses on secondary bolometers, but also decay time of corresponding pulses on the main bolometer. In fig. 5 it is shown the decay time as seen from the main TeC-2 crystal of a normal detector and of a detector with SSB.

References 1. 2. 3. 4. 5. 6.

Elliott, Steven R. and Vogel, Petr, Ann. Rev. Nucl. Part. Sci. 52, 115 (2002) A. Faessler, and F. Simkovic, J. Phys. G24, 2139 (1998). Moe, M. K., Nucl. Phys. Proc. Suppl. 38, 36 (1995). E. Fiorini, and T. O. Niinikoski, Nucl. Instr. Meth. A224, 83, (1984). A.Giuliani, Proc. SPIE, 4507, 203, (2001). S.H. Moseley, J.C. Mather and D. McCammon, J. Appl. Phys. 56, 1257, (1984). 7. M.Barucci et al., J. Low Temp. Phys. 123, 303, 2001. 8. R. Ardito, and CUORE collaboration, hep-ex/0501010. 9. R. Ardito, and Cuoricino collaboration, Phys. Rev. Lett. 95, 142501, (2005) 10. Klapdor-Kleingrothaus, H. V. and Dietz, A. and Krivosheina, I. V. and Chkvorets, O., Nucl. Instrum. Meth. A522, 371 (2004). 11. L. Foggetta et al., Appl. Phys. Lett 86, 134106, (2005)

TIMING CALIBRATION OF THE NEMO OPTICAL SENSORS M. CIRCELLA, C. DE MARZO 1 , R. MEGNA, M. RUPPI 2 for the NEMO Collaboration INFN, Sezione di Bari and Dipartimento di Fisica dell 'Universita di Bari, Italy e-mail: martino. ruppi@ba. infn. it This paper describes the timing calibration system for the NEMO underwater neutrino telescope. The NEMO Project aims at the construction of a km3 detector, equipped with a large number of photomultipliers, in the Mediterranean Sea. We foresee a redundant system to perform the time calibration of our apparatus: 1) A two-step procedure for measuring the offsets in the time measurements of the NEMO optical sensors, so as to measure separately the time delay for the synchronization signals to reach the offshore electronics and the response time of the photomultipliers to calibration signals delivered from optical pulsers through an optical fibre distribution system; 2) an all-optical procedure for measuring the differences in the time offsets of the different optical modules illuminated by calibration pulses. Such a system can be extended to work for a very large apparatus, even for complex arrangements of widely spaced sensors. The NEMO prototyping activities ongoing at a test site off the coast of Sicily will allow the system described in this work to be operated and tested in situ next year.

1.

Introduction

Very large volume underwater or underice Cherenkov detectors represent nowadays the most common approach to neutrino astrophysics. These detectors consist of very large (up to the order of 1 km3), three-dimensional arrays of photomultipliers which detect the Cherenkov light emitted by high-energy muons which are created in neutrino interactions inside or around the apparatus. The capabilities of these experiments are strictly dependent on how well time measurements are performed in the detectors. In fact, the reconstruction of the physical events, extracted from the large environmental background typical of the marine abysses, is based on the possibility to properly align in time the measurements performed by a large number of widely spaced sensors. In all cases in which the signals are digitized and time-stamped offshore, two different tasks are required for aligning in time the measurements: synchronization of the electronics, i.e. the delivery of common clock signals to the whole apparatus,

1 2

Deceased. Presenting author.

292

293 and timing calibration of the sensors, i.e. the measurement of how the local time measurements performed by the sensors compare to the time measured onshore. In this paper we illustrate the timing calibration system of the NEMO neutrino telescope, focussing on the approach to the solution of this problem, the overall design of the system and the validation measurements performed in the laboratory. 2.

The approach of the Timing Calibration in NEMO

The NEMO apparatus is described in detail elsewhere [1]. The Timing Calibration in NEMO project requires an embedded system in order to track the possible drifts of the time offsets during the operations of the apparatus underwater. The task of this embedded system is essentially to measure the offsets with which the local time counters inside the optical modules are reset on reception of the reset commands broadcasted from the shore, i.e. the time delays for such commands to reach the individual optical modules. All time measurements are in fact referred to the readout of such counters. Due of the effect of the timing calibration, we propose to realize a completely redundant system: 1) a two-step procedure for measuring the offsets in the time measurements of the NEMO optical sensors; 2) an all-optical procedure for measuring the differences in the time offsets of the different optical modules. In the first system the needed measurements are performed in two separate steps: - with an 'echo' timing calibration; - with an "optical" timing calibration. The former will allow us to measure the time delay for the signal propagation from the shore to the Floor Control Module (FCM) of each floor; the latter will allow us to determine the time offsets between the FCM and each optical module connected to it. The second system is an extension of the "optical" timing calibration, as is discussed in the next section. 3.

Optical Timing Calibration in NEMO

The "optical" timing calibration system will allow to measure the time delays between each FCM and the optical modules connected to it. The scheme of the system to be implemented on each floor of the towers is shown in Figure 1. In this scheme a blue light pulse is generated from a calibrated optical source (inside each FCM) upon reception of the calibration command from the shore. This pulse is then injected into four optical fibres of known lengths in order to simultaneously illuminate the optical modules of the floor. This simple system represents an efficient way to distribute common calibration signals to groups of optical modules. In fact it is important to point out that the use of beacons which deliver light flashes into the water, as is made in other detectors, is not a convenient solution due to the peculiar arrangements of the optical modules on the NEMO tower and the large distances among them.

294 Optical Module

yr Optical fibre

[illigt^Sil

V

splitter

Optical source

FCM (FloorControl Module)__

V.

FCM electronics Interface

Figure 1. The optical calibration system of NEMO.

The pulser indicated in Figure 1 has been implemented and has undergone a long characterization activity. The schematic of this circuit, which was evolved from the circuit discussed in [2], is shown in Figure 2.

Figure 2. Schematic of the pulser circuit.

This circuit features a LED Agilent HLMP CB15, which was carefully selected to work as the source of the fast light pulses with a blue wavelength as we need. In view of the fact that the light pulses from this LED are not very well focussed, we introduced a commercial optical collimator Thorlabs F230FC-A to increase the optical power picked up by the fibre. In order to cope with attenuation introduced by the cable and connectors we have developed a remote control to manage the pulse intensity of the LED by means of the regulation of the diode voltage. This voltage is set from two levels of regulations respectively for the coarse and fine tuning of the intensity of the light pulses. So, it will be possible to tune the intensity emission of the LED from the shore. In the Figure 3 it is possible to see a photograph of the board equipped with the pulser and digital circuit controller.

Figure 3. LED pulser complete of collimator and fiber coupling.

295 In order to measure the fast and faint light pulses emitted by the pulser, fast photomultipliers Hamamatsu H6780 were used in laboratory tests. These devices offer very appealing characteristics: they are compact, fast (rise-time less than 1 ns) and reliable; in addition, they are internally equipped with a high voltage supply. The coupling of the photocathode to the fibre can be performed easily with a commercial adapter. By means of these detectors, we performed an extensive evalutation of the performance of different multimode and singlemode fibre for the transport of the fast light pulses in the wavelength range of our interest, over the required length. We finally selected the Thorlabs ASF50/125 multimode fibre, which will be used for the implementation of the system in NEMO. The pulser performance is illustrated in Table 1. Table 1. Performance of the LED pulser of the timing calibration system of NEMO. Falling time

- 2 . 5 ns

FWHM

<7ns

Standard deviation of time from trigger

<100 ps

The pulser has also been coupled to the large photomultiplier Hamamatsu R7081SEL which will be used in the activities of NEMO Phase 1. Due to the large dimensions and high sensivity of this photomultiplier, the measurements were performed in a dedicated dark room. An illustration of the signals measured in these conditions is in Figure 4.

Figure 4. Light pulse from the pulser as viewed with a photomultiplier Hamamatsu R7081SEL (blue trace: trigger signal; black trace: photomultiplier output).

4.

Conclusions

The timing calibration is a delicate operation to perform for assuring the full functionality of a large underwater neutrino telescope. In this paper we have

296

illustrated the main features of our project for a timing calibration system tailored to the needs of the NEMO apparatus. We point out however that the approach proposed can be easily adapted to work for any underwater neutrino telescope. We want also to mention a possible extension to the system discussed so far, which offers increased reliability for the whole apparatus. In fact, the pulser that we have developed can deliver very intense light flashes which can therefore serve a larger number of photomultipliers than 4 as considered so far. It is therefore possible to distribute the signals from the same calibration pulser to the optical modules of more than one floor of a NEMO tower, as illustrated in Figure 5. Moreover, with the new design of the pulser board it is now possible to accurately control the intensity of the light pulses. This solution will provide an important redundancy to the 'echo' calibration procedure mentioned in Section 2.

O-

To upper floor

E V_.-'

•> I I

e

r

E

From lower floor

Figure 5. Extension of the optical calibration system of Figure 1 in which calibration signals are distributed to the optical modules of more than one floor of the NEMO towers.

5.

Acknowledgments

This work is dedicated to the memory of Carlo De Marzo. Carlo was an enthusiastic supporter of neutrino telescope in the Mediterranean Sea. The NEMO Collaboration misses the clever scientist and a good friend. References 1 2

E. Migneco, these proceedings. J. S. Kapustinski, Nucl. Instr. Meth. A241: 612, 1985.

D E S I G N , P R O D U C T I O N , A N D FIRST RESULTS F R O M T H E I C E C U B E DIGITAL OPTICAL M O D U L E

0. TARASOVA FOR THE ICECUBE COLLABORATION* DESY Zeuthen Platanenallee 6, D-15837 Zeuthen, Germany E-mail: [email protected]

The first string of IceCube - a kilometer-scale, high energy neutrino detector - was deployed in January this year deep in the Antarctic ice. At completion, IceCube will consist of at least 70 strings each with 60 Digital Optical Modules (DOMs) and a surface array of 280 DOMs. 2004 was the first year of real production and testing of these DOMs: 400 DOMs were produced in the US, Sweden and Germany. A series of tests such as basic functionality, phototube dark noise, optical sensitivity, linearity, time resolution, and high voltage tests were performed in special Dark Freezer Laboratories at temperatures ranging from +25 °C to -55 °C. 280 DOMs were sent to the South Pole for deployment and tested there again prior to deployment. Now frozen in the ice, the DOMs are showing exceptionally promising functionality. Here DOM production and testing procedures and corresponding results are discussed and data from the first in-ice string are presented.

1. Physics Motivation The main goal of the IceCube experiment is to reach an understanding of the origin of high-energy cosmic rays as well as to test a fundamental lows of physics. Low fluxes of HE neutrinos require kilometer-scale detector volumes to provide information about the sources of these particles with reasonable statistics. IceCube is designed to search for sources of high-energy neutrinos such as Active Galactic Nuclei (AGN), Supernova Remnants (SNR), microquasars or Gamma Ray Bursts (GRB). The sensitivity of IceCube to astroparticle sources is discussed in Ref.1. Due to low ambient noise in the ice, IceCube has the opportunity to detect low-energy (MeV) neutrinos from supernovae of your galaxy. In addition to high-energy astronomy IceCube physics extends to the field of particle *http://icecube. wisc.edu

297

298

physics by searching for neutrinos from possible cold dark matter candidates, weakly interacting massive particles (WIMPs), magnetic monopoles etc. 2 ' 3

2. The IceCube Detector IceCube is a kilometer-scale, high energy neutrino observatory located at the South Pole in Antarctica (Fig. 1). The construction of the detector has started in January 2005 and at completion it will consist of two subdetectors made of more than 4200 optical modules 5 . The in-ice part includes up to 70 strings mounted at depths of 1450 to 2450 m in the ice. Strings are regularly distributed 125 m apart. Each of them carries 60 optical sensors vertically spaced by 17 m. Above the in-ice part of the detector an air shower array IceTop is located at the surface. IceTop modules are being build on top of in-ice part at the surface. IceTop consists of 160 stations (2 modules each) and covers an area of 1 km 2 . These two sub-detectors can be used either in anti-coincidence mode where IceTop provide a veto for down-going muons (main background for IceCube) or in coincidence mode synthesizing information on chemical composition of cosmic rays for energies up to 10 18 eV. It also can be used for detector cross-calibration. The detector will provide an effective detection area of 1 km 2 for upward-going muons in the TeV range. Above 100 TeV detection of down-going neutrinos (i.e. observation of the southern hemisphere) will be possible. The effective area for PeV muons is above 0.6 km 2 and increases towards the horizon.

IceTop

Snow layer

South Pole Station

IceCube

Figure 1. The IceCube detector at the South Pole.

299

3. Digital Optical Module (DOM) 3.1. DOM

Design

The Digital Optical Module (DOM) (Fig. 2) is the most important component of the IceCube detector. The main principle of the DOM is a combination of large area photo multiplier coupled with support electronics for PMT power and digitization of the PMT anode pulses. PMT signals are digitized inside the DOM and get a global time stamp with a precision of better than 5 ns. This is a key concept for IceCube as it overcomes many problems associated with analog signal transmission over long distances, up to 3.5 km in the case of the IceCube detector array. DOM technology as well as the installation procedure of IceCube are built on the wealth of experience gained with its smaller prototype - the AMANDA detector 4 .

penetrator arcl pressure t •! •

•

•

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L Figure 2.

. metal cast-

The IceCube Digital Optical Module components.

For IceCube the Hamamatsu R7081-02 photomultiplier tube (PMT) has been chosen. It has low noise (200-300 cps @ -40 °C) and high gain as well as fast PMT pulse risetimes (4 ns). PMT high voltage bias is supplied by a HV generator board that is mounted on the PCB stack around the PMT's neck. It produces voltage from 0 up to +2048 V controllable from the surface. The divider circuit is a high impedance (130 Mfi) resistive bleeder (hias — 10 fiA nom) with a toroidal transformer at the anode signal output to block the DC high voltage of the PMT. The phototube and electronic is potted into the glass pressure sphere using a clear RTV gel. Gel holds the PMT providing a good optical coupling and mechanical

300

support. The magnetic metal cage shields the terrestrial magnetic field to improve photoelectron collection efficiency. Each DOM contains 12 individually selectable LED's installed on a DOM flasher board. This is a powerful tool to obtain optical properties of the surrounding ice and geometrical calibration information. One LED is capable to emit 1010 photons per pulse. This light can be detected by modules located few hundred meters away.

3.2. The Data Acquisition

System

(DAQ)

The core of the IceCube data acquisition system (DAQ) is the DOM Mainboard. Mounted inside the DOM affixed to the neck of the PMT, it contains the electronic circuits for controll, readout, digitizing, processing, and buffering of PMT signals (Fig. 3). The PMT signals are fed to the front end electronics where they are split into two paths: one travels directly to a trigger discriminator system which can start the acquisition when the pulse height exceeds a programmable level; the other path is delayed and then is further branched into several independent waveform capture channels. Two parallel Analog Transient Waveform Digitizer (ATWD) chips capture fast waveforms with 10-bit resolution and with sampling speeds programmable from 250 MHz to 1 GHz. Each ATWD has 4 input channels. Three of them are used with different gain paths, 16x, 2x, and 0.25x resulting in a large dynamic range and giving an effective resolution of 14 bits. The fourth channel is used for monitoring and calibration purposes. A 40 MHz, 10-bit flash ADC digitizes slow waveforms from distant high-energy events which are spread out over timescales of microseconds by scattering in the ice. Digitization is controlled by the Altera Excalibur FPGA and a hardcore ARM CPU on a single die. The FPGA provides the real-time interfacing to the digitizers, various on-board electrical and optical calibration devices, flasher board, and the communications DACs and ADCs. Waveform signal processing is also done in firmware. The CPU executes application codes such as DOM self-test and self-calibration applications and data acquisition programs that buffer the waveform data from the digitizers, packages it with time stamps from the local oscillator and other local state information, and send the data packets to the surface DAQ. System software can be downloaded to the DOM at any time before and after modules are deployed in the ice.

301

Digital Optical Module Block Diagram {^9

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3.3. DOM

•

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DOM Block Diagramm.

Integration

DOM assembling is supported at 3 institutions of the IceCube collaboration: UW - Madison (USA), DESY (Germany), Stockholm University (Sweden) and each production site is provided by all necessary DOM components. The procedure of DOM integration requires special equipment and includes several steps: (1) A molded plastic collar is attached to the PMT neck to provide a base for electronics installation. (2) The HV divider is soldered onto the PMT flying leads. (3) The phototube is placed into the lower hemisphere of the glass pressure vessel surrounded by the magnetic shield and the RTV gel, properly mixed and degassed. (4) In 24 hours the gel is cured and all electronics components such as the delay board, mainboard, HV control board, and flasher board are installed. (5) The penetrator cable is soldered to the mainboard. (6) Finally the glass sphere is sealed with an internal pressure of 0.5 atm. Those DOMs which pass the final acceptance testing are attached with a harness assembly and sent to the South Pole.

302

4. Final Acceptance Testing (FAT) Before the DOMs are sent to the South Pole series of tests such as basic functionality, dark noise, optical sensitivity, PMT calibration, time resolution, and linearity are performed on all of them in the dark freezer laboratory (DFL). This is final acceptance test (FAT) lasts for about three weeks. During this time DOMs experience a temperature ranging from -55 °C to +25 °C. This proves the operativeness of the DOMs ander real in-ice conditions. Each FAT includes also a long stability test (at least 180 hours) at a temperature of -45 °C what is very close to the temperature of the antarctic ice at the depth of IceCube.

Figure 4. Dark Freezer Laboratory Setup.

DOM test stations are arranged on shelves inside the DFL. Each DOM is placed on a plastic cylinder which is foiled on the inside to reflect more light and compensate for the bias towards illuminating the center. Three light sources are used during the FAT: a 405 nm diode laser, monochromatortuned tungsten lamp, and a fast LED pulser. Light is distributed to the station via optical fibers which are packed together into a tight bundle at the input. Light from each source is focused directly onto the ~mm-sized spot of bare fiber ends (Fig. 4). A computer-controlled filter-wheel attenuator is installed just in the front of the bundle to regulate overall intensity. At the station end, the light exiting the fiber is diffused using a holographic diffuser material to distribute light uniformly over the cylinder. The optical fiber system was designed to be invariant over the wide temperature range.

303 5. South Pole Deployment During the last austral summer season the first in-ice string of IceCube was deployed. About 300 DOMs were shipped to the South Pole and tested there again prior to deployment. To drill IceCube holes an enhanced hot water drill (EHWD) was developed. Using this technology it takes slightly less than 50 hours to drill holes of 2.5km depth and 60 cm diameter in the ice. In addition 4 IceTop tanks (8 DOMs) have been installed on the surface what makes a total of 76 deployed DOMs which are functioning and producing physics data. All DOMs are operational and meet the design requirements. Obtained noise rates for in-ice modules are about 700 Hz on average. Time resolution measured with flashers is better than 2 ns. Data taking had started after the first surface tanks and the in-ice string were deployed and will continue during the detector construction. The plan for the next season is to deploy up to 10 in-ice strings and 32 IceTop stations. Continuing at a rate of 16-18 strings and 32-36 IceTop tanks per year IceCube will be completed within the next 5 years. Acknowledgments This material is based on work supported by the following agencies: U.S National Science Foundation (NSF); German Ministry of Research and Education; Swiss National Fund; Federal Office for Scientific, Technical and Cultural Affairs, Belgium; Funds for Scientific Research (FNRS and FWO), Belgium; Institute for the Promotion of Innovation by Science and Technology in Flanders, Belgium; German Research Foundation (DFG), Germany; Inoue Foundation for Science, Japan; Knut and Alice Wallenberg Foundation, Sweden; Swedish Polar Research Secretariat, Sweden; The Swedish Research Council, Sweden; Department of Energy, Office of Science, USA; University of Wisconsin Alumni Research Foundation, USA. References 1. 2. 3. 4. 5.

J. Ahres et. al., Astropart. Phys. 20, 507 (2004). F. Halzen, astro-ph/0311004. C.Spiering for AMANDA collaboration, Proc 27th ICRC (2001) 1242. http://amanda.wisc.edu J. Ahrens et. al., The IceCube Proposal to NSF (2000), J. Ahrens et. al., PDD:IceCube Conceptual Design Document (2001).

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Calorimetry Organizer: C. Leroy

S. Bimbot P. Bloch A. Bornheim D. Bryman I. Evangelou M. Groll E. Giilmez S. Hagopian V. Hagopian M. Ippolitov A. Krokhotin P. Meridiani M.M. Obertino

C. Oliver J. Pinfold R. Ruchti R. Salerno

Study of Electromagnetic Calorimeter Trigger-Primitive Performances for the CMS Data Selection The CMS Electromagnetic Calorimeter The CMS ECAL Laser Monitoring System Pointing Calorimeter for Measuring K°L -> K^VV Decay and Development of Extruded Scintillator CMS Preshower In-situ Absolute Calibration Dedicated Front-End Electronics for an ILC Prototype Hadronic Calorimeter with SIPM readout Using Single Photoelectron Spectra in the Calibration of the CMS-HF Calorimeter Jet Energy Scale Determination for the Run II DO Calorimeter The Hadron Calorimeter of the CMS Experiment at the Large Hadron Collider Properties of a 256-channel Prototype of the ALICE/PHOS Calorimeter Source Calibration of the CMS Quartz Fiber Forward Calorimeter The Calibration Strategy of the CMS Electromagnetic Calorimeter The Very Front-End Cards for the CMS Electromagnetic Calorimeter: Description, Calibration and Performance Uniformity of Response of the ATLAS Electromagnetic Calorimeter Series Modules Plans for the Very Forward Region of ATLAS - the LUCID Luminosity Monitor Fast, Long-Wavelength Scintillators and Waveshifters Test Beam Results of the CMS Electromagnetic Calorimeter 305

M. Schram C. Serfon A. Topkar A. Ulyanov K.-C. Voss

Electron and Pion Results from the ATLAS Foward Calorimeter 2003 Test Beam Linearity, Energy and Position Resolution of the ATLAS Electromagnetic Calorimeter Series Modules Preshower Silicon Strip Detectors for the CMS Experiment at LHC Test Beam Results of the CMS Forward Quartz Fibre Calorimeter Inter-Calibration and Offline Compensation Studies of the ATLAS Calorimeter Using Beam Tests and Monte Carlo

STUDY OF ELECTROMAGNETIC CALORIMETER TRIGGERPRIMITIVE PERFORMANCES FOR CMS DATA SELECTION STEPHANE BIMBOT On behalf of CMS ECAL Collaboration Laboratoire Leprince-Ringuet, Ecole Polytechnique, Route de Saclay 91J28PALAISEAU, France CMS will be one of the LHC detectors at CERN. The flux of data will be so high that a drastic on-line selection is needed. The Trigger Primitives (TP) are a basic element of this selection. In the electromagnetic calorimeter (ECAL), they are used to estimate the transverse energy, and to find the bunch crossing corresponding to the event. The study presented in this paper shows that the linearity and the resolution of the ECAL-TP response are satisfactory, and that the efficiency of the bunch crossing identification is perfect for particles above 1 GeV.

1. Introduction The Large Hadron Collider (LHC) is a proton-proton collider under construction at CERN (the European Laboratory for Particle Physics). It will be able to generate collisions at energies up to 14 TeV in the center of mass frame. The nominal luminosity of this collider will be 1034 cm"2 s"1. The Compact Muon Solenoid (CMS) [1] will be one of the general-purpose detectors operating at LHC. It is designed to observe the Higgs Boson, and to study physics beyond the standard model. Because of its high resolution, the electromagnetic calorimeter (ECAL) [2], made of 75,848 lead-tungstate crystals, is a key-element to identify the Higgs boson (through gamma-gamma decay, or ZZ* into four electrons decay channels). At LHC, a bunch crossing will happen every 25 ns. For the ECAL only, the data volume to store would be 80,000 Gbytes per second. How to transmit such a data flux? As most of these data will not contain any interesting information for Physics purpose, an on-line selection will be done to transmit only useful events: the Level-1 (LI) Trigger [3]. For the ECAL, the aim of the trigger is to select the events in which high transverse momentum electrons or photons are identified. For this selection, both the transverse energy and the compactness of the shower (which signs the * For each crystal, a vector is evaluated, the norm of which corresponds to the value of the energy deposited in the crystal, and the direction of which is given by the axis linking the vertex (here

307

308

presence of an electron or photon) have to be estimated on-line. This is precisely the task of the Trigger Primitive Generation (TPG). 2. Trigger Primitives 2.1. The Trigger Primitive Generation: Hardware Description The ECAL is made of a barrel and two end-caps. The following study has been done in the barrel only, where a trigger tower is defined as a group of 25 crystals. Each trigger tower is divided into five strips of five crystals (along the phi coordinate). Each strip is linked to an electronic board called "Very Front End" (VFE). The 5 VFE of each trigger tower are linked to another board called "Front-End" [4]. This board contains six ASICS involved in the Trigger Primitive (TP) calculation called "Fenix". Five of them are configured as "FenixStrip" and one as "Fenix-TCP". The light produced in the scintillator crystals is collected by two Avalanche Photo Diodes (APD). The signal is then shaped with a multi-gain amplifier, and digitized by an ADC (Analog to Digital Converter). LMMrUM PiW»

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Figure 1. The different steps of the signal propagation as performed by each Fenix Strip.

A Fenix-Strip takes as input the digitized samples coming from each crystal to estimate the transverse energy in each strip. As seen on Figure 1, the signal is first linearized for each crystal: the ADC samples are multiplied by a factor taking into account the gain value and the position of the crystal, making the signal amplitude proportional to the transverse energy. Next, the "Adder" sums

approximated by the zero of CMS) and the center of the crystal front face. The norm of the transverse component (with respect to the axis of the beam) of this vector is called "transverse energy". This notion is interesting because it gives, at high energy, an evaluation of the transverse momentum.

309 the values found for the five crystals. Then, a filter reconstructs the transverse energy of the strip using a weight-method (linear combination of digitized samples), and finally, a peak-finder identifies the maximum output of the amplitude filter. The time position of this maximum makes it possible to identify the bunch crossing corresponding to the event. The outputs of the five Fenix-Strips are used by the Fenix-TCP to add up the transverse energy from the five strips (ETot), and to find out the maximum transverse energy in two consecutive strips. These two outputs are used to estimate the compactness of the electromagnetic shower characterized by a single bit [3]. This bit and ETot are combined into one word of 11 bits for each tower: the TPG output, transmitted to the Level-1 trigger processor. 2.2. Validation of the Simulation using Electron-Beam Data To study the TP response, data taken in an electron beam in November 2004 were used. However, the amount of data was not sufficient for a detailed study, especially at low energy. Moreover, the condition of the data taking was different from the conditions at LHC: in test beam operation, the phase between the ADC clock and the signal pulse is random. So, for the global study, new sets of data had to be simulated. This implied first to simulate the energy deposited in each crystal (using the Monte-Carlo Simulation "Geant 4" [5] and real pedestal runs without beam to have a realistic electronic noise), and next, to have a perfect simulation of the response of the electronics. The simulation of this electronics response follows closely the hardware description given in the previous section. In order to check it, we used digitized samples from real data sets as input. The response of this electronics simulation was compared to the Trigger Primitives measured in these real data sets. This simulation reproduces perfectly (bit to bit) the real electronics response. Table 1. Comparison between Data and Simulation t^Bcam

(GeV) 15 20 30 50 120





39.93 ±0.07 55.10 + 0.08 85.55 ±0.10 147.10±0.17 369.67 ±0.63

39.91 ±0.06 55.22 ± 0.08 86.10 ±0.12 148.18 ±0.20 367.24 ±0.51

a Data 1.72 ±0.04 2.29 ±0.04 3.13 ±0.06 5.31 ±0.10 12.20 + 0.44

a Sim 1.79 ±0.04 2.26 ±0.04 3.22 + 0.06 5.46 ±0.13 12.87 ±0.39

To check the validity of the global simulation, its results were compared with the real data at the beam energies of 15, 20, 30, 50 and 120 GeV. For each energy, we used about 1000 events (data and simulation) for the comparison. Data were

310 selected such as the incident beam could be considered as point like. For each beam energy, the distribution of the energy of the TP has been fit using a gaussian function. In Table 1, the mean value and the standard deviation are shown for the data and the simulation. Clearly, data and simulation are in good agreement within 1% for the mean values and 5.5% for the standard deviations. This last difference is only observed at 120 GeV, the agreement being much better at lower energies. The overall agreement between data and simulation is satisfactory for the purpose of this study, focused on low energies. 3. Trigger Primitive Results based on Simulation For the following study, the global simulation (Monte-Carlo plus electronics simulation) has been used, with the conditions of CMS in situ (random phase between the ADC clock and the signal pulse). The study has been done for two different impact points in the trigger tower: one in the center, and one in the corner. A simulated beam of 2 mm around the center of the crystal front face has been used. Figure 3 shows the transverse energy estimated thanks to the TP by the LI Trigger algorithmt [3] versus the energy of the incident particle up to 120 GeV. The parameters in the electronics simulation were set to get an optimized resolution at low energies. These parameters imply a saturation of the TP response around 130 GeV. pTPB Linearity!

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Figure 3. Response of TP algorithm (sum of 2 trigger towers) as function of the incident energy for 2 impact points: center crystal (plain line) and corner crystal (dashed line).

A linear fit has been applied in Figure 3 for the 2 impact points. It shows that the response of the TP is linear over the energy range considered within the statistical errors (%2/NDF=4.l/8). For a given energy, the value reconstructed is slightly lower when the impact point is in the corner because of the ECAL LI ^The trigger algorithm uses the sum of the TP values in the tower hit by the particle, and in one of the adjacent towers; among the four possibilities, the tower with the maximum energy is used.

311

Trigger which associate only two trigger towers. The resolution of the TP response measured by the ratio between the width and the mean value of the TP distributions for a given energy is shown in Figure 4 as function of the energy. The measured resolution fully satisfies the CMS trigger requirements: for example, it gives a resolution of 2% at 23 GeV (the threshold of the LI trigger for single electrons). The resolution is almost independent on the impact point. [

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Figure 5. Bunch crossing assignment as function of the incident energy.

The TP can also be used to identify the bunch crossing in which the event occurred: the time position of the maximum of the signal collected by a trigger tower (response of the peak finder, see section 2.1) makes it possible to determine this bunch crossing. The efficiency, i.e. the probability to assign correctly the bunch crossing to the event, is shown in Figure 5 versus the incident energy of the particle. This efficiency is almost 100% above 1 GeV. This result corresponds to CMS trigger requirements (see [3] p 89). 4. Conclusion The simulation performed in this work matches the data within the statistical error. Its use to the test of TPG suggests a satisfactory linearity and resolution. It also indicates that this TPG system allows an excellent bunch crossing identification for particles with energy higher than 1 GeV. References 1. CMS Technical Proposal, CERN/LHCC 94-38 2. CMS Collaboration, The Electromagnetic Calorimeter Project, Technical Design Report, CERN/LHCC 97-33, CMS TDR 4, 1997

3. CMS Collaboration, Trigger and Data Acquisition Systems, Technical Design Report, CERN/LHCC 2000-38, CMS TDR 6.1, 2000 4. N. Pastrone, in proceedings of Workshop on Electronics for LHC and future experiments, Heidelberg 2005, CERN-LHCC-2005-038 5. Geant4 collaboration MM A 506, 2003

T H E CMS ELECTROMAGNETIC CALORIMETER

P. B L O C H PH Department,

CERN

O n behalf of t h e C M S E l e c t r o m a g n e t i c Calorimeter G r o u p The CMS electromagnetic calorimeter (ECAL) aims at measuring with high precision the photons and electrons produced in the collisions at LHC. It contains « 76000 Lead-tungstate crystals. The main challenges are to measure the electromagnetic shower energy with a precision better than 1% above 100 GeV and to operate reliably in the harsh radiation environment of LHC. We present a short overview of the calorimeter design, recent progress in its construction and results of system tests performed recently with fully assembled Barrel supermodules.

1. Introduction The prime motivation of the LHC is to discover the nature of symmetry breaking. The Higgs mechanism is the currently favoured process. In the mass interval 114-130 GeV the two-photon decay is the most promising Higgs boson decay channel. The natural width of the Higgs boson is small and the measured width of a signal will be entirely dominated by the instrumental two-photon mass resolution. High resolution electromagnetic calorimetry is therefore a central design feature of the Compact Muon Solenoid (CMS), a general purpose detector to be installed at the LHC proton-proton collider 1. The best performance in terms of energy resolution is obtained using fully active calorimeters. CMS has chosen lead tungstate (PbW04) crystals. These crystals are fast, radiation tolerant and have a short radiation length (XQ = 0.89 cm) and Moliere radius(2.1cm), allowing the design of a compact and highly granular detector. The main challenge is to build a detector which can reliably perform during more than 10 years in the high radiation levels of the LHC interactions region. The use of lead tungstate imposes further, specific requirements. The low light yield requires use of photo-detectors with intrinsic gain that can operate in a magnetic field; silicon avalanche photo-diodes (APDs) have been chosen for the barrel crystals and vacuum phototriodes (VPTs) for the endcap crystals. The change of

312

313 response with temperature due to both the variation of the scintillation efficiency and of the APD gain, puts strong requirements on the temperature stabilisation. Finally, the small, but non negligible variation of transparency of the crystals during irradiation requires a precise light monitoring system. In section 2 , we present a brief overview of the ECAL design. Section 3 is devoted to the status of construction of the various elements. In section 4 we report on the results of system tests using fully assembled sections of the detector in the laboratory or in test beams at CERN. crystals in a

preshower

EC crystals

Figure 1. The CMS Electromagnetic Calorimeter

2. C M S E C A L design The layout of the CMS ECAL 2 is shown in Fig. 1. In the Barrel, the 23 cm long crystals (25.8Xo) are grouped into 5 x 2 matrices, held in a glass fibre alveolar submodule, of which 40 or 50 are mounted into a module. Four modules are assembled together in a supermodule, which contains 1700 crystals. Eighteen supermodules form a half barrel covering the range of pseudorapidity |?7| from 0 to 1.48. Two APDs, embedded in a plastic capsule, are glued to the back of each crystal. They are connected in parallel to the read-out electronics, which is mounted on the outside of the mechanical structure. The electronic read-out chain follows a modular structure whose basic elements are matrices of 5 x 5 crystals corresponding to a trigger tower. The APDs are connected to the Very Front End (VFE)

314 boards which shape, amplify, and digitise the signals. The digitised data are then stored in the FE board and the trigger primitives (the energy sum in a tower) are generated and transmitted to the trigger electronics by a 800 Mbit/s optical link. On receipt of a level-1 accept, the data are transmitted to the off-detector electronics by a separate optical link. The Endcaps, covering the rapidity range \r]\ from 1.48 to 3, consist of 4 Dees containing 3662 crystals each. The 22cm long crystals are held in a carbon fibre alveolar, forming a 'supercrystal' containing 25 identical crystals. A VPT is glued on the back of each crystal. The supercrystals are countilevered from a thick Aluminum back plate. The readout electronics is the same as for the Barrel. To improve the 7—7r° separation, a Preshower covers most of the Endcap area. It contains 2 planes of silicon (1.9 mm pitch) strip sensors, positioned after two lead absorbers of 2 and 1 radiation length thickness respectively.

3. Construction progress Crystals: Sixty-seven percent of the Barrel crystals have been delivered. The majority of the crystals is produced at the Bogoroditsk plant (BCTP) in Russia, but recently a second source of supply has been opened with the Shanghai Institute of Ceramics (SIC) in China. The last Barrel crystal will be delivered by the end of 2006, and the last Endcap crystal one year later. Each crystal undergoes dimensional and optical checks in fully automated equipment located at CERN and in Rome. For BCTP crystals, the radiation hardness can be inferred from optical properties. The inferred radiation hardness is checked regularly by performing irradiation measurements on a sample of crystals. For SIC crystals, no such correlation has been found yet and every crystal is pre-irradiated. Photodetectors and electronics: All the APDs and 80% of the VPTs have been produced and tested. In 2002, the electronics chain has undergone a major, but very successful revision using the 0.25/um CMOS technology. All the custom ASICs have now been produced. About 75% of the Barrel VFE boards are at hand. The serial integration of electronics in Supermodules has started. Assembly: Twenty-two of the 36 Supermodules have been mechanically assembled. By the end of 2005, all the large mechanical pieces for the Endcaps will be delivered. Preshower: The production of the Preshower silicon sensors is 90% completed, the specifics ASICS have been produced and are being tested.

315

The electronics hybrid production is launched. 4. R e s u l t s of s y s t e m t e s t s In the past year, several Supermodules have been fully assembled including the final electronics and its associated cooling. One of them was thoroughly studied during Fall 2004 in the H4 electron beam at CERN. Most of the tests focused on the energy resolution and particularly on effects which could affect the constant term, which limits the resolution at high energy. T e m p e r a t u r e stability: the variation with temperature of the calorimeter response was measured to be « —4%/°C. Given the stability of the cooling water the only source of temperature variation within the ECAL is a possible variation of the power dissipated by the electronics. To investigate the sensitivity to this, the temperatures of thermistors in the APD capsules were measured with the electronics switched on and switched off. The maximum measured change in temperature AT on _ 0 fr was 0.1°C, with a mean change of 0.056°C (Fig. 2). Therefore the contribution to the constant term of the energy resolution due to thermal fluctuations will be negligible, even without temperature corrections.

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C r y s t a l t r a n s p a r e n c y corrections: the validation of the light injection system with its final laser and read-out electronics and the feasibility of determining the short-term transparency corrections were the goals of dedicated beam tests. The channel response to laser light without any incident beam, normalized to reference diodes, was stable to 0.1%. During the

316

tests with beam, crystals were exposed to a high intensity 120 GeV electron beam with dose rates from 0.2 to 0.4 Gy/h at the shower maximum, larger than that expected in the barrel at high luminosity. Under these conditions, the signal loss is expected to saturate at around 5%. The correlation between the electron and laser signal is shown for one crystal in Fig. 3. It allows one to correct for the signal loss with a precision < 0.3 %. Electronics performance: the electronics noise was studied using random triggers. The noise per channel ranges from 35 to 45 MeV with very little spatial correlation between channels. Finally, the energy resolution was measured using data taken with electron beams of momenta ranging from 20 to 250 GeV/c. The Supermodule was mounted on a rotating table, allowing the beam to cover almost uniformly the front face of each crystal. Fig. 4 shows the energy resolution as a function of energy for nine crystals. The energy resolution above 100 GeV is typically 0.5%, meeting the goal set for the CMS ECAL.

Erwrgy(GeV)

Figure 4.

Energy resolution as a function of energy for 9 crystals

5. S u m m a r y The construction of the CMS Lead tungstate crystal calorimeter is well advanced, with 60% of the Barrel Supermodules already mechanically assembled. The procurement of the Barrel electronics will be completed by spring 2006. All major mechanical components for the Endcap ECAL are also available. The construction schedule is driven by the crystal delivery, which should allow completion of the Barrel before LHC startup and of the Endcaps for the first physics run in 2008. System tests and test beam results with fully equipped Supermodules confirm that the CMS ECAL will

317

meet its demanding design goals. Acknowledgments The author would like to acknowledge the large dedicated community of scientists, engineers and technical staff who have worked extremely hard to ensure the success of this project. References 1. http://cmsdoc.cern.ch/docLHCC.shtml 2. The Electromagnetic Calorimeter Technical Design Report, CERN-LHCC 9733 (1997).

THE CMS ECAL LASER MONITORING SYSTEM

ADOLF BORNHEIM On behalf of the CMS ECAL

Collaboration

Lauritsen Laboratory, CALTECH, 1200 E California Blvd. Pasadena, CA 91125, USA The CMS detector at LHC will be equipped with a high precision lead tungstatc electromagnetic calorimeter. To ensure the stability of the calorimetric response at a level of a few per mille, every channel of the detector is monitored with a laser system. This system allows to correct for fluctuations in the detector response with high precision, in particular the expected radiation induced changes of the crystal transparency, which are on the level of several per cent at nominal luminosity. We report results from recent tests on a super module of the CMS ECAL equipped with final design components. The performance of the monitoring system is demonstrated and details of the monitoring strategy will be discussed.

1. Introduction One of the main physics goals of the CMS detector at LHC is the search for the Higgs boson. In the mass range around 125 GeV, the decay H—>yy is of particular interest because of its clean final state. This mode however requires a very good energy resolution for the electromagnetic calorimeter to take advantage of the expected narrow decay width of the Higgs boson in this mass range. To fulfill this requirement, the CMS experiment features a high resolution crystal calorimeter utilizing lead tungstate (PWO) crystals [1, 2]. These crystals do experience a very slight, dose rate dependent decrease of their transparency under irradiation, which recovers to a large extend in irradiation free periods. In CMS, the variation of the crystal transparency under irradiation and the corresponding decrease in light out put of the crystals will be monitored with a high precision light monitoring system. In this paper we describe the experience with the final design monitoring-system in the test beam at CERN and discuss the strategy for correcting the variation in the crystal response during LHC operation.

318

319

2. Radiation Induced Transparency Change in PWO4 Crystals The mass produced lead tungstate crystals used for the ECAL are radiation hard to high integrated dosage, but they experience a dose rate dependent transparency loss, as shown in Figure 1 (left). Although there is no damage to the scintillation mechanism, color centers are created during irradiation. These radiation induced color centers may annihilate at room temperature. During irradiations both annihilation and creation processes coexist and the color center density reaches an equilibrium at a level depending on the dose rate applied, as shown in Figure 1 (center) [3].

Figure 1 : In the left plot the transparency loss in a lead tungstate crystal is shown as a function of the wavelength. The loss is most pronounced close to the peak of the emission spectrum at around 440 nm. On the right, the transparency loss is shown as a function of the irradiation time. The transparency loss saturates after an initial decrease.

During operation of the accelerator the does rate for the crystals in ECAL will vary significantly, depending on the instantaneous luminosity, the beam quality and the location in the ECAL. A typical rate for the barrel crystals will be 15 rad per hour and up to a factor 100 higher for the crystals closest to the beam pipe. The transparency loss will recover to a large extend when the irradiation stops, typically in a fast initial recovery with a time constant of several tens of hours followed by a slower recovery on the time scale of hundreds to a few thousand hours (Figure 1, right). This is to be compared with the time cycle in the LHC operation of 12 hours. It should be noted that the exact amount of the transparency change as well as the time constants vary from crystal to crystal. 3. The CMS ECAL Laser Monitoring System The laser monitoring system must track the change in the transparency precisely enough to maintain the constant term in the ECAL resolution of 0.55%. This requires a measurement of the transparency with an accuracy of better than 0.2%. The transparency in each crystal will have to be measured approximately every 30 minutes during LHC operation to account for the fast initial change at the beginning of an irradiation cycle. To achieve this, the monitoring has to be

320

performed while the data taking of the detector is ongoing. This can be realized by taking advantage of gaps in the LHC bunch train which occur every 90 [is.

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moi»™ni laser monitoring source. Two laser Figure 2 : Scheme of the continuous laser monitoring S y Ste ms provide light at 440 nm and during LHC operation. Taking advantage of a 3 LIS gap in goo nm as primary and secondary the LHC bunch train laser pulses are injected at a rate of monitoring wavelengths. 100 Hz.

To allow this operational mode the laser source has to fulfill a number of design parameters. In particular the laser pulse width must be shorter than 40 ns and stable to better than 10% to match the shaping of the ECAL electronics. Furthermore must the pulse timing be stable to a few ns to allow a synchronization which the LHC clock and the detector readout. Finally, the pulse energy must be on the order of lmJ to provide sufficient light to large groups of crystals to ensure that the entire ECAL can be monitored in 30 minutes. The pulse energy also has to be stable to better than 10% to prevent possible non-linear effects in the reference system against which the signals of the individual crystals are compared. To achieve all these performance requirements a dual-stage laser system is used [4, 5]. Two such laser systems provide two different wavelengths, on at the peak of the scintillation light wavelength at 440 nm, one far away in the red wavelength range to allow for systematic cross checks. These two systems distribute the light pulses via a system of optical switches to the ECAL super modules, as shown in Figure 3.

4. Performance of the CMS ECAL Monitoring System The feasibility of the radiation damage monitoring with the desired precision has been repeatedly demonstrated in the test beam. This is done by irradiating crystals in a high energy electron beam with dose rates similar to the ones expected in CMS. During the irradiation the crystals are calibrated with the electron beam in regular time intervals, followed by laser monitoring runs. The

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Figure 4 : The left plot shows the correlation between the transparency loss measured by the laser system and the signal loss of high energy electrons under irradiation. The right plot shows the slope of this linear correlation for a group of 19 crystals. It is relatively uniform among crystals from the same producer.

The correlation can be parameterized by a linear function with slope a. As shown in Figure 4 (right), the spread of a among different crystals was measured to be 6.3% in a sample of 19 crystals. Given the average signal loss for a CMS-like radiation environment, such a small dispersion would allow to use the same correction parameter for all crystals. It should be noted that the change in the ECAL response during normal LHC operation might be relatively small because the recovery time of the crystals is much longer than one LHC cycle, as detailed in section 2. . The slope of the correlation is known to vary for crystals from different producers and is different for the barrel and the end cap of the ECAL, due to the slight differences in the crystal geometry and the light injection. Methods to determine a in-situ from physics events used for calibration are being considered. 11.004

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322

To ensure that the constant term of the resolution can be maintained it is crucial that the transparency measurement itself is sufficiently stable. The careful design of the entire monitoring system ensures that the transparency can be monitored with a precision of typically 0.1% over several hundred hours. In Figure 5 we show the stability of an individual channel of a fully equipped super module over 450 hours. Each point comprises a transparency measurement with the laser monitoring system. In Figure 6 we show the stability of a group of 500 crystals over the same time period. 5. Summary The CMS ECAL laser monitoring system has been tested at CERN for several thousand hours. All design performance parameters have been achieved. In particular it has been shown that the radiation induced transparency change in the CMS ECAL can be monitored with a precision of typically 0.1% over several hundred hours. The effect of the transparency change expected for normal LHC operation can hence be corrected and the effect reduced to a level which allows to maintain the constant term of the calorimeter resolution within the design specifications. Acknowledgments 1 would like to thank the entire ECAL collaboration for their outstanding effort, R.-Y. Zhu as well as the CALTECH team for their superb work on the laser system and the organizers of the 9th ICATPP Conference at Como. References 1. "The Electromagnetic Calorimeter Technical Design Report", CMS Collaboration , CERN/LHCC 97-33, 1997 2. "The CMS ECAL", P. Bloch, these proceedings. 3. "Radiation Induced Color Centers and Light Monitoring for Lead Tungstate Crystals", X. Qu, L. Zhang, R.-Y, Zhu, IEEE Trans. Nucl. Sci. NS-47 (2000). 4. "Performance of the Monitoring Light Source for the CMS Lead Tungstate Crystal Calorimeter ", IEEE Trans. Nucl. Sci.Vol. 52 No. 4 (2005) 1123— 1130. 5. "ECAL Monitoring Light Source at H4", D. Bailleux, A. Bornheim, L. Y. Zhang, K. J. Zhu, R.-Y. Zhu, and D. Liu, CMS IN 2003/045.

Pointing Calorimeter for Measuring K[ -» TTVV Decay and Development of Extruded Scintillator

DOUGLAS BRYMANf, JOSS IVES Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T1Z, Canada PIERRE AMAUDRUZ, YURI DAVYDOV, ROBERT HENDERSON, NAIMAT KHAN, CHAPMAN LIM, ANDREW MILLER, TOSHIO NUMAO , ALEKSEY SHER, DAVID WONG TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3Canada

A sampling calorimeter based on plastic scintillator-drift chamber sandwiches was designed to measure the angles, positions, energies, and times of medium energy photons with good resolution and high efficiency. Techniques for manufacturing extruded plastic scintillators with multiple holes for wave length shifting fibers have been developed. Light output comparable to commercial scintillator and good dimensional tolerances have been achieved for 8 mm x 70 mm x 2.5 m planks which can be glued into large sheets.

1. Calorimeter for the Measurement of K°L —> n°vv The single most incisive measurement in the study of CP violation (CPV) in the context of the Standard Model (SM) would be the branching ratio B( KL —> n vv) which uniquely allows direct access to the SM CPV phase. Since the

KL—*TCVV

reaction is predicted to be extremely rare with a

branching ratio of B(K° —> TT°VV) = ( 2 . 9 ± 0 . 6 ) x l 0 ~ u , an experiment designed to observe and study it needs unprecedented sensitivity. In the KOPIO concept for this measurement, shown schematically in fig. 1, a low energy, time-structured neutral beam is used for determining the incident K momentum using time-of-flight. The detection system for observing n —> yy decay is designed to allow a fully constrained reconstruction of the decay vertex, and mass, energy, and momentum measurements in the K

center of

mass system. This is accomplished by measuring the position of interaction, Corresponding author: [email protected]

323

324

angle, and energy of each individual photon in a fine grained preradiator (PR) detector (2.7 radiation length, X0) followed by an efficient shashlyk [1] calorimeter (15 XQ ).

Figure 1. Concept of the KOPIO experiment. Photons from n —> yy decay are detected in the preradiator (PR)-calorimeter system which measures the energy, position, time, and angle of incidence.

1.1. PR Design The PR-calorimeter combination was designed to be a high resolution, high efficiency gamma ray imaging device consisting of 64 sandwiches of scintillators and cathode strip drift chambers for photons in the range of 50 to 500 MeV. Each layer had a thickness of 0.04 Xo of which about half was scintillator. The resolutions expected for the PR-calorimeter combination were

2.7% 250um, ,

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= • for position, energy, angle (at

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250 MeV), and time, respectively, based on prototype detector measurements and simulations. The conversion efficiency for photons in the PR was estimated to be 88%.

325

The PR design was arranged in four quadrants of modules, each of which had eight 1.5 m x 1.5 m x 7 mm drift chambers and nine, 8-mm thick sheets of plastic scintillator. The drift chambers measure the induced charge distribution on cathode strips and drift time of ionization to the anode wires to obtain the position of e e pairs. Since both particles contribute to the induced charge distributions, an accurate measure of the average position of the pair may be obtained using the cathode information. On the anode wire, only the earliest time of arrival of the primary ionization can be measured so it is expected that anode drift time will give poorer position resolution for an e+e~ pair than from the cathode strip signals. However, the resolution depends on the angle of incidence for the strip readout whereas the anode information is relatively independent of angle. Consequently, the chamber layers have alternately vertical (horizontal) and horizontal (vertical) wires (strips). The expected precision for measuring photon angles was demonstrated experimentally using 100-300 MeV tagged photons at the BNL light source.

1.2. Extruded Scintillator During the past three years we developed the technique for production of lowcost multi-hole extruded polystyrene scintillator with CELCO Industries (Surrey, BC). Readout was intended to be done with wave length shifting (WLS) fibers which are inserted into the holes without glue or optical coupling. Using holes in the extruded scintillator rather than grooves into which the WLS fibers are glued facilitates assembly and insertion of the fibers after assembly of the chamber-scintillator modules, and results in comparable light output. The polystyrene chemistry and extrusion technique were modeled after work done at Fermilab [2] which produced single hole scintillators. The extrusion planks we developed had dimensions 8 mm x 70 mm x 2.5 m with six 1.4 mm diameter holes separated by 10 mm. Photographs of planks are shown in fig. 2. To assemble a large area sheet, the plank edges are machined with a tongueand-groove shape before gluing in a jig.

326

Figure 2. Scintillator during extrusion process (left) and planks glued to form a sheet. Techniques were developed to reliably extrude multiple hole planks with uniform dimension holes, and reasonable dimensional tolerances of <0.15 mm for flatness and <0.4 mm for excursions from "straightness" over 2 m lengths. We have studied properties including hole dimensional tolerance, light output, uniformity, and attenuation. During these tests, scintillators were read out by 1 mm dia. KURARAY Yl 1(200) S fibers. The light from the fibers was collected at one end by a HAMAMATSU 3178X photo-multiplier. The other end of the WLS fiber was blackened. The tests indicated that the amount of the scintillation light produced by the extruded scintillator was comparable to that of the commercial cast scintillator BC408. The single end light yield using WLS fibers (see below) was found to be 16 photo-electrons (p.e.)/MeV, which would correspond to statistical energy 1% resolution of 0F < , =•. The measured time resolution for a single

VE(GeV) 90 ps sheet was found to be consistent with

At = - .

= with a 700 ps

ylE(GeV) constant term. The light attenuation length in the 6 hole extruded material including surface effects was found to be 150-200 mm. This is considerably shorter than in BC408 but is of minor importance in the WLS readout geometry studied. The variation of the light yield across the scintillator planks and across the joints when the scintillators are glued together with optical cement was found to be about 1.5%. In addition, it was found that the quality (surface imperfections and diameter variations) of the extruded holes had little effect on the light output.

327

Following a recent production run of 500 planks the light output dependence of various wrapping materials and coatings was studied using a 400 mm long piece of extruded scintillator with a 1.4 m WLS fiber. We observed a 60% improvement in the light output using 3M enhanced specular reflector (ESR) [3] and Avian-D [4] paint compared to wrapping with aluminized mylar. Single end readout results for these and other materials are shown in Table 1. After testing the dependence of the light yield on the Avian-D paint thickness, we found that one layer of paint (0.11(4) mm) would provide sufficient light output (88% of the maximum) and with two layers of paint (0.28(3) mm) the maximum light output was obtained.

Material Black absorber (paper) Aluminized mylar Plastic-backed aluminum foil Tyvek 3M enhanced specular reflector Avian-D paint

Light Yield (p.e./MeV) 3.63(2) 10.4(1) 10.1(1) 9.1(1) 16.4(1) 16.2(2)

Table 1. Effect of wrapping materials on the single end light yield (p.e./MeV) from the extruded scintillator read out with WLS. fibers. The tests were done with cosmic rays.

Acknowledgments This work was supported by the Natural Sciences and Engineering Research Council, the National Research Council, and the Canada Foundation for Innovation. References 1. G. S. Atoian et al. ,Nucl.Instrum.Meth. A531, 467 (2004) 2. A. Pla-Dalmau et ah, Nucl.Instrum.Meth. A466,482 (2001). 3. See http://products3.3m.com/. 4. See http://www.aviantechnologies.com/.

CMS P R E S H O W E R / i V - S m / ABSOLUTE CALIBRATION IOANNIS EVANGELOU* University ofloannina, Physics Dept., Panepistimioupoli, 45110 Ioannina, Greece

This paper describes the in-situ absolute calibration of the Preshower detector of CMS. The Preshower is based on silicon strip sensors that will be installed in the endcaps of CMS in front of the crystal Calorimeter. Energy deposited in the lead of the Preshower is estimated by the silicon sensors, allowing a re-scaling of the energy measured by the endcap crystals. Measurement of the charge deposited in the silicon strips to a few per cent accuracy is required. A precise in-situ absolute calibration of the Preshower using minimum ionizing particle signals from physics events is examined. It is shown that sufficient calibration accuracy can be obtained by using muon or pion events, and that the time required for the calibration is of the order of a few days at low luminosity

1. The Preshower at CMS The CMS Preshower detector ("ES" [1]) is based on silicon strip sensors and will be installed in the endcaps (1.653
328

329

— > z Figure 1. The Preshower detector in front of the endcap ECAL of CMS

The ES sensors measure deposited energy, normally expressed in terms of mips (minimum ionizing particles). The total deposited energy in the EE+ES system is given by E=Eg+Epresh where Eg is the energy measured by EE and Epresh= y(Ex+aEy) is the energy measured by ES. Ex,y are the deposited energies in the planes X and Y of ES respectively, a is the relative weight of the two planes and 7 is the slope of Eg[GeV] = f(Ex+aEY)[mips] plot, which effectively relates the energy deposited in the lead to that measured by the silicon sensors. Typical values of a = 0.6-0.8 and y = 0.024GeV/mip have been evaluated from beam test data and simulation. The readout electronics of ES have a large dynamic range in order to measure the energy deposited by very high energy electron-photon showers. The front-end electronics are organized as an analogue pipeline, where events are stored in a front end ASIC called PACE3 [2], followed by an on-detector analogue to digital conversion stage and a non-zero-suppressed data transmission system to the counting room. PACE3 can operate in two modes: Low Gain (LG) for normal running with a large dynamic range (1 to 400mips with a S/N ratio for single mips of ~2) and High Gain (HG) for calibration purposes (0.1 to 50 mips with a S/N ratio for single mips of -10). A measurement of single mips in HG, together with the zero point, is then complemented by the use of a charge injection circuit (inside the PACE3) to inter-calibrate the two gains, thus allowing a calibration over the full dynamic range and between channels of an individual sensor. Details of this inter-calibration procedure can be found elsewhere [3].

330

2. The absolute calibration method The strip occupancy of the ES is relatively low - a fraction of a percent at low n and low luminosity, rising to a few percent at high r| and high luminosity. This means that it is necessary to use tracks pointing to particular strips in order to obtain efficient mip distributions. The CMS simulation framework [4] was used with two approaches: a) Muon tracks from inelastic physics events reconstructed with the "GlobalMuonReconstruction" algorithm of the CMS software packages. This is a standalone method using the muon chambers (RPCs & CSCs in the endcaps) to find its own "seed" or hit in the outer region. The muon trajectory is then evaluated by following the hits all the way to the primary vertex with the "Kalman Filter" method using all muon chambers and full Tracker information. Cuts on the quality of the muon tracks reconstructed are applied at this stage. It should be noted that the fiducial coverage of the muon chambers extends to |r)|<2.4 b) b) Charged pion tracks from minimum bias or jet events. Only tracker information is used for reconstruction with the "CombinatorialTrackFinder" method. Also here, reconstruction quality cuts are applied. The CMS Tracker extends to |r||<2.5 The impact points of the reconstructed trajectories at the ES are stored for both planes (X and Y) as well as the coordinates of the strips of the ES having signals above threshold. The impact point coordinates are then compared to all found "strips with signal" coordinates for each plane of the ES and in both directions (forward and backward). A strict match is required for the first plane (|AX(X-plane)|<0.095cm - i.e. strip pitch) and the same for the other coordinate, in the second plane (|AY(Y-plane)|<0.095cm). Looser matches are required on the other coordinates (i.e. |AY(X-plane)|<3.05cm and |AX(Y-plane)|<3.05cm respectively, corresponding to the strip length). The tracks that have matched hits in both planes are recorded and their deposited energies in the ES layers are added to the mip distributions. 3. Results To evaluate the code, Monte Carlo event samples were used from the CMS production data base. In these events Gaussian noise is included in the simulated

331 signal output of the Preshower strips and threshold is applied. Initially single track events were tested of either muons or pions (with fixed p T and flat |n|<2.5). Then several fully simulated muon event samples were tested, such as: tt->/jju + X, Z/y* Z/y*->unun + X, Drell-Yann DY->uu or B°s -> JI *¥ + O , with JI *P —> MM > with pileup included corresponding to low luminosity LHC running. The tracks pointing to the ES fiducial region are first selected. Then, reconstruction quality cuts are applied (i.e. x2/ndf<1.5) and the remaining tracks are checked for matched strips with signal on both layers of the ES, as described previously. The signal distribution of the first plane of the Preshower for the collected muons of the representative it -> MM + X events sample is shown on Fig.2. A fit of a Landau distribution convoluted with a Gaussian is also shown on the same plot with a Landau "most probable value" of 86.6±0.9keV. This is in good agreement with the expected deposited energy of mips traversing the 320um silicon of the Preshower with impact angle of 10°-20° (approximately equivalent to 3.7fC). 1

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The selection efficiency of the calibration method for muon tracks varies with the type of events used. In general, about 50% of the muon tracks crossing the ES layers are selected. Note that the geometrical acceptance of the Preshower for flat eta is about 30% of the total tracks, resulting in a "global efficiency" of about 15%.

332 Fully simulated pion event samples were also tested, for example jets with 2y, jets with 1 electron or minimum bias events. Looser cuts on the reconstruction quality were applied (i.e. x2/ndf<2.0). In all cases the mip distributions obtained are similar. In the case of pions the acceptance efficiency is similar to muons if high p T pions (in jets) are considered. With low p T pions (i.e. from minimum bias) the selection efficiency is significantly lower, about 5.5%, as many are not reconstructed in the Tracker. The total time required for the full Preshower calibration, taking into account the CMS high level trigger rates [5] and the LHC duty cycle, is around one month using only muon events. This can be reduced to a few days if some geometrical regularities are included, such as the forward/backward endcaps (factor 2) and using "rings" of sensors at the same pseudorapidity (factor >10). In the case of pions, the minimum bias and jet events will be a good source of mips. The time for full calibration in this case is estimated to be of the order of a few days. A combined selection of muon and pion tracks will be the best solution for fast and effective Preshower calibration. A few days of data taking at the initial run of CMS at LHC luminosity of 2xl0 33 crnV 1 . Acknowledgments This work was partially supported by CERN-PH and GSRT (Greek General Secretariat for Research and Technology). References 1. ECALEDR-04 Report Volume 2, Preshower (ES), https://edrns.cern.ch/document/l 15565/2/TAB3 2. PACE3 Digital Specifications, P.Aspell, Nov.2002, http://proikchip.web.cern.ch/proi-Kchip/preshower/doc/P ACE3DigitalSpecV3.pdf 3. Preshower in-situ absolute calibration, I.Evangelou, CMS Note in preparation 4. CMSIM, CMS simulation framework, see: http://cmsdoc.cern.ch/cmsim ORCA, Object-Oriented Reconstruction for CMS Analysis, see: http://cmsdoc.cern.ch/orca OSCAR, Object Oriented Simulation for CMS Analysis and Reconstruction, see: http://cmsdoc.ceni.ch/oscar 5. TDR, CMS The Trigger/DAQ project, Data Acquisition and High-Level Trigger, Technical Design Report, Volume 2, CERN/LHCC 02-26

D E D I C A T E D F R O N T - E N D ELECTRONICS FOR A N ILC P R O T O T Y P E H A D R O N I C CALORIMETER W I T H SIPM READOUT

M. GROLL* University of Hamburg / DESY, 22603 Hamburg, Germany E-mail: [email protected]

A prototype hadronic calorimeter for the ILC detector is under construction at DESY. It will consist of 8000 scintillator tiles with analog readout performed via SiPM directly mounted on each tile. A dedicated ASIC has been developed to match the requirements of large dynamic range, low noise, high precision and large number of readout channels needed. Particular care is given to the optimization aspects of the ASIC in the SiPM readout chain. To fully exploit the self-calibrating potentiality of this photodetector device, capable of recording single photoelectron signals, an externally selectable gain and shaping time are included in the design. The calorimeter system is described and the first calibration data are presented.

1. I n t r o d u c t i o n The requirements imposed by the high precision physics at the International Linear Collider (ILC) set high demands on calorimetry. For excellent separation of WW jets from ZZ jets, which plays a key role in Higgs boson studies, a jet-energy resolution oias/E ~ 0.3/y/E is required * 2 . This can be achieved by combining the potentials of the particle flow approach with high granularity in transverse and longitudinal directions for both electromagnetic (ECAL) and hadronic (HCAL) calorimeters. The ECAL design is based on a Si/W sampling structure with sensitive silicon pads of 1 cm2 surface. The prototype will consist of approximatly 10,000 channels 3 . A sub-group of the CALICE collaboration3, has developed the analog op*On behalf for the calice (scintillator-heal) collaboration T h e institute members of the group are: DESY, Hamburg U, ICL (London), I T E P (Moscow), LAL (Orsay), LPI (Moscow), MEPHI (Moscow), Northern Illinois U., RAL,

a

333

334

tion for the HCAL based on a sampling structure with scintillating tiles (of smallest size 3x3x0.5 cm 3 ) individually readout by Silicon PhotoMultiplier (SiPM) 4 mounted on each tile 5 . The SiPM is a pixelated avalanche photo-diode operated in limited Geiger mode. The detector surface of l x l mm 2 is divided into 1024 pixels. The analog output is obtained by adding the response of all pixels fired as independent digital counters. The SiPM are operated at 2-3 volts overvoltage. Given an internal pixel capacitance CPixei of typically 50 fF, the charge collected for one photoelectron signal is ~160 fC (or ~10 6 electrons). The SiPM offer a very fast response with a typical rise time of 3-5 ns. The dynamic range is determined by the finite number of pixels and is ~200 pC. In order to test the feasibility of the particle flow approach prototypes of both ECAL and HCAL are being built by the CALICE collaboration and will be tested in a combined test beam experiment. To meet the needs of the analog HCAL prototype it was decided to adapt the design developed for the ECAL front-end electronics 6 to read the SiPM signal. In the following the main features of the ASIC are described and its properties analyzed. 2. Very front-end electronics description and properties An 18-channel ASIC dedicated to SiPM readout has been developed starting from the design, in AMS 0.8 [im CMOS technology, already in use for the Si-W ECAL prototype readout. The integrated ASIC components allow 16 selectable preamplification gain factors between 0.67 and 10 V/pC, and 16 CR(RC) 2 shaping times between 40 and 180 ns. After shaping the signal is held at its maximum amplitude with a track and hold method and multiplexed by an 18-channel multiplexer to provide a single analog output to a 16 bit ADC. In Fig: 1 a schematic view of the ASIC chip ILC-SiPM is given. The longest shaping time has been introduced in the ECAL design to match the long delay of the beam trigger («150 ns) in a test beam environment and to minimize electronic noise. For gain calibration using LED signal, pile-up from thermal noise-induced signals can be reduced by exploiting the fast SiPM response with a shorter shaping time. An adjustable 8 bit DAC (0-5 V) in the ASIC allows individual adjustment of the SiPM bias voltage for each one of the 18 channels. UCL (London).

335 The ASIC chip properties relevant for the SiPM operation were tested with a dedicated test board. The linear range of the ASIC with the longest shaping time proves to be sufficient to read out SiPM up to its saturation. The ASIC chip output saturates between 1.7 and 1.9 V depending on the gain. A linearity of 3% in the dynamic range up to 200 pC and a negligible channel-to-channel crosstalk of 0.1-0.2% are measured with good reproducibility for all 18 channels. Fig: 2a) shows the effect of the ASIC preamplification and fast shaping on a SiPM signal. The noise introduced by the ASIC preamplifier has been found to be « 0.25 Me in this mode. Considering a SiPM gain of 10 6 = 1 Me this would give a S/N ratio of 4 for the single photoelectron signal. Fig: 2b) shows a single photoelectron peak spectrum from SiPM with ASIC readout. The DAC output voltage is found to be linear in the range 0.24 - 4.7 V, with a noise of 2 mV. The variation of the DAC voltage output for the 18 ASIC channels is below 5% (250 mV) on the full DAC range. These values are acceptable considerung that the dependence of the SiPM signal amplitude on voltage is of the order of 7%/100 mV. We conclude that the ASIC chip can be used for SiPM-like signal readout.

SiPM input

test Binput

Figure 1.

Schematic view of ASIC components.

Exploiting the channel reduction and the individual voltage steering of the ASIC chip it was possible to design the very front-end electronics for the 8000 channels of the HCAL prototype. Each of the 38 active layers is readout by 216 SiPM. On the side of each layer two base boards for the very front-end electronics are mounted. A base board hosts 6 analog boards, each equipped with one ASIC and the control and calibration electronics

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necessary for its operation. Bias and low voltage as well as all communication signals are distributed to all analog boards from the main base board. One HCAL layer is readout on one front-end port of a VME 64X board and processed in series with the other layers by a fast DAQ capable of 100 Hz average readout rate.

3. Calibration method For the operation of the HCAL prototype each channel is calibrated to its MIP response. The single photon detection capability of the SiPMs allows in addition the calibration of each channel to pixels per MIP. This quantity is used to apply a unique non linearity function to correct each SiPM response. In addition the stability of the pixel separation (SiPM gain) is used to monitor voltage and temperature induced fluctuations of the SiPM response. Due to the properties of our front-end electronics it is mandatory to use two different readout modes for collecting beam data and LED data. The first mode is used to determine the MIP response and the second to determine the pixel separation. Therefore an intercalibration between both modes is necessary, which is obtained by the ratio of the signals recorded in the two modes for a fixed LED light amplitude. Each channel is adjusted to a pixels/MIP value of 15 by adjusting the SiPM bias voltage. The first beam data taking period in a DESY electron testbeam was dedicated to establish this lightyield calibration method. The first MIP beam calibration data are shown in fig: 3. The determined mean value is around 13 pix per MIP with a RMS of « 2.

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Outlook

T h e combined ECAL-HCAL prototype test in hadronic test beams will provide experience with the operation, readout, calibration and long t e r m stability of a medium size detector based on the new SiPM technology. Hadronic shower d a t a collected with unprecedented granularity will form a basis to validate and improve the simulations of shower development and prove the particle flow approach, which to large extent determines the architecture of the LC detectors.

References 1. J.-C. Brient Proc. of Snowmass 2001 C010630 (2001) E3047. 2. F. Sefkow Proc. of Calor 2004, Perugia, Italy (28 Mar.-2 Apr. 2004) Published in Perugia 2004, Calorimetry in particle physics 435-444 3. J.-C. Brient Proc. of Calor 2004, Perugia, Italy (28 Mar.-2 Apr. 2004) Published in Perugia 2004, Calorimetry in particle physics 452-467 G. Bondarenko, P. Buzhan, B. Dolgoshein, V. Golovin, E. Guschin, A. Ilyin, V. Kaplin, A. Karakash, R. Klanner, V. Pokachalov, E. Popova, K. Smirnov, Nucl. Instrum. Methods A 442 (2000) 187. V. Andreev et al. Nucl. Instrum. Methods A430 (2004) 22. S. Manen, G. Bohner, J. Lecoq, J. Fleury, C. . D. la Taille, G. Martin and c. democrite-00023563 [the FLC Collaboration Proxy] arXiv:physics/0501063.

U S I N G SINGLE PHOTOELECTRON S P E C T R A IN T H E CALIBRATION OF T H E CMS-HF CALORIMETER

E. GULMEZ, M.DELIOMEROGLU, AND K. DINDAR Bogazici University, Bebek 34342 Istanbul, E-mail: [email protected]

Turkey

U. AKGUN, A. S. AYAN, Y.ONEL, AND I.SCHMIDT University of Iowa, Iowa City 52242 IA, USA Relative gains of the PMTs installed in the CMS-HF calorimeter were measured at the University of Iowa PMT Test Station. Using the source data taken during the HF Test Beam period 2004 at CERN, PMT gains were determined from the single photoelectron spectra. Comparing these data provides an effective pedestal value and shows a very high correlation (r=0.994). PMT gains obtained in this way can be used for final calibration of the HF calorimeter.

1. Introduction Calibration of the HF Calorimeter requires the gains of the PMTs used in the calorimeter. One way of measuring the PMT gains would be to use the single photoelectron spectra. Such a set of data were taken during the test beam 2004 period with some of the HF wedges using a 5mCi 60 Co source. The following is a report of the study made on the wedge 2-13 data to compare the relative gains obtained at the University of Iowa PMT test station and the gains obtained from the single photoelectron spectra. Then, these gains are used to calibrate the calorimeter with 100 GeV electron beam. 2. University of Iowa Measurements As part of the quality control checks performed at the University of Iowa PMT test station, the relative gain of each tube was measured. The same reference tube was used throughout the measurements and the absolute gain of the reference tube was also measured accurately. 1 ' 2 Even though all the measurements were performed at 1100 V, it was shown 3 that the

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Figure 1. A sample fit of a single photoelectron spectrum at 1350V.

relative gains stayed flat as the HV increased. Each HF wedge is divided into three sectors; inner, middle, and outer towers. PMTs are installed into each of these sectors according to their relative gains. Tubes with lower relative gains are installed in the inner sector (closer to the beam), medium gain tubes in the middle and the higher gain tubes in the outer sector. PMTs in each sector are also selected so that they have almost the same relative gains within a few percent. With three sets of PMTs in each wedge for both type of fiber lengths, EM and HAD,we have a total of six group of PMTs providing us with six different relative gain values. 3. C E R N Measurements During the test beam period at CERN in 2004, some of the wedges were tested under various beams and also with a 60 Co source. The source was inserted into each tower one at a time and the pulse height histograms obtained through the QIE readout system were recorded. The histograms accumulated when the source was outside were used to determine the pedestals. In this study, the source runs for the HF wedge 2-13 at three different HVs, 1150, 1250, and 1350 V, were used. For each run, pulse height histograms were fitted to a gaussian to determine the peak position corresponding to the single photoelectron distribution. Then the absolute gain

340

could be obtained from the single photoelectron peak position after correcting for the pedestals. The best results were obtained from the data taken with the 1350 V (Fig.l).

25 r 20 c

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Figure 2. Single photoelectron peak positions or "uncorrected absolute gains in arbitrary units" (CERN) versus relative gains (Iowa). Intercept on the absolute gain axis is the effective pedestal value.

The single photoelectron peak positions were grouped according to the corresponding Iowa relative gains into six groups. When the peak positions are corrected for the pedestals, fitting the average of each group versus the Iowa relative gains to a straight line should yield a line passing through the origin. However, this does not happen when we determine the pedestals from the pedestal histograms. Instead of subtracting the pedestals obtained from the pedestal histograms, peak position versus the Iowa relative gain data were fitted to a straight line (linear correlation coefficient r=0.994) (Fig. 2). The intercept on the peak position axis would be the effective pedestal value. At 1350 V, the effective pedestal value is higher than the average pedestal value obtained from the pedestal histograms (Table 1). At lower HVs, the effective pedestal gets closer to the value obtained from the pedestal histograms. The effective pedestal value at each HV was subtracted from the single photoelectron peak positions to obtain the corrected PMT absolute gains.

341 Table 1. Effective pedestal values obtained from the single photoelectron peak positions. P M T HV

Pedestal from Histograms

Effective Pedestals

1150 V

5.0±0.3

not enough statistics.

1250 V

4.9±0.2

4.1±1.1

1350 V

4.9±0.2

7.3±1.6

4. Energy response of the calorimeter During the same test beam period, the HF wedges are exposed to several different beam types and energies. Response of the individual towers are determined by fitting the PMT pulse height distributions obtained with a 100 GeV electron beam. Since the PMTs in each of the three sectors have different gains, the PMTs show different responses. A simple approach to calibrate the calorimeter would be to correct the energy responses with the corresponding PMT gains. Figure 3 shows the individual response of the wedges corrected with the PMT gains obtained as explained above.

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342 5.

Conclusion

In this study, we show t h a t we can calibrate the H F Calorimeter and monitor the calibration by using the P M T gains obtained from the single photoelectron spectra. Then the energy response of the individual towers to a specific beam energy can be corrected with these gain values. Such a simple method results in a calibration with an accuracy of a b o u t 10%. We also show t h a t the relative gains measured at the University of Iowa P M T test station and the gains determined from the Test Beam 2004 source d a t a are highly correlated (r=0.994). However, determining the pedestal values seems to require a more careful study.

Acknowledgements This work was supported by the US Department of Energy (DE-FG0291ER-40664), Turkish Atomic Energy Commission (TAEK) and the Scientific and Technical Research Council of Turkey ( T U B I T A K ) . We also t h a n k the University of Iowa, Office of Vice-President of research and Bogazici University Scientific and Technological Research Fund (04B301) for their support.

References 1. U.Akgun, A.S.Ayan, P.Bruecken, F.Duru, E.Gulmez, A.Mestvirishvilli, M.Miller, J.Olson, Y.Onel, and I.Schmidt, Complete tests of 2000 Hamamatsu R7525HA Phototubes for the CMS-HF Forward Calorimeter,Nucl. Instr. and Meth. A, 550, 145 (2005). 2. U.Akgun, W.Anderson, A.S.Ayan, E.Gulmez, M.Miller, Y.Onel, I.Schmidt, D.Winn, Evaluation of PMTs from three different manufacturers for CMSHF Forward Calorimeter, IEEE Iran, on Nuclear Science, 51,1909, (2004). 3. E.Gulmez, U.Akgun, Y.Onel, Variation of relative gains as a function of HV for the Hamamatsu R7525HA PMTs used in the HF Forward Calorimete, CMS Internal Report CMS IN-2005/006,(2005).

J E T E N E R G Y SCALE D E T E R M I N A T I O N FOR T H E R U N II D 0 CALORIMETER

NORMAN BUCHANAN Fermilab, M.S. 352 P.O. Box 500, Batavia, IL, 60510 USA E-mail: [email protected] SHARON HAGOPIAN Dept. of Physics, Florida State University, Tallahassee, FL 32306, USA E-mail: [email protected] for the D0 Collaboration

The D0 Calorimeter system consists of three uranium/liquid-argon calorimeters and an inter-cryostat detector. The central calorimeter covers |ij| up to ~ 1, and the two forward calorimeters extend the coverage up to |»j] of ~ 4. In order to determine the particle jet energy from the measured detector jet energy, corrections must be made to account for the energy offset not associated with the hard interaction, the calorimeter response to the jet, and losses due to out-of-cone showering. These corrections will be presented as a function of detector eta and measured jet energy.

1. Introduction The determination of the relationship between the energy of a jet of particles measured in a detector and the original energy of the particles produced in the collision is crucial in order to compare measurements to theoretical models. The uncertainty in this determination is often one of the largest sources of systematic error in the measurement of physical properties such as invariant mass and production cross sections. This paper describes the procedure the D 0 experiment uses to determine the Jet Energy Scale (JES) and gives preliminary values of these corrections and corresponding uncertainties as a function of detector eta and measured jet energy.

343

344

2. The D 0 Detector The D 0 Calorimeter system consists of three uranium/liquid-argon calorimeters and the intercryostat detector. The central calorimeter (CC) covers \rj\ up to ~ 1, and the two end calorimeters (EC) extend the coverage to |r?| ~ 4. Each calorimeter contains an electromagnetic section closest to the beam line followed by fine and coarse hadronic sections. Scintillators between the CC and EC cryostats provide sampling of developing showers for \r)\ between 1.1 and 1.4. A detailed description of the calorimeter and the Run II D 0 detector can be found in the NIM paper 1 . 3. Jet Energy Scale The jets in this note are reconstructed using the D 0 Run II cone algorithm 2 , which is an iterative seed-based algorithm (including mid-points) with radius RCone = 0.7. The conversion from measured jet energy E^faa to particle level jet energy Ev^t is performed using the following correction: rPtd^

E%r-E0(nr,£) cone

(line) where Eo(Tir]C) is the offset energy, which includes multiple interactions, underlying event energy, electronic noise, uranium noise, and pile-up from previous bunch crossings, Rjet(1Zr)£) is the calorimeter response to the hadronic jet, and Rcone(R,r)C) is the fraction of particle jet energy contained within the algorithm cone. 3.1. Offset

Factor

Any energy that is not associated with the hard interaction must be accounted for. Contributions to this correction are electronics noise, pileup, uranium noise, the energy that can be attributed to the underlying event and additional minimum bias interactions. The uranium and electronics noise can be determined using zero bias events, which have the single requirement of a bunch crossing. Minimum bias events are those where the luminosity counters on each side of the detector are hit. These events are used with the information obtained using zero bias events to determine the contribution of the underlying event, and thus the offset energy distribution. Figure 1(a) shows the offset energy density, summed over 0.177 rings for one, two, and three primary vertices. The offset energy density has been

345

parameterized by the number of primary vertices, which largely removes the dependence on instantaneous luminosity. The bump in the offset energy density data comes from large weighting factors used for the inter-cryostat detector and coarse hadronic layers. 3.2. Jet

Response

The energy deposited in the calorimeter is not equal to the measured energy. This is due to the fact that the calorimeter is not completely compensating, there is dead material in front of, and within the calorimeter, there are response fluctuations between calorimeter modules, etc. The balancing of transverse energy in 7+jet events is used to obtain the response of the calorimeter to jets. For an ideal calorimeter the energy of the photon would exactly balance that of the recoiling jet. In reality, the following balance results: RemPT'y + RhadPThad

= ~$T

(2)

where Rempr-y is the electromagnetic response of the calorimeter and ^hadPThad is the hadronic response of the calorimeter. The Z mass is used to determine the electromagnetic calorimeter response and the hadronic response can then be measured in back-to-back 7+jet events. The jet response is plotted for a jet cone size of 0.7 in figure 1(b). The response values for the end-caps are scaled to account for the corresponding cryostat walls. The dominant error in the jet response comes from the differences in photon response for the forward and central calorimeters. A correction must be made for particles that scatter into, or out of, the jet cone. Particles that have trajectories taking them outside of the algorithm cone, will only have a fraction of their energy accounted for and particles that travel into the cone, under the influence of a magnetic field for example, will artificially increase the measured jet energy. In addition to the instrumental effects of out of cone scattering, physics processes can also legitimately contribute. The physics out of cone processes are determined using Monte Carlo studies. The shower corrections are given in table 1 for cone sizes of 0.5 and 0.7. Dijet and 7+jet events are used to determine the shower correction factor. The energy density is measured outward from the cone center in a radial direction (in r)- space). The offset energy density, by r?-ring, was subtracted off of the energy densities used to calculate this correction. The final correction comes from the ratio to the energy density inside the algo-

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rithm cone to the total summed energy. The uncertainty in the corrections is approximately 5% and primarily attributed to statistics and model dependence.

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Combining the corrections for the offset energy, jet response, and out of cone showering gives the final jet energy scale correction factors plotted in figure 2. The correction values are plotted as a function of detector 77 and transverse, uncorrected, jet energy, for a cone size of 0.7, and for events with only one primary vertex. The T42 noise suppression algorithm 3 has been applied during jet reconstruction.

347 2

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4. Conclusion The jet energy scale correction has been measured for the D 0 experiment and varies in value between 1.5 at low jet transverse energies, to 1.1 at large transverse jet energies. The uncertainty in the jet energy scale is largest, approximately 18%, at low jet transverse energy and decreases to about 6% at 500 MeV uncorrected jet energy. Work is continuing to improve the D 0 jet energy scale determination. Further refinement of these corrections will lead to smaller uncertainties in measured physical quantities. 5. Acknowledgements Thanks to the D 0 Operations Group and to the D 0 Calorimeter Algorithms Group. Special thanks to the D 0 Jet Energy Scale Group for determining the jet energy scale corrections and uncertainties. The work of these and many others has led to the results presented in this paper. References 1. D0 Collaboration, "The Upgraded D 0 Detector", submitted to Nucl. Inst, and Meth. A; hep-physics/0507191; Fermilab Pub-05/341-E. 2. G.C. Blazey et al, Proceedings of "QCD and Weak Boson Physics in Run II" Workshop, ed. U. Baur, R.K. Ellis, and D. Zeppenfeld, Batavia, Illinois (2000) p. 47. See Section 3.5 for details. 3. J-R Vlimant et al, D 0 Note 4146, (2003).

THE HADRON CALORIMETER OF THE CMS EXPERIMENT AT THE LARGE HADRON COLLIDER* VASKEN HAGOPIAN Department of Physics, Florida State University Tallahassee, Florida, 32306, USA The construction of the Hadron Calorimeter is now complete. The major components of the detector have been assembled and are now in the process of installation, integration and calibration. The hadron calorimeter inside the CMS detector is made of scintillator and copper absorber covering the |nj range of 0 to 3.0. The forward calorimeter made of quartz fibers and iron absorber covers the |r|| range of 3.0 to 5.0. Segments of the hadron calorimeter have been tested in the CERN test beam with electrons, u's and n's. 1.

Introduction

The Hadron Calorimeter1 is a critical sub-system of the CMS detector. The Hadron Calorimeter will detect jets, single hadrons and n's. It is required for the discovery of the Higgs between masses of 100 to 1,000 GeV and for searches of Dark Matter. Astronomical measurements of the rotation curves of galaxies as well as the distribution of microwave background of the sky require the existence of Dark Matter. The neutralino predicted by SUSY is a Dark Matter candidate. The CMS experiment will search for Dark Matter by triggering on missing transverse energy events. SUSY searches also require the hadron calorimeter and in one year of data taking at a luminosity of 10 cm" sec" the mass limit of supersymmetric particles will be extended from existing value of 350 GeV to about 2 TeV. Extra Dimensions can also create Dark Matter candidates and again the Hadron Calorimeter is crucial for these searches. The Hadron Calorimeter will be installed in the CMS detector, calibrated and ready to take data on day one of the experiment2. The Hadron Calorimeter (HCAL) Central Barrel (HB) is 9 meters long, one meter thick and 6 meters in outer diameter. The HB has 36 wedges of 20° segments made of brass and scintillator. The two End Caps (HE) are also made of brass and scintillator, with a diameter of 0.8 to 6.0 meters and a thickness of 1.8 meters. Both HB and HE are inside the 4-tesla solenoid coil. The n-<}) segmentation of HB and HE is 0.087 x 0.087, except near n = 3.0, where the size of the segmentation is doubled. The longitudinal segmentation for HB is one unit while for HE, varies from one to *For the CMS HCAL Collaboration

348

349 three. The two forward calorimeters (HF) are constructed to measure forward jets. Since HB is only 6.5 interaction lengths thick at n=0, additional scintillators are placed inside the muon barrel system, outside of the solenoid coil, to measure the HB energy leakage. This system is called HO and both Monte Carlo studies and test beam results show that it improves the energy measurement by reducing the tails of the energy distribution substantially. To correct for the radiation damage at n above 2.0, HE has extra longitudinal segmentation to allow correction for signal loss. The light from scintillators is transported by means of plastic fibers to the Hybrid Photodetectors (HPD's) for HB, HE and HO. The signals for HF are Cherenkov radiation in quartz fibers read by conventional phototubes. The essential electronics elements are QIE's (charge integrator and encoder), HTR's (trigger and readout module) and event builder card (DCC- data concentrator card). The detector is presently in the assembly area of the CMS above ground hall, where final integration is being done. That means the front end photodetectors and electronics are being installed and connected to services, including high voltage for HPD's, low voltage, signal cables, cooling water, nitrogen purges, etc. Eighteen wedges of HB, making a half barrel of the Central Calorimeter, have been assembled and integrated and are now detecting cosmic u's.

2.0 Calibration ofHCAL The calibration of the detector is a complex process using several radioactive wire sources, nitrogen laser at 337 nm, Light Emitting Diode (LED), u's from cosmic rays and test beams as well as charge injection. The energy scale is determined in the CERN test beam using electrons and n's. Final calibration will be accomplished using p-p collision data and will not be discussed in this paper. 2.1 Radioactive wire source Co60 radioactive source implanted at the tips of a wires passes over each scintillator segment and next to several quartz fibers (HF). Even though the signal size is small, by taking millions of readings per scintillator segment, a 3% calibration is achieved. Each sub-system has permanent radioactive wire source units that can be used when the detector is closed. There are several portable units that can be used when the detector is open. This is an absolute calibration that will be used to calibrate each scintillator segment about once a year. Figure 1 shows the readout over a single tile segment. For the final calibration a stronger Co60 source will be used, which gives a signal of about 0.5 fC.

350

2.2 Nitrogen Laser for Calibration and Monitoring A nitrogen laser is a sealed 300 micro Joule unit at X.=337 nm, built by Spectra Physics, Inc. The laser system includes two neutral density filters wheels, a quartz lens focusing system, a commutator with 20 output paths and 3 pin diodes to monitor laser light, both before and after the filters. The system can be pulsed 10 times a second and will be used for both calibration and monitoring the calorimeter system. In addition the laser pulse is used to time the various components of the detector that mimic an actual event that occurs at the center of the CMS detector. The whole calorimeter can be timed before LHC start. The laser intensity is controlled by neutral density filters and the signal can be sent to various paths using a commutator. The various paths include excitation of one scintillator segment in each calorimeter tower, lighting up each pixel of every HPD and measuring radiation damage. The laser is also used for extrapolating the energy calibration from 250 GeV measured in the test beam to 3 TeV. The laser also simulates Cherenkov light in the hadron Forward Calorimeters (HF). For the laser signal to reach scintillator tiles or HPD pixels, various splitters were designed and constructed. The splitter design uses one large diameter quartz fiber butted to several smaller diameter quartz fibers. Each side is polished and attached to each other by simple pressure. For example to split a signal six ways, the large diameter fiber is 800 microns, while the smaller fibers are 250 microns. This form of splitting makes the light in each smaller fiber uniform within 10% of the average. The fiber splitters constructed are 1->18, 1->16, l->6 and l->4. All laser signal fibers are made of quartz with quartz cladding to reduce radiation damage. 2.3 Light Emitting Diode (LED) In addition of the laser lighting each pixel of every HPD as well as each photomultiplier tube in HF, fast LED's also illuminate the pixels and PMT's. For the HB, HE and HO green LED have been used that have a color spectrum that is very similar to the green light of Kuraray Y l l fibers used inside the calorimeter. For HF, blue LED's are being used. LED's have the advantage that they can be pulsed over 100 times a second with a time jitter of about one nano second. In contrast the laser has a time jitter of about 25 nsec, but all segments of the detector get the laser signal at the same time. Each LED has its own pulser and the signal cable length to each pulser varies from unit to unit.

351

700

BOO

900 CM

Figure 1. Readout of radioactive (Co60) source passing over a scintillating tile.

Figure 2. On line display of a cosmic muon through the hadron calorimeter.

2.4 Pin Diodes The laser signal and LED light are monitored by several pin diodes. There are also pin diodes inside each Calibration Module on each 20° wedge. Pin diodes have very high quantum efficiency and a gain of one and are very stable. The laser light output varies from pulse to pulse with a sigma of about 10%. To obtain calibration constants of a few percent, the laser has to be normalized. 3.0 Test Beam Two 20° HB wedges, a 40° segment of HE, and a 40° portion of HO have been permanently installed in the H2 test beam at CERN on a rotating table. The system also has 7 x 7 ECAL module and several wedges of HF. The beam included e's up to 150 GeV, u's and n's from 3GeV to 250 GeV. The resolution, shower shape and ECAL versus HCAL responses have been

352

measured at various energies and compare well with Geant4 Monte Carlo predictions. In 2006 the plan is to measure a full complement with an ECAL production module. 4.0 The Cosmic Muon Challenge A half barrel of HB has been installed with electronics, services and front end trigger. The detector is in a free trigger mode detecting cosmic muons. Figure 2 shows an example of a cosmic muon traversing the whole HCAL detector. This is a challenge, as it requires all the various components to work together. Next, the hadron calorimeter barrel will be inserted inside the 4 tesla magnet. We plan to detect cosmic muons during the magnet test, in early 2006. Acknowledgments The CMS Hadron Calorimeter has been designed, constructed and assembled by physicists and engineers from several tens of institutions from about 10 countries. It is a monumental task that has been ongoing for 10 years. The author acknowledges the hard work of all the people involved in this task. Special thanks to the management of this task. We will be ready to take data on day one. References 1. The Hadron Calorimeter Technical Design Report CERN/LHC 97-31 and P. de Barbara and V. Hagopian, Proceeding of the 8' International Conference on Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications, ICATPP-8, ed. Barone, Borchi, Leroy, Rancoita, Riboni and Ruchti; p. 186 (World Scientific, Singapore, 2004). 2. The CMS HCAL Collaboration. Design, Performance and Calibration of CMS Hadron-Barrel Calorimeter Wedges, submitted to Nucl. Instrum. and Methods (2005).

PROPERTIES OF A 256-CHANNEL PROTOTYPE OF THE ALICE/PHOS CALORIMETER* MIKHAIL IPPOLITOV, ANREY VASILIEV FOR THE ALICE/PHOS COLLABORATION Russian Research Center "Kurchatov Institute",Kurchatov sq.l, Moscow, 123182, Russia

We discuss in this paper the measured characteristics of a 256 - channel prototype of the ALICE/PHOS lead-tungstate electromagnetic calorimeter. The prototype has been tested with electron and charged pion secondary beams delivered by the CERN PS and SPS accelerators. Energy, time of flight and two-photon invariant mass resolutions are presented. Data were taken at electron and pion momenta from 0.6 GeV/c up to 150 GeV/c. An overview on the characteristics of the ALICE mass production crystals are presented.

1.

Introduction

ALICE (A Large Ion Collider Experiment) [1] is one of the four experiments under construction at the CERN Large Hadron Collider (LHC). This experiment is optimized for the study of heavy-ion collisions at a centre-of-mass energy -5.5 TeV. At these conditions the creation of a new state of matter, called quark-gluon plasma (QGP), in which quarks and gluons behave almost as a free particles, is expected. The PHOS (PHOton Spectrometer) [2], is a high resolution electromagnetic calorimeter consisting of 17920 detection channels of lead-tungstate crystals, PbW0 4 (or PWO). It will be positioned on the bottom of ALICE set-up at a distance 460 cm from the interaction point and will cover the pseudorapidity range -0.12 < r\ < 0.12 and 100° in azimuthal angle. PHOS is subdivided into five independent modules. Each module is segmented into 3584 detection channels. The detection channel consist of 2.2x2.2x18 cm3 PWO crystal, coupled to 5x5 mm2 Avalanch Photo-Diode (APD). The PHOS is dedicated to the search for electromagnetic signals from the QGP in nucleus-nucleus interactions at high energies. Approving PWO crystals as scintillating material for a large number of highenergy physics experiments has stimulated the research on this material and the optimization of its fabrication at industrial scale. The Russian Research Center

This work was supported by Russian Federal Atomic Energy Agency.

353

354

Kurchatov Institute together with North Crystals Company performed a R&D programm on development of the technology of mass production of high quality lead-tungstate detectors. In this paper we are presenting beam-tests results of the 256-channel PHOS prototype in beam momentum range 0.6 -150 GeV/c. The tests were performed at the CERN PS and SPS accelerators in years 2002-2004. Also, an overview on the characteristics of the ALICE mass production crystals are presented. 2.

Crystals

PWO was chosen for the PHOS detector due to its basic properties. This scintillating crystal has low value of radiation length, 0.9 cm, low value of Moliere radius, 2.2 cm, and it is fast. Its emission spectrum has a good correspondence to the spectral sensitivity curves of the most commonly used photo detectors. As a main crystal production facility for the ALICE PHOS project, North Crystals Company, Apatity, Murmansk region, Russia has been chosen. Crystals have been grown by Czochralski method in a resistant ovens. About 12000 ALICE/PHOS crystals were produced in years 2001-2005 and delivered for testing at the Kurchatov Institute. Light yield (LY), optical transmission (OT), emission and scintillation decay time spectra were measured. As an example, in Figure 1 the distributions of LY and OT at 420 nm of the crystals are shown. Double structure in LY distribution is due-to

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improvements in production technology in the middle of year 2003. Only crystals which fulfilled ALICE crystal's requirements (LY >8 photoelectrons/MeV, nonuniformity of LY <5%, longitudinal optical transmission (OT) >25%, 55% and 65% at the wavelengths of 360nm, 420nm and 620nm, 96% of light are collected within 300ns gate) were accepted.

355

3.

Prototype description

A single channel of the prototype consists of a crystal, an APD and a low noise charge sensitive preamplifier. HAMAMATSU S8148 APD of 5x5 mm2 sensitive area were used as readout. A row of 8 crystal detector units constitutes the basic assembly of a crystal strip unit. The detectors were stacked to form 16 x 16 array. The LY of PWO crystals is relatively low and depends on temperature. The temperature coefficient is ~-2%/°C in the temperature range between +20 °C and -30 °C [2]. The temperature dependence gives the possibility to increase the light yield at low temperatures, and -25 °C is chosen as the working temperature for the PHOS detector. The prototype was cooled down to -25°C with cooling system providing long-temperature stability within 0.1 °C. Monitoring of the detection channels was carried out by Light Emitting Diodes (LED) monitoring system [2], More detailed description of the prototype one can find in [3]. 4.

Beam-test Results

The beam tests were performed with electron, ri and 7t+ secondary beams at the CERN PS and SPS synchrotrons in the energy range 0.6-150 GeV. The beam momentum spread was less than 1%. The typical beam intensity was ~2-5xl0 3 particles/spill during calibration runs and 2xl0 4 n /spill during neutral mesons measuring runs. 4.1. Calibration, Energy Resolution and Linearity A set of thin scintillator beam counters and a gas Cherenkov counter were used to define beam particle type. The calorimeter was calibrated with 2 GeV/c and 10 GeV/C electrons during PS and SPS tests respectively. During calibration beam was defined by a thin 10 x 10 cm2 area scintillator counter. After the calibration, the beam was centered on several chosen detectors in the matrix and the energy scan was performed. In this case the beam was defined by a small lxl cm2 area detector placed in the front of the prototype. The energy resolution was determined from a Gaussian fit to the electron peak obtained by summation of the gain corrected signals in the crystals of the 3x3 matrix around a given central detector. The experimental data are well fitted by the usual function:

£.= « © A e c E

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(i)

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where a=3.5%, b=13 MeV and c=l.l% are stochastic, electronics noise equivalent and constant terms, respectively. Response of the PbW0 4 matrix in the energy range of 0.6-150 GeV is linear with a precision better than 1%.

356

4.2. Time-of-FUght Resolution For the time-of-flight resolution measurements START signal was taken from external trigger detectors with intrinsic TOF resolution -150 ps, STOP signals were taken from detectors of the PHOS prototype. The time resolution as function of energy deposited by electromagnetic shower in a single channel, is shown in Figure 2. The resolution is better than 0.5 ns was observed for deposited energies higher than 1 GeV.

5

e

Energy, GeV Figure 2. Time resolution as a function of deposited energy in a single channel of the calorimeter.

43. Two - Photon Invariant Mass Resolution The experimental setup for two-photon invariant mass measurements is shown in Figure 3. Inclusive TI" + 12C -»7i°+X reaction was used. A large area VETO scintillator counter was set to reject charged particles in the detector acceptance. A n beam with 6 GeV/c momentum was used and the calorimeter was set at a distance of 203 cm from 10% of interaction length carbon target. As an example, in Figure 4, invariant mass spectra of all photon pairs from the same event are shown. Clean peak around the n" mass is observed. This peak was fitted with Gaussian curve and position of the peak at 135.6 ± 0.2 MeV/c2 with invariant mass resolution of 4.7 ± 0.2 MeV/c2 was obtained (see Figure 4).

-He-^ Target

From PS and SPS gamma2 PiO-trigger: S L x S2 x Veto

Figure 3. Experimental setup for the measurements of the two photon invariant mass resolution of the calorimeter.

357

| Invariant mass vs E1+E2 | Chi2(ndf = 13.arM7 Constant = 41.57 ±2.439

£0 5GeV<E1+E2<6GeV

Mean

= 135.S± 0.241

Sigma =170.1 ±0.211

40

30

20

10 , l.-^ll.,ft Jll.
20

40

60

Lull ml i-mlnJl i 80 100 120 140 160 180 200 220 240

Mn, MeVfc"2 Figure 4. Two - photon invariant mass distribution from 6 GeV/c 7t" + 12C ->7t°+X reaction. Line is results of the fit by Gaussian. Only events with two photons with energies between 5 GeV and 6 GeV are taken into cionsideration.

5.

Conclusions

A test of the 256-channel prototype of the PHOS detector has been performed. An energy resolution, ranging from -3.9% at 1 GeV to - 1 . 1 % at 150 GeV, has been found. Time resolution better than 0.5 ns was found for deposited energies in crystal higher than 1 GeV. 7t° mesons have been measured via its two - photon decay channel. Invariant mass resolution of 4.7 MeV/c2 was found. An overview of the ALICE/PHOS crystals properties was presented. Acknowledgments The author wishes to thank all the members of the ALICE-PHOS group for their help during test and preparation of the prototype. We also wish to express our gratitude to the CERN accelerator division for excellent performance of the PS and SPS complex. References 1. ALICE Collaboration, Technical Proposal, CERN/LHCC/95-71 (1995) 2. ALICE Collaboration, Technical Design report of the Photon Spectrometer (PHOS), CERN/LHCC 99-4, ALICE TDR 2, 5 March 1999 3. M.Ippolitov, A.Vasiliev in Proc. Of the 11 Int. Conf. on Calorimetry in Particle Physics,Perugia, Italy, 2004, World Scientific, p. 103

S O U R C E CALIBRATION OF CMS QUARTZ F I B E R FORWARD CALORIMETER

N. AKCHURIN, M. SPEZZIGA Texas Tech University, Department of Physics, Lubbock, U.S.A. V. GAVRILOV, A. KROKHOTIN, S. SEMENOV, A. ULYANOV, E. VLASSOV Institute for Theoretical and Experimental Physics, Moscow, Russia A. ERSHOV Skobeltsyn Institute for Nuclear Physics, Moscow, Russia S. PAKTINAT Institute for Studies in Theoretical Physics and Mathematics,

Tehran, Iran

We show three different methods for the calibration of the CMS forward calorimeter (HF) using a radioactive source (Co60) and compare them against the calibration obtained with high-energy electrons in the test beam. About 5% calibration precision (rms) was obtained for the five HF wedges tested.

1. Introduction The Hadron Forward (HF) calorimeter is a part of the CMS detector which will be installed at the LHC collider at CERN. HF covers pseudorapidity range 3.0 < |r?| < 5.0. It should provide the detection of jets at the very small angles and improve the missing transverse energy measurement. HF is made from steel absorber with embedded quartz fibers [1,2]. The signal is registered by Cherenkov light produced by charged particles in quarts fibers. The choice of the design is driven by the requirement to construct the detector able to operate in the extremely hard radiation conditions (up to lOOMRad/year). While known physics channels could be used for HF calibration during the running, an opportunity to calibrate HF before starting operation is very important. In this paper, we discuss the calibration procedures using the radioactive Co60 sources.

358

359 2. Detector Description Each HF is assembled from 18 identical wedges. HF wedge is constructed from steel absorber with embedded quartz fibers running parallel to the beam axis. There are two types of fibers: long (L), running the entire lengths of the absorber and short (S). L and S fibers are separately grouped into the bundles to form L and S readout cells. Through light guides bundles are connected to PMTs, located at the rear side of the wedge. Every wedge contains 31 source tubes which are inserted in place of either L or S fibers. Each cell contains either one or two source tubes. 3. Calibration Setup 5 mCi Co60 source is used for the calibration. PMT signals are digitized every 25 ns time slice by front end electronics with QIE digitizer measuring charge in non linear scale with minimal step size equal 2.6fC. Data are automatically sampled in histograms and readout with frequency 20 Hz. Fig. 1 shows typical QIE distributions for L and S readout cells in terms of linearized QIE counts. Solid (dashed) line corresponds to the case when the source is inside (outside) the wedge. Both distributions are obtained for the same time interval and hence are normalized to the same number of entries.

Figure 1. Typical QIE distribution for L (left plot) and S (right plot) cells; solid (upper) line - source is inside the absorber; dashed (lower) line -source is outside the wedge.

From Fig. 1, it can be seen that the number of noise entries from the pedestal peak is about a few orders more than signal entries. Moreover, the gammas from the source are emitted randomly so the signal from single photoelectron (SPE) collected in QIE could be shared between two consecutive time slices and give two entries in histogram. During the test beam five wedges were exposed to lOOGeV electron

360

beam. Source data are also available for these wedges. Combined analysis of the source and test beam data allow to make an absolute normalization of the calibration done with source. 4. Calibration Procedure and Results Several methods of source data analysis were tested. They differ in the way how to extract charge related to the source activity from the data. 4.1. "Mean Charge"

Method

Charge (per sec) is calculated for the signal and background histograms: r\{S/B),~.T„i

v

40MHz

32

r-^

where i - the source tube number, qj - charge value expressed in terms of linearized QIE counts and rij - the number of entries in j t h bin. The value of Qf - Qf gives charge Qi related to the source activity. Calibration coefficient
-

U(^Ci)e(GeV)Ri Qi{QIEjs)

(2)

where U(mCi) - source activity, e(GeV) is the average signal as it would be collected by large size cell (no leakage from cell borders) for 1 mCi source and Ri - geometrical scale factor. Ri show what part of the signal is collected by fibers of the ith cell relatively to the cell of infinite size. They depend on a distance from source tubes to the cell boundaries and on the source tube position relatively to readout fibers. For each calorimeter cell a single calibration coefficient Cfource should be derived. It's equal to corresponding g if the cell contains only one source tube and mean value of two g if there are two tubes. To calculate Cj, the only unknown parameter e in Eq. 2 should be determined. It can be fixed from the requirement < c?°urce/C$eam >= 1, where average is calculated over all cells of tested wedges and C\eam calibration coefficients derived with 100 GeV electron beam [3]. RMS of the < dource IC\eam > distribution defines the precision of the calibration method. It's 7.0% (12.4%) for L (S) cells. The precision of this method is limited by pedestal stability. A small fluctuation in the pedestal position may strongly affects calculated mean charge because both the signal and background histograms consist mostly from pedestal entries.

361 4.2. "Fixed QIE Bin Range"

Method

In order to reduce the influence of the pedestal stability on the calibration precision, charge can be calculated in some predefined QIE bin range. The calibration procedure follows to the method described in Sec. 4.1, but in Eq. 1 charge is calculated in the range of 10-30 bins. The precision of the method (as defined in Sec. 4.1) is 5.4% (8.2%) for L (S) cells. Due to possible sharing of signal from SPE between two consecutive time slices the signal is distributed from maximum value to zero. Charge calculated in a fixed bin range is a part of the total charge which is different for different towers. This limits precision of the method. 4.3. "Signal Extrapolation

Under the Pedestal"

Method

Fig. 2 (solid line) shows the results of subtraction of the signal and background histograms. It gives the pure source activity at the region of large QIE values, while it's not the case for small QIE values where the shape of the histogram is determined by subtraction of pedestal entries. Large negative peak in the small QIE region results from the normalization of the signal and background histograms to the same number of entries. To estimate the part of the signal under the pedestal peak, an extrapolation should be applied. In this paper, the simplest constant extrapolation is used (Fig. 2, dashed line).

1

• subtraction extrapolation

Linearized QIE Counts

Figure 2. Solid line - QIE distribution after subtraction of signal and background histograms. Dashed line - the result of the extrapolation (see text for details).

Using the extrapolated histogram a charge related to the source is calculated as: _

AQMHz n

Ej=i i

32

^

j=iP+i

(3)

362

where rij - the number of entries in j bin of histogram, P - pedestal position, ip - bin containing pedestal value P. Then the procedure described in Sec. 4.1 is applied starting from Eq. 2. This method combines the advantages of the first two methods: the signal charge under the pedestal is taken into account and the collected charge is not strongly affected by the pedestal position fluctuations. The precision of this method is 4.9% (5.9%) for L (S) cells (Fig.4.3) It is limited by uncertainty of the extrapolation to the region of small QIE values.

13 z

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5. Conclusion An opportunity to use source calibration for HF initial calibration is studied. The developed methods allow to achieve calibration precision about 5%, where the precision of calibration is determined relatively to calibration derived from 100 GeV electrons. This value is comparable with intrinsic lateral nonuniformity of the calorimeter response. References 1. N. Akchurin et al., Nucl. Inst, and Meth. A Vol. 399, pp. 202-226 (1997). 2. V.B.Gavrilov and M.V.Danilov, Physics of Atomic Nuclei, Vol. 67, No. 7, pp. 1390-1397 3. A.Ulyanov, "Test Beam Results of the CMS Quartz Fiber Calorimeter", talk given at "9th ICATPP Conference".

T H E CALIBRATION S T R A T E G Y OF T H E CMS ELECTROMAGNETIC CALORIMETER*

P. MERIDIANI INFN Romal Calibration is one of the main factors that set a limit on the ultimate performance of the CMS electromagnetic calorimeter (ECAL) at LHC. In situ calibration with physics events will be the main tool to reduce the constant term of the energy resolution to the design goal of 0.5%. In the following the calibration strategy will be described in detail.

1. The CMS Electromagnetic calorimeter global calibration strategy The physics reach of the CMS electromagnetic calorimeter (ECAL) 1, in particular the discovery potential for a low mass Standard Model Higgs in the two photons decay channel, depends on its excellent energy resolution. Crystal inter-calibration precision goes directly into the energy resolution constant term with very little scaling, due to the fact that most of the energy of an electromagnetic cluster is contained within a single crystal (around 80%); this means that crystals' calibration will represent a key aspect in order to reach the design performance of the electromagnetic calorimeter. Inter-calibration, however, it is not the only calibration task, since calibration should be naturally seen as composed of a global component, giving the absolute energy scale, and the inter-calibration component. Schematically the reconstructed energy might be decomposed to three factors: Eei~f = G x F X ^ C J x A,

(1)

i

where G is a global absolute scale. depending on the type of particle, clustering algorithm used: this is containment of the shower or the

The function F is a correction function its position, its momentum and on the necessary to correct either the partial energy loss due to bremsstrahlung or

* On behalf of the CMS Collaboration

363

364

conversions in the material in front of the calorimeter. The c, factors are the inter-calibration coefficients while the A, are the signal amplitudes, in ADC counts, which are summed over the clustered crystals. 2. Inter-calibration at Startup The natural spread of the channel-to-channel response variation in the calorimeter barrel is the crystal-to-crystal variation of the scintillation light yield which has an RMS of «8%. In the endcap the VPT signal yield, the product of the gain, quantum efficiency and photo-cathode area, has an RMS variation of almost 25%. The tight CMS production and assembly schedule will allow inter-calibration only of a limited number of crystals (few super-modules) with electron beams of known energies; these crystals will have an initial inter-calibration precision better than 1%. For all the other crystals there will be two other sources to compute initial value of the inter-calibration coefficient: • Laboratory measurements • Cosmic events During the assembly phase, all the detector components are characterized 2 ' 3 . The crystal light yield (LY) is measured in the laboratory with a photo-multiplier tube, exciting the crystal with a 60 Co source. An independent LY measurement can be obtained exploiting the correlation between the crystal LY and the Longitudinal Transmission at 360 nm 5 . The precision of the inter-calibration which can be reached using the average of the direct light yield measurement and the light yield predicted from the longitudinal transmission is 4.2% and has been evaluated with the 2004 ECAL test beam data; using only the direct light yield this precision is 4.6%. After the electronics integration, all the super-modules will be commissioned using cosmic events for about one week. Aligned cosmic rays can be selected by vetoing on signals above a rather low threshold in the adjacent crystal with highest signal, avoiding the need for external tracking. Inclining the super-module by ~10° (in order to have a rather uniform number of cosmic events along the super-module length) an overall precision of 2-3% is achievable for all channels. 3. In-situ calibration Since the startup inter-calibration precision will be around 2-3%, in-situ calibration techniques will be the fundamental tool to reach the 0.5% goal precision. Over the period of time in which the physics events used to

365

provide an inter-calibration are accumulated, the response must remain stable and constant to high precision. For example, the changes in crystal transparency are tracked and corrected using the laser monitoring system 6 . Several methods have been proposed to obtain inter-calibration factors. At the start-up, the 0-independence of the energy deposited in the calorimeter can be used to rapidly to inter-calibrate regions at the same pseudo-rapidity (T/). Once the tracker will be fully functional, the abundant electrons, mainly from W —> ev, can be used for local inter-calibration in region with uniform tracker material exploiting the E/p peak. A complementary method, not relying on tracker momentum measurement, is based on 7T° —> 77 and rp —» 77 mass reconstruction. The Z mass constraint in Z —> e+e~ can be used instead for region intercalibration, for example as a complement of the 77 ring inter-calibration at the start-up, or as invaluable tool to tune the electron reconstruction algorithms and to fix the energy scale. A similar use can be made of Z —> /U/U7, where the photon is coming from inner bremsstrahlung, constraining the 7 energy using the di-muon system. 3.1. <j> Symmetry Inter-calibration can be obtained by comparing the total energy deposited in each crystal with the mean of the distribution of total energies for all crystals at that pseudo-rapidity, assuming that the energy deposit should be uniform in
Using Single

Electrons

Once the tracker is fully operational and well aligned, inter-calibration of different crystals can be obtained using the momentum measurement of isolated electrons. The main difficulty in using electrons for inter-calibration is that they radiate in the tracker material in front of the ECAL, and both the energy and the momentum measurement are affected. Moreover the

366

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average amount of bremsstrahlung varies with tracker material thickness 9 . The single crystal inter-calibration is obtained minimizing the difference between the energy and momentum measurements. The event selection is based on variables which are sensitive to the amount of bremsstrahlung emission, and consequently measure the quality of the energy and momentum reconstruction. Due to the variation of the average value of E / p peak with 77, caused by the variation of the amount of material in front of the ECAL, inter-calibration is obtained locally in region over which the average value of the E/p is rather constant. In a second step the regions are intercalibrated with each other. The calibration precision achievable is strongly dependent on the available statistics, as shown in Figure 2; the fit function represents the sum of a statistical and a constant term.

3.3. Inter-calibration

using n0 and rf

The possibility of inter-calibrating the ECAL using mass reconstruction of 7T°, and rj° -+ 77 is also being investigated. It was found that about 1.4% LI triggered events have a usable TT° in the barrel and that almost all of these are tagged by the isolated electron Level-1 trigger information. A 7r° mass peak can be well identified in all the ECAL barrel with relatively little background, the mass resolution being about 8%. If we assume a Level-1 trigger rate of 25 kHz, and that 1.4% of these triggers have usable 7r°s, then about 100 7r°s per crystal can be obtained in less than 5 hours running.

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Figure 2. Calibration precision versus the number of events per crystal for different 77 regions. Upper curve: the last 10 crystals in the ECAL Barrel, Middle curve: 10 crystals in the middle of ECAL Barrel, Lower curve : the first 15 crystals in the ECAL Barrel. The third point along each line gives the precision for 5 f b - 1 of integrated luminosity.

3.4. Use of Z decays 3.4.1. £ - > e + e A number of different uses for Z —> e + e~ decays are envisaged, from the tuning of the electron reconstruction algorithms (for example to obtain or verify algorithmic corrections, F, in Equation 1), to the inter-calibration of ECAL regions. An iterative calibration method has been tested taking as calorimeter regions rings of crystals (at fixed rj) in the ECAL barrel: there are 170 such rings of 360 crystals. Z decays with low-radiating electron should be selected for this purpose. The calibration precision as a function of the number of events per ring is shown in Figure 3. The x-axis is the average number of events per ring, and the plotted point before the highest, at an average of 370 events per ring, corresponds to an integrated luminosity of 1.0 fb- 1 . 3.4.2. Z -> wy Radiative decays Z —• (j,fj,j can be used in a similar way to the Z —* e+e~; the photon energy is here constrained from the di-muon system and the Z mass. The available rates have been estimated using a sample of events generated using a full matrix element calculation of radiative decays. A cross-section of 65 pb over the region \r]\ < 2.5 has been obtained, selecting events with a loose cut on the di-muon mass and searching for a photon with pT >15 GeV within a radius of AR < 0.8 of either muon. Thus for

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Figure 3. Ring to ring calibration precision versus average number of events per ring; the fit function represents the sum of statistical and constant term. an integrated luminosity of 1 fb * an average of nearly 1 such p h o t o n per crystal will be collected. 4.

Acknowledgments

T h e a u t h o r would like to t h a n k the entire CMS E C A L e / 7 community and the organizers of 9th ICATPP conference. References 1. CMS Collaboration, "The CMS Electromagnetic Calorimeter Technical Design Report", CERN/LHCC 97-33, 1997. 2. E. Auffray et al, "Performance of ACCOS, an Automatic Crystal Quality Control System for PWO crystals of the CMS calorimeter", Nucl. Inst, and Meth. A 456 (2001) 325-341 3. S. Baccaro et al, "An automatic device for the quality control of large-scale crystal's production", Nucl. Inst, and Meth. A 459 (2001) 278-284 4. S. Baccaro et al, "Precise determination of the light yield of scintillating crystals", Nucl. Inst, and Meth. A 385 (1997) 69 5. L.M. Barone et al., "Correlation between Light Yield and Longitudianl Transmission in PbWC"4 crystals and impact on the precision of the crystal intercalibration", CMS Rapid Note 2004/005, article in preparation 6. A. Bornheim, these proceedings 7. D. Futyan, C. Seez, "Inter-calibration of ECAL crystals in <j> Using Symmetry of Energy Deposition", CMS Note 2002/031 8. D. Futyan, "Inter-calibration of the CMS Electromagnetic Calorimeter Using Jet Trigger Events", CMS NOTE-2004/007 9. CMS Collaboration, "Data Acquisition & High-Level Trigger Technical Design Report", CERN/LHCC 2002-26, 2002.

T H E VERY F R O N T - E N D C A R D S FOR T H E CMS E L E C T R O M A G N E T I C CALORIMETER: D E S C R I P T I O N , CALIBRATION A N D P E R F O R M A N C E

M. M. OBERTINO ON BEHALF OF THE CMS ECAL CALORIMETER GROUP. I.N.F.N. Via Giuria 1, 10125 Torino, Italy E-mail: [email protected]

The CMS electromagnetic calorimeter is made of about 76000 PbW04 crystals. It has been designed to measure energy and location of electromagnetic showers with high resolution and over a 16 bit dynamic range. Part of the read-out chain (the Front End Electronics) will be installed within the detector in a high radiation environment and inside a 4T magnetic field. The heart of the system is the Very Front End card (VFE), the board where the signal coming from the photodetectors is amplified and converted into digital form. After a brief description of the Front End Electronics, the results of the tests performed during the production, including the calibration procedure will be presented.

1. Introduction The CMS electromagnetic calorimeter (ECAL) [1] is a challenging detector designed to perform high precision measurements in the very demanding environment of the CERN Large Hadron Collider (LHC). Design philosophy was strongly driven by the requirement of excellent energy and angular resolution for the detection of Higgs boson through its two photon decay. The large proton-proton inelastic interaction rate (10 9 s _ 1 ), the 25 ns bunch crossing interval, the high value of the center of mass energy at LHC (14 TeV) and the calorimeter location inside a 4T magnetic field set the other constraints: low noise, wide dynamic range (50 MeV to 1.5 TeV), high processing speed of detector signals, radiation hardness and insensitivity to the magnetic field. To fulfill all these requirements it was decided to build a homogeneus calorimeter made of lead tungstate crystals. The detector is organized as one barrel, covering the pseudorapidity region \i]\ < 1.48, and two endcaps extending the acceptance to \r}\ < 3. The barrel is subdivided into

369

370

36 supermodules, each composed of 85(7j)x20(^) crystals; the endcaps are made of 14648 crystals arranged in 4 Dees. The PbWC>4 was chosen as active medium because of its excellent radiation hardness, short decay time (80% of the light collected in 25 ns) and the small value of radiation length (0.89 cm) and Moliere radius (2.2 cm), allowing for compact design and fine granularity. The disadvantages of the lead tungstate are the low light yield ( 50 ph/MeV) and the strong temperature coefficient (-2%/°C). The first effect is compensated by use of photodetectors with internal gain (APDs in the barrel and VPTs in the endcaps), the second by keeping the temperature inside the calorimeter stable within 0.1 °C. The ECAL electronics is organized in two logical layers, one positioned inside the detector (the Front End System), immediately behind the crystals, the other in the counting-room. The connection between the two parts is made by optical fibers. The building block of the Front End Electronics is the trigger tower (TT) designed to read out 25 crystals. It consists of a Mother Board (MB), 5 Very Front End Cards (VFE), a Low Voltage Regulator Board (LVB) and a Front End Card (FE). The MBs are passive devices which provide the flat surface where the VFEs and the LVB are mounted. They distribute the regulated analog and digital voltage provided by LVB to the VFEs. In the barrel they also filter and distribute the high voltage to the APDs and drive the signals to the electronic chain; the connection with the photodetectors is made by kapton cables. In the endcaps an HV board is interposed between the MB and the VPTs. The Very Front End Card, which is described in more detail below, shapes, amplifies and digitizes the photodetector signals. On top of the trigger tower, the FE board provides the sampling clock to the 5 VFEs, generates the Trigger Primitives (TT transverse energy and bunch crossing assignment) and stores the raw data while waiting for the level 1 trigger decision.

2. The Very Front End card As shown in Fig 1 each VFE card consists of 5 identical channels, each consisting of a multi gain amplifier (MGPA), a 12-bit 40MS/s 4-channels ADC (AD41240) and a buffer. The MGPA has 3 different gains: 1, ~6 and ~12. Their number and ratio are adequate for both barrel and endcaps. It consists of a low noise

371

Figure 1. VFE card.

preamplifier buffered by source followers to 3 gain channels in parallel. The total noise is ~ 8000 (gain 6 and 12) or ~28000 (gainl) e~~ for the barrel and ~3500 e~ for the endcaps. Each gain channel is connected to one of the ADCs which are inside the AD41240. A digital logic following the analog to digital conversion stage automatically selects as output the signal of the highest, not saturated gain. The multiple gain approach allows to achieve the requirement of a 90 db dynamic range without using radiation hard 16 bit ADCs. The buffer at the end of the chain adapts the LVDS signal coming from the ADC to the single ended inputs of the FE. A Detector Control Unit (DCU) measures the APD leakage current and the crystal temperature. All the chips mounted on the Very Front End Cards are manufactured in a 0.25 yum CMOS process which guarantees radiation hardness and low power consumption.

3. V F E production and test The VFE production (12880 cards for ECAL barrel and 3000 for the endcap) was started in summer 2004 and will be finished by the first quarter of 2006. Each board is characterized and calibrated before being mounted in the supermodule, following three steps: power-on test, burn-in and calibration. 3.1. The power-on

teat

The power-on is the first electrical test of the VFE cards. It is performed by the manufacturer and repeated at CERN. The test system powers the

372

VFE and measures the total power consumption, pedestal and noise. The internal MGPA test pulse is used to inject a charge into each channel and perform a fast pulse shape analysis. The correct behaviour of the DCU is also verified. Up to now 8880 cards were tested at CERN and only 79 failed.

3.2. The burn-in

test

In order to eliminate infant mortality failures, the cards which have passed the power-on are kept for 3 days at 60 °C, powered and clocked while the current is constantly monitored. So far all the cards survived this test.

3.3. The

calibration

The calibration test system measures the gains of each channel sending a charge pulse between 100 fC and 50 pC and reading the corresponding ADC response. By fitting the output signal peak versus injected charge, a slope is obtained as measurement of the gain (in ADC counts/pC). The distribution of gain ratio (G6/G1 and G12/G1) for the first 2320 VFE tested this year is reported in Figure 2a and shows a good homogeneity of the VFE performance (a/mean ~ 1%). Since more than one year is required to test all the boards the calibration system stability is a strong requirement. The dispersion of the gain ratio was measured over a period of 6 months and found less than 1%. The linearity of ADC and MGPA was measured during the chip production and found within specification (see [2] and [3] ). An indication of the linearity of the VFE card can be extracted looking at the residuals calculated with respect to the calibration line. Their average (estimated for the 2320 cards mentioned before) was found within 2 ADC count for all charges and gains (see Fig. 2b) indicating that the linearity is satisfactory over the entire range. Pedestal and RMS noise of each VFE channel are measured with the calibration system and after the installation in the supermodules. The measured noise for gain 12 was found 1.1 ADC counts corresponding to about 40 MeV, fully complying with the requirement and in good agreement with the results obtained from the test beam of the same supermodule [4]. The measured noise for gain 1 and 6 are respectively 0.54 and 0.73 ADC counts.

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(a)

(b)

Figure 2. (a) Gain ratios G12/G1 and G6/G1 distribution for 2320 VFE. (b) Mean value of the residuals calculated respect to the calibration line versus injected charge for gain l(top), 6(middle) and 12(bottom). 4.

Conclusion

The E C A L Very Front E n d cards are under construction. Each board is tested and calibrated before the installation in the supermodule. T h e test results show t h a t the design goals have been achieved. References 1. CMS collaboration,"The Electromagnetic Calorimeter Project Technical Design report" CERN/LHCC 97-33, CMS TDR 4, 15 Dec. 1997. 2. M. Raymond et al.,"The M G P A Electromagnetic Calorimeter readout Chip fro CMS", Proceeding of the ST Workshop on Electronics for the LHC Experiment and IEEE 2004, Rome. 3. K. Kloukinas et al.,"Characterization and production testing of a quad, 12 bits, 4 0 M s / s A / D converter with authmatic digital range selection for calorimetry", Proceeding of the 11 Workshop on Electronics for the LHC and Future Experiments, Heidelberg Sept. 12-16, 2005. 4. ECAL group,"Test b e a m results for the C M S Electromagnetic Calorimeter", Proceeding of the Sr Conference on Astroparticle, Particle, Space Physics, Detector and Medical Physics Applications, Como, Oct 17-21, 2005.

UNIFORMITY OF RESPONSE OF THE ATLAS ELECTROMAGNETIC CALORIMETER SERIES MODULES C. OLIVER* Department of Theoretical Physics, Universidad Autonoma de Madrid, Madrid, 28049, Spain

Cantoblanco

During the construction of the ATLAS electromagnetic calorimeter, four barrel modules, and three end-cap modules, were scanned with electron beams of fixed energy. Final results on the uniformity of each module, and comparisons between modules are given. The contributions to the constant term expected in ATLAS at the LHC are analyzed.

1. Introduction The ATLAS electromagnetic calorimeter1 will play a central role in providing accurate measurements of energy, position and time of electrons and photons at the future LHC proton-proton collider. Its performance requirements have been defined studying the benchmark channels H-^yy, H-> 4e. A 1% resolution on a low mass Higgs requires a constant term less than 0.7% over the full detector coverage devoted to precision physics (rj< 2.5). To achieve this resolution the response non-uniformity has to be less than 0.6%. The EM calorimeter is based on a lead-liquid argon sampling technique which guarantees intrinsic linear behavior, stability of the response and radiation hardness. The accordion geometry of the electrodes and of the absorbers avoids uninstrumented areas in the azimuthal angle ^. The gap, filled with liquid argon, allows for the drifting of ionization electrons under high voltage. The calorimeter is composed of three parts, a barrel (|^| < 1.475) closed by two endcaps (1.375 < |?7| < 3.2) and segmented in depth in three compartments. The Front sampling is made of narrow strips in TJ for the precise position measurements needed in J/T? separation. The Middle sampling has a depth of 16 to 18 XQ and collects most of the shower energy. The Back sampling recovers high energy tails and helps to separate hadronic from electromagnetic particles. In addition, a thin presampler corrects for energy losses due to dead material for

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On behalf of the ATLAS EM group.

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The barrel is subdivided into two half barrels centered over the z-axis, in their turn subdivided into 16 modules. Each module is composed of 64 absorbers and 128 electrodes2. A 2kV high voltage is applied by the readout electrodes with respect to the grounded absorber. The end-cap3 is composed of two wheels with a projective crack at rj = 2.5. It is subdivided into 8 modules. In one module, 96 absorbers and electrodes are stacked in the outer wheel and 32 in the inner wheel. The gap increases with the radius and therefore decreases with r\. The detector signal is proportional to the sampling fraction and the drift velocity and inversely proportional to the liquid argon gap thickness so that an ^-independent detector response could be achieved with a continuously varying HV with the pseudorapidty. However, for technical reasons, a high voltage varying by steps was chosen, defining seven (two) high voltage sectors in the outer (inner) wheel. 2. Signal Reconstruction and Calibration The amplitude of the ionization signal after shaping and amplification is reconstructed from the linear combination of five samples, digitized at 40 MHz, located around the peak, using the optimal filtering (OF) technique4 in order to minimize the noise. The OF coefficients are calculated from the noise autocorrelation matrix, the pulse shape and its derivative. The physics pulse is obtained from the calibration one, which is corrected for the difference in the input current and injection point. Unlike the final situation in ATLAS with its fixed delay between trigger and sampling clock the beam test trigger comes asynchronous w.r.t the sampling clock. Therefore the OF weights are calculated in steps of 1 ns in order to fill the 25 ns trigger window. The energy in one cell is obtained by applying the electronic gain to the reconstructed amplitude in ADC counts, including a small non-linear behavior. The electron energy is reconstructed by summing the clusters formed in each compartment around the cell with maximum energy deposit. To compensate for energy losses in front of the calorimeter and leakage beyond it, weights are applied to the presampler and Back compartment measurements. The energy resolution and the total energy are extracted from a Gaussian fit starting 1.5
* There are two types of electrodes in each gap: one for tj < 0.8 and the other one for JJ > 0.8

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3. Test Beam Results 3.1. Test Beam Setup In 2001 and 2002, four barrel and three end-cap series modules of the ATLAS electromagnetic calorimeter were extensively tested under electron beams. Electron/positron secondary and tertiary beams are derived from SPS protons and their energies range from 10 to 245 GeV. Scintillators for triggering and timing and 4 multiwire proportional chambers with horizontal and vertical wire planes for beam position reconstruction were present at several positions along the beam line. 3.2. Energy Corrections Due to the finite cluster size, an observable lateral leakage is produced for events when the electrons is incident far from the centre of the cluster. This dependence is fitted and corrected for with a parabola (second order polynomial) for EMEC (EMB) modules. In <j> direction, the non-containment effect is convoluted with the local nonuniformities of electric field and sampling fraction, due to the accordion geometry. This translates to periodic non-uniformities on the cluster energy along the ^-direction, where the period is the distance between two consecutive absorbers. The ^-modulation is corrected using a Fourier polynomial (for the barrel only odd coefficients are considered). Additional corrections are needed to obtain the electron energy for EMEC modules. The specific setting of the high voltage by finite sectors in the end-cap induces a linear variation of the energy response as a function of 77 in each sector. To correct for the slope of the energy response inside HV sectors, tjdependent weights are applied on each cell. The HV correction parameters are obtained from a fit to the test beam data and their values are in good agreement for all EMEC modules. The energy measured in test beam shows an unexpected non-uniformity along ^-direction for the EMEC modules. This effect is correlated with the middle cell capacitance measured along this direction. Therefore, this leads to interpret this asymmetry in the calorimeter response as a consequence of the fluctuations of the gap thickness, generated during the module stacking. The most accurate capacitance measurements for each module are used to correct this effect. Since there is no accurate measurement for ECC1 module, an ad-hoc correction has been extracted from the ^-dependence of the energy averaged over 77.

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The transition region (77 = 0.8) between the two kinds of electrodes in a barrel module is 2 mm long and it is not instrumented, leading a loss of energy up to 15%. This effect is well understood by MC simulation and is corrected multiplying the energy reconstructed by a 77-coefficient. Finally, not related to the calorimeter itself but the test beam setup, the two feedthroughs used in the barrel test are not identical: FT-1 was a prototype and FTO a final element. It has been observed that FT-1 degraded with time and some resistive crosstalk between connectors developed, producing a higher energy dispersion in the region covered by FT-1. To correct this effect, a fraction of the measured current in a ^-adjacent cell is subtracted to the current of the excited cell. 3.3. Uniformity Results The mean reconstructed energy after all corrections for the tested end-cap and barrel modules is shown in Table 1 and Table 2, respectively. The dispersion (RMS/mean) of the distribution of the mean cluster energy reconstructed at every scan point is the non-uniformity of the module response. The non-uniformity was found to be less than 0.6% for the whole barrel and end-cap modules (results from CPPM5 and UAM6 groups), thus ensuring a global constant term below 0.7%. Table 1. Mean energy and non-uniformity of the three tested end-cap modules over the whole analysis region. For the outer (inner wheel), statistical errors on the mean energy are around 0.02 GeV (0.1 GeV) and statistical errors on non-uniformity are around 0.02 % (0.1 %).

Module <E> RMS/<E>

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ECC5

188.8GeV

119.1 GeV

119.3GeV

119.1 GeV

0.52%

0.57%

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0.59%

Inner wheel ECC5

Table 2. Mean energy and non-uniformity of the three tested barrel modules over the whole analysis region. Module <E> RMS/<E>

P13 233.3 GeV 0.57%

P15 235.8 GeV

M10 232.6 GeV

0.62%

0.62%

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4. Conclusions The uniformity of the electromagnetic end-cap and barrel calorimeter response to a electron beam has been studied on several production modules. Corrections for leakage and geometrical effects are applied on calibrated energies. For endcap modules, additional high voltage and capacitance correction, are needed. All correction are reproducible from one module to another. The results meet the challenging ATLAS requirements despite the complex geometry of the detector. References 1. ATLAS Collaboration, ATLAS Calorimeter Performance Technical Design Report, CERN/LHCC/96-40 (1996). 2. ATLAS Electromagnetic Liquid Argon Calorimeter Group, Nucl. Inst. Meth. A500, 202 (2003). 3. ATLAS Electromagnetic Liquid Argon Calorimeter Group, Nucl. Inst. Meth. A500, 178 (2003). 4. W.E. Cleland and E.G. Stern, Nucl. Inst. Meth. A338, 467-497 (1994). 5. F. Hubaut, C. Serfon, Response uniformity of the ATLAS electromagnetic endcap calorimeter, ATL-LARG-2004-015 (2004). 6. C. Oliver, J. Del Peso, Outer Wheel uniformity of the electromagnetic endcap calorimeter, ATL-LARG-PUB-2005-002 (2005).

PLANS FOR THE VERY FORWARD REGION OF ATLAS THE LUCID LUMINOSITY MONITOR JAMES PINFOLD t Physics Department, University of Alberta, Edmonton, Alberta T6G 2N5, Canada. The ATLAS collaboration is actively pursuing a program to develop the detection capability of the ATLAS detector. A prime motivation for this development is to allow the measurement of the absolute luminosity as well as, for example the study of diffractive physics and two photon physics. It is envisaged that the measurement of elastic scattering in the Coulomb region could be used to calibrate a dedicated luminosity monitor, LUCID, that will be used to measure the luminosity of ATLAS across the full range. The challenges of the design, simulation and testing of the LUCID detector are discussed below.

1. Introduction The general purpose LHC detectors, ATLAS [1] and CMS [2] have excellent central coverage in the Ecm, up to a pseudorapidity (|TJ|) ~ 5 because of the dominating interest in high-Pr physics at the LHC. However, ATLAS is now implementing additional forward detectors, which aim to cover the forward diffractive regions with tracking and/or calorimetry [3]. The CMS detector has very similar capabilities to ATLAS with the TOTEM [4]. The approved design for the upgrade to the forward system of ATLAS incorporates a relative luminosity monitoring using LUCID, a system of 400 (2 x 200) Cerenkov tubes, surrounding the beam pipe at 2=±18 m. Absolute calibration of LUCID is performed using Coulomb scattering measured in runs with special machine optics and Roman Pot detectors at z=±240 m reaching very close to the circulating beams to intercept small-angle protons. A measurement of the elastic cross section, extending down to the Coulomb scattering region (\t\ a 6 x 10"4 GeV2), is proposed for ATLAS and should provide a 2% determination of the luminosity, as well as a direct measurement of cjei, atot, the

t Representing the Luminosity and Forward Physics Working Group.

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p parameter, and the nuclear slope parameter B. We also plan to use wellcalculable physics processes (e.g. W/Z production, pp->uu) for calibration. It is also envisaged that the LUCID detector would be useful in defining a rapidity gap trigger for a wider program of diffractive physics [5]. In addition it could, in principle, be useful in determining the "event-plane" when the LHC is operating in heavy-ion collider mode.

2. A Dedicated Luminosity Monitor for ATLAS - LUCID Traditionally, for hadron colliders, the luminosity has been measured using dedicated scintillation counters which measure the fraction of bunch crossings with no interactions [6]. Such detectors, typically placed at relatively large pseudorapidities, can have large acceptance for inelastic pp(p) interactions. However, the design luminosity of the LHC is so high that the fraction of crossings with no interactions will be small. Also, radiation levels are high enough to render scintillation counters unusable. At the LHC it is necessary to utilize a dedicated detector to measure the average number of interactions per bunch crossing, directly. In order to monitor with precision the average number of pp interactions we have proposed a robust, fast, radiation hard, dedicated detector that is mostly sensitive to particles from inelastic pp collisions and insensitive to backgrounds. This detector will monitor the average number of inelastic pp interactions by

Figure 1. A sketch of the proposed positioning of one of the two LUCID detectors. The other detector would be placed in the symmetrically opposite position.

measuring the number of particles, and their arrival time, in each bunch crossing. The proposed detector is called LUCID. The design of the LUCID

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detector is largely based on that of the Cerenkov Luminosity Counter (CLC) that is currently operating at CDF [7]. 2.1. The Design of the LUCID Detector The LUCID detector consists of two modules that are located in available space between the beam pipe and the conical beam-pipe support structure. This places LUCID in the forward shielding, after the ATLAS Endcap Toroids. A schematic depiction of the proposed placing of LUCID is given in Figure 1. The beampipe inner diameter at the position of LUCID is 123mm with thickness 1.5 mm. The range of pseudorapidity covered by LUCID is |5.4 > 6.1| when the front of the detector is placed at z = ±16.98 m from the IP. An artist's impression of the LUCID detector in place is given in Figure 2. Each LUCID detector module consists of 200 thin walled (0.5mm), 1.5 m long, cylindrical, gas filled Cerenkov counters. These counters are arranged around the beampipe in five concentric rings with 40 tubes each, pointing to the centre of the interaction region. The tubes are 0.5 mm thick aluminium pipes, with a polished interior. The tube diameters are, from the inner to the outer layer of tubes: 13.28, 15.22, 17.43, 19.97, and 22.89 mm. A cross-section though LUCID is depicted in Figure 3. The light from each tube is collected by a Winston cone machined from aluminium. The ength of the Winston Cones vary from around 80 mm for the inner layer of tubes to 130 mm for the outer layer of tubes.The wall thickness of the Winston cones is envisaged to be 0.5 mm. The light from the Winston cone is collected by a bundle of clad 1mm fused silica fibres. Fused silica is chosen for its radiation hardness. Each bundle is routed through the forward shielding

Figure 2. An artists impression of the LUCID detector in place between the beampipe and the conical support tube of the beampipe. to remote photo-detectors. We will utilize 7 fibres to cover the egress of the Winston Cone for all layers. The complete detector structures are enclosed in a

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thin, aluminium pressure vessel filled with C4F10 or isobutane. The nominal operation point is at atmospheric pressure, however the vessel will be designed to operate up to one atmosphere above atmospheric pressure. This allows us to increase the gas radiator pressure in the event that more light is required. The high radiation environment requires that the Cerenkov light from the LUCID detector is taken out to remote photo-detectors housed on the top of the nose shielding using fused silica fibres. These fibres penetrate the pressure vessel containing the detector through leak proof connectors. At this point magnetic fields are not a problem and there are two candidates for the read-out technology currently under investigation. The first is the baseline solution: multi-anode photo-multiplier tubes with quantum efficiency -20%. The second, solution is provided by avalanche photo-diodes (APDs) with quantum efficiency -80%. It is envisaged that the readout crates servicing the MaPMTs will be placed adjacent to the "nose" of the forward shielding, as shown in Figure 4. An LED system is planned to enable light to be introduced into the mouth of each Cerenkov tube, for the purpose of calibration.

Figure 3. A cross-section through LUCID at the mouth of a detector (z = ±16.98m).

We are proposing to use C4F10 (or isobutane) as a radiator as it has one of the largest indice of refraction, n=l. 00137 (n=l.00143), at atmospheric pressure for commonly available gases and good transparency for photons in the UV region where most of the Cerenkov light is emitted. The Cerenkov light cone half angle is ~3°, and the momentum threshold for light emission is 9.3 MeV/c for electrons and 2.7 GeV/c for pions. Prompt particles coming from the IP (primaries) will traverse the full length of a Cerenkov tube and generate a large amplitude signal in the photo-detector. Particles originating from secondary interactions of prompt particles in the

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Figure 4. The positioning of the readout structures for LUCID.

detector material and beam-pipe (secondaries) are softer and will traverse the counters at larger angles, with shorter path lengths. In addition, the light from these particles will also suffer a larger number of reflections. The signals from these secondaries are therefore usually significantly smaller than those from the primaries and can be discriminated using amplitude thresholds in the electronics and in the offline data analysis. This is not feasible with scintillation counters. Also, at higher occupancies, when two primary particles traverse a single counter the resulting signal is twice that of a single particle. Given that there are no Landau fluctuations the counter's amplitude distribution should show distinct peaks for the different particle multiplicities hitting the counters. This enables us to count the actual number of primary particles hitting LUCID and helps prevent the counting rate from saturating at high luminosity, allowing a more precise measurement of the average number of interactions [8]. The LUCID detector counts tracks from minimum bias events that consist of elastic scattering events, diffractive events, double diffractive events, low-pT scatters and semi-hard QCD (2—>2) processes. The total cross-section for these processes is estimated to be ~100 mb (PYTHIA, msel=2). Secondary particles from primary interactions as well as beam-halo and beam-gas particles are expected to form the main backgrounds.

2.2. Simulating the L UCID detector A GEANT4 simulation of the main features of the LUCID detector has been written. A complete simulation awaits the details of the support structure and pressure vessel design. The GEANT4 simulation includes a simulation of the complete path of a photon in the LUCID detector, from generation in the

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Cerenkov light produced by an incident particle, through transmission down the Cerenkov tube to collection in the Winston cone attached to the tubes and subsequent transmission down a fused silica fibre-optic cable to the PMT. A GEANT4 simulation of a single muon incident along the axis of a Cerenkov tube of LUCID reveals some important characteristics of the LUCID detector. A high momentum particle incident on a LUCID Cerenkov tube gives the maximum light output at an angle of 0° to the axis of the tube. The light is then transmitted over ~5m of fused-silica fibres. Light is lost in the fibres due to the geometrical acceptance of the fibre bundle and attenuation in the fibre. For readout fibres with a numerical aperture 0.37 we expect roughly 200 photons with wavelength between 200 nm and 800 nm to reach the photo-detector Assuming the light is detected by the photomultiplier tube with an average quantum efficiency of approximately 10%, we would expect to generate around 20 photo-electrons per track. Twice that number of photons would be detected for two tracks in the same tube. .- *

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The LUCID detector performance was verified using minimum bias (MB) events generated by PYTHIA-6.152 [8] using the standard ATLAS settings for MB event generation. The distribution of the number of photons in the wavelength range 200nm-^800nm that reach the PMT (assuming a readout fibre with NA = 0.37 is utilized) is shown in Figure 5. As can be seen from Figure 6. there is a linear relationship between the luminosity and the number of tracks detected in LUCID. The results shown in Figures 5. & 6. are not appreciably disturbed by the presence of the beam pipe.

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A initial study of background light generated in the fused-silica readout fibres have been carried out. This problem is ameliorated by the fact that the large Cerenkov angle of the light in the fused silica fibre, with moderate to low NA ( < 0.37), precludes efficient trapping of the Cerenkov light from particles traveling at small angle to the fibre axis. Another ameliorating factor is that for efficient trapping of Cerenkov light particles must be moving through the fibre at large angle, with small path lengths and thus smaller light emission. In addition, we shall be reading out each fibre This will allow us to use pattern recognition to reduce the effect of this background. Fully detailed Monte Carlo (MC) data with test volumes corresponding to fibre optic readout in various positions is currently being generated to allow more detailed estimates of the background. Beam-tests of fibre-optic cables with a view to studying backgrounds in the readout fibres are also envisaged.

2.3. Prototyping the LUCID Detector Six aluminium Cerenkov tubes with diameter roughly 2 cms have been equipped with Winston cones and mounted in a test pressure vessel capable of taking a 2 Bar overpressure. The gas vessel has been provided with fibre-optic readout cables with 42(7x6) fibres that take the light to a 64-channel Hamamatsu (H7546B-03) MaPMT Isobutane gas will be employed as the radiator. This six tube prototype will be tested using an electron beam at DESY in October 2005. The main purpose of this first beam test is to provide data for the validation of the MC simulation. To do this the variation of the light output with angle, position in tube, pressure, temperature and different fibre type will be measured.

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2.4. The Radiation Environment Around the LUCID Detector The radiation environment at the LUCID detector is severe, at 105 Gy/yr total ionizing dose. In addition, there is a flux of 1013 (1 MeV equiv.) neutrons/ (cm2.yr) through the same region. Of particular concern are the epoxy joints that provide the gas seals for LUCID. As, the LUCID detector will be relatively inaccessible it is of great importance that all elements of the LUCID detector be tested for the equivalent of 10 years of running at these radiation levels. The radiation level in the region of the MaPMT readout is estimated to be only a few to several Gy/yr The ionizing dose expected for the primary read-out crates is of the order of 0.1 Gy/yr. The neutron flux for the same region is approximately 1 kHz/cm2, or ~1010 neutrons/(cm2.yr). The electronics and photo-detectors will be tested for radiation levels equivalent to ten years of running in this less critical radiation environment.

3. Calibrating the LUCID Detector The elastic scattering data will be recorded at an luminosity of 1027 c m ' V and thus the luminosity monitor will be calibrated at this luminosity. The luminosity for normal running of ATLAS will be in the range 1033-1034 cm"V. This implies that the calibration has to be carried over six to seven orders of magnitude. Roughly, there are three orders of magnitude from the difference in p., two orders from the difference in the number of bunches and two orders of magnitude from the lower bunch intensity. Thus, there is an advantage in using a luminosity monitor that can resolve the individual bunches i.e. with a time resolution better than 25 ns. This implies that the six to seven orders of magnitude reduces to four or five as the number of bunches factorize out in the extrapolation from low to high luminosity. In practice this means that the calibration has to be transferred from running conditions with 2xl0"4 interactions per bunch crossing to conditions with about 20-30 interactions per bunch crossing. These numbers illustrate the two main requirements for any luminosity monitor to be calibrated via special low luminosity runs: there has to be very good background rejection to avoid counting of fake interactions at low luminosity and there has to be a dynamic range of at least 20-30 to avoid saturation of the monitor at high luminosity. The LUCID concept addresses both these questions. Using Cerenkov tubes pointing to the interaction point implies possibilities of rejecting low energy secondary particles and thus being sensitive to mainly primary particles from the interaction point. Furthermore, should be possible to separate one and two particle peaks in the amplitude distribution from each tube, allowing counting of particles instead of counting only hits. If these double tracks were counted as a single hits a non-linearity in the response of roughly 5-10% would result.

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Particle counting will be of importance to: avoid saturation at high luminosity; obtain the necessary dynamic range; and, ensure linearity of response. The cross-calibration of LUCID with the elastic scattering data will take place at a luminosity of 1027 c m ' V . Based on Figure 5, we can expect approximately 1.7 tracks to be detected per arm of the LUCID detector per pp collision, where we include single diffractive processes. At the luminosity of 1027 cm"V, the collision rate is 100 Hz for a 100 mb cross-section, which thus corresponds to 170 Hz tracks per arm. The double-arm rate is slightly less. The rate of 170 Hz per arm in LUCID is to be compared with the elastic rate in the RP of about 30 Hz. Hence, the calibration of LUCID in a high p run can count on at least the same order of numbers of events being available from LUCID. There are (at least) three other methods of luminosity determination that could be employed to give cross-checks and useful consistency criteria when used with the detectors based approaches discussed above.. For example the machine measurement of the luminosity by the Van de Meer method [9] could well be used. However, it is envisaged to provide only a 5%->10% accuracy. In principle, physics processes could give a better measurement. Two main methods are being considered by ATLAS at present [10]. The first uses a QED process: the production of a muon pair by double photon exchange[ll]. This process is experimentally clean but has a very small observable cross section because of the necessary pT trigger conditions imposed on the muons. Another process that has been proposed, and has been studied in some detail by ATLAS, is the QCD production of W/Z gauge bosons, and the measurement of their production rate times leptonic branching fraction [12]. This process measures the parton luminosity directly, and hence may serve to normalize the parton luminosity in many other production processes of interest. Alternatively, W/Z boson production is one of the theoretically best investigated QCD processes, and the parton distribution functions participating in the production process are already well and continue to improve further. Moreover, this process has a high enough rate that it could serve as a luminosity monitor as well as an absolute luminosity gauge. 4. Conclusion The LUCID approach to luminosity monitoring, based on the CLC detector at CDF provides a practical, robust and economical approach to luminosity monitoring across the full luminosity range. The LUCID detector is also relatively light (~40Kg/end) and should be supportable in the available space between the beampipe and the its conical support vessel without any costly changes to the beampipe or beam line elements. The fast timing characteristics of this detector allow a bunch by bunch luminosity determination. Also, LUCID monitors MB events with a very large cross-section (~100 mb) allowing an online luminosity measurement. However, more work needs to be done in

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prototyping, simulation and modeling to prove that LUCID can meet the challenges of precise luminosity monitoring in the ATLAS environment. This work is well underway.

References 1. 2. 3.

4. 5.

6. 7. 8. 9. 10.

11. 12.

The ATLAS Collaboration, "Detector and Physics Performance Technical Design Report", CERN/LHCC/99-15, 1999. The CMS Collaboration, "The Compact Muon Solenoid, Technical Proposal, CERN/LHCC/94-38", 1994. The ATLAS Luminosity Lol CERN/LHCC/2004-010 (22 March 2004); ATLAS Collaboration: ATLAS-TDR-15, Vol I, 438-447, 25 May 1999; http://atlasinfo.cern.ch/Atlas/GROUPS/PHYSICS/TDR/access.html; http:// sbhepl.physics.sunysb.edu/wrijssenbeek/ATLAS-LumiForwPhys.html. TOTEM Technical Proposal, CERN/LHCC 1999 - http:// totem, web. cern. ch/ totem; CERN-LHCC/2004-002, TOTEM TDR. FP420 : "An R&D Proposal to Investigate the Feasibility of Installing Proton Tagging Detectors in the 420m Region at LHC", CERN-LHCC2005-025, LHCC-I-015 F. Abe et al., Phys. Rev. D 50, 5550, (1994); D. Cronin-Hennessy, A. Beretvas, P.F. Derwent, Nucl. Instr. & Meth. A 443, 37 (2000). D. Acosta et al., Nucl. Inst. & Meth. A461, p540, (2001); D. Acosta et al., Nucl. Inst. & Meth. A 494, 57, (2002). T. Sjostrand, Computer Physics Commun. 82, 74, (1994). K. Potter, CERN Yellow Report, CERN 71-01 (1971). Eg: The ATLAS Luminosity Lol CERN/LHCC/2004-010 (22 March 2004); J. L. Pinfold, "Determining Luminosity Using Physics Processes", ATLAS Physics Workshop, Rome, June 2005. G.Shamov and V.I. Telnov, Nucl. Instrum. Methods A 494 (2002) 51. M. Dittmar, F. Pauss & D. Zurchner, Phys. Rev. D36, 7284, (1994). 2002.

FAST, LONG-WAVELENGTH SCINTILLATORS AND WAVESHIFTERS* K. ANDERT, B. BAUMBAUGH, A. BROTHERS, A. DAVID, H. GUNTHER, J. GURROLA, D. KARMGARD, T. MADLEM, J. MARCHANT, P. MCGOUGH, M. MCKENNA, R. RUCHTIf , J. THOMPSON AND M. VIGNEAULT Department of Physics and QuarkNet Center, University of Notre Dame Notre Dame, Indiana 46556, USA L. HERNANDEZ AND C. HURLBUT Ludlum Measurements Inc. 501 Oak Street Sweetwater, Texas 79556, USA

Studies are presented of new Hue-green to red emitting scintillator and waveshifter materials for tracking and calorimetric applications for the detection of ionizing radiation. Materials include plastic scintillators, liquid scintillators, and plastic scintillating and waveshifting fibers. Program goals are to develop faster and more efficient detection media for a variety of experimental applications. 1.

Introduction

This paper focuses on the development of scintillating and waveshifting materials for calorimetric and tracking applications. Ultimate objectives are to identify and produce materials with high quantum efficiency, fast fluorescence decay and that afford tolerance to radiation exposure. For calorimetric detectors, a common detector technique is the scintillation sampling calorimeter. 1 ' 2 ' 3 Plates or "tiles" of blue-violet emitting scintillator are read out with embedded multi-clad waveshifting fiber containing the dye Y l l . Light emission is near X = 510nm and the fluorescence decay time is relatively long > 7ns. This paper provides preliminary measurements of scintillating organic plastic and liquid materials containing new fluorescent dyes that are potential replacements for conventional scintillator and waveshifter. In a high radiation environment, scintillator-based liquids may be flushed from a system * This work is supported in part by the U. S. Department of Energy STTR Program, the U. S. National Science Foundation and Department of Energy QuarkNet Program, the National Science Foundation RET Program and the University of Notre Dame. + Current address is the National Science Foundation, Arlington, VA 22230 USA.

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390 and replaced by new material as needed. Additionally several of these dyes have been incorporated as waveshifters in multiclad fiber for the readout of tile scintillators. 2.

Scintillators and Wavelength Shifters

Bulk plastic scintillator and waveshifter samples were polymerized in either polystyrene or polyvinyltoluene. Liquid samples were prepared from aromatic, organic liquid-scintillator base. Fiber waveshifters were produced with doublecladding. 2.1. Optical Characteristics of Scintillators and Waveshifters Initially the materials were characterized optically. Emission and excitation spectra for each of the samples were measured with a Varian Cary Eclipse Fluorescence Spectrophotometer attached to a custom data acquisition system. Figure 1 displays a sample of emission spectra for three new shifter dyes, DSFl and AH328B in organic liquid base and DSF2 in a polyvinyltoluene base. The fluorescence decay times of several samples were measured with a Horiba Jobin Yvon Pulsed Diode Laser Excitation System with TBX-04 Picosecond Photo Detection Module. Samples were excited at X = 377 nm and the fluorescence decay observed at emission maximum. As can be seen in Figure 2, the photo-excited fluorescence decay time of DSFl (T = 1.3ns) is significantly faster than standard Y l l (T = 7.4ns) by a factor of over 5. The decay of AH238B is found to be slower (T = 11.5ns), but the emission wavelength for this dye is considerably longer (in the yellow/red).

2.2. Scintillation Efficiency Studies Efficiencies of the scintillator samples were characterized by exposing them to a Sr beta source. Signals were detected with a Hamamatsu R1104 photo multiplier connected to a LeCroy QVT pulse height analyzer and read into a custom computer data acquisition system. Optical filtering was used between the samples and the phototube to cut out emission from the bulk material and other primary dyes (if used). 4 Table 1 displays the measured scintillation efficiencies as a function of wavelength for several scintillation materials prepared for this study. The efficiencies are corrected (scaled) for the wavelength dependent quantum efficiency of the Rl 104 PMT. 90

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650

Figure 1. Emission spectra for several waveshifters. DSFl waveshifter organic liquid base (upper left). DSF2 waveshifter in PVT base (upper right), AH238B waveshifter in organic liquid base flower left).

Figure 2. Fluorescence decay times excited by a Diode Laser at 377 nm. Light is detected at the emission maximum for the samples. Fits to the curves indicate the following: DSFl in organic liquid (upper left, 1.3ns); Y l l in PVT (upper right, 7.4ns); AH238B in organic liquid (lower left,

11.5ns).

392 Table 1. Scintillation Efficiency of Samples Composition

Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid

3.

L1 L1 L1 L1 L1 L1 L1 L2 L2

Primary

Secondary

Dye

Dye

AH238B P1 P1 P2 P3 P3 P3 P3 P3

AH238B BBQ DSF1 DSB1 DSB2 DSF1 DSF2

Emission Wavelengt h (nm)

Efficiency Relative to BBQ

600 450 600 470 480 500 510 480 480

0.84 1.05 1.42 1 1.02 1.11 1.25 1.08 1.02

Tile-Fiber Measurements

Those secondary dyes that revealed promising scintillation performance and rapid fluorescence decay were then incorporated into double-clad fiber for waveshifter studies. The dye materials so chosen included DSFl, DSF2, DSB1 and DSB2 and these were compared to the standard dye Yl 1. All were used to read out scintillation tiles of 10cm x 10cm dimension and 6mm thickness of scintillation material (EJ200) with emission maximum in the blue-violet (X = 425nm). The detection system has been described elsewhere4 and includes a three-fold coincidence of trigger counters, including a PMT that views the scintillation tile itself and two other thin independent counters one above and one below the tile. The coincidence of these counters was used as a gate for the signal from the waveshifter fiber, located in a groove in the tile. The optical signal from the fiber was read out using a Hamamatsu H7422P-40 GaAsP PMT. The signals from the PMT were averaged over 2048 pulses and displayed and recorded on an HP54502A digitizing oscilloscope and read into a computer. Such signals were recorded for triggers that occurred in response to a 90 Sr beta source. Figure 3 displays the recorded signals from the tile/fiber combinations. As can be seen, the best performance is observed for DSB1 shifter and it is superior to Yl 1 in both overall signal as well as signal response time. DSFl and DSF2 have lower overall signal, but have very fast response. The data show that for DSFl, the full width at half maximum (FWHM) is 9.0ns and a fit to the tail with a single exponential function reveals a decay time of T = 6. Ins. The values of these quantities for Yl 1 are FWHM = 17ns and T = 12.1ns, indicating the

393 superiority of the DSFl in time response. Table 2 summarizes the results of these measurements and others. r0465 DSF1 f.02-stc01 Ru106 140mV/div 10-12-05

r0466 DSF2 f.02-stc01 Ru106 140mV/div 10-12-05

h

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C 6 T«+.'• 0.3S93 B: 1.S3+.'-0.0320 RMSE 0.463

1 /

\ \

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**J

1

Tims (ta'ni J f a f l 2 n s « c )

i

T

Time c'fcris aw 0.2 n 3 K | JOB, « . * »

r0468 DSB1 f.13-stc01 RU106 140mV/div 10-13-05

r0467 Y-11 f.17-stc01 RU106 140mV/div 10-13-05

AuloFrtFor; [<S$7.Cate C a ( o m n T c y = A'exp(-»t;6ttB A: 358 +/- T75 C 12.1 -.'-0.0B94 6 1 07 +,'- 0.0343 fiMSE 0 353

'^0F\t?'Qt'''r0ttB:C'aic7Coiu'ffii™i E A 3 32E-0G3 ->i- 123 C:6.21 4V-0C5W

•e

/ ,

•f

„,

B

i

I i I

.

Time [bins ire 0 2 nsac)

Figure 3. Averaged output pulses from various waveshifters in double-clad fiber of 940mm diameter and lm length reading out EJ200 scintillating tile of 6mm thickness and 10cm x 10cm area. Photosensor is a GaAsP PMT. DSFl (upper left); DSF2 (upper right); DSBl (lower left); Y l l in PVT (lower right). Values are summarized in Table 1 below. Table 1. Summary of Waveshifting Efficiency for Sample Materials. Waveshifter

Emission wavelength

Efficiency rel .to Yl 1

Decay Time Source Exc.

FWHM Source Exc.

Decay Time Laser Exc.

Yll DSBl DSFl DSF2

529nm 500nm 480nm 480nm

1 1.02 0.84 0.79

12.1ns 6.2ns 6.1ns 6.8ns

17ns 9.4ns 9.0ns 9.6ns

7.4ns 1.3ns

394

4.

Conclusions

From the studies underway in this program, several new scintillation and waveshifter materials have been produced and studied with fluorescence emission wavelengths in the red to green and with rapid fluorescence decay times and reasonable efficiency. These have been incorporated into multi-clad fiber and tested as waveshifters in tile-fiber geometry (DSF1, DSF2, DSB1, DSB2). Additionally, several of these have been used as secondaries in liquid scintillation cocktails (DSF1, DSF2, DSB1, DSB2, and AH238B). Initial measurements indicate that DSB1, DSF1 and DSF2 offer superior performance in fluorescence decay time to conventional Y l l waveshifter. DSB1 and DSF1 offer comparable integral fluorescence efficiency. These materials are therefore potential replacements for Y l l in any application where fast timing is required, or where improved signal to noise would be an advantage by utilizing short integration times.

Acknowledgments We would like to thank P. Kamat and the staff of the Notre Dame Radiation Laboratory for the use of the Diode laser spectrometer, E. Hahn of Fermilab for fiber end-polishing, and to the University of Notre Dame for support facilities and resources for high school students and teachers at the Notre Dame QuarkNet Center. References 1. The CDF II Technical Design Report, Fermilab-Pub-96/390-E. 2. CMS The Hadron Calorimeter Project Technical Design Report, CERN/LHCC 97-31. 3. ATLAS Tile Calorimeter Technical Design Report (TDK), CERN/LHC/9642. 4. M. Albrecht, et al, Scintillators and Wavelength Shifters for the Detection of Ionizing Radiation, Astroparticle, Particle and Space Physics, Detectors ad Medical Physics Applications, ICATPP-8, M. Barone, E. Borchi, C. Leroy, P.-G. Rancoita, P.-L. Riboni, R. Ruchti, Eds, World Scientific, 502-511 (2004).

T E S T B E A M RESULTS OF T H E CMS E L E C T R O M A G N E T I C CALORIMETER

ROBERTO SALERNO On behalf of CMS ECAL

collaboration

The CMS (Compact Moun Solenoid) experiment is a general purpose detector designed to explore the physic of proton-proton collisions at the centre-of-mass energy of 14 TeV. The CMS electromagnetic calorimeter (ECAL) will play a major role in the study of the electroweak symmetry breaking, particularly through the exploration of the Higgs boson sector. In order to test the ECAL performance, a full supermodule with the final mechanics, cooling and electronics was exposed in a test beam during the summer 2004. The most relevant results obtained at this test beam are presented together with the calibration procedure and its comparison with the precalibration obtained from the laboratory optical measurements and from cosmic rays.

1. Introduction One of the major goals of the Large Hadronic Collider is to elucidate the origin of the mass, in particular by discovering the Higgs Boson predicted by the Standard Model. For a low Higgs mass (< 120 GeV) the main channel is H —> 77 whereas for a higher mass range the channels H —> WW and H —> ZZ become important. In both cases the electromagnetic calorimeter plays a central role in the selection of these events with high efficiency and low background. In particular an excellent energy and angular resolutions are mandatory for the detection of electrons and photons. The above considerations have led the CMS collaboration to opt for an electromagnetic calorimeter built of Lead Tungstate (PbW04) crystals. The low time constant, as well as the short radiation length and Moliere radius of PbW04 allow the building of a rapid, compact and granular detector. However the relative low light yield of the crystals entails the usage of avalanche photodiodes with an intrinsic gain. Since both the gain of the APDs and the light yield of the crystals are very sensitive to the temperature fluctuations, a precise control of the temperature is required. Exhaustive detector description can be found in [1] and [2].

395

396 The energy resolution of ECAL can be parametrized as a function of the energy as
_

S

N

£ ~7I®£

'

(1)

where S is called the stochastic term which includes the shower fluctuations and the photo-statistics, N is the noise term which comprises the electronic noise and C is the constant term which includes the calibration precision, the longitudinal non-uniformities of the light collection and the fluctuations due to the temperature. 2. The testbeam setup During the summer 2004 a complete supermodule of ECAL barrel (SM10), built accordingly to the final production specifications, was tested with an electron beam at the H4 facility in the CERN North Area. The laser monitor system, the HV and LV distribution systems and the cooling system were built in the final design. At the test facility the supermodule was positioned on a moving table allowing to center each crystal on the beam. The position of the impact point of electrons on the crystal surface is measured, event by event, by a set of scintillating fibers, with a resolution of 145 /xm. SM10 had 1700 channels equipped with the Multi Gain PreAmplifier chip in the VeryFrontEnd cards.

3. Amplitude reconstruction The whole procedure for reconstructing the particle energy starting from the electromagnetic calorimeter has been investigated at the test beam. The signal from the ECAL-APD is digitized at 40 MHz and 10 samples are recorded, including 3 samples before the start of the signal to allow an estimation of the baseline. The signal amplitude A is obtained with a general technique as a weighted sum of the information of the samples (Si):

A = Y^ SiWi .

(2)

i

The weight Wi are obtained by minimizing the variance A. The additional requirement Sw, = 0 allows an event by event baseline substraction since any costant level does not contribute to A. This procedure and the resulting expression for the weights are described in detail in [3].

397 4. Noise A measurement of the electronic noise can be made by applying the amplitude reconstruction procedure to data with non signal, taken with a random trigger. The distribution of the noise measurements in a single channel is about 39 MeV while in a 3 x 3 and 5 x 5 clusters is about 120 MeV and about 200 MeV, this corresponds to 3 times and 5 times the single channel noise, demonstrating the removal of the small component of noise which is channel-to-channel correlated. 5. Calibration A precise intercalibration of the CMS ECAL is mandatory to keep the constant term in energy resolution at the level of 0.5%. The final calibration of the CMS ECAL will do obtain in situ at LHC from physical events. Nevertheless it is important to have a preliminary initial intercalibration at LHC startup. This can be obtain with a good accuracy from calibration with a beam of electrons, from laboratory measurements of the crystal light yield and from the calibration with the cosmic ray. In the testbeam the intecalibration (d) can be defined as the ratio of the response of each channel to impinging electrons of a given energy (Mj) over the response of a reference one (Mref). The position of the peak in the energy distribution measured in one single crystal is chosen as the estimator of the channel response. Detail studies have demonstrated that accuracy of the intercalibration coefficient is at the level of the 0.3% [4]. An initial intercalibration for the whole detector can be obtained from the APD and electronics gain measured in the laboratory and the light yield determined by combining the measurements of the trasparency and the response to 7 from a mCo source. The agreement with the intercalibration is at the 4% level [5]. The ECAL supermodules will be intercalibrated using cosmic rays before the installation in CMS. A statistical precision of 2 — 3% on all crystals is achievable [6]. 6. Energy Resolution The shower, for energetic electrons, is not contained into a single crystal. Therefore the energy of the electron is reconstructing as the energy deposited in a 3 x 3 and 5 x 5 clusters of crystals. The sum of the energy deposited in each the 9 or 25 crystals in the array, is carried out by using the calibration coefficients obtained with the procedure explained in Sec. 4.

398

CMS ECA^ Preliminary Resolution 3x3 «/E "0.44* 0.01%

Preliminary Resolution SxS IT/E "0.45 ± 0 . 0 1 *

Figure 1. Distribution of the energy reconstrued in 3 X 3 crystals (left) and in 5 X 5 crystals (right) at 120 GeV/c.

6.1. Resolution

for central

impact

Restricting the impact to a 4 x 4 mm2 region, over which the containment remains almost unchanged, it gives a measure of the intrinsic resolution of the calorimeter without any corrections. In figure 1 the energy resolution in the case of central impact are presented for a 3 x 3 and 5 x 5 clusters of crystals [7]. 6.2. Cluster

containment

corrections

Since the sum of 9 or of 25 crystals are vary with position of the incident particle, a correction for this effect is implemented. The position of the impinging particle is evaluated comparing the amplitudes measured in the crystals to the left and to the right of the hit. The Log(E\/(E2) method used has been extensively tested with simulated events and beam data [8]. The effect of the cluster containment on the energy resolution obtained displacing the beam to a crystal corner is in perfect agreement with the resolution for central impact. 6.3. Energy resolution

as a function

of

energy

The energy dependence of the energy resolution has been fitted by the expression 1. Data was taken at a series of beam momenta corresponding to the energies: 20, 30, 50, 80, 120, 180 and 250 GeV. The resolution as a function of energy is extracted for the case of uniform impact across 20 x 20 mm2 area of the trigger. The cluster containment corrections described in the Sec. 6.2 are applied. The figure 2 shows the energy resolution when summing the energy in a 9 and 25 crystals arrays, the errors quoted

399

1.4

- \\ : l\

CMS ECAL Preliminary Resolution for 704

1.2 1 O.B

r

0.6

^\

S=2.35V-0.1% fV=0.25OV- 3.0 GeV C'0.33-/- 0.01 % 3x3 $=2.96*/-0.1 % N=0.166*/- 3.4 GeV C'0.32*/- 0.01 %

0.4 0.2 0

. . . .

1 . . . .

200

I

250 Energy (GeV)

Figure 2. Resolution as a function of t h e energy reconstrued by summing 5 x 5 crystal (black line) compared to the sum of 3 x 3 crystals (red line).

on the fitted parameters include b o t h the statistical and the estimated systematic uncertainties. T h e resolution is better t h a n 0.5% for electrons energy greater t h a n 100 GeV. 7. C o n c l u s i o n T h e CMS electromagnetic calorimeter collaboration has achieved relevant results in the test beam 2004, all of t h e m consistent with the target values established by the collaboration to deal with the challenging physics program of C M S a t L H C . T h e C M S E C A L aims a t an excellent energy resolution for uniform impact. The comparison with the testbeam calibration have demonstrated t h a t the initial pre-calibration of t h e whole detector by laboratory and cosmic ray measurements is accurate at the 4% and 2-3% level. References 1. CMS collaboration, The electromagnetic calorimeter project. Technical Desing Report, C E R N / L H C C 97-33. 2. CMS collaboration, Data Acquisition and High-Level Trigger technical Design Report, C E R N / L H C C 2002-26. 3. R. Bruneliere and A. Zabi,Amplidute reconstruction in the CMS ECAL with the weights method", C M S N o t e in preparation 4. G. Franzoni at al, Intercalibration for the CMS ECAL at 2003 H4 testbeam, C M S IN 2 0 0 4 / 0 2 3 .

400 5. L.M. Barone at al., Correlation between Light Yield and Longitudinal Trasmission in PbWOi crystals and impact on the precision of crystal intercalibration, CMS R N 2004/005. 6. A. De Min at al., Pre-calibration of the CMS electromagnetic calorimeter with cosmic rays, C M S note in preparation. 7. R. Bruneliere, P. Jarry and A. Zabi, Energy resolution performanca of the CSM electromagnetic calorimeter, C M S note in preparation. 8. J. Descamps and P. Jarry, Test beam results on the performance of the cluster containment correction for the CMS electromagnetic calorimeter barrel, C M S note in preparation.

ELECTRON A N D P I O N RESULTS FROM T H E ATLAS FORWARD CALORIMETER 2003 T E S T B E A M

M. S C H R A M * Carleton

University 1125 Colonel By Drive, Ottawa, K1S 5B6, CANADA E-mail: mschram@physics. carleton. ca

The ATLAS Forward Calorimeter (FCal) modules have been exposed to beams of electrons, pions, and muons within the energy range of 5 GeV ^ E ^ 200 GeV at the CERN SPS, in Geneva, Switzerland. These modules are unique calorimeters with fine transverse segmentation allowing for a significant improvement in the hadronic resolution and reconstructed energy. Presented in this paper are leptonic performance results. Also included are the hadronic performance results using a transverse weighting technique.

1. ATLAS Forward Calorimeter Within the ATLAS experiment, the calorimeters will be used to reconstruct the electron, photon, jet, and missing transverse energy 1 . In particular, the ATLAS Forward Calorimeter (FCal) has been designed to provide complete forward coverage for jets in the pseudo-rapidity range of 3.2 < \rj\ < 4.9. Results of the 2003 test beam are presented in this paper. 2. The 2003 Test Beam Setup For the 2003 test beam, the full ATLAS FCal modules were placed in a cryostat in the H6 test beam area at CERN. The FCal modules consist of three longitudinal segments: one electromagnetic copper-liquid argon section and two hadronic tungsten-liquid argon sections. These modules have been exposed to beams of electrons, pions, and muons within the energy range of 5 GeV < E < 200 GeV delivered by the CERN SPS. The positioning of the modules was such that the beam impact point corresponded to T] « 3.8 in ATLAS. The 2003 beam line downstream of the last bending *on behalf of the ATLAS FCal group

401

402

magnet includes several scintillation counters and multi-wire proportional chambers followed by the cryostat containing the FCal modules, a warm tailcatcher, a concrete beam stop, and a muon scintillation counter. 3. Energy Reconstruction 3.1. Pulse

Shape

The pulse shape in the FCal modules is generated by a charged particle which ionizes the active medium (liquid argon). The initial pulse shape caused by the ionized liquid argon is triangular. After passing through the electronics, the signal peak is rounded, followed by a small depression. In the 2003 test beam, the pulse shape was sampled every 25 ns for a total of seven samples. The first sample was used to determine the pedestal (internal offset) and the signal peak was typically located in the fourth sample. To determine the current deposited in each channel the amplitude of the pulse was calculated using the optimal filtering (OF) technique 2 . 3.2. Optimal

Filtering

The pedestal-subtracted signal was processed by applying the OF method using five of the seven samples. The OF weights were determined using the physics pulse shape and the autocorrelation matrix. The physics pulse shape was derived from the physics events and the autocorrelation matrix was derived from the pedestal events. The OF weights were calculated with one nanosecond binning. 3.3.

Clustering

For electrons, a three-dimensional topological cluster algorithm 3 was used to determine which readout cell had sufficient energy to be included in the analysis. Three noise dependent parameters of the topological cluster algorithm were investigated in this study: (1) The seed threshold (crseed): clusters are seeded by cells with a signalto-noise ratio above the seed threshold. (2) The neighbor threshold (aneighbor)'- cells with an absolute value of their signal-to-noise ratio above the neighbor threshold are asked for neighboring cells. (3) The cell threshold (
403

For pions, no clustering was used, hence all readout channels were included in the analysis. 4. Test B e a m 2003 Results 4 . 1 . Energy

Resolution

The energy resolution of the FCal modules was determined by measuring the width of the reconstructed energy for particles of the same type and energy. 4.2. Electron

Results

For the electron analysis, the energy ranged from 5 GeV to 200 GeV. The energy resolution was parametrized using the following equation: a E

a y/E

b E

,. w

Here a is the sampling term, b is the noise in the electronics, and c is all remaining uncertainties of the detector. The results of the electron study are presented in Table 1. At high energy the weighted average of the three settings gives an energy resolution of (3.10±0.05)%. Table 1. Test beam 2003 ATLAS Forward Calorimeter electron energy resolution using different topological clustering settings. The cluster parameters are ®seed I & neighbor

I®cell

Cluster Parameters

Sampling [% • \/GeV]

4/3/2 4/3/1 4/2/0

4.3. Pion

Noise [% • GeV]

Constant [%]

29.594±0.741

91.84±2.47

3.07±0.07

28.066±0.825

106.95±2.31

3.12±0.07

30.407±0.917

115.33±2.68

3.13±0.08

Results

Due to the non-compensative nature of the FCal modules, two software techniques were investigated to minimize the difference between the detector response to electrons and hadrons. The flat weighting technique applies a single calibration term for each FCal module. The transverse technique 4 applies weights based on the radial distance from the cluster center. In

404

this study, the transverse technique has 16 calibration constants for each module, one for each radial centimeter spacing. For this analysis the pion energy ranged from 40 GeV to 200 GeV. The energy resolution was parametrized using the following equation:

E^VZ®0

(2)

Here the terms are the same as in Section 4.2, with the exception of the noise term which was accounted for before the fit. The results of this study are presented in Table 2 and show that the transverse weighting technique improves both the sampling and constant term. Moreover, at high energy the energy resolution is (2.74±0.34)% which is within the ATLAS design requirements. Table 2. Test beam 2003 ATLAS Forward Calorimeter pion noise subtracted energy resolution using different weighting methods. Sampling [% • VGeV]

Constant [%]

Flat Weights

96.81±8.52

5.24±1.25

Transverse Weights

90.44±0.59

2.74±0.34

In addition to the improvement in the energy resolution, the transverse weighting technique nearly achieves compensation with a e/7r-ratio ranging from 1.05 at 40 GeV to 0.98 at 200 GeV. 5. Outlook In the future, a full Monte-Carlo study will be performed to help understand properties such as the energy deposited in the upstream material and shower containment. In addition, a study on jet reconstruction using the transverse weighting technique will be performed. References 1. The ATLAS Collaboration, ATLAS Calorimeter: Performance Design Report, 1997, CERN-LHCC-96-40.

405 2. W. E. Cleland and E. G. Stern, Nucl. Instrum. Meth. A 338 467 (1994). 3. C. Cojocaru et al. "Hadronic calibration of the ATLAS liquid argon end-cap calorimeter in the pseudorapidity region 1.6 < \q\ < 1.8 in beam tests," Nucl. Instrum. Meth. A 531 (2004) 481 [arXiv:physics/0407009]. 4. A. Savine, Calorimttry in High Energy Physics 595 (2000).

LINEARITY, E N E R G Y A N D POSITION RESOLUTION OF T H E ATLAS ELECTROMAGNETIC CALORIMETER SERIES MODULES

C. SERFON* * Centre de Physique des Particules de Marseille 163 avenue de Luminy - case 902, 13288 MARSEILLE Cedex 09 FRANCE, E-mail: [email protected]

During the construction of the ATLAS Electromagnetic Calorimeter, several series modules were exposed to electron beams of energies between 10 and 245 GeV. Final results on the linearity and energy resolution are given, together with the precision obtained on position and angular measurements.

1. Introduction The ATLAS 1 experiment on the future LHC 2 (proton-proton collider) is dedicated to the search of the Higgs boson, the study of the Standard Model and the search of physics beyond the Standard Model. To reach these goals, high performances of the detector are needed. The Higgs boson decay H - • 77 (one of the most promising discovery channel for low mass Higgs) imposes some constraints on the energy and position resolution of the electromagnetic calorimeter 3 . Moreover, to extrapolate the absolute energy scale from Z° mass (used for calibration) to other energy domains, a good linearity (non-linearity less than 0.1%) is needed. 2. The ATLAS electromagnetic calorimeter The ATLAS electromagnetic calorimeter is a lead-liquid argon (LAr) sampling calorimeter with an accordeon geometry which allows a good hermiticity. It is divided in a barrel section (|^| <1.4) and in two end-caps *Now at department fur physik, LMU Munchen. Am Coulombwall 1, D-85748 Garching, Germany tQn behalf of ATLAS Liquid Argon group

406

407

(1.375< I77I < 3.2) and segmented longitudinally in three samplings (see figure 1). The sampling 1 (front) is finely segmented into the rj direction in strips which allow to perform precise position measurements (see section 3.2) and 7/71-° separation. The sampling 2 (middle) is the one where the electromagnetic shower deposit most of its energy. The sampling 3 (back) allows to recover the longitudinal leak of the most energetic electrons or photons. For \r)\ <1.8 a presampler is located in front of the calorimeter.

Towers in Sampling 3 A<|>xAr| = 0.0245)0.05

Figure 1. Cut view of a sector of the EM calorimeter. The typical granularity of the samplings is indicated.

3. Test-beam studies To study the performances of the calorimeter, 4 (resp. 3) modules of the barrel (resp. end cap) have been tested under beam. The studies performed with these data concern in particular linearity and energy resolution, angular and position resolution, and uniformity4. The two first analyses are detailed hereafter.

408

3.1. Linearity

and energy

resolution

The first kind of study made with test-beam data is the study of the linearity of the EM calorimeter 5 . This study relies on the Monte-Carlo simulation. It needs in particular a good description of the matter in front of the calorimeter, as well as the as a good knowledge of the calorimeter itself (dead material, inactive layers, e t c . ) . This simulation was tuned with the test-beam data, and all the energy distributions obtained are in good agreement between data and MonteCarlo. A calibration scheme has therefore been developped on the simulation and applied to the data leading to linear response within 0.1% in the energy range 15 < E < 180 GeV (see figure 2). 1.006 LU

1.004 -

E Hi

1.002 -

ate gearn energy uncertainty

uncorrellated beam energy uncertainty correllated ~"

0.9960.994 0.992 0.99

^ 50

100

150

200 EbeamtGeV]

Figure 2. Ratio of reconstructed electron energy to the beam energy as a function of the beam energy at the point (r/=0.687,0=0.282). The ratio is normalised to 1 at 100

GeV. The calibration is made according to equation (1) :

Erec =

a(E) + b{E)E™ + c(E)

EvisEms

+

d(E)f;samp

£

._ i=l,3

E

f°

•fcell impact(A^) • (1 + /jea*a,e((0))(l)

where a, b, c are parameters extracted from MC to take into account energy loss in front of the presampler, and between the presampler and the calorimeter, d is used to correct for variation of sampling fraction. The sampling term and the local constant term of energy resolution obtained after this procedure are respectively !0%y/GeV and 0.20% (see

409

figure 3) and reach the ATLAS requirements.

0.05 "^0.045 m

0.04 0.035-e 0.03 0.025

Sampl. Linear term [%/^(GeV)J [%] 10.0±0.1 0.20±0.04 ~

Data Data noise subtracted Noise

0.02 0.015 0.01 0.005 0

100

200 Ebean, [ G e V ]

Figure 3.

Energy resolution as a fuction of the energy.

It has to be underlined that the linearity study has been made in only one i] position in a setup different of the ATLAS one (no magnetic field, material in front of the calorimeter different). Study to transport this calibration scheme to ATLAS is being carried on.

3.2. Position

and angular

resolution

The position and angular resolution have been studied in the barrel as well as in the end cap 6 . The position measurement in the front and middle section is made using energy-weighted bary center. Because of the finite size of the cells leading to the so-called S-shape effect (bias in the reconstructed position), corrections have been applied. The position resolution in the front (resp. middle) along the r\ direction is 240 /im (resp. 540 Mm)- By combining the position of the particle reconstructed in these two samplings, the polar angle of the incoming particle can be reconstructed. The corresponding angular resolution is plotted as a function of the energy on figure 4. An average resolution in the range 50-60 mx&d/y/E(GeV) is obtained on the whole calorimeter, in agreement with the design expectations.

410

60

+

u755 •

•

^

t

•

t

t

1

+

250 E, t?45

•

40

; b

25 50 75 100125150175200225250 Energy (GeV)

20

40

60 80 100 120 140 160 Energy (GeV)

Figure 4. Variation of polar angle resolution with energy for barrel at 77=0.69, <j>=0.28 (a) and end-cap at TJ = 1.74, 0=0.18 (b). 4.

Conclusion

T h e tests beam performed on the series modules of the ATLAS E M calorimeter have shown t h a t the performances required (linearity, energy and position resolution) by the benchmark physics channels (H —)• 7 7 , H —> 4e, and W —> ev especially) could be obtained on series modules. These studies have now to be carried out in simulation with the real ATLAS design, quite different from the test-beam setup.

Acknowledgements I want to t h a n k my colleagues of the ATLAS liquid argon group for providing me results and figures.

References 1. The ATLAS Technical Proposal, CERN/LHCC/94-43 (1994). 2. The Large Hadron Collider, CERN/AC/95-05 (1995). 3. ATLAS Calorimeter Performance, Technical Design Report, CERN/LHCC/96-40 (1996). 4. C. Oliver, Uniformity of Response of the ATLAS EM Calorimeter Series Modules, these proceedings. 5. J. Colas et al., Energy Linearity and Resolution Performance of the ATLAS Electromagnetic Barrel Calorimeter in the CERN Electron Test-Beam, to be submitted to Nucl. lustrum. Meth. A. 6. J. Colas et al, Position resolution and particle identification with the ATLAS EM calorimeter, Nucl. lustrum. Meth. A 550, 96 (2005).

PRESHOWER SILICON STRIP DETECTORS FOR THE CMS EXPERIMENT AT LHC ANITA TOPKAR, PRAVEENKUMAR S, BHARTI AGARWAL & S K KATARIA Bhabha Atomic Research Centre, Trombay Mumbai, 400 080, India Y P PRABHAKAR RAO, N SHANKAR NARAYANA & NIKHIL SHARMA Bharat Electronics Limited, Jalahalli Bangalore, 560 013, India, A specific research and development program has been carried out in India to develop the technology for 32-strip silicon strip detectors for application as a preshower detector for CMS experiment at LHC, CERN. The production of strip detectors is already underway to deliver 1000 detector modules and 80% production is completed. In this paper, we summarize this research & development work carried out to develop the detector fabrication technology. The results of various tests carried out to evaluate the performance of detectors have been presented. The quality control procedures used for detector qualification during the production have been described.

1. Introduction The preshower silicon detectors will be used in the CMS experiment for the n°/y rejection. The detector has a geometry of 63mm x 63mm and it incorporates 32 P+strips with width of 1.78 mm with a pitch of 1.9 mm [1]. The fabrication technology to produce such silicon strip detectors with very good uniformity over a large area of ~40mm2, low leakage currents of the order of 10nA/cm2 per strip and high breakdown voltage of > 500V has been developed in India. The strip detectors have been characterized using IV and CV measurements to obtain the leakage currents, breakdown voltages and full depletion voltage. The performance of the detectors has been evaluated after exposure to fast neutrons and protons to ensure their radiation hardness for operation in the harsh radiation environment of LHC. A strict quality control procedure is being used for qualification of detectors during the production. In the subsequent sections, we discuss the fabrication technology development, detector characterization and present the performance of the strip detectors.

411

412

2.

Detector Design & Fabrication Process Development

In view of the expected radiation damage during ten years of operation in the LHC environment, the electrical specifications of the detectors are quite stringent i.e. low leakage currents and high breakdown voltage. Therefore, the strip detector mask has been specially designed to incorporate features such as strips with rounded corners and floating field P+ guard-rings [2]. The floating field guard rings reduce the surface electric fields and hence increase the breakdown voltages of the strips. Figure 1 shows the schematic cross section of the detector. windows In the passivation

Scribe-line

"^j

1000

Passivation

^

Figure 1 Schematic cross section of the strip detector (All dimensions are in microns)

The detector has been fabricated using a four layer mask process. The process for the detectors has been developed by Bhabha Atomic Research Centre (BARC) in a close collaboration with the foundry, Bharat Electronics Limited (BEL) [2-3]. The 32-strip silicon detectors have been fabricated using 4", retype, <111>, 3-5 kQ-cm, 300 urn thick silicon wafers. The front P + strips and the back N+ contact have been obtained by ion implantation The silicon surface of the strip detectors is passivated with a thick silicon dioxide layer. The PSG layer is used as the top passivation layer over the front metal. This layer is resistant to contaminants which could diffuse and degrade the detector performance with time. The bad performance of a strip i.e. high leakage, low breakdown voltage is known to be due to pin holes in the implant & bad oxide quality. The increase of current around the full depletion voltage is attributed to the injection from the back plane [4]. The desired specifications of high breakdown voltage and low leakage currents have been achieved by strict contamination control, incorporating implant damage at the back side for gettering, having an uniform and thick >T layer at the back for suppressing back injection. Critical process parameters such as temperature, time, ambient for processes such as oxidation,

413 implantation anneal and drive-in, etc have been optimized. Generation of defects in the bulk of the silicon and the quality of the oxide which passivates the silicon were of more concern in order to reduce leakage currents and improve breakdown voltage. The fabrication process has been optimized in several batches to tune the process parameters so as to achieve the desired specifications. 3.

Performance of Strip Detectors

The reverse IV and CV characteristics of the 32 strips of the detectors have been measured simultaneously using PC based automated measurement setups. The static characteristics of the detectors are as shown in Figure 2 and Figure 3. The characteristics show that the strip leakage currents are of the order of 5-10 nA at high voltages of the order of 500V. The CV curves are uniform for all the strips and the full depletion voltages are about 110V. The radiation hardness of the detectors have been tested by irradiating them to neutrons in a reactor (at BARC and at Dubna, Russia) or using a 24 GeV proton beam (at CERN). Figure 4 shows the strip currents measured at -18 °C and at a voltage of 300V after irradiation to a total neutron fluence of 1.36x 1014n/cm2. The detectors have been found to meet the specifications and show a stable behavior after the irradiation. 2.0E-10 T

1.5E-10

1.0E-10

5.0E-11

0.0E-KJ0 200

400

50

Bias Voltage (Volts)

Figure 2 TV Characteristics of all 32 strips of the strip detector

4.

100

150

Bias voltage (Volts)

Figure 3 CV characteristics of all 32 strips of the strip detector

Production of Strip Detectors and Quality Control

Detectors with very good uniformity and at a production yield of 40%-50% are being produced at BEL and 80% of the production has been completed. During production, the quality control is being carried out as per the qualification procedures specified by CERN - IV & CV measurements, geometrical tolerance

414

of 100 microns over detector length and width and thickness of the detector (within 300um-340um). Various parameters such as i. Leakage current of strips at full depletion voltage and at 300V, ii. Total leakage current at full depletion voltage at 300V and at 500V, iii. Breakdown voltage of strips and iv. Full depletion voltage are extracted from the static measurements and are used to qualify the detectors. All the tests are being carried out in a class 10,000 environment with controlled humidity and temperature. Figure 5 and Figure 6 show the statistical distribution of breakdown voltage and total leakage current at 300V for about 800 sensors. As can be seen, the breakdown voltage exceeds 500V for majority of the detectors. The total leakage current of the detector is less than 1.0 uA for most of the detectors though this tolerance is lO.OuA 800i

8.0E-06

600

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I

400

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Figure 4 Leakage currents of the 32 strips after irradiation to neutrons

5.

350

425

500

7 10 13 16 19 22 25 28 31 Breakdown Voltage (V)

Figure 5 Distribution of the breakdown voltags for the detectors

Assembly of Detectors

The silicon strip detectors are assembled in the form of detector modules so as to integrate the front end electronics hybrid to the detector. The assembly of detectors involves very precise mechanical alignment of the detectors to the ceramic tile which carries the hybrid. The detector mounted in this form is finally glued to the aluminum tile which subsequently will be used to make ladders. The quality control procedure for the detector modules involves monitoring the degradation of the detector leakage current due to assembly and the mechanical damage such as chipping at the edges. Prototype modules have been made at India and the detector performance has been observed to be stable during assembly (Figure 7). Full scale production of micro modules has been initiated for delivering 1000 micro modules for the preshower detector of the CMS.

415 6.

Summary

We have developed the demanding technology for fabrication of strip detectors 250

1.0E-07 o initial D ceramic A A! tile

9

wjmmujmmimutu 0.1

2.1

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6.1

8.1

Total Current at 300 V (M icroamps)

Figure 6 Distribution of the total current for the detectors

0

5

10

15

20

25

30

35

Strip Number

Figure 7 Leakage current of 32 strips of the 300V detector at various stages during assembly

with good uniformity, low leakage currents and high breakdown voltage. The detectors also meet the radiation hardness requirements for operation at LHC. The detector fabrication process has been briefly described. Subsequent to the development of technology, the full scale production of the detectors has been started in India. More than 800 detectors have been produced so far and the detector production will be completed by the end of 2005. Acknowledgement We acknowledge Dr Anna Peisert, CERN EP Division for her valuable suggestions during this development work and for evaluating the radiation hardness of detectors. References 1. Ph Bloch, Y H Chang, et al, Nucl. Instr. and Meth., A479, 265 (2002) 2. S K Kataria, Anita Topkar, et al , Proceedings of the DAE-BRNS Symposium on Intelligent Nuclear Instrumentation, 8 (2001) 3. Anita Topkar, Proceedings of the DAE-BRNS symposium on compact nuclear instruments and radiation detectors, 441 (2005) 4. Ph Bloch, A Cheremukhin, et al, IEEE Trans. Nucl. Sci. NS-49, 321 (2001)

TEST B E A M RESULTS OF THE CMS FORWARD QUARTZ FIBRE CALORIMETER

A. ULYANOV* Institute

for Theoretical and Experimental B. Cheremushkinskaya 25, 117218 Moscow, Russia E-mail: [email protected]

Physics,

The forward calorimeter extends the pseudorapidity coverage of the CMS calorimeter system from 77=3 to 77=5.2. It is composed of quartz fibres embedded into a steel absorber and measures Cherenkov light radiated by shower particles. Six production modules of the forward calorimeter have been tested and calibrated using particle beams at CERN. An outline of the calibration procedure and results on the energy resolution, response uniformity and linearity for electrons and pions are presented in this paper.

1. I n t r o d u c t i o n The forward calorimeter (HF) in the CMS experiment extends the pseudorapidity coverage of the calorimeter system from 77=3 to 77=5.2 which improves jet detection and the missing transverse energy resolution. 1 ' 2 HF calorimeter is especially important for identification of forward tagging jets associated with the Higgs boson production via vector boson fusion. The CMS forward calorimeter is based on detection of Cherenkov light radiated by shower particles in optical quartz fibres embedded into a steel absorber. The choice of quartz fibres as the active medium was dictated by the extremely hostile radiation environment in the region of high rapidities: at the nominal LHC luminosity the calorimeter will be absorbing up to 100 MRad/year. Several calorimeter prototypes were built in the past and successfully tested with high energy particle beams. 3 ' 4,5 ' 6 ' 7 In this paper are reported recent test beam results obtained with final production modules.

* On behalf of the CMS Collaboration

416

417 2. Design of the Forward Calorimeter The forward calorimeter is located at 11.15 m on either side from the CMS interaction point. It is a cylindrical structure with a hole for the beam pipe. This structure is physically composed of 18 wedges. A wedge (schematically shown in Figure 1) is made of 5 mm thick steel plates with grooves separated by 5 mm. These grooves are parallel to the beam line and are filled with 600 (im core-diameter quartz fibres (fused-silica core and polymer hardclad). There are two types of fibres which alternate in grooves: long fibres run through the full length of the absorber (165 cm), and short fibres start 22 cm deep from the front face of the calorimeter and thus are sensitive mostly to hadronic showers.

E.

£ E

I

|

473 mm

1

I

Figure 1. Transverse view of a wedge. The readout segmentation is shown by dashed lines. Open and filled circles represent long and short fibres.

The fibres are bundled to form 0.175 x 0.175 (A77 x A) towers and connected to photomultipliers (Hamamatsu R7525) by means of 40 cm long air-core lightguides with highly reflective walls. Long and short fibres are bundled and read out separately. 3. Test B e a m S e t u p Six assembled wedges of the forward calorimeter were extensively tested at the H2 beam line of the Super Proton Synchrotron at CERN. The wedges were placed one by one on a table which could move horizontally and vertically across the beam. The results presented here were obtained with the

418 wedges positioned at an angle of two degrees with respect to the beam that corresponded to rj « 4. The trigger was provided by a set of scintillator counters in front of the calorimeter with an overlapping area of 5 cm x 5 cm. A muon veto counter was installed behind an iron block downstream the calorimeter. Five multiwire proportional chambers upstream the detector were used for accurate tracking of incoming particles. 4. Results 4.1. Energy

Calibration

Hadronic showers are subject to large transverse leakage effects in a single wedge, especially close to the beam pipe. That is why electrons were used to calibrate the six wedges in question. The entire front face of each module was scanned with a 100 GeV electron beam. For all electrons hitting tower i, we calculated Aij as the average signal in the long segment of tower j . The full wedge response was then required to be equal to the beam energy: ^AijWj

= 100GeV,

where Wj is the unknown calibration coefficient for tower i. Solving these 24 linear equations (there are 24 towers per wedge) we determined the calibration coefficients for all towers. With equal PMT gains the signal produced by 100 GeV electrons in the short segment was typically 30% of the signal obsereved in the long segment. Therefore the same calibration procedure was applied to the short segment but the full response was normalized to 30 GeV. Events with the particle impact points within 2 cm range from the module edges were excluded from the calibration procedure to reduce the leakage effect. The beam to radioactive source response ratio was measured that will allow radioactive sources to be used for the calibration of the rest of the calorimeter. 4.2. Response

Linearity

and Energy

Resolution

Calorimeter response to electrons and pions measured at different energies is shown in Figure 2. The long segment response to electrons is linear within 2%. The short segment response is non-linear indicating deeper penetration of electromagnetic showers into the calorimeter at higher particle energies. The quartz fibre calorimeter is sensitive mostly to the electromagnetic component of hadronic showers. As a result, the response to pions is lower than

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the response to electrons. The 7r/e ratio increases at higher particle energies due to growing fraction of neutral pions in hadronic showers. Response difference between different particles contributes to the jet energy resolution and can be reduced by using a sum of long and short segment signals as illustrated on the right plot in Figure 2. Optimization of additional weights for long and short fibre readouts is a subject of Monte-Carlo simulations. The electron energy resolution (Figure 3) is dominated by photoelectron statistical fluctuations. The stochastic term is in a good agreement with the light yield of 0.28 pe/GeV and the 40% fractional width of a single photoelectron peak. The constant term in the electron energy resolution is

420

dominated by structural non-uniformities: fiber-to-fiber distance is comparable to the transverse size of electromagnetic showers. The electron energy resolution is slightly improved when responses from the two segments are added together, mainly because the light yield is increased. The pion energy resolution is dominated by fluctuations of the fraction of neutral pions in a hadronic shower. However, in the 30-300 GeV range the resolution can be parametrized as a sum of stochastic and constant terms as shown in Figure 3 (right plot). 4.3. Response

Uniformity

In order to check the uniformity of the calorimeter, the wedges were scanned with 100 GeV electrons and pions across their front face. Apart from response oscillations with a step corresponding to the fibre-to-fibre to distance, these scans revealed response non-uniformities of a larger dimensional scale which were related to imperfections of the detector construction. In order to quantify the effect of these non-uniformities on the energy resolution, the wedge surface was divided into 2 cm x 2 cm cells and the average response was calculated in each cell. The cell-to-cell response fluctuations were about 5% for electrons and about 3% for pions. These are small contributions to the single particle energy resolutions. The effect on the jet energy resolution is even less important because the jet size is essentially larger than 2 cm. 5. Conclusions Six (out of 36) calorimeter modules were extensively tested and calibrated with high energy particle beams. These tests revealed no particular problem in the detector construction and assembly. Performance results were in agreement with the tests of prototype calorimeters. A radioactive source will be used to transfer the energy calibration to the rest of the calorimeter. References 1. 2. 3. 4. 5. 6. 7.

CMS Collaboration, Technical Proposal, CERN/LHCC 94-39 (1994). CMS Collaboration, Heal TDR, CERN/LHCC 97-31 (1997). N.Akchurin et al., Nucl. Instr. and Meth. A379, 526 (1996). N.Akchurin et al., Nucl. Instr. and Meth. A399, 202 (1997). N.Akchurin et al., Nucl. Instr. and Meth. A400, 267 (1997). N.Akchurin et al., Nucl. Instr. and Meth. A408, 380 (1998). N.Akchurin et al., Nucl. Instr. and Meth. A409, 593 (1998).

INTER-CALIBRATION A N D OFFLINE C O M P E N S A T I O N STUDIES OF T H E ATLAS CALORIMETER U S I N G B E A M TESTS A N D M O N T E CARLO

K.-C. VOSS* University of Victoria CERN PH, 1211 Geneve 23 Switzerland E-mail: [email protected]

The inter-calibration of the different ATLAS calorimeters has been studied using beam tests at the CERN SPS accelerator and also with Monte-Carlo techniques. Particularly important in these studies has been the development of techniques allowing the offline compensation of the hadronic response of the ATLAS calorimeter system. Results on calorimeter inter-calibration, offline hadronic compensation, and comparisons of data with Geant 4 simulation, are reported from beam tests in the ATLAS barrel, end-cap and forward calorimeter regions performed from 2002-2004. Results on ongoing studies extrapolating these techniques to the full ATLAS calorimeter are also discussed.

1. Introduction 1.1. The ATLAS Calorimeters

Hadronic

and

Electromagnetic

The ATLAS calorimeters are designed to provide an accurate measurement of the energy and position of electrons, photons and jets in the pseudo rapidity range \r]\ < 5. This is achieved by several sub detectors (EMEC, EMBARREL, HEC, FCAL, TILE) [1] [2] which use different techniques for the shower detection. For all calorimeter components the response to hadrons differs significantly from that for electrons of the same energy. Therefore for hadrons a correction has to be applied to the signals obtained on the corresponding electromagnetic scale.

*on behalf of the atlas liquid argon collaboration

421

422

1.2. Electromagnetic

and hadronic

showers

Electromagnetic (EM) showers are purely based on electromagnetic processes. Due to the short distance between the individual EM processes the showers are very compact and have high energy densities. Showers from electromagnetic particles are known to a high precision and therefore can be simulated very well. A hadronic shower on the other hand consists of EM and hadronic energy. It is usually much larger than an electromagnetic shower and has large fluctuations. The hadrons in the shower can react with the nuclei of the calorimeter. Energy deposited in nuclei excitation is not detected and hence is regarded as detector invisible energy. Also particles like neutrinos which escape the detector are produced for example in pion decays. These are effects which the calibration has to account for.

2. Hadronic calibration cell weighting The different response of the calorimeter to electrons and hadrons can be corrected using so-called software compensation. The goal is to get an equal response for the electromagnetic as well as purely hadronic component of a hadronic shower. Thus fluctuations in the hadronic shower on the energy reconstructed can be minimized and the energy resolution can be substantially improved. The technique of signal weighting has been successfully used in previous experiments (see [4]). The deposited energies in individual read-out cells of a segmented calorimeter can be identified as of electromagnetic or hadronic origin on a statistical basis. The goal is to reconstruct calorimeter final state objects (clusters) first and calibrate those using a "local" normalization based on locally deposited energies in the calorimeter. For ATLAS these weighting constants have to be derived from MonteCarlo (MC) simulations, and the procedure is tested proving that beam test data are well described. The essential Monte-Carlo information in this tuning process is the knowledge of all the shower components as the visible energy and the total energy in each read-out cell. Thus the weights could be directly derived from the MC simulation and applied to beam test or Monte-Carlo data. 3. 2002 Liquid Argon End-cap Beam Test setup The beam tests were carried out in the H6 beam at the CERN SPS. This beam provides pions, electrons or muons in the energy range 6 < E <

423 200GeV. Positioned in the liquid argon (LAr) cryostat were one EMEC module, three HEC1 modules and two HEC2 rear modules. Cerenkov counters were used for the trigger up to 80 GeV. The impact position and angle of particles have been determined from four multi wire proportional chambers (MWPC) with vertical and horizontal planes. A veto wall rejected beam halo particles and muons. 3.1. Energy reconstruction

- cluster

algorithm

A cluster algorithm has been developed to reconstruct the energy of the impact particle. In each layer the topological neighbours of a given cell are considered: they have to share at least one common corner. A seed is chosen if the energy is significantly above noise: Eaeed > 4crnoise. The general cut-off at the cell level is Eceu > 2crnoise, the cluster is expanded as long as one neighbour has an energy of Eneighbour > 3<jnoise. In [5] and [6] it was already presented that it is possible to extract the cell weights as a function of the energy density using the beam test data alone. In these publications it is also shown that the Monte-Carlo simulation and the real data agree very well. In addition to this data driven method the cell weights for the local hadronic calibration have been extracted using the Monte-Carlo simulation. In the simulation of single pions the amount of EM, non-EM, visible and escaped energy per read-out cell as it is calculated by GEANT4 [3] is stored in so-called calibration hits and can be used for analyses. With this knowledge the weight per cell can be calculated e.g. as a function of the cell energy density. The application of these weights give a better energy resolution of the reconstructed pions and electrons and is compatible with the calibration scheme based purely on the beam test data. 4. First implementation of local hadronic calibration scheme A first version of the local hadronic calibration scheme was implemented in ATLAS. First the clusters in the calorimeter are classified as hadronic or electromagnetic. In case they are tagged as hadronic, cell weights are applied to each cell in the cluster in order to correct the energy scale. The weights and criteria for the classification were extracted from 400k simulated single pion events in the full ATLAS simulation. They were produced in batches of 40k for 1,3,5,10,20,50,100,200,500,1000 GeV keeping all the information of the true energy deposits in the calorimeter cells stored

424 in t h e calibration hits. Afterwards t h e cluster algorithm was carried out and different cluster moments were calculated. For the cluster classification look-up tables in 120 bins of energy and pseudo-rapidity were created. In every table t h e t r u e electromagnetic energy (EM) fraction is stored as a function of the average cell energy density < Pceii > and t h e depth of the cluster center Xcenter in the calorimeter. At t h e classification step for each reconstructed cluster the corresponding electromagnetic energy fraction is determined by extracting t h e value using the two moments < pCeii > and Xcenter and the look-up tables. If the E M fraction is below 85%, t h e cluster is classified as hadronic. T h e weights were calculated using t h e calibration hits. In the simulation the weights for each cell in each event can be calculated using the formula: T?l

—

,., . p w

„

-arhprp

II!

—

/pem i p n o n - e m v i s , r^non — eminvia ^l-Ar+Ab^^LAr+Ab. -re
• rpescaped \ +aLAT+Ab,<

"ceil ~ Menu wnere w — _M„_emt,„ m T h e weights were calculated from the simulation as function of the cluster energy and the cell energy density and are also stored in tables. All cells in hadronic tagged clusters are t h e n weighted with t h e weights which correspond t o the cluster energy and the individual cell energy density. Applying the described local hadronic calibration scheme consisting of the cluster classification and cell weighting to a pion test sample results in a significantly (about 10%) improved energy reconstruction. At the same time the the energy resolution is only slightly improved. Analysing the t r u t h information in t h e simulation reveals t h a t an improved weighting scheme should also be able t o improve the resolution. References 1. The ATLAS Collaboration, ATLAS liquid argon calorimeter: Technical design r e p o r t C E R N - L H C C - 9 6 - 4 1 , 1996. 2. The ATLAS Collaboration, ATLAS calorimeter: Performance design report, C E R N - L H C C - 9 6 - 4 0 (1997). 3. S. Agostinelli et al. GEANT4 Collab., Nucl. Instrum. M e t h . A 506 , (2003) 4. B. Andrieu et al. (HI Calorimeter Collab.),Nucl. Instrum. M e t h . A 336,1993 5. C. Cojocaru et al. [ATLAS Liquid Argon EMEC/HEC Collaboration], Hadronic calibration of the ATLAS liquid argon end-cap calorimeter in the pseudorapidity region 1.6 j \eta\ j 1.8 in beam tests, Nucl. Instrum. Meth. A 531 (2004) 481. 6. S. Menke [ATLAS Liquid Argon Collaboration], Energy-calibration of the ATLAS hadronic and electromagnetic liquid-argon endcap calorimeters, arXiv:physics/0310127.

GEANT4 and Software Applications Organizers: S. Giani and P. Binko

S. Giani M. Asai K. Amako M.J. Boschini A. Brahme M. Ellis F. Gargano P. Goncalves V.M. Grichine V. Ivantchenko

0 . Okudaira

I. Papadopoulos D. Rebuzzi A. Salzburger G. Santin J.P. Wellish

Symposium on the Applications of the GEANT4 Simulation Software GEANT4 Applications for the NASA Space Missions GEANT4 Application to Hadrontherapy in Japan AMS02 Italian Data Transfer System: Real Life Experience Application of GEANT4 in the Development of New Radiation Therapy Treatment Methods G4 Accelerator Applications A Full Monte Carlo Simulation of Silicon Strip Detectors Simulation of Radiation Monitors for Future Space Missions Electromagnetic Physics Modeling in GEANT4 Package Perspectives in Medical Applications of Monte Carlo Simulation Software for Clinical Practice in Radiotherapy Treatments GEANT4 Monte Carlo Simulation to estimate Gamma-Ray Emissions from Lunar Surface and SELENE Spacecraft CORAL, a Software System for Vendor-Neutral Access to Relational Databases GEANT4 Simulation of the ATLAS Muon Spectrometer Track Extrapolation with Intrinsic Navigation in the New ATLAS Tracking Scheme Applications of GEANT4 for the ESA Space Programme Detector Simulation in High Energy Physics 425

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SYMPOSIUM ON THE APPLICATIONS OF THE GEANT4 SIMULATION SOFTWARE

SIMONE GIANI PH, CERN Geneva 23, CH-1211, Switzerland The following executive summary includes an introduction to the Symposium on the GEANT4 applications, as well as the key elements of its program, and the related overall conclusions.

1. Executive Summary GEANT4 was born as an R&D project to develop object-oriented engineered simulation software. Laboratories and institutes such as CERN, KEK, SLAC, TRIUMF, ESA were amongst the main founders of the project, which was managed by the author from conception to the release of the first production version (1994-1998). A MoU then formalized the creation of an international collaboration (including also members such as INFN-Italy, IN2P3-France, JeffersonLab-Usa, Karolinska-Sweden, LPI of Russian Academy of Science, Tera foundation, W.Goethe Inst.-Germany, Budker Inst.-Russia, Pittsburg Univ.-Usa), which has continuously evolved and supported the software to date. GEANT4 simulates the propagation and interactions of elementary particles in matter and vacuum within an energy range varying from the eV (down to thermal and cold energies for neutrons) to the TeV (up to PeV for muons). The research phase granted to GEANT4 the possibility to study and develop innovative features: particle-production cuts in range, processes' final state computation in space and time per simulation step (e.g. with applications in gas detectors and optics), multiple models per physics process, tracking of particles until zero-range (with extended validity range for energy losses and multiple scattering), automatic secondaries creation below production-threshold near geometric boundaries (and handling of their thickness), integration of equation of motion in arbitrary fields, 3D voxelization algorithm for optimized navigation in geometry databases, parameterized volumes by material, solid models moving 427

428

in time, numeric precision tolerances, system of units and physics constants, user actions interface (e.g. including triple-stacks mechanism and biasing techniques). Most of these features have made the tool applicable and exploitable for a wide variety of advanced technological and scientific fields: radiation damage in space, astronauts safety, dosimetry, radiotherapy, detector simulation, design of imaging devices, accelerators, beam lines, radiation and neutron background, astrophysics. The value of scientific software is not evaluated on the metric of the number of customers, but rather on the relevance and impact of its applications. The Symposium focuses on multidisciplinary applications of GEANT4, in the domains of: • • • • •

INDUSTRY - including at General Electrics and Siemens in the field of medical instrumentation. SPACE - including for missions of the ESA space program, of NASA, and the International Space Station. MEDICINE - including radio/brachi/hadro-therapy, at Karolinska Inst., IST-Italy, LIP-Portugal, CHU Quebec-Canada, and other medical centers. TECHNOLOGY - including PET research by the international OpenGate collaboration and ion-therapy developments at KEK-CHIBA (Japan) PHYSICS - including detectors (LHC experiments, BaBar, AMS) and accelerators (ILC, CLIC, MICE, T2K).

The relative weight of the different applications indicates that the potential of the GEANT4 software is exploited for science beyond the physics domain.

GEANT4 APPLICATIONS FOR NASA SPACE MISSIONS MAKOTO ASAI Scientific Computing and Computing Services, Stanford Linear Accelerator Center, 2575 Sand Hill Road, Menlo Park, CA 94025, USA

ROBERT A. REED, ROBERT A. WELLER School of Engineering, Vanderbilt University VUStation B 351553, Nashville, TN37235, USA Geant4 is nowadays widely adopted as the simulation engine for NASA space missions. We will review three major application areas of Geant4: apparatus simulation, including pre-launch design and post-launch analysis; planetary scale simulation, including radiation spectra; micro-dosimetry simulation, including single-event effects on electronics components.

1. Apparatus Simulation 1.1. The GLAST mission GLAST1* will measure the direction, energy and arrival time of celestial gamma rays (launch foreseen for 2007). The LAT (Large Area Telescope) instrument measures gamma rays in the energy range from about 20 MeV to greater than 300 GeV (there are no other detectors in operation covering this range). In addition, the GBM instrument provides correlative observations of transient events in the energy range between 20 KeV and 20 MeV. GEANT4 was adopted since 1999 for the test beam simulations and the balloon flights2'3'. The geometry of the system was described and managed via a dedicated repository. The GEANT4 simulation was then integrated in the GAUDI framework to manage the event loop, source and hits generation, digitization and reconstruction. At present, massive Monte Carlo productions have been run. 1.2. Cassini-Huygens mission to Saturn and Titan The low-energy magnetospheric measurement system (LEMMS) has been simulated using GEANT44). All the sensitive elements of the LEMMS sensor contained in the aluminum and platinum inner and outer cylinders, as well as the 429

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full collimator geometry, have been reproduced and simulated. Simulations of 2 MeV electron pencil beam, 50 MeV/n Oxygen pencil beam, and 2 MeV electrons circular plate distributions have been validated in comparison to calibration data taken on 22-micron foils. Channel efficiency and pass band determination have also been simulated and reproduced. 1.3. ACE/EPAM solid state detector system The NASA Advanced Composition Explorer EPAM detector is simulated with GEANT4 to study the effect of solar energetic electrons scattering5'. Single detector configurations have been simulated using a 91 KeV mono-energetic electron beam onto a 200-micron silicon detector (preceded by !4 micron dead layer). The double detector configuration has been simulated using an incident beam of 126 KeV. The study of the energy deposited in the SSD has shown that a percentage of the incident electrons will trigger channels lower than the incident energy because some electrons scatter out of the SSD. 1.4. Messenger mission to Mercury The Messenger Energetic Particle Spectrometer engineering specifications were reproduced via GEANT4 geometric modeling, including the EPS collimator, the MCP high voltage cup, the SSD mounting bracket, the PCB board, the SSD array, the large and the small pixels, and the Al flashing. In particular, the four rings of the EPS collimator have been reproduced including all holes, as well as the MCP screws and bolts. Placing the EPS model inside a spherical shell has simulated an isotropic environment: such simulation was produced generating 10 millions of 200 KeV electrons. The gamma ray spectra at the MCP from energetic electrons have thus been calculated via the GEANT4 simulation described above. GEANT4 simulations under development include the computation of background rates due to solar energetic particle events, the determination of the EPS geometrical factor, the examination of the channel pass-bands and of the SSD rates due to solar X-rays, and the study of the efficiency of the foil flashing to discriminate electrons for low energy ions6). 1.5. Combined ion and neutron spectrometer for space applications The National Space Biomedical Research Institute CINS (Combined Ion and Neutron Spectrometer) instrument is designed and developed using GEANT4 simulations7'8'9'. The first simulations for the ion spectrometer have allowed computing the energy deposition in the last silicon SSD in function of the

431 energy deposition in the thick BGO detector. Results show that the vast majority of the protons can be separated from the electrons, and that with the BGO protons up to about 300 MeV can be uniquely identified; results also show that a proton depositing 80 MeV in the BGO yields primary and penetrating depositions of 1 MeV resolution in the fourth SSD. The initial simulations for the energetic neutron detector were compared to balloon flight observations. Results show that balloon flight data match GEANT4 simulations of incident neutron of E"3 spectra. Significant simulations are under way to test the instrument concept, the rejection efficiency requirements and the expected rates. 2. Planetary Scale Simulation 2.1. Validation We recall that the near-Earth regions are usually subdivided in three zones, defined as LEO (Low Earth Orbit, at about 400 Km X 51.6 deg), MEO (Medium Earth Orbit, at about 20200 Km X 55deg), and GEO (Geo-stationary orbit, at about 36000 Km X Odeg). LEO is the typical orbit of the International Space Station, MEO is the typical orbit of GPS satellites, and GEO orbits are of course also of wide interest for space missions. Thus LEO electron and proton spectra are particularly relevant for the ISS. At NASA they are based on the AE-8MAX electron integral and differential spectra, and on the AE-8MESI proton integral and differential spectra10). AE8 and AP8 models of Earth radiation belts have known limitations11,12,13^ (limited range of observations from the '60s, simplified account of 11-years solar cycle), thus making even more crucial a proper modeling of the radiation environment via simulations. A validation of GEANT4 dose rates has been conducted at Boeing. At GEO orbits, GEANT4 simulation results for electron dose rates have been validated for different Aluminum thicknesses, ranging from 0.01 to 10.0 g/cm2. At LEO orbits, GEANT4 simulation results for 200 MeV incident protons, as well as for 1.0 MeV incident electrons, have been validated onto equivalent Aluminum thicknesses14). 2.2. Modelling of the LEO radiation environment Theoretical modelings of the inner zone using GEANT4 simulations as inputs were conducted at The Aerospace Corporation and at CalTech15\ As sources, the Cosmic Ray Albedo Neutron Decay, and the cosmic ray protons modulated by solar activity (F10.7), were taken into account16,17'18*. The Geomagnetic access to the atmosphere was modeled using vertical rigidity cutoffs plus

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transmission function to account for off-vertical variation. Mono-energetic protons have been simulated incident isotropically at an altitude of 200 Km. The atmosphere was modeled with average of NRLMSISE-00 N. O. and Ar densities over latitude, longitude, local time, and season, with typical geomagnetic conditions: F10.7 = 150, Ap = 419). GEANT4 physics list LHEP_PRECO_HP was selected. Neutrons exiting top of simulation were tabulated. Neutron production was convolved with incident modulated/cutoff proton spectrum. Results were shown at 41 deg geomagnetic latitude (4.83 GV), obtaining a vertical neutron flux (cm2 s MeV)"1 in agreement with observations20'2^, and especially good at the crucial high energies from 100 MeV to 8000 MeV. Future improvements foreseen for the LEO modeling include the addition of neutrons from cosmic ray He to the CRAND source, and the light-ions secondaries (deuterium, tritium, alphas, He3) that have been observed with AP8 inputs: both this improvements will be based on GEANT4 simulation. 3. Micro-dosimetry Simulation 3.1. Physics case for the RADSAFE concept Any CCD image of the sun (SOHO observatory, LASCO experiment) reveals the great activity of particles that can reach electronics components. From radio and microwaves (COBE, WMAP), to x-ray (CHANDRA, XMM) and gammarays (INTEGRAL, GLAST) astronomy, to gravity waves physics (LISA), electronics devices required for imaging need to be protected from particle upsets. Three sources in the natural space environment contribute to various effects on microelectronics22), solar particles (protons and heavier ions), galactic cosmic rays, trapped particles (in the Earth's belts). There are three basic effects that occur when components are exposed to these environments: Single Event Effects (SEEs)23'24), total ionizing dose25,26), and displacement damage27). It is noted that these effects can occur when components are exposed to any radiation environment; the space environment is just one of those environments. Single Event Upset (SEU) is one element of a set of SEEs that occur when components are exposed to a radiation environment, others are single event latch ups, transients, gate ruptures, and functional interrupts. Single-Event Upset (SEU) observations recorded by the ISS show clear patterns of temporal and special dependence (this is true for all space radiation effects). SEUs are mainly due to two ionization cases: direct (incident particle generates electron-hole

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pairs) and indirect (secondary particles add to generated electron-hole pairs). In all cases, charge collected on a sensitive node of electrical circuit causes unwanted change in the information stored in the system. Commercial Technology Computer Aided Design (TCAD) tools are unable to predict indirect ionization case. Also, classical models have given unsatisfactory performance when addressing emerging technologies28'. Hence the requirements for the development of a tool capable of taking into account orbit information, interfaced to electronic engineering design tools (detailed device and circuit response and approximation models), and equipped with full microscopic physics modeling were set. The RADSAFE system is currently being developed by researchers at Vanderbilt University and is focused on addressing the farreaching needs of the spacecraft design community. Researchers at NASA, SLAC, and other government and industrial facilities are integrally involved in the development. The next two sections give an overview of the RADSAFE concept and applications. 3.2. The RADSAFE concept The RADSAFE system is based on advanced software design and technologies. RADSAFE is a revolutionary, modular computational system for predicting the radiation response of electronic devices, circuits, and systems from basic device structure, circuit topology, and fundamental radiation characteristics. Its goals to be informed by the best available physics, implemented by robust algorithms, enabled by supercomputer technology, and calibrated by data. It allows for input of geometry and material specifications from TCAD systems for electronics devices. Given any source of radiation environment, the energy deposition and partitioning are computed. This energy partitioning information is returned to the TCAD tools so that the effects of ionization on electrical parameters can be modeled, thus leading to the calculation of device and/or circuit performance degradation. At the core of the system lies the GEANT4-based MRED component (MonteCarlo Radiative Energy Deposition tool), developed at Vanderbilt University: it includes run-time selectable physics lists features and a Python interface for providing output with a great degree of flexibility. 3.3. RADSAFE applications We have demonstrated the RADSAFE concept on a simple Silicon diode, showing excellent agreement with expect values. A Si diode was reproduced as a 5-micron cube, then MRED was used to simulate energy deposition profiles for 1 million of 12C ions with 1 GeV per nucleon. A single energetic event was

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selected and passed to the TCAD tool. The charge collected on the anode and cathode of the diode was comparable to what is expected assuming all the generated charge is collected. The RADSAFE concept has already been applied to investigate single event upsets in SRAMS and single event transients in IR focal plane arrays29,30'3''32'33).

4. Spin-off to Medicine The requirements for medical imaging, dosimetry, and treatment planning software have strong synergy with space simulation software, both in terms of relevant physics processes and in terms of energy ranges. Applications include external beam radiotherapy, beam-line design and quality assurance, absolute dosimetry, patient dose calculation, time-dependent simulations (to account for patient breathing phase), radiation risk and protection, brachyterapy, iso-dose distributions, DICOM interface and voxelized geometry compression, PET and SPECT design. In order to provide a reference point for the rapidly growing GEANT4 medical user community in North America, and to meet regional certification needs, the G4NAMU (GEANT4 North America Medical Users Organization) has been created in May 200534). It currently involves 27 institutes throughout Unites States and Canada, including Harvard, UCSF, U. Laval, CHU Quebec, Stanford, Louisiana U., McGill, Memorial Sloan-Kettering N.Y, TRIUMF, Johns Hopkins, U. Arkansas, Jefferson Lab, U. Wisconsin, U. Alberta, WRA Medical Center, Texas A&M, U. Washington, Hopital Maisonneuve-Rosemont, U. Minnesota and others. Conclusions We gave an overview of GEANT4 applications in NASA missions, particularly for apparatus simulation, planetary scale simulation, and micro-dosimetry simulation. Related application areas include spacecrafts charging, radiation shielding for spacecrafts, astronauts' safety, and bio-medical dose calculation. Comprehensive coverage of physics models and powerful geometrical modeling capability provided by GEANT4 were proven to be suitable for mission critical simulations.

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References 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

L. Latronico, for the GLAST collaboration, "Como 2003, Astroparticle, particles and space physics, detectors and medical physics applications" 69 (2003) W. Atwood et. al. "Perugia 2004, Calorimetry in particle physics" 329 (2004) T.H. Burnett et. al., IEEE Trans. Nucl. Sci .49, 1898 (2002) D.K. Haggerty and S. Livi, Adv. Space Res. 33, 2303 (2004) D.K. Haggerty and E.C. Roelof, Adv. Space Res. 32, 423 (2003) D.K. Haggerty, private communication. J.D. Kinnison, R.H. Maurer, D.R. Roth and R.C. Haight, Radiation Research 159, 154(2003) R.H. Maurer, J.D. Kinnison and D.R. Roth, accepted for publication in Radiation Protection Dosimetry. R.H. Maurer, C. Zeitlin, D.K. Haggerty, D.R. Roth and J.O. Goldson, 2005 Nuclear Science Symposium, October 24-27, 2005. J.I. Vette, NSSDC/WDC-A-R&S 91-24, 1991. E.J. Daly, J. Lemaire, D. Heynderickx, and D.J. Rodgers, IEEE Transactions on Nuclear Science 43, 403-415 (1996). S.F. Fung, Radiation Belts: Models and Standards, AGU Geophysical Monograph 97, 79-91 (1996). S.L. Huston, G.A. Kuck, and K.A. Pfitzer, Radiation Belts: Models and Standards, AGU Geophysical Monograph 97, 119-122 (1996). R. Reddell and B. Atwell, SPENVIS & Geant4 Space Users' Workshop, October 3-7, 2005. M.D. Looper, R.S. Selesnick and R.A. Mewaldt, SPENVIS & Geant4 Space Users' Workshop, October 3-7, 2005. D. Heynderickx, M. Kruglanski, V. Pierrard, J. Lemaire, M. D. Looper and J. B. Blake, IEEE Trans. Nucl. Sci., 46, 1475, (1999) G. Kanbach, C. Reppin and V. Schonfelder, J. Geophys. Res.79, 5159, (1974) I.G. Usoskin, K. Alanko, K. Mursula and G. A. Kovaltsov, Sol. Phys., 207, 389, (2002) J.M. Picone, A.E. Hedin and D.P. Drob, J. Geophys. Res., 107, 1468, doi:10.1029/2002JA009430, (2002) A.M. Preszler, S. Moon and R. S. White, J. Geophys. Res., 81,4715, (1976) R.S. Selesnick and R. A. Mewaldt, J. Geophys. Res., 101, 19,745, (1996) J.L. Barth, C.S. Dyer and E.G. Stassinopoulos, IEEE Trans. Nucl. Sci.,50, 3, 466 (2003). P.E. Dodd and L.W. Massengill, IEEE Trans. Nucl. Sci., 50, 3, 583 (2003). P.E. Dodd and L.W. Massengill, IEEE Trans. Nucl. Sci., 50, 3, 603 (2003). T.R. Oldham and F.B. McLean, IEEE Trans. Nucl. Sci., 50, 3, 483 (2003).

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26. R.L. Pease, IEEE Trans. Nucl. Sci., 50, 3, 539 (2003). 27. J.R. Srour, C.J. Marshall and P.W. Marshall, IEEE Trans. Nucl. Sci., 50, 3 653 (2003). 28. R.A. Reed, J. Kinnison, J.C. Pickel, S. Buchner, P.W. Marshall, S. Kniffin and K.A. LaBel, IEEE Trans. Nucl. Sci., 50, 3, 622 (2003). 29. K. M. Warren, et al., accepted for publication in IEEE Trans. Nucl. Sci., 2005 30. A. S. Kobayashi, et al., accepted for publication in IEEE Trans. Nucl. Sci., 2005 31. C. Howe, et al. accepted for publication in IEEE Trans. Nucl. Sci., 2005 32. J. Pickel, et al., accepted for publication in IEEE Trans. Nucl. Sci., 2005 33. D. Ball, et al., accepted for publication in IEEE Trans. Nucl. Sci., 2005 34. J. Perl, et al., http://geant4.slac.stanford.edu/g4namu/

GEANT4 APPLICATION TO HADRONTHERAPY IN JAPAN KATSUYA AMAKOt High Energy Accelerator Research Organization, KEK Institute of Particle and Nuclear Studies, 1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japan The Geant4 simulation toolkit provides a wide variety of physics models which describe interactions of elementary particles and ions with matter in a broad range of energy. In this article, an overview of an activity in Japan to apply the toolkit for simulation of hadrontherapy is given. Also an experiment to collect ion-matter interaction data applying the emulsion technique to validate Geant4 physics models is introduced.

1. Introduction Hadrontherapy is a radiation therapy technique for tumor treatment using hadron beams from accelerator machine. Historically this technique was proposed by R. Willison [1], who recognized the merit of applying proton and ion beams to radiotherapy. For details of hadrontherapy including historical overview, see the article by P. L. Petti and A. J. Lennox [2] and also U. Amaldi and G. Kraft [3] for more recent advance. According to the information provided by Particle Therapy Cooperative Group (PTCOG) [4], currently there are 27 hadrontherapy facilities (23 for proton beams and 4 for ion beams) in operation in the world and the number of patients treated so far by proton and ion beams is more than 46,000. There is a history of more than 20 years in Japan in the practical tumor treatment using proton and ion beams; Proton Medical Research Center in Tsukuba University (PMRC) started the first treatment in 1983 using the proton booster in High Energy Accelerator Research Center (KEK). Currently there are 5 proton therapy facilities in Japan as shown in the Fig. 1. For ion beam therapy, the HIMAC synchrotron (the maximum energy is 800 MeV/u) at National Institutes of Radiological Sciences (NIRS) in Chiba started its operation in 1994 and more than 1,800 patients have been treated so far using the 12C beam. Also Hyogo Heavy Ion Medical Center (HIBMC) in Hoyogo has a synchrotron machine, which can accelerate ions up to 320 MeV/u, This work has been partially supported by CREST of JST (Japan Science and Technology Agency).

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for iontherapy (Fig. 1). Currently there are only four iontherapy facilities in the world and the half of them are in Japan.

Tha Energy Research Cenlar Wakaaa Bay (Touruoa 200 MaV)

U ofTsukubaPMRC (Taukiiba 250 MaV) Hyogo Ian Baam Madlcal Centar . (N,.b,-H.r1m. 32.M.V,,,, *

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_ „__ _ . u ^ ^ ^ S - L " ' , " , ? ^ (Kashhva 235 MaV) • NIRS (Chlba proton 90 MaV, lona 400MeWu) Shbuoka Cancar Cantor (Mlshlma 230 MaV) ' 0

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Figure 1. Hadrontherapy facilities in Japan.

The choice of the 12C beam for iontherapy treatments in Japan comes from the study of several competing factors of the characteristics of various ion beams. To obtain a large Relative Biological Effectiveness (RBE) and a small Oxygen Enhancement Ratio (OER), a heavier ion is better. Also to get a well collimated beam a heavier element is better because the effect of multiple scattering is smaller. For a small dose at the entrance channel and a tolerable fragmentation production, a lighter ion is better. An optimal choice taking into account these competing factors is the 12C beam [5]. The clinical results from HIMAC support this choice. NIRS is now promoting a project for a compact carbon accelerator for iontherapy [6]. 2. Geant4 and Hadrontherapy in Japan As described in the previous section, the research community of hadrontherapy in Japan has more than 20 years of clinical experiences. Based on these experiences medical physicists committing to hadrontherapy in Japan want to have a simulation tool which can be used to study a performance of their treatment devices, to study a new design of treatment elements, and to evaluate the validity and performance of their treatment planning systems. Meanwhile, the Geant4 [7] development team in Japan is a founder of the Geant4 International Collaboration and has more than 10 years of experience in developing the Geant4 toolkit. The developers in the team want to see their

439 product used not only in the high energy physics domain (HEP) but also in medical, radiology, space and other domains. These requirements from the two completely difference research domains triggered in year 2002 a formation of a collaboration, which consists of Japanese researchers who belong to Geant4 Japan development team, medical physicists in the hadrontherapy facilities, physicists in the space science related institutes and high energy physicists in universities/institutes. This collaboration spawned two projects; a project to create a generic Geant4 toolkit for simulation of radiotherapy (the CREST project) and a project to perform a series of experiments to collect basic data of interactions of ions with material for validation of the Geant4 physics implementations (the PI52 experiment). 3. The CREST project The goal of this project is to develop a Geant4 simulation framework which can be generally utilized for researches in the radiation therapy domain. The activities included in the project are development of G4-DICOM interface, visualization and interactivity, parallel processing under GRID environment. Also included is the validation of Geant4 physics processes. The project is founded by the Core Research for Evolutional Science and Technology (CREST) program organized by Japan Science and Technology Agency (JST). Under this project there are 11 institutes from HEP, medical and space domains in Japan; High Energy Accelerator Research Organization (KEK), Ritsumeikan Univ., Kobe Univ., Naruto Univ. of Education, Toyama National College of Maritime Technology, National Institute of Radiological Science (NIRS) , National Cancer Center-Kashiwa, Gunma Univ., Faculty of Medicine Hyogo Ion Beam Medical Center (HIBMC), Kitasato Univ., and Japan Aerospace Exploration Agency (JAXA). The project started in 2003 and will continue till the end of 2008. For the details of this project, see the presentation given at the 2005 Geant4 User Conference [8]. 4. Critical Aspect in Applying Geant4 to Hadrontherapy One of the most critical requirements when we want to apply Geant4 to hadrontherapy (and also true for other radiation therapy) is the reproducibility of the depth dose distribution in human body with the accuracy of about 1mm. This requires Geant4 to provide a high-reliability description of not only electromagnetic processes but also hadronic processes and nuclear decay processes in the relevant energy regions and particle types. To attain this level

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of accuracy, it requires extensive elaboration of G4 physics descriptions through validations by experiment data and tuning the program codes. In this respect the most critical aspect in the project is validation of Geant4 physics description. We are currently executing extensive studies of comparing available hadrontherapy related data with Geant4 simulation. One of such studies is the comparison with the data of projectile fragments' fluence and linear energy transfer from H 2 0 with various ion beams measured at HIMAC/NIRS [9]. It is still under the study but some preliminary results demonstrate that Geant4 reproduces the relative dose distribution including the Bragg peak of the 12C beam in H 2 0 quite well, though there are some discrepancies in the fluence distributions of projectile fragmentations [10]. Another study is the validation using the dose distribution data from the treatment beam line at Hyogo Ion Beam Medical Center HIBMC (NishiHarima/Japan). The data used for the validation are observed (spread-out) Bragg peak distributions with the proton beams in the water phantom. It demonstrates that Geant4 reproduces the data with the accuracy which nearly satisfies the medical requirement [11]. 5. The P152 Experiment Albeit its importance, a systematic accumulation of ion interaction data relevant to ion-therapy has not been established yet. This is probably due to the fact that there are not many therapeutic ion accelerator machines available in the world. Another reason might be the priority of usage of an ion accelerator machine - it is clear that treatment has much higher demand and priority than collection of experimental data. However, the shortage of reliable data causes a difficulty in Geant4 physics validation. The second project, spawned from the effort to establish a collaboration between the Geant4 development team and medical physicists, is to perform a series of experiments to collect basic data of interactions of ions with material for validation of the Geant4 physics implementations. The PI52 experiment was approved as a research project using the HIMAC/NIRS machine in 2003. The purpose of the experiment is to collect data of heavy ion interactions with H, C, O, Ca, P, etc in the energy region of iontherapy (several hundreds Mev/u). Also it is to collect data of heavy ion interactions which are important in the radiation shielding of space laboratories. The number of institutes joining the experiment is twelve; High Energy Accelerator Research Organization (KEK), Nagoya Univ., Ritsumeikan Univ., Kobe Univ., Naruto Univ. of Education, Toho Univ., Aichi Univ. of Education, Univ. of Tokyo, Stanford Linear Accelerator Center (SLAC),

441 National Institute of Radiological Science (MRS), Faculty of Medicine Gunma Univ. and Japan Aerospace Exploration Agency (JAXA). The key technology employed in this experiment is the emulsion chamber. The major reason we decided to use this technology is its excellent characteristics; it has 4K acceptance, a high spatial resolution (~l(J.m), capability of recording a very short track and charge identification. The detector has no active parts, and it is also easy to handle and very compact. Thanks to the development of the automatic scanning machine for the emulsion chamber by Nagoya Univ. [12, 13], we do not need to scan emulsion films for the track reconstruction with a manually handled microscope as we did in the old days; the scanning machine automatically does the all necessary works of track and vertex findings. Fig. 2 shows the detail of the emulsion chamber (water-emulsion chamber) which was used in the experiment last year. It consists of water layers sandwiched by emulsion films.

11

!•

i

Figure 2. The upper figure shows the overall view of the water emulsion chamber. It consists of multiple layers of water sandwiched by emulsion files. The lower figure shows the enlarged view of a sandwich of water with two emulsion films.

The water-emulsion chamber consists of 87 layers of emulsion films put in a water tank in an equal spacing. A single sheet of emulsion film consists of a film-base (TAC) and two 44um emulsion layers at the front and back of TAC. The ion beam was injected from the left in the Fig. 2. A typical density of beam particles injected to the chamber is the order of 10,000 particles/cm2. The allowable maximum density is limited by the performance of the automatic track reconstruction algorithm to separate one track from other. The automatic scanning machine reconstructs all tracks in a single emulsion film with the

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speed of ~10 hours/plate. A reconstructed track has the spatial resolution of ~lum and the angular resolution of ~5mrad. The efficiency of finding a track is -100% (tan9<0.4). For charge identification of a track we use the technique so called 'refresh method'. This technique has been newly developed and established for the PI52 experiment. It is known that a film is put under the environment of a high temperature and humidity, a latent image center in the film is destroyed. This is sometimes called 'force fading'. The refresh technique applies this fact and forces fading a track image in the film by controlling temperature and humidity so that the saturation of the grain density by highly charged particle can be resolved [14]. Using this technique it has been demonstrated the charge identification up to Z=6 is possible for particles with (3y~0.8. So far we have performed about 10 times of beam exposures using emulsion chambers. The most of them are for the study of establishing the 'refresh method', though we also have taken data of interactions of 12C with H 2 0. A preliminary result of the measurement of the total charge-changing cross section of 12C with H 2 0 is already obtained and now its systematic errors are under the study. Also available are charge multiplicity distributions of secondary particles. These results will be published soon. 6. Summary Recently the Geant4 developers in Japan established with domestic medical physicists two projects. The first one is a project to create a generic Geant4 toolkit for simulation of radiotherapy (CREST Project). The second project is to perform a series of experiments to collect basic data of interactions of ions with material (P152 Experiment). The CREST project is to develop a G4-DICOM interface, visualization and interactivity, and parallel processing under GRID environment. Also included is the validation of Geant4 physics processes. Currently the Geant4 validation using existing data measured at NIRS is on-going. The PI52 experiment at NIRS aims to collect systematically data of heavy ion interactions with H, C, O, Ca, P, etc. in the energy region of ion therapy with the HEP emulsion chamber technology. The major reason for choosing this technology is that it provides a detector with excellent characteristics; a wide acceptance, a high spatial resolution, etc. Also the refresh method newly developed provides the capability of charge identification of ions. The basic technique to apply emulsion technology to measure ion interaction has been successfully established and some preliminary data of the total charge-changing

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cross section of C with H 2 0 has been obtained; more data including the charge multiplicity of secondary particles, angular distributions, energy distributions, etc. will come soon. Acknowledgments I would like to thank to the following persons for providing information and data to prepare the presentation; T. Aso, N. Kanematsu, T. Koi, K. Murakami, T. Sasaki, T. Toshito and K. Yusa, References 1. RR. Willsion, "Radiological Use of Fast Proton", Radiology 47, 487 (1946). 2. P. L. Petti and A. J. Lennox, "Hadron Radiotherapy", Ann. Rev. Nucl. Part. Sci, 44, 155 (1994). 3. U. Amalidi and G. Kraft, "Radiotherapy with beams of carbon ions", Rep. Prog. Phys. 68, 1861 (2005). 4. Particle Therapy Cooperative Group: url; http://ptcog.mgh.harvard.edu/. 5. HIMAC/NIRS: url; http://www.nirs.go.jp/tiryou/himac/himac_t.htm. 6. K. Noda, T. Fujisawa, T. Furukawa, et al., European Particle Accelerator Conference 9, 2631 (2004). 7. S. Agostinelli, J. Allison, K. Amako, J. Apostolakis, H. Araujo, et al., "Geant4 - a simulation toolkit", Nucl. Instru. Methods A506, 250 (2003). 8. T. Sasaki, "Status and plan for the hadron therapy simulation project in Japan", Talk in Geant4 2005, 10th User Conference and Collaboration Workshop. 9. M. Komori, N. Matsufuji, T. Kanai, A. Fukumura, M.Hirai, et al., "Comprehensive Study of the Fluence and LET Distribution of Projectile Fragmentation Produced from Heavy Ion Therapeutic Beams", Submitted to Atomic Data and Nuclear Data Tables, Oct.,2003. 10. K. Yusa, Gumma University, Japan; Private communication. 11. K. Aso, A. Kimura, S. Tanaka, H. Yoshida, N. Kanematsu et al., "Verfication of the Dose Distributions with Geant4 Simulation for Proton Therapy", IEEE TNS 52, 896 (2005). 12. S. Aoki et al., Nucl. Instrum. Methods B51, 466 (1990). 13. K. Kodama et al., Nucl. Instrum. Methods A493, 45 (2002). 14. T. Toshito, K. Kodama, K. Yusa, M. Ozaki, K. Amako, et al., "Charge identification of highly ionizing particles in desensitized nuclear emulsion using high speed read-out system", Nucl. Instrum. Methods, the paper accepted in Nov., 2005.

AMS02 ITALIAN DATA T R A N S F E R SYSTEM: REAL LIFE EXPERIENCE

M. J. B O S C H I N I CILEA Via R. Sanzio, 4, Segrate, Mi-Italy E-mail: [email protected]

D. GRANDI, R. MANGONI AND P.G. RANCOITA INFN-Milano P.zza Scienza, 3, Milano, Italy

In this work we present "real life" experiences with the AMS Italian Ground Segment Data Transfer System. Its goal is moving data for the AMS-02 experiment, which will take data on board of the International Space Station, from the Central Facility, located elsewhere, to the Italian Ground Segment, which will act as Master Copy of the whole experiment data sample. The system has been in use since 2004 to transfer MonteCarlo data from various remote sites to AMS Science Operation Center, located at CERN. Furthermore the system is in use to transfer the whole MonteCarlo data sample from AMS Science Operation Center to the Italian Ground Segment. Data Transfer proves to be fast and reliable, achieving the goal of 3 times the expected DAQ rate and fully consistent book-keeping mechanisms. Technical implementation, experiences and "lessons learned" will be described and discussed.

1. Introduction Accurate measurements of cosmic ray spectra, particularly of protons, are important because of their close connection with the foundation of the theories of elementary particles, Grand Unified Theory and Dark Matter. The Alpha Magnetic Spectrometer AMS is a space-bourne experiment dedicated to investigate and measure with high precision such spectra, with particular attention to the detection of primordial anti-matter. AMS-01 was flown on the Space Shuttle Discovery on flight STS-91 in June 1998. This was primarily a test flight that enabled the AMS team to gather data on background source and adjust and verify the spectrometer performance for the

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actual data taking, known also as AMS-02. The AMS-02 experiment, described in greater detail elsewhere1, is scheduled to start data taking by the second half of year 2008 on board of the International Space Station Alpha (ISSA) and continue to operate through at least 2010. 2. Data Flow The data flow, which is described in greater detail elsewhere2, can be summarized as follows: data will be recorded on board of International Space Station (ISS) and transmitted initially to Marshall Space Flight Center (MSFC) Alabama and then to AMS ground centers: Payload Operation and Control Center and Science Operation Center. From there, data will be sent over the Internet to the Science Operation Center and Central Production Facility (CPF), located at CERN, where it will be processed and analyzed. Furthermore, a Master Copy of the whole data sample, both raw, reconstructed and simulated, will be stored at the Italian Ground Segment Data Storage (IGSDS). The IGSDS will act not only as a master copy, but also as a " distribution center", in order to allow fast access to data to all collaborators without interfering with the CPF activity. The expected Data Sample for the ISS Data taking is summarized in the following table:

Table 1.

AMS02 related data transmitted to Earth - NASA-POCC

Stream

BandWidth (Avg.)

High Rate Slow Rate NASA

Data Category

Volume (GB/day)

Volume (TB/year)

3-4 Mbit/s

Scientific

30-43

11-15

16 kbit/s

House Keeping

0.16

0.06

Auxiliary Data

0.027

0.01

Facing the long running time and the way how the data will be transmitted from the detector to the analysis and production facilities, the development of a secure and reliable way of data transferring and organization is one of ground data handling vital issues. In particular, consistency between the CPF and the IGSDS data samples and data catalogs is of prominent importance. 3. AMS02 Italian Data Transfer One major component of AMS Ground Data Handling system will be the Data Transfer (DT). Its goal will be moving data efficiently from CPF to IGSDS without interfering with CPF's activities.

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The main goal of the Data Transfer System is to move data from CPF to IGSDS and keep track of what has been moved and if successfully or not. There are thus 2 components, which, for the sake of portability, we addressed separately and then merged together in the general architecture: • data transfer protocol (DTP): part of DT in charge of actually moving the data. • book-keeping mechanism (BK): part of D T in charge of keeping track of what has been moved and how (successfully or not). The "natural" solution to integrate the two parts is a client/server architecture: move data from CPF to IGSDS , keep track at CPF of what/how was moved, notify IGSDS about moving, IGSDS receives data, IGSDS keeps track of what/how has been received, and so on... This approach decouples the technical solutions adopted for the 2 components, making the whole system more portable and maintainable. BK has been taken care of by means of a Relational Data Base. For the whole system we adopted OpenSource solutions, which are: Linux O.S., MySQL5 for the RDBMS, Perl5 and C as programming languages*, OpenSSL3 for encrypted socket communication and bbftp 4 for the DTP. Inconsistency of the book-keeping can be divided in two components: first-level inconsistency, which occurs whenever a file does not appear in one of the two databases 5 . The AMS02 Data Transfer System provides a mechanism that checks for first-level inconsistencies and triggers a retransmission of the file. These inconsistencies are mainly due to network outages or hardware hangs at one of the sites. Any inconsistency that still remains, and which has to be fixed by human intervention, is defined as second-level inconsistency. The general requirements can be summarized as follows: low CPU load, sustain 3 times max data-rate, both in terms of bandwidth and of number of files, optimize bandwidth usage and reduce second-level inconsistency. Thorough tests, described elsewhere6, showed that the system satisfies the requirements and is suited for use in a production environment. In particular, it has been put in use since February 2004 to transfer MonteCarlo data from various remote locations to CPF . Furthermore, in 2005 it a Perl is used for the main programs, while C is used for the core of the DB communication system. b There is a DB at the sending site, keeping track of what and how has been transferred, and one at the receiving site, keeping track of what and how has arrived.

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has been used to transfer the whole MonteCarlo data sample from CPF to IGSDS. 4. Real Life Experience As already mentioned, the system is in production since February 2004 to transfer MonteCarlo data from various remote locations to CPF. Locations, scattered across Europe, have used the AMS-02 Data Transfer System to transfer, totally, 3.3 Tbyte of data, split into 5200 files. The system's architecture implements configuration in such a way that TCP (mainly WIN.SIZE) and bbftp parameters are automatically configured for the location, and dinamically modified, according to run-time environment. This choice proved to be an essential issue in order to provide maximum effective throughput. This because the remote sites had very different networking and storage capabilities, and fixed parameters (as demonstrated during the tests 6 ) would have degraded performance of the single remote site and lower the level of consistency of the book-keeping systems. In fact, total first-level inconsistencies, obtained by summing the inconsistencies of all sites, is of 0.02%, meaning that only 10 files required human intervention. Nevertheless, this number, as will be shown later on, could be drastically reduced if remote sites provide some administrative privilege over storage and network configuration and maintenance. Altough very useful, both as a testbench of the system itself and as a tool for the AMS collaboration, this layout0 is not considered fully compliant with the original system design, which is transfering data from the CPF to the IGSDS d . The latter layout has been used in 2005 to transfer the whole 2005 MonteCarlo production, stored at the CPF , to the IGSDS . This first unicast implementation, besides the obvious goal of making a master copy of all data, had also the purpose of testing the system's integration with offline storage systems like CASTOR - CERN Advanced STORage Manager7, deployed at both locations. Main goal was measuring how well the throughput requirement of 3 times the maximum expected data rate (which, for the time being has been set to 12 Mbit/sec) e has been fulfilled. The nominal bandwidth of the CPF to IGSDS link is 100 Mbit/sec, but since it is not a dedicated link, a meac

Sometimes referred to as multicast. Sometimes referred to as unicast. e Which means that the throughput requirement is 36 Mbit/sec. d

448

surement of the available bandwidth of the link was necessary. We relied on pathloariP to perform such measurements. In fact, if the available bandwidth of a network path is defined as the maximum throughput that can be provided to a flow, without reducing the throughput of the cross traffic, one needs a non-intrusive tool 8 to measure it, and pathload is such a tool. We thus deployed this technique, after a first period of tuning, measuring pathload's estimate of the available bandwidth every three transfers. Differences between this value and the effective DT throughput indicate an inefficient use of the network link, due to bad configuration of DT's parameter, or eccessive overhead introduced by some component of the DT (main candidates beeing the book-keeping, data validation and interaction with off-line storage like CASTOR). The following plot shows the effective DT throughput and pathload's measurements for a subset of the data sample transfer. 1

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Defined as total transferred data / RealElapsedTime of the whole DT + Book-keeping mechanism. g In some cases, pathload's measurement is lower than the correspondig DT value. This

449 eating an optimal bandwidth usage. Average I T effective throughput is 45 Mbit/sec. We also evaluated the throughput of saving data to the off-line storage system, which we performed using CASTOR's rfio APIs; this never proved to be a problem, being the average throughput approximately 80 Mbit/sec, thus ruling CASTOR out as a possible bottleneck. A second goal was measuring the second-level inconsistency in a fullycontrolled layout: the full data transfer had a second-level inconsistency of 0.01%, which fully satisfies the DT systems requirements. Finally, the overall Real Elapsed Time has been analyzed. The following plot shows the incremental time line of the transfered data sample, and different plateaus are evident. wuu

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Figure 2. Incremental time line of the transfered data sample.

These inactive periods are due to major hardware downtimes at CPF or IGSDS , which were still in a setup phase. The overall throughput, including these periods, is around 6 Mbit/sec, which is too low with respect to the requirement of 36 Mbit/sec, especially if we compare it with the average effective throughput of 45 Mbit/sec. The overall throughput, calculated including only 5% of downtime periods, which we believe can can be due to network glitches or some other application heavily using the link.

450

be considered a good estimate of a "natural" performance of such sites, is around 40 Mbit/sec, perfectly consistent with the requirements. 5. Conclusions We presented experiences and results of the AMS02 Italian Data Transfer System during nearly two years of full operation. In particular, we showed that the DT system is able to achieve an effective throghput that maximizes the bandwidth usage, thanks to the adaptive TCP and bbftp run-time tuning. This leads to an effective throughput of 45 Mbit/sec on a nominal 100 Mbit/sec link, which fully satisfies the initial requirements. The system also shows a very low second-level inconsistency when there is administrative control at both sites. Finally, the overall throughput shows a high dependence on the receiving and/or sending sites reliability: if we assume a "natural" downtime of 5%, an overall throughput of at least 40 Mbit/sec is expected, fully satisfying the requirements. These results clearly show that the AMS02 Italian Data Transfer System is reliable and efficient, and can thus be adopted by the AMS02 experiment. Acknowledgment s We wish to aknowledge the continuous support received by CILEA and its director, Prof. A. Cantore. References 1. AMS Collaboration Physics Reports, 366/6, 331 (2002) 2. V. Choutko, A. Klimentov Proceedings of Computing in High Energy Physics (CHEP 2001), Sep. 2001. 3. 4. 5.

http://www.openssl.org http://doc.in2p3.fr/bbftp/ http://www.mysql.com

6. M. J. Boschini et al, Proceedings of 8th ICATPP Conference (2004). 7.

http://castor.web.cern.ch/castor/

8. M. Jain, C. Dovrolis, Proceedings of Passive and Active Measurement Conference (2002). 9.

http://www.cc.gatech.edu/fac/Constantinos.Dovrolis/pathload.html

APPLICATION OF GEANT4 IN THE DEVELOPMENT OF N E W RADIATION THERAPY TREATMENT METHODS

ANDERS BRAHME, IRENA GUDOWSKA, SUSANNE LARSSON, BJORN ANDREASSEN, RICKARD HOLMBERG, ROGER SVENSSON Karolinska Institutet and Stockholm University Stockholm, S-J71 76, Sweden VLADIMIR IVANCHENKO Budker Institute for Nuclear Physics Novosibirsk, 630090, Russia ALEXANDER BAGULYA, VLADIMIR GRICHINE, NIKOLAY STARKOV Lebedev Physical Institute Moscow, 119991, Russia

There is a very fast development of new radiation treatment methods today, from advanced use of intensity modulated photon and electron beams to light ion therapy with narrow scanned beam based treatment units. Accurate radiation transport calculations are a key requisite for these developments where Geant4 is a very useful Monte Carlo code for accurate design of new treatment units. Today we cannot only image the tumor by PET-CT imaging before the treatment but also determine the tumor sensitivity to radiation and even measure in vivo the delivered absorbed dose in three dimensions in the patient. With such methods accurate Monte Carlo calculations will make radiation therapy an almost exact science where the curative doses can be calculated based on patient individual response data. In the present study results from the application of Geant4 are discussed and the comparisons between Geant4 and experimental and other Monte Carlo data are presented. 1.

Introduction

The increasing cancer incidence demands more advanced and cost effective treatment techniques in busy radiotherapy departments. Accurate, safe and fast dose delivery is required to shorten the treatment time at the same time, as the treatment outcome should be maximized. A high accuracy and flexibility in dose delivery is therefore of major importance in the clinical application. A detailed spatial and energy distribution of the narrow photon beam generated by the almost point monodirectional and monoenergetic electrons incident on the target

451

452

can be calculated for the complex therapy head configuration of a modern medical accelerator, which uses advanced field shaping techniques with multileaf collimators and electromagnetic scanned beams and purging magnets to remove primary and secondary electrons as shown in Fig. 1 [1], [2]. This information is of particular importance both for accurate treatment planning as well as for intensity and energy modulation of narrow elementary beams operating in scanning mode to deliver almost any desirable intensity distribution in the target volume [3].

Figure 1. Exploded view of the Treatment Head of an advanced scanned electron and photon beam accelerator. The scanning magnets in the bending plane and across it can scan both the photon and electron beam over the patient to deliver arbitrary intensity modulated beams.

Monte Carlo (MC) simulation of therapy beams allows accurate prediction of the radiation dose distribution delivered to a patient. An optimization of the

453

therapy beam delivered by different treatment head configurations can be preformed with this type of simulation. Additionally, the MC technique represents a powerful tool to quantify possible sources of scattered dose and radiation transport in the heterogeneous tissues of the patient. In this study the result of such MC simulations are discussed. The simulations have been done using the object-oriented Monte Carlo simulation toolkit Geant4 [4]. Geant4 is a general toolkit for simulation of particle transport through matter, originally designed for applications in high energy physics. Through the years Geant4 has been extended, redefined and new physics models have been developed for applications in different fields like particle and nuclear as well as medical, accelerator and space physics [5]-[7]. However, in some areas validation of the code for transport of particles used in radiation therapy is still required and this is the subject of the present study. The electron-photon transport through a treatment head has been studied for the therapeutic photon beam of energy 50 MV from the MM50 racetrack microtron at the Karolinska Hospital, Stockholm, as shown in the exploded view of Fig. 1. Light ion beam transport in tissue equivalent media was simulated for ions of Z = 1-6 and energy range up to 400 MeV/u. The Geant4 results are compared with experimental results from the radiation therapy facilities in Sweden, Germany and Japan and other Monte Carlo calculations. 2. Radiation Transport Calculations for scanned Electron and Photon Beam Therapy A new thin transmission target technique for dose delivery using narrow scanned photon beams has been developed [1], [2], [8], [9]. The aim of this technique is to allow very fast intensity modulated photon dose delivery especially useful for radiobiological optimized radiation therapy. High-energy, 50-70 MeV, electron beams of low emittance incident on thin low-Z targets produce very narrow intense high-energy bremsstrahlung beams. However, the electrons transmitted through the target have to be removed from the therapeutic field with a purging magnet and an electron collector.

The 50 MV racetrack microtron (MM50) installed at the Karolinska Hospital, Stockholm, delivers electron and photon beams with maximum energies in the range 2-52 MeV. Two different composite targets for bremsstrahlung production were used: one almost fully absorbing target made of 9 mm beryllium plus 6 mm tungsten (9Be6W) and one thin transmission target made of 3 mm beryllium

454

(3Be). The radial dose distribution of the 50 MV photon beam have been measured. The details of the experiment and simulation are described in Refs. [13], [14]. A special Geant4 application called GammaTherapy was designed for the simulation of the experiment, which is now a part of the Geant4 toolkit as an example of medical applications.

Bremsstrahlung thin Be target

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radius(mm) Figure 2. Radial energy deposition normalized to the integral in a water phantom 1000 mm from the 3Be target experimental data (dots) and Geant4 calculations: solid histogram - the Standard subpackage, dashed line - the Low energy subpackage. It is seen that most of the bremsstrahlung photons are emitted within a few degrees from the incident electrons.

A comparison was made between different Geant4 models for electronphoton interactions both for thin and for thick target. The models used in this comparison are the Standard [11] and Low energy [12] implementations of the electromagnetic processes. The results show that the radial dose profiles inside the phantom calculated by these two models are similar. For the case of the 3 mm Be target, these predictions are in good agreement with the measurements (Fig, 2). The result is shown on a logarithmic scale to demonstrate validity of Geant4 predictions over three orders of magnitude. Below this level, the experimental uncertainty becomes significant but the results are still realistic. For

455 the case of the 9Be6W target, the agreement between simulation and the experimental data is also good [10], [14]. However, some difference exists at small radii, which was also observed in comparisons with MCNP4B [13]. The results of this work confirm that Geant4 is quite accurate and can be used to accurately simulate also very extreme photon therapy beams. Figure 2 clearly shows that it is possible to make very narrow bremsstrahlung beams with high energy electrons and thin transmission targets. By increasing the energy to about 70 MeV and treating the patient at 750 mm from the target the half width of the photon beam is decreased to about 12 mm, which is very useful for Intensity Modulated Radiation Therapy (IMRT). 2.2. Simulation of Electron Collection with the purging magnet In the thin transmission target technique only a fraction of the electron energy is converted to the narrow useful photon beam. Electrons transmitted through the target have to be bent away from the therapeutic beam by a strong purging magnet and effectively absorbed in an electron collector. The choice of collector material is essential to achieve maximum absorption of the transmitted electrons and minimal secondary bremsstrahlung contamination of the therapeutic beam. A crude design of the collector is shown in Figs. 1 and 3. It is made of a 9 cm thick block of tungsten, and it is placed immediately below the purging magnet, displaced from the central axis in y-direction since the purging magnet deflects the electron beam in this direction. The thickness of the collector in the y direction is varied to study leakage of bremsstrahlung photons. In the simulations with three different collector designs most of the electrons are stopped in the collector only leaving a few low energy electrons scattered mainly in the opposite direction to the patient [14]. Geant4 is a powerful toolkit that allows detailed study of complex beam configuration with different absorber geometry, materials and magnetic field distributions. Also the fluence and dose contribution due to scattered protons can be studied in order to minimize their contamination at the location of the patient.

456

Figure 3. Cross-section through the purging magnet and electron collector of Figure 1 showing the influence of the magnetic field on the primary electrons (left panel). In the right panel a large number of forward bremsstrahlung photons as well as back scattered photons from the electron collector are seen. The electron beam is deviated in positive y-direction hitting the collector from above, about 2 cm from the edge. The incident mean electron energy is close to 50 MeV.

2.3. In Vivo Dose Delivery verification using Photonuclear reactions inside the body The new thin transmission target technique [14] with narrow high-energy bremsstrahlung beams of high intensity in the forward direction is also suitable for 3D in vivo verification of dose delivery using PET-CT imaging of the induced /f activity [1], [2]. This approach is based on photonuclear reactions in the human tissues to convert normal 12C and 16 0 to n C and 15 0 by knocking out neutrons with the high energy photons. The generated PET emitters that are

457

ideally suited for in Vivo dose delivery verification, which is an important clinically task that never before has been possible by non invasive methods.

Figure 4. This figure illustrates the activity distribution induced by the narrow photon pencil beam in a water phantom. The PET activity distribution (/T(z)), the absorbed dose (D(z)), the fluence (<2>(z)) and the energy fluence (Viz)) in the water phantom are shown. The good agreement between b* activity absorbed dose and the energy fluence is particularly clear. The fluence profile is broader because of the background of low energy photons, which does not contribute much to the energy fluence.

Today we are using Geant4 with an extensive photonuclear cross section library to calculate the density of positron emitters in the irradiated medium or patient. Figure 4 illustrates the photonuclear PET emitter activity due to the narrow photon pencil beam in Figure 2. Interestingly, this distribution will be spread out over the whole tumor volume during therapy (cf. Fig. 5) so all regions being irradiated can be accurately traced by PET-CT imaging in relation to the normal tissue anatomy depicted by the CT part of the PET-CT camera. A broad thick target bremsstrahlung beam would not be so useful here since the mean photon energy will vary considerably over the patient and make it very difficult to interpret the true dose delivery in the patient. In Figure 5 it is seen that if the broad scanned photon beam has a uniform dose (upper right panel) both the activity distribution and energy fluence are over compensated, whereas the low energy photon fluence is strongly forward directed

458

Figure 5. Same as Fig. 4 but using scanned photon beams so that the absorbed dose is closely uniform. The intensity or energy fluence (Viz)) is then slightly over compensated and so is the PET activity.

3. Simulation of Light Ion Therapy Beams To be able to treat well also the most severe radiation resistant tumors of complex local spread, intensity modulated ion beams combined with biologically based therapy optimization methods [15] are under development and belong to the most advanced treatment techniques that are possible. The development is carried out at the Karolinska Institute and Hospital [16]. The ultimate step in the therapy development process is therefore to optimize the beam quality for the relevant tumor and normal tissue situation. To facilitate beam quality modulation and treatment optimization light ions are really needed and the Monte Carlo method is a very important tool to develop this approach. The Geant4 code is today very suitable for calculations of several important problems relating to the use of particle beam transport for new treatment facilities.

459

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Carbon ions 195MeV/u in water

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Figure 6. Comparison of the central axis depth absorbed dose distributions in a carbon ion beam, measured in a water phantom [19], calculated by Geant4 and by the dedicated light ion code SHIELD-HIT [20]. A generally good agreement is seen between all the three data sets.

As a first step of this work the comparison between Geant4 calculations and the experimental data for different ion beams has been performed [17]. Figure 6 shows the results of simulation for carbon beam of 195 MeV/u from the carbon therapy facility at GSI, Germany [19]. It is seen that Geant4 calculations reproduce well the experimental depth-dose distribution. The calculated position of the Bragg peak is about 2 mm shallower, which indicates the necessity of implementation of more recent stopping power values such as those from ICRU 73 (2005) in the Geant4 code. On the other hand the Geant4 Binary Cascade model [18] well describes the fragmentation tail of the carbon ion beam. Conclusions The possibility to combine advanced Monte Carlo calculations with the influence of strong electromagnetic fields makes Geant4 a very flexible transport code far beyond standard MC programs. This property has been essential in the present study where the magnetic field of the purging magnet

460

severely affects the multiple scatter of the electrons in the air. A good agreement between Geant4 and experimental data was seen for all cases studied. The very narrow bremsstrahlung beam is indeed a very challenging test where experimental data are well described almost over 3 orders of magnitude. Also Geant4 shows good performance for carbon ion transport at least in the energy range up to 200 MeV/u. Acknowledgements This work was supported by VINNOVA and the EU project IOTAS (grant INTAS-2001-0323). References 1. A. Brahme, In: Radiation Therapy Physics, Ed. Smith A., Berlin Springer, 209 (1995). 2. A. Brahme, Acta Onco.l 39, 579 (2000). 3. A. Brahme, Acta Oncol. 42, 123 (2003). 4. Geant4 Collaboration (S. Agostinelli et al), Nucl. Instr. Meth. A506, 250 (2003). 5. M. Ellis, Talk at this conference. 6. G. Santin, Talk at this conference. 7. M. G. Pia, Talk at this conference. 8. A. Brahme, T. Kraepelien and R. Svensson, Acta Radiol. Oncol. 19, 305 (1980). 9. A. Brahme, Acta Oncol. 26,403 (1987). 10. I. Gudowska, V. N. Ivanchenko, S. Larsson and B. Sorcini, IEEE Nucl. Sci. Symp. Conf. Record*, 2148 (2004). 11. H. Burkhardtetal,/£££'7Vwc/. Sci. Symp. Conf. Record!, 1907 (2004). 12. S. Chauvie et al, IEEE Nucl. Sci. Symp. Conf. Record 3, 1881 (2004). 13. I. Gudowska, B. Sorcini, and R. Svensson, In: Proc. of MC 2000 Conf., Lisbon, 23-26 October, 2000, p 317. 14. S. Larsson et al, In: ICRS 10 /RPS 2004 Conference, Madeira Island, Portugal, 9-14 May 2004, accepted to Radiat. Prot. Dosim. (2005). 15. A. Brahme, Int. J. Rad. Oncol. Biol. Phys. 58, 603 (2004). 16. H. Svensson, U. Ringborg, I. Naslund and A. Brahme, Radiother. Oncol, Suppl. 2, 73 (2004).

461 17. I. Gudowska, A. Bagulya, V. Ivanchenko and N. Starkov, In: Geant4 10 User Conference and Collaborative Workshop, 3 - 1 0 Nov., Bordeaux, France, 28 (2005). 18. G. Folger, V. N. Ivanchenko and J. P. Wellisch, Eur. Phys. J. A21, 407 (2005). 19. M. Kramer et al, Phys. Med. Biol. 45, 3299 (2000). 20. I. Gudowska, N.M. Sobolevsky, P. Andreo, D. Belkic, and A. Brahme, Phys. Med. Biol. 49, 1933 (2004)

G4 ACCELERATOR APPLICATIONS MALCOLM ELLIS High Energy Physics Group,Imperial

College London, South Kensington SW7 2AZ, UK

GRAHAME BLAIR* Dept of Physics, Royal Holloway Univ of London, Egham, Surrey TW20 OEX, UK YAGMUR TORUNt Illinois Institute of Technology, 3300 South Federal St, Chicago, IL 60616, USA TOM ROBERTS t t Muonslnc,

Batavia, IL 60510, USA

This paper will review GEANT4 applications and extensions for accelerator studies, both as a general framework and for the simulation of specific accelerators. G4Beamline is a Geant4-based program intended to easily simulate many different aspects of beam-line design. G4Beamline implements accelerator components via an object-oriented ASCII input format. For instance, it allows the definition of a quadrupole magnet by its physical geometry, materials, and also magnetic field (including several models for fringe fields). The Muon Ionization Cooling Experiment (MICE) poses some challenges for simulation not normally found in a particle physics experiment. The G4MICE package is under development to allow a complete simulation of the MICE cooling channel (including the simulation of magnetic and RF fields) and instrumentation. BDSIM is a Geant4 based code that implements fast accelerator-style tracking for particles within the beam-pipe together with the full set of Geant4 physics processes for particles that leave the beam-pipe. The program has been used to simulate and optimize the design of proposed electron positron linear colliders (ILC and CLIC). The input to the code is based on the MAD format and allows definition of geometries, fields and materials. Geometry descriptions common to those used by the ILC detector groups have

This work is supported in part by the Commission of the European Communities under the 6th Framework Programme "Structuring the European Research Area", contract number RIDS011899 and by the British Council Alliance programme. * Work supported by the U.S. Department of Energy and the Minoise Board of Higher Education. " Work supported by the U.S. Department of Energy.

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463 enabled common studies of the interaction region, specification of the required collimation depths and optimization of the extraction lines.

1. Introduction It has been common to separate the simulation and optimisation of an accelerator system from that of the detectors that use the supplied beam. Research and development towards future accelerator systems (the International Linear Collider and Neutrino Factory) has resulted in the development of Geant4-based software packages that provide tools to accurately simulate the accelerator system as well as the detectors. These software tools can be used for some of the design and optimization tasks normally performed by dedicated fast accelerator simulation packages, which typically have a minimal set of material description and physics processes. These tools can also be used to simulate the interaction and passage of the beam through the relevant detector system in the more conventional manner. The merging of these abilities to design, model and simulate a beamline and detector system in one consistent package can be particularly beneficial when there is a very strong coupling between the two. 2. G4Beamline G4beamline [1] is a general-purpose simulation program based on the Geant4 [2] toolkit that is specifically optimized for the simulation of beam lines and other systems. By implementing a large repertoire of beamline elements it relieves the user of C" programming, but permits the realistic simulation of many interesting systems. G4beamline is open source and easily extensible, and users can add their own elements when necessary, via C* programming. G4beamline uses a single ASCII input file to define all aspects of the simulation, including the geometry, beam definition, physics use-case, program control parameters, output NTuples, and visualisation. As it is inherently object oriented, and beam elements include both physical materials and electromagnetic fields, describing the system is usually a matter of selecting the objects to use, setting their parameters, and placing them into the beamline. Geant4's exampleN02 consists of ~900 lines of C ^ code; Figure 1 shows it described in 17 lines of G4beamline input. This enormous reduction in size and complexity is common, as unlike C ^ code the G4beamline input is no more complex than the system being simulated. Note this includes visualisation using most visualisation drivers supported by Geant4, plus histograms using HistoScope [4]; additional histogram packages are planned.

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G4beamline implements "centerline coordinates" that easily permit the use of bending magnets. Figure 2 shows the MICE muon beamline in G4beamline. As it is designed for flexibility, G4beamline has been used for simulating many different systems. An example is the helical cooling channel summarised in Figure 4. The meaning of "beamline" is quite flexible, and G4beamline includes a cosmic-ray "beam" used in the simulation of muon tomography of a shipping container shown in Figure 3. # select physics use-case, define materials and beam physics QGSP material Pb A=207.19 Z=82 density=l 1.35 material Xe A=131.29 Z=54 density=0.005458 beam gaussian beamZ=-200 sigmaX=0 sigmaY=0 sigmaXp=0 sigmaYp=0

\ meanMomentum=3000 sigmaP=0 particle=proton nEvents=5 # define elements of the simulation box Target width=50 height=50 length=50 material=Pb color=l ,0,0 virtualdetector Detl width=480 height=480 length=100material=Xe virtualdetector Det2 width=1344 height=1344 length=100 material=Xe virtualdetector Det3 width=2208 height=2208 length=100 material=Xe virtualdetector Det4 width=3072 height=3072 length=100 material=Xe virtualdetector Det5 width=3936 height=3936 length=100 material=Xe # place elements into the simulation "World" volume (which expands) place Target z=0 place Detl z=800 place Det2z=l 600 place Det3 z=2400 nlace Det4 7^3200 Figure 1. ExampleN02 in G4beamline, units are mm and MeV.

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Figure 3. Cosmic-Ray exposure of a shipping container.

465 Evolution of Beam emittance ICOOL

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Z(m)

Figure 4. Emittance reduction (cooling) along a helical cooling channel.

3. G4MICE G4MICE is the Geant4-based software suite that provides Simulation,. Reconstruction and Analysis tools for the MICE experiment [3]. The MICE experiment requires the construction, operation and measurement of the performance of an ionisation cooling channel. The simulation of MICE therefore requires a combination of accelerator components (201 MHz RF cavities, liquid hydrogen absorbers, quadrupole triplets and solenoid magnets) and instrumentation (Scintillating Fibre tracker, Time of Flight and Cherenkov detectors and a Calorimeter), shown in Figure 5. G4MICE provides an accurate model of the components of the downstream section of the muon beam line as well as the full cooling channel and instrumentation. Magnetic fields (including fringe fields) can be calculated directly from magnet currents (supplied as input parameters) or read in directly from an external file. An important aspect of particle tracking through the channel is the correct simulation of the time varying EM field associated with the 201 MHz RF cavities. The simulation of the production of X-rays and electrons by the high accelerating gradient of the RF cavities has also been implemented and was crucial to the validation of the design of the tracking detector. The process of field emission from RF cavities has been studied with a high frequency cavity and the results extrapolated to the MICE cavities. The resulting extrapolation was used to create a spectrum of particles at the RF cavities which are then propagated through the experiment by GEANT4. Figure 6 shows the shower of

466

Figure 5: Muon Ionisation Cooling Experiment (MICE). Instrumentation and cooling channel comprising eight 201 MHZ RF cavities and three Liquid Hydrogen absorbers.

Figure 6: Visualisation of the passage of one muon through the MICE experiment with the production of X-rays and electrons by the RF cavities.

X-rays and electrons produced by two RF cavities overlaid with a single muon passing through the channel. The simulation of the tracking and particle identification detectors in G4MICE is as detailed as can be reasonably achieved. An example is the scintillating fibre tracker model, which includes the active core and two layers of cladding for each fibre. Each fibre is explicitly represented and is contained inside a layer of glue and supported by a carbon fibre frame. The digitisation parameters have been taken from previous experience and updated in light of the results of test-beam experience as part of the ongoing research and development efforts for the experiment. G4MICE has been used to simulate the performance of the full experiment with a realistic set of accelerator components and a range of input beams. Ten

467

different beams (each matched to the cooling channel and of a given transverse emittance) were simulated passing through the experiment. The digitised events were reconstructed and analysed using one potential analysis technique under study. The results show the expected performance of a transverse cooling channel and are summarised in Figure 7. Cooling Measurement

Cooling Measurement Resolution

x Truth EmJttanca Difference + Reconstructed Emit-tance Dtlference

Initial Enlluiice (pi ran rail)

Generated Eialttanee (pi n

Figure 7: Left: The reconstructed (+) and simulated (x) change in emittance as a function of the input beam emittance. Cold beams are seen to be heated (positive change) and hot beams are seen to be cooled (negative change). Right: The difference between the reconstructed and simulated change in emittance is plotted as a function of the input beam emittance.

4. BDSIM BDSIM is a Geant4-based program created originally to simulate the beam delivery system (BDS) of the International Linear Collider (ILC), with early application to the CLIC [5]. However BDSIM is designed flexibly in order to track particles through any beam-line and includes particle interactions and production of secondary particles in materials. The accelerator input to the program is controlled by a file that is based on the MAD format, but also includes additional descriptions of geometries, apertures and materials [6]. The code is maintained under CVS and is available freely [7]. Inside the beam-pipe a new approach is adopted. Rather than track the particles using a locally defined magnetic field strength, the approach more commonly adopted by accelerator tracking codes is followed, using an analytic solution to the equations of motion. By using the analytic solutions directly within a G4Stepper, a significant time saving is obtained over the more usual Geant approach of solving locally for each step in a magnetic field. In addition, an

468

interface to full field maps has been implemented for the specialised magnets such as are needed in the ILC interaction region and extraction line. The accurate tracking performance was tested in detail as part of a comparison of a wider set of tracking codes [8] and showed that the BDSIM technique has comparable accuracy to other accelerator trackers. The magnitude of the step length is provided by Geant4 and depends on which processes are present. For most cases, the step length is equal to the length of the beam-line element because the particles are traveling in high vacuum. Scattered particles, or halo particles in outer positions of phase-space, can leave the beam-pipe; once outside the beam-pipe the tracking defaults to the usual Geant approach. BDSIM naturally has access to all the impressive range of processes that are included in the Geant4 toolkit. These processes include multiple scattering off beam-gas particles or in detector elements and the usual electromagnetic shower processes such as electron-positron pair creation and bremsstrahlung. The beam-gas is specified by a normal Geant4 material with a pressure that can be defined by the user. Neutron generation from photo-production can also be included using the Geant4 photo-nuclear processes. Synchrotron radiation (SR) from electrons in the magnet elements can be generated and tracked down the beam-line. An example is shown in Fig.8 (Left) where the subsequent absorption of the SR is obtained as a function of position in the CLIC BDS.

Figure 8: Left: The SR absorption in GeV/m as a function of length along the CLIC BDS showing impacts on limiting apertures such as collimators. Right: muons created in electro-magnetic showers in the (TESLA) collimation system are transported down the tunnel to the interaction point (IP). Only the tracks of those muons that actually reach the IP are shown.

Additional processes can also be included as options. One example is tracking of beam halo and its collimation in the spoilers and absorbers of the ILC. The resultant generation of muons within electromagnetic showers in the collimation

469 sections, and the tracking of these muons down the accelerator tunnel, is illustrated in Fig.8 (Right). BDSIM is particularly useful for the detailed design of accelerator components and layouts. A common interface has been set up with the ILC detector groups so that detector geometries can be included rapidly for performance studies and optimisation. An example is shown in Fig. 9 (Left) where the ILC interaction region has been modeled and is currently being studied [9] to quantify and minimise the backgrounds in the detector from reflection of SR off downstream components (Fig. 9,Right).

M

0

0.01 0,02 0.03 0.04

0.05 0.06

0.07

0.08

0.00 0.1 XlmJ

Figure 9. Left: The Interaction Region for one of the ILC detectors ("LDC") as modeled in BDSIM, the IP is beyond the left of the figure. Right: The SR impinging on the first quadrupole downstream of the IP for various collimation depths; the black outline is the air-pocket of the quadrupole.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

T. J. Roberts, http://g4beamline.muonsinc.com The Geant4 Collaboration, http://geant4.web.cem.ch/ueant4/ The MICE Collaboration, http://mice.iit.edu/ http ://fermi tools, fnal. gov/abstracts/histoscope/abstract .html G. Blair CERN-CLIC-NOTE-509 (2002). G. Blair, Proc. LCWS05, Stanford (2005). The BDSIM code can be obtained from the website: cvs.pp.rhul.ac.uk. S. Redaelli et al Proc. EPAC 2002. J. Carter, Univ. of London PhD thesis (in preparation).

A F U L L MONTE CARLO SIMULATION OF SILICON STRIP DETECTORS

M.BRIGIDA, C.FAVUZZI, P.FUSCO, F.GARGANO*. N.GIGLIETTO, F.GIORDANO, F.LOPARCO, B.MARANGELLI, M.N.MAZZIOTTA, N.MIRIZZI, S.RAINO', P.SPINELLI DipartimentoInterateneo diFisica "Michelangelo Merlin"andINFNBari, Orabona4, 70126 Bari, Italy

Via

We have developed a full Monte Carlo simulation code to evaluate the electric signals produced in silicon strip detectors (SSDs) by absorbed X-rays and by thoroughgoing charged particles. This simulation can be applied in the design stage of a SSD, to study its performance as a function of the main detector parameters, like the strip pitch and the silicon thickness. All the physical processes leading to the generation of electronhole (e-h) pairs in silicon have been taken into account. Induced current signals on the readout strips are evaluated by applying the Shockley-Ramo's theorem to the charge carriers propagating inside the detector volume. Detailed maps of the electricfieldand of the weighting fields have been implemented to describe the motion of charge carriers and the time development of the induced current signals. A simulation of the front-end electronics, including the main sources of noise, allows to convert the current pulses generated on each strip into voltage signals. This simulation code can be very useful in studying the performance of different kind of silicon detectors (pixel, strip, drift) tailored for different applications (particle identification, tracking, X-ray detection) 1.

Introduction

Silicon strip detectors (SSDs) were first introduced as high precision tracking devices for high energy physics experiments in the early 1980s. Since then, impressive technological progress has been made, and SSDs have been used both in accelerator and space experiments. SSDs allow to obtain space resolutions up to a few um and, thanks to the reduction of their costs, large detector areas can be equipped with these devices. For these reasons, SSDs will be widely adopted in the next generation detectors, that will be mounted at the colliders (LHC, RHIC) or that will be flown on satellites. During the design phase of a detector, there are some parameters that need to be optimized in order to ensure the performance required by the experiment where the detector will be operated. An accurate detector model is therefore

* Corresponding author: fabio.gargano(a),ba.infn.it Tel. +390805443172

470

471 essential for understanding the behaviour of the device. We have written and developed a full Monte Carlo simulation code that simulates all the physical processes taking place in a SSD and allows to calculate the electrical signals generated by ionizing particles. A detailed description of the simulation code can be found on Ref.l. , where the ionization cross-section using the algorithm described in Ref.l.

2. Energy loss of a charged particle in silicon A charged particle crossing a silicon layer leaves an ionization track along its trajectory. The most probable energy loss Ap is between 180 and 300 eV/um, increasing with the silicon thickness. Since the energy required to produce an e-h pair in silicon is about 3.6eV, these values correspond to about 50 e-h pairs/um and 80 e-h pairs/um respectively. A complete and accurate calculation of the differential cross section cr(E) as a function of the energy loss E in silicon is given in Ref.2. The cross-section shows three peaks at energies of about 17, 150 and 1840eV, that are associated with the M, L and K shells of silicon. The energy transfer in a single collision is mainly due to the contribution of the M shell, while the contribution from energies above the K shell is less than 1%. A Monte Carlo method is used to calculate the straggling functions that describe the distributions of energy loss for different silicon thicknesses. The energy loss E; in the ith collision is determined by extracting a random number from the differential probability distribution F(E). Individual energy losses are added up to give the total energy loss of the particle. The distance travelled between collisions is sorted according to an exponential collision probability, P(x)=exp(-xA,)M, where X is the number of ionizing collisions per unit path length. The calculation is repeated for N particles and the energy loss distribution is thus generated. The charge carriers are produced both in the primary collision and by the subsequent energy losses of the secondary e-h pairs. Usually, the energy loss in aprimary collision is larger than the energy W needed to produce an e-h pair («3.6eV). Thus, for each collision of the incident particle, several e-h pairs will be produced. The primary collision occur with the exchange of "virtual" photons between the incident charged particle and the medium. The absorption of these photons and the subsequent relaxation of the excited atoms

472

yield the photoelectrons, the Auger and Coster-Kronig "primary electrons" and the corresponding "primary holes" in the valence band. All primary electrons and holes lose part of their kinetic energy by phonon scattering and another part by impact ionizations, producing secondary e-h pairs. When photons are absorbed by an inner shell (KjLijI^), the photoelectron energy is determined by the photon energy, whereas the energies of primary electrons and primary holes depend only on the vacancies created in the valence band by the photoionization process. Photons having energies less than Egap are not absorbed. A primary electron (hole) with energy E large compared to the semiconductor band gap can interact both by ionization, i.e. creating secondary e-h pairs, and by phonon emission. The generation of secondary e-h pairs is a cascade process, that is simulated with a Monte Carlo method. All secondary electronhole particles produced by the primary particle are followed up with the same procedure until all particles have residual energies below the threshold. These particles are the total charge carriers produced by the primary particle. In Fig. 1 are shown the results of the simulation for the number of carrier produced and for the pair creation energy W.

Fig. 1 LEFT: Average number of secondary e-h pairs, np (black circles) and carriers, n,; (open circles) as a function of the photon energy. The inset shows an expanded view of the behaviour of np in the low energy region. RIGHT: Electron-hole pair creation energy, W, as function of the photon energy. The inset shows an expanded view of the low energy region.

3.

Signal simulation

Silicon detectors can be described as p-n junction diodes, operated with a reverse bias voltage that forms a sensitive region depleted of mobile charge

473

carriers, in order to allow the collection of free charge carriers generated by radiation. The motion of a carrier (an electron or a hole) produced by ionization inside a silicon detector can be described by the equation v = u(E)-E where v is the carrier velocity, E is the electric field and u is the mobility, that depends on the electric field. During their drift, electrons and holes are diffused by multiple collisions, and their distribution follows a Gaussian law. In our Monte Carlo code the diffusion effect is taken into account during the solution of the equation of motion. The current induced at the time t by a moving carrier on the kth electrode can be evaluated with the Shockley-Ramo theorem ik(0=-qo-v-Ek where q0 is the charge of the carrier, v is its velocity and Ek represents the weighting field associated to the k* electrode. The induced currents are evaluated during the motion of the carriers, and the total currents are computed by adding the contributions from all carriers. A typical detector front-end consists of a preamplifier, that provides a gain, followed by a shaper, which tailors the overall frequency response for optimum noise performance, and limits the pulse length to accommodate the signal rate. The analytical relation between input and output signals can be derived using the Laplace transform. We have considered a standard front-end in which each induced current signal ik(t) is fed into the electronic chain formed by a preamplifier followed by a shaper. In our simulation we also taken into account different sources of electronic noise: • the leakage current Ij of the semiconductor detector, that can be represented by a current noise generator in parallel with the detector, and is a shot noise; • the noise due to the bias resistor, that can be assumed as a current generator in parallel with the detector, and is a thermal noise; • the noise from the feedback resistance, that can be described by a current generator and is a thermal noise; • the electronic noise of the amplifier, that is described by a combination of a voltage source and a current source at its input.

474

In this way we can simulate the voltage output event by event. In Fig. 2 are shown the result of the electronic simulation.

Time Ins)

Timeout

Fig. 2 LEFT: Simulated current signal for the hit strip and the adjacent ones. RIGHT: simulated shaper voltage output for the hit strip with noise added.

4. Conclusion We have developed a new Monte Carlo full simulation code that can be used for designing SSDs both for high energy and space physics. Our code includes all the physical processes taking place in a SSD and allows to study its performance. It can be used for different detector geometries and front-end electronics. A complete calculation of the e-h pairs is performed to generate the charge carries produced by the absorption both of virtual, e.g. exchange photons of charged particles with the silicon, and real photons. The signal simulation includes a noise simulation, that takes into account the detector and the electronic noise contributions.

References 1. M.Brigida et al. Nucl. Instr. and Meth. A 533 (2004) 322-343 2. H. Bichsel, Rev. Mod. Phys. 60 (1988) 663.

SIMULATION OF RADIATION MONITORS FOR FUTURE SPACE MISSIONS PATRICIA GONCALVES, MARIO PIMENTA AND BERNARDO TOME LIP -Laboratorio

de Instrumentacao e Fisica Experimental Av. Elias Garcia, n°14,l° Lisboa, 1000-149, Portugal

de

f

Particulas,

A GEANT4 based simulation of a compact lightweight radiation monitor to be included in the payload of future space missions is presented. The instrument must meet severe mass and power constrains, satisfy mission safety requirements and it is also required to perform as a scientific instrument. GEANT4 is a powerful tool for developing and optimising such detector concept thanks to its capabilities for describing the complex detector geometries and simulating the passage of particles through matter. Moreover, the presence of an optical physics process category in GEANT4 is of the utmost importance in the development of a scintillation based detector concept.

1. Introduction Radiation monitors are becoming an essential component in space missions, providing crucial radiation environment information for the in-flight protection of the spacecraft and instruments onboard. In addition, the data acquired during the mission are a valuable input for space environment models; in particular data gathered in missions to other planets (e.g. particle fluxes and spectral distributions as a function of the distance to the Sun) are essential for models describing the injection and propagation of particles from the Sun. Several of the future space missions (e.g. LISA, Gaia, JWST, BepiColombo) are planned to carry radiation monitors. Given the limited resources on mass, power and accommodation onboard a spacecraft, a new generation of compact and lightweight general purpose energetic particle detectors are being explored. Monte Carlo simulation tools are essential to develop and optimize a given detector concept and to explore alternative detector concepts. The ability to predict the detector performance in realistic radiation environments is also crucial. This requires the implementation of software models in order to perform an end-to-end simulation, including particle propagation from the source to the spacecraft location, followed by a detailed simulation of the detector response. The GEANT4 toolkit is an ideal framework for simulating particle interactions with detector materials in complex geometries. A GEANT4 based ^is work was supported SFRH/BPD/11547/2002.

through

FCT

475

grants

SFRH/BPD/11500/2002

and

476

simulation of a compact lightweight radiation monitor concept for future space missions is presented. 2. Space Radiation Environment The high energy ionizing radiationt environment in the solar system consists of three main sources: the radiation belts, galactic cosmic rays and solar energetic particles. The radiation belts are composed of electrons and protons trapped in the magnetosphere of the planets. In Earth's radiation belts (Van Allen belts), electrons have energies smaller than about 6-10 MeV and protons have energies up to 250 MeV [1]. Earth's radiation belts are mostly relevant for low earth orbit (LEO) missions. The galactic cosmic rays (GCR) component consists of a continuous flux of electrons, protons and ions, arriving from beyond the solar system, which are originated mainly in the supernovae. The GCR flux is maximal at energies around 1 GeV/n, decreasing as a power law for higher energies. Lower energy particles are strongly affected by the 11-year solar cycle. Although the GCR flux is comparatively low, energetic heavy ions can locally produce significant energy depositions originating single-event upsets. Solar energetic particle events (SPE) consist of a sudden and dramatic increase in the flux of particles from the sun, including electrons, protons and heavier ions. SPE are associated with impulsive solar flares and Coronal Mass Ejections and occur with higher probability during the solar cycle maxima. Although the energies of SPE rarely exceed few hundred MeV they give rise to potential risks for space missions due to the high fluxes attained. On the other hand their study is relevant for developing and testing solar particle propagation models [2].

3. The GEANT4 toolkit GEANT4 [3,4] is a Monte Carlo radiation transport simulation toolkit, with applications in areas as high energy physics, nuclear physics, astrophysics or medical physics research. It follows an Object-Oriented design which allows for the development of flexible simulation applications.

t In the context of space radiation effects, particles are considered energetic if their energy is high enough to penetrate the outer skin of the spacecraft. This means electrons with energies above 100 keV and protons and ions with energies above 1 MeV.

477

GEANT4 includes an extensive set of electromagnetic, hadronic and optics physics processes and tracking capabilities in 3D geometries of arbitrary complexity. The electromagnetic physics category covers the energy range from 250 eV to 10 TeV (up to 1000 PeV for muons) while hadronic physics models span over 15 orders of magnitude in energy, starting from neutron thermal energies. The optical physics process category allows the simulation of scintillation, Cherenkov or transition radiation based detectors. A distinct class of particles, the optical photons, is associated to this process category. The tracking of optical photons includes refraction and reflection at medium boundaries, Rayleigh scattering and bulk absorption. The optical properties of a medium, such as refractive index, absorption length and reflectivity coefficients, can be expressed as functions of the photon's wavelength. The characteristics of the interfaces between different media can be defined using the UNIFIED optical model [5]. Full characterisation of scintillators include emission spectra, light yields, fast and slow scintillation components and associated decay time constants. 4. Simulation of a generic space radiation concept The simulation of a simple space radiation monitor concept [6] was implemented using the GEANT4 toolkit. The detector consists of a tracker made of two position sensitive silicon planes followed by a scintillating crystal surrounded by photodetectors (Fig. 1.). An aluminium foil placed on the backside of the crystal is used to tune the energy range of the detector. In the present simulation the silicon tracker planes were 0.065 mm and 0.5 mm thick, the crystal size was 3x3x3 cm3 and the Figure 1. View of the simulated aluminium absorber was 2 mm thick. The particle detector. From left to scintillation properties of the CsI(Tl) crystal right: the two silicon planes, the were used. These included a light yield of scintillating crystal surrounded by 65000 photons per MeV of deposited energy photodetectors, the aluminum and two scintillation components with decay plane followed by a charged particle sensitive detector. time constants of 0.68 us (64%) and 3.34 us (36%). The scintillation light is readout by large area silicon PIN photodiodes, with a spectral sensitivity matching the CsI(Tl) emission spectrum. Since the silicon photodiodes are sensitive both to photons and charged particles, this makes them suitable to be used also as anti-coincidence shield. Figure 1 displays the fraction of the initial particle energy deposited in the crystal for protons with energies ranging between 25 MeV and 125 MeV.

478

Protons with energy amounting to 25 MeV or 50 MeV are absorbed in the crystal. For 100 MeV protons the effect of straggling is visible. 10'

10'

10'

""r 10" .I 1

Figure 2. Total energy deposited in the crystal by incident protons expresses as a fraction of the incident particle energy.

2

3

.. «

I 5

~tL^_ 6

/

I

S

••••

>

'

Figure 3. Simulated distribution of the photodetectors' hits time spectrum.

The principle of the anti-coincidence operation mode is illustrated in Fig.2. where the timing of the simulated hits in the photodiodes, produced by a 100 MeV proton that is not stopped inside the crystal is shown. The fast signal at t=0 is due to the direct ionization produced by the proton in the photodetector, while the slower signal comes from the scintillation light emitted by the crystal. Thus, by a suitable signal processing it is possible to disentangle the direct ionisation from the scintillation. This allows the identification of events where particles entered the detector from the sides or were not fully absorbed in the crystal.

Incident Energy (MeV)

Figure 4. Energy deposited in the first (left) and second (right) silicon planes by electrons, protons and alphas as a function of the energy of the incident particle.

Particle identification is performed by measuring the energy loss in the thin silicon trackers. Figure 4 shows the simulated energy loss in the first (left) and second (right) silicon planes, as a function of the initial particle energy, for electrons, protons and alphas.

479

5. Conclusions A new generation of compact, lightweight, general purpose radiation monitors needed for future space missions (e.g. BepiColombo) is under study. A simple concept based on a scintillating crystal was presented. LIP will be responsible for implementing the required Geant4 based detector simulations for the described setup in the context of a contract with EFACEC/Portugal. References 1. 2.

3.

4. 5.

6.

E.R. Benton and E.V. Benton, "Space Radiation dosimetry in low-Earth orbit and beyond", Nucl. Instrum. Methods B184, 255 (2001). D. Maia, " Particles from the Sun", 5th international workshop on New Worlds in Astroparticle Physics, 8-10 January 2005, Universidade do Algarve - Faro, Portugal. S. Agostinelli, J. Allison, K. Amako, J. Apostolakis, H. Araujo, P. Arce et al, "GEANT4-a simulation toolkit", Nucl. Instrum. Methods, 506, 250 (2003). M. G. Pia, "The Geant4 Toolkit: simulation capabilities and application results", Nucl. Phys. B - Proc. SuppL, 125, 60 (2003). A. Levin and C. Moisan, "A more physical approach to model the surface treatment of scintillation counters and its implementation in DETECT, in Proc. IEEE 1996 NSS, 2 (1996). A. Owens et al. (ESA-SCI/A), private communication.

E L E C T R O M A G N E T I C PHYSICS MODELING I N G E A N T 4 PACKAGE

V.M. G R I C H I N E P.N. Lebedev Physical

Institute

of RAS,

Moscow

119991,

Russia

Electromagnetic physics processes of GEANT4 package are considered in terms of model approach. The approach allows one to define the priority of models in different GBANT4 regions and to test their predictions versus the region cuts. The examples of ionisation process model implementation are discussed. Recent developments for GEANT4 X-ray transition radiation package are presented. The simulation results are in agreement with experimental data.

1. Geant4 Ionisation Models 1

uses by default so called standard ionisation model. It is essentially Landau-like fluctuation model with additional Poisson-like fluctuations for soft energy transfers. The model parameters relate with the Bethe-Bloch truncated energy loss with a number of corrections. Another approach is based on photo-absorption ionisation (PAI) model allowing one to estimate the cross section of ionizing collisions. PAI-model reads: GEANT4

a

(PNx Mw dx

T

irhc Im

»-£]ta(irjji;.l>-

<»

and dPNn hdudx

aN nh/32u

. . , 2mv2 1 [u . ... , In — 1— / c 7 [u ) aw - " - ' ~ niij •=-• ' w • • 'J0 ~ " ' ' r (w)

^

where a is the fine structure constant, /? = v/c, dx — v dt, m is the electron mass, N is the atomic density, hw is the energy transfer, e is the permittivity, and
480

481 0 . 9 3 A r + 0 . 0 7 C H 4 . 1.5 c m < 2 0 0 C , 2 a t m )

^ 900 "3

g> soo

600 500 400 300 2O0 IOO O

Figure 1. Ionisation energy loss distributions of pion with the momentum 9 GeV/c in the gas mixture 93%AT -f 7%CHA with the thickness of 1.5 cm (STP). Histogram is the experiment, points are GEANT4 simulation according to the PAI model, PAI model with photons and standard ionisation model. The cut value is 1 mm.

Si, 0.032 m m

I

I

6O0 O 4k Z^ O

SOO

PAI model: IO e v e n t s P A I w i t h p h o t o n s : IO4 e v e n t s G E A N T 4 s t a n d a r d : IO4 e v e n t s experiment: positron, p = 2 Gev/c

400

3O0

2O0

IOO

O

2

4

6

8

IO

12

14 16 18 20 Energy Loss (keV)

Figure 2. Ionisation energy loss distributions of positron with the momentum 2 GeV/c in silicon with the thickness of 0.032 mm. Histogram is the experiment, points are GEANT4 simulation according to the PAI model, PAI model with photons and standard ionisation model. The cut value is 0.1 mm. charge e crossing the interface between two media against an additional electric fields induced by the charge in the vicinity of the interface. This field is defined from the boundary conditions. Using the methods developed in 3 one can derive t h e relation describing t h e mean number of X T R photons, JVjn , generated per unit photon frequency OJ and 92 (0 is the emitting angle), d?Nin/hduidO2, inside radiator

482

for the most general XTR radiator consisting of n different absorbing media with fluctuating gap thicknesses tj (j = 2 , 3 , . . . , n — 1): fc-ln-l

atuO2

d?Ni,

2 2 Re[Yt(Zi-Zi+1) +2^2^2(Zi-Zi+1)l[Hj(Zk

Mcjde2 whc

i=l

•>k+l

)]•

i = l k=2

(3) In the case of gamma distributed gap thicknesses tj the correlation factors Hj are:

Hi = r

dti

Jo

t:

-l

r(^)

exp

t

l

2Zj

l+» 2Zu

jVj

(4) where T is the Euler gamma function, tj is the mean thickness of the j - t h medium in the radiator and Vj > 0 is the parameter roughly describing the relative fluctuations of tj. In fact, the relative fluctuation of tj is, $i = V\A^- ^ n e complex formation zone of XTR in the j-th medium 3 ' 4 , Zj (j = 1,2,..., n) is defined by: -l

1 - iLj/lj'

(5)

where i2 = —1, 9 is the angle between the charged particle velocity and the direction of XTR photon, 7~ 2 = 1 — /3~ 2 , /? = v/c is the ration of the particle velocity v and the speed of light c; uij and lj are the plasma frequency and the photon absorption length in the medium, respectively. In the case of a transparent medium, lj —>• oo, the complex formation zone is reduced to the coherence length Lj of XTR, which is real number. In many high energy physics experiments the XTR is detected by the arrays of straw tube gas proportional counters, Fig. (3), inserted in radiator. The G E A N T 4 class G4StrawTubeXTRadiator describes the generation of XTR in straw tube. In the particular case of n foils of the first medium interspersed with gas gaps of the second medium, the summation of (3) results in: cPNin hdw d62

2a

_we2Re{{R(n)}^ (6) H = H1H2, nhc2 " (1 - #i)(l - H2) , ( l - f f ^ ^ l - f T * ) (RW) = (Z, - Z2)2 1-H (1 - H)2

This approach allows us to implement XTR as G E A N T 4 as standard electromagnetic process. The integration of (6) in respect to 62 can be simplified for the case of regular radiator {vi$ -> oo) with transparent in terms of

483

Straw tube crossed by relativistic charges

^ZBBtu'

••-"'"

(P^\' ^^^r i

Figure 3. The diagram of straw tube crossed by charged particles with the charge e generating XTR on the straw tube surface interfaces. The medium 1 is air, the medium 2 is the straw tube wall, and the medium 3 is the detector gas mixture.

dE/dx and X-ray TR (Li/He 300 folis): 0.9 Xe + 0.1 CH,,, 3 cm (STP) 450 histogram: PAI+XTR model 7 = 4xl0 3 , 5 x l 0 3 events • experiment: electrons, p = 2 GeV/c £• 400

_

s 2

*

5 350 300

^

250 200

Fb

150 100

n

'^W

v++r>

50 40 0

50 6(1 Energy Loss (keV)

Figure 4. The ionisation energy loss distribution produced by electrons with a momentum of 2 GeV/c in a gas mixture 0.9Xe + O.ICH4 with a thickness of 3 cm at pressure 1 atm. Closed circles are the experimental data, the histogram is simulation according to PAI and XTR models. XTR radiator: 300 • ( Li / He = 0.04/0.126) mm.

XTR generation media, and n » 1 5 ' 6 . The frequency spectrum of emitted

484

XTR photons reads: f]N-

/•~107

J2|\f-

hdudO = TTHUJ ^4^k ^.(c1 + c2f

hdu -J J — o n

k

* E ( - 'l) (

(K

fc C

(-Jrnin)

2 fc

_ ti, 2 ((»?-fa;|) "J

~ ,

„;„2

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however numerically, only few tens of terms contribute substantially to the sum, i.e. one can choose kmax ~ kmin + 20. The comparison between 7 G E A N T 4 simulation and experimental data is shown in Fig. (4). Some of G E A N T 4 models show satisfactory agreement between experimental data and simulation and other require more experimental details for comparisons. This investigation was partly supported by Software Development Group of Physics Division of CERN in the framework of GEANT4 Collaboration and INTAS-2001-0323 grant. References 1. 2. 3. 4. 5. 6. 7.

Collaboration, GEANT4 : A Simulation Toolkit, Nucl. Instr. and Meth., A506 250 (2003). J. Apostolakis, S. Giani, V. Grichine et al., Nucl. Instr. and Meth., A453 597 (2000). V.M. Grichine, Physics Letters, B525 225 (2002). J. Apostolakis, S. Giani, V. Grichine et al., Comput. Phys. Commun., 132 241 (2000). G.M. Garibyan, Sov. Phys. JETP 32 23 (1971). C.W. Fabian and W. Struczinski Physics Letters, B57 483 (1975). Y. Watase, Y. Suzuki, Y. Kurihara et al., Nucl. Instr. and Meth., A248 379 (1986). GEANT4

PERSPECTIVES IN MEDICAL APPLICATIONS OF MONTE CARLO SIMULATION SOFTWARE FOR CLINICAL PRACTICE IN RADIOTHERAPY TREATMENTS MATTEO BOSCHINI SyS&Net dpt.,CILEA, via. R. Sanzio 4, Segrate (MI) Italy SIMONE GIANI PHdpt., CERN, Geneva 23, CH-1211, Switzerland VLADIMIR IVANCHENKO PHdpt., CERN, Geneva 23, CH-1211, Switzerland PIER-GIORGIO RANCOITA INFNsez.

di Milano, piazza della Scienza 3 Milan.20126, Italy

We discuss the physics requirements to accurately model radiation dosimetry in the human body as performed for oncological radiotherapy treatment. Recent advancements in computing hardware and software simulation technology allow precise dose calculation in real-life imaging output, with speed suitable for clinical needs. An experimental programme, based on physics published literature, is proposed to demonstrate the actual possibility to improve the precision of radiotherapy treatment planning.

1.

Physics Background

The physics of the electromagnetic processes in presence of material, density, atomic number Z, and discontinuities is well measured and understood since years [1]. The key effects are on the energy distribution in zones of sensitive materials followed and/or preceded by higher density and higher Z materials (exactly what happens in the human body). It has been shown since over a decade that standard MonteCarlo programs (EGS [2], GEANT3 [3]) take into 485

486

account and reproduce those processes and effects. However the computing time necessary to maintain the physics quality were prohibitive for clinical usage. On the contrary, the analytical and numerical dose calculations conventionally used for radiotherapy provided adequate speed for clinical needs. The question, whether the physics effects described above were properly taken into account or not, was purely academic, because de facto the speed requirements left no alternatives. The combination of different factors [4] has progressively started to allow performing the highest precision MonteCarlo calculations at speed compatible or superior than what is required for conventional radiotherapy treatment planning. The following key factors can now bring MonteCarlo computational methods to be exploited for clinical practice (assuming that the patient positioning precision will be performed with a sufficient accuracy): 1.GEANT4 [5] expert features created during its related R&D [6] phase (such as combination of production threshold in range and tracking to 0 range, dynamic generation of secondary particles below threshold near boundaries, geometry navigation in 3D voxelized models, volumes parameterization by material, time movement/variation of volumes). 2.1ncrease of CPUs speed up to 3 GHz on the market. 3.Parallelism using commodity hardware (50GHz fit in a 50cm box). Thus it can now be appropriate to re-evaluate the physics performance (and/or limitations) of classical analytical tools, for which we propose a detailed programme of benchmarks. 2. Computing Hardware and Software Performance The experimental setup used to prove the feasibility of real-time dose evaluation, is based on a cluster of high-end computers-off-the-shelf. In particular, the system was made out of 4 dual-cpu nodes, with a cpu clock of 3GHz each, amounting to a total of 8000 SPEC CFP2000. Nodes are connected through a dedicated 1 Gbit/sec LAN. The nodes share all software applications by means of NFS, but jobs are run locally, using a sand-box mechanism. The GEANT4 toolkit was installed, together with the related medical applications software, onto the CPU farm described above. The simplest parallel architecture has been observed to maximize speed performance: the simulation software was run on a master-slave parallelism architecture, where i/o was constrained just to the jobs (and random seeds) distribution at initialization phase, and to the merging of the physics results at finalization phase. The performance of the system was found to scale proportionally to the number of processors. It has to be emphasized that the full system fits within standard

487

office furniture; moreover, current dimensional specifications allow including 50 GHz in a box 50 cm high, which allows standalone-dedicated systems to be used as commodity PC hardware. An ion-therapy dose-calculation has been chosen as first case study. Scanner output with granularity of 1 cubic millimeter has been reproduced. A 3D simulation accuracy of at least 100 micron was enforced to maximize physics quality (Figure 1), including the exact determination of the Bragg peak width and height, of all background sources contributing to shorter-range dose release, and of the higher-range radiation tail due to fragmentation. Configurations with incident Carbon, Neon and Helium ions at different energies have been simulated, in all cases with a performance equal to or better than 7 minutes using 24 GHz. An alternative case study has been preliminarily tested, involving a complete accelerator beam line for photon-based radiotherapy in a leading European medical center. Target extraction, beam collimation, and patient phantom are included. For a typical 5 cubic millimeter scan granularity, 10 million Monte-Carlo events generate accurate dose distribution in 3 dimensions. Preliminary results induce the expectation that results can be computed in about 5-10 minutes using order of 30 GHz. 700

250 300 Depth (mm) Figure 1. Energy deposition of Carbon ions in water.

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3. Validation and Benchmarks The following set of measurements can be performed using dose calculation tools such as Varian [7], Plato [8], BrainScan [9], or other treatment planning software used in radiotherapy. 1."Virtual Calorimeter": this measurement would consist in inputting a geometry model into the fore mentioned programs without using a scanner (e.g. via XML, ASCII, CAD...); in such a case it would be extremely interesting to describe therein the SICAPO [10] calorimeter geometry (Fe-Si-Pb sampling), for which all dimensions and materials (and results) are published; thus a dosimetry calculation could be done directly using those software programs on such a calorimeter configuration; this measurement would be almost at no-cost, because it would be entirely computer-based. 2."Inertial Calorimeter": this measurement is certainly possible at a limited cost, because it would not require irradiation nor operation of detector read-out electronics; it would consist in constructing a "passive detector" built by few sandwiches of lead/silicon/iron layers (just passive material); since data are available and published for different energies and SICAPO configurations, it would be possible to manufacture the passive detector according to the geometrical and chemical specifications of the original SICAPO calorimeter; then, the full scanning imaging system should be used to produce the 3D output of SICAPO (i.e. the calorimeter should be put on the bed of the CT scanner as if it was the patient); such 3D output should finally be given in input to the classical treatment plan software programs as before; and again the results produced could be evaluated. 3."MonteCarlo runs": the same procedures as above should be repeated by executing GEANT4 simulation calculations, both in full geometry mode (case 1.) and in scanner output mode (case 2.); the maximal performance achievable and physics quality should be exploited using GEANT4 expert features and hardware parallelism; all results can finally be compared to the experimental data. 4. Conclusions and Perspectives In case results will demonstrate advantages in favor of the MonteCarlo approach, then one could move to easily show how the discontinuities (in atomic number, interaction and radiation lengths, density) in a SICAPO-like calorimeter actually correspond to the real life situation in the human body composition

489 [11]. Ultimately it can be proposed to investigate how to use CCD technology to take measurements also in corpses, whose composition in terms of bones, soft tissues, air, water, allows to cross-check the predictions. It must be emphasized that the imaging systems must maintain a central role during the computation of results, because all calculations in clinical practice are ultimately done in what is "seen and reconstructed" by the diagnostic apparatus, and not in the ideal engineering geometry/materials models. Computing hardware architecture and software optimization also play a key role, because decisions in clinical practice need to be taken within human-driven time constraints: if results will confirm our expectations, there is a perspective for providing superior dose precision calculations at speed compliant to clinical needs, today. In all cases, we consider interesting to investigate how an integrated solution, including all the aspects discussed in this document, might realistically be put into operation. Such a system could integrate in a unique framework the imaging and diagnostic devices, the GEANT4-based dosimetry for treatment planning, and the treatment hardware. References 1. C. Leroy and P. G. Rancoita, Rep. Prog, Phys. 63, 505 (2000). 2. W. R. Nelson, H. Hirayama and D. W. Rogers, SLACReport 165, (1985). 3. S. Giani, Nucl. Phys. B32, 361 (1993). 4. F.Foppiano et al., Distributed Processing, MonteCarlo and CT Interface for Medical Treatment Planning, Proc. of the 8th ICATPP, World Scientific, Singapore, 324 (2004). 5. S. Agostinelli et al., Nucl. Instr. Meth in Phys Res.. A506, 250 (2003). 6. RD44 Collaboration, CERNLHCC 98-44, (1998). 7. www.varian.com, (2005). 8. www.nucletron.com, (2005). 9. www.brainlab.com, (2005). 10. G. Barbiellini et al., Nucl. Instr. Meth. in Phys. Res. A235, 55 (1985). 11. Particle Data Group, Phys. Rev. D66, (2002).

GEANT4 MONTE CARLO SIMULATION TO ESTIMATE GAMMA-RAY EMISSIONS FROM LUNAR SURFACE AND SELENE SPACECRAFT O. OKUDAIRA, N. YAMASHITA, N. HASEBE, M. MIYACHI, M. MIYAJIMA, M. HAREYAMA, K. SAKURAI, H. YAMAMOTO, M. -N. KOBAYASHI, E. SHIBAMURA, T. HIRAMOTO Advanced Research Institute for Science and Engineering, Waseda Univ, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, JAPAN Using Geant4 Monte Carlo simulation code, background gamma rays for Gamma Ray Spectrometer onboard SELENE were estimated. Gamma ray emissions from the spacecraft body by leakage neutrons from the Moon and GCR protons were calculated as well as ones from the Moon. It is found that a BGO background reduction system works so well that the background gamma rays produced from the spacecraft body is small in the energy spectrum to be observed by the spectrometer in SELENE.

1. Introduction Determining the chemical composition of lunar surface is an essential part of the investigation of the Moon. Gamma ray lines emitted by various isotopoes allow the abundances of many elements to be determined by gamma ray spectroscopy [1,2]. A Japanese lunar orbitar, SELENE, is to be launched in 2007. A gamma ray spectrometer (GRS) is onboard SELENE to study the chemical composition of the lunar surface by the remote sensing method [3,4]. In the present spectroscopic method, it is essential to identify gamma ray lines. Gamma ray measurement in space is inevitably carried out in a severe environment with background due to natural and cosmic ray induced gammas as well as prompt gamma rays induced by secondary neutrons from the Moon. Quantitative understanding of the sources of the background gamma rays is important in order to derive the chemical composition of the lunar surface material with high precision. However, the background gamma rays are difficult to be measure directly during the mission. By the use of Geant4 Monte Carlo code, the background gamma rays from the spacecraft body were calculated and the simulation was made for on the performance of the gamma ray spectrometer.

490

491 2. Gamma Ray Spectrometer onboard SELENE The GRS complies of a cylindrical germanium detector (252 cm3, 66mm and 78mm of diameter and length, respectively) as a main detector, a BGO scintillator (1255 cm3) and a plastic scintillator (5mm in thickness). The BGO scintillation detector serves to reduce Compton background events in the Ge detector. An energy window of the GRS ranges from 100 keV to 12 MeV, with an energy resolution of 3 keV at 1.33 MeV (FWHM) [4]. Such a high energy resolution can identify a number of elements in comparison with those of Apollo and Lunar Prospector. 3. Background Gamma Rays The GRS is mounted on the spacecraft body and looks at the lunar nadir. Therefore, the measurement of lunar gamma rays is interfered by background gamma rays emitted from the spacecraft body. It is important to estimate not only gamma rays from the Moon but also background gamma rays from the spacecraft for increasing the identification capability of elements in the lunar surface. The origin of gamma rays from the spacecraft body are categorized as (1) GCR, (2) neutrons from the Moon, (3) Compton-scattered continuum in the germanium detector and its ambient materials. The germanium detector is surrounded with the BGO scintillator which works as a passive shield against gamma rays from the spacecraft body and as well as an active shield for the anticoincidence system, which rejects signals of energy deposition exceeding 100 keV. The BGO is expected to effectively reduce gamma rays from spacecraft body, Compton-scattered gamma rays and charged particles. 4 Simulation by Geant4 Using Geant4 Monte Carlo simulation codes, we studied gamma rays emitted from the Moon and the spacecraft body, as categorized in (1) and (2) in section 3. Then, by considering the passive/active shield of BGO and plastic scintillators for background suppression, the energy spectrum of the Ge detector was derived, which contains the background occurred in (3), in order to estimate the performance of the spectrometer. To simulate the processes in lunar surface, we used a chemical composition of ferroan anorthosites as a model of the lunar surface material. It consisted of 45.8 wt% Si0 2 , 28.2 wt% A1203, 17.0 wt% CaO, 4.81 wt% FeO, 3.89 wt% MgO, 0.29 wt% Na 2 0 [5]. On the other hands, GCR flux model was based on the equation:

492 1.244xl0 6 £ , (g + 1876)(.E + M + 780exp(-2.5xl0 4 g))(E + M)(E + W6 + M)

J(E,M)[protons/cm2 I si MeV] =

where E is the initial energy of GCR, M is a modulation coefficient [6,7]. The coefficient M is 950 MeV at solar maximum, 550 MeV at solar cycle averaged, 375 MeV at solar minimum. Here, we adopted the M value at 550 MeV. For simplicity, the energy distribution of the primary GCR was divided into 20 bins each 0.5 GeV. For this simulation, a simplified model of spacecraft body (1650 kg, 2.1 X 2.1 X 4.2 m) and detectors was assumed. In this model eight aluminum panels with a thickness of 11.5 mm and three propulsion fuel tanks were incorporated. One of the tanks was spherical and filled with liquid N2O4, the others were cylindrical and filled with liquid 77 26, 30 . nt I ^ » N2H3. Hydrogen in the fuel t contributes to producing _ 16 %S% 10 0 ) l f 66 background gamma rays because it a acts as an effective moderator for S^^^^ 50 neutrons. In this model the z , 140 detectors, as in Fig. 1, was placed vJ on the spacecraft body facing to the Fig.l Simplified model of GRS detectors. lunar surface.

^GeJ

m

• • m

5. Results and Discussions Neutron intensity at the orbital altitude leaked from the Moon was simulated by assuming that GCRs are isotropically incident to the lunar surface materials. A spectrum of gamma rays leaked from the spacecraft body (only lunar side) produced by the lunar 10" neutrons was •fiiel:full calculated. In • fiiel:empty 10"2 addition, a leakage gamma ray spectrum - 1 0 by GCRs directly o Is NU incident to the 1 1 0 spacecraft body was 10" also calculated. Then, both the spectrum 10"6 summed as in Fig.2. 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Energy fceV] From the Fig.2, in the Fig.2 The comparison of gamma ray emissions from the spacecraft with case of empty and without full fuel.

k

s

493

propulsion fuel, the total number of incident background gamma rays was approximately 4.7 times more than that in the case of full fuel. In addition, peaks originated from nitrogen and hydrogen disappeared, whereas peaks from aluminum remain. The background spectrum obtained by GRS was calculated. A spectrum obtained in BGO active shield mode and one in passive shield mode are shown in Fig. 3. It is clear that gamma rays from the spacecraft are substantially suppressed by approximately one order of magnitude by the BGO active shield. l

10"6 0

1000

2000

3000

4000 5000 6000 Energy [keV]

7000

8000

9000

10000

Fig.3 Background spectra of gamma rays detected by GRS with and without anticoincidence. The case of full fuel is shown.

As the results, some peaks appear clearly. An energy spectrum expected to be measured by SELENE GRS in the lunar orbit was calculated and the result is shown in Fig.4. In this estimation, gamma rays from the Moon was re-calculated by taking into account gamma rays on the lunar surface as mentioned by Reedy [7]. BGO active shield was enabled and from tile M oon and spacecraft b.g. from spacecraft

-a cur* SwA^W,

^^^H.

10"3

"sLulJU

wwm

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1000

2000

3000

4000

5000 6000 Energy DteV]

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8000

9000 10000

Fig.4 Expected gamma ray spectrum by SELENE GRS with anticoincidence (upper). The lower spectrum is the same as one with anticoincidence at Fig.3. The fuel tanks are filled.

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the fuel was full. Figure 5 shows energy spectra for 1809 keV gamma rays of 27A1 (left) and 2313 keV gamma rays of 14N (right). From this result, the counting rate of the former gamma rays from the lunar surface is much enhanced in comparison with that from the spacecraft body. We can easily obtain the flux of the gamma rays from Al-composed | ftwn th« Moon and spavawafi• • - - from thw spacecraft | material in the Moon after subtracting the A |AI 1BQ9 [ ft IN 2313 1 background gamma rays from the spacecraft. For the latter, most of them come from the spacecraft 7VA7\A J '•" \ V/\"A/ " ' \>v' s* i \ body. So far as this peak is concerned, it is hard to 27 4 detect gamma rays from Fig.5 Gamma ray spectra around A1 1809 keV ancP N 2313 keV nitrogen-composed material in the Moon amid the background.

i-

!

Vi - /

6. Conclusion We estimated the emission spectrum of gamma rays from the Moon and the spacecraft body with or without anticoincidence system. In the case of full fuel, the number of gamma rays registered by the germanium detector from the spacecraft was reduced to about 1/8 with anticoincidence measurement. The BGO background reduction system worked effectively. Actually, since the fuel left at the beginning of observation is considered to be about 20 - 30% amount before the launch at the beginning of observation, the background level of gamma rays will be considerably small. Further detailed studies of simulation using a more realistic model of the spacecraft structure are in progress. References 1. R.C.Reedy, Proc. Lunar Planet. Sci. Conf. 9, 2961(1987) 2. C.M.Pieters and P.A J.Englert, "Remote Geochemical Aanalysis: Elemental and Mineralogical Composition", Cambrigde Univ. Press, Cambridge, 1993 3. N. Hasebe et al., Lunar Planet. Sci. Conf. XXX, 117(1999). 4. M. -N. Kobayashi et al., Nucl. Instr. and Methods, Res. A548, 401(2005). 5. P.H.Warren, E. Jerde and G. W. Kallemeyn, J. Geophys. Res. 92, E303(1987) 6. G. Castagnoli and D. Lai, Radiocarbon 22, 133(1980). 7. KCReedy, J. Geophys. Res. 92, E697(1987).

CORAL, A S O F T W A R E S Y S T E M FOR V E N D O R - N E U T R A L ACCESS TO RELATIONAL D A T A B A S E S

I. PAPADOPOULOSf R. CHYTRACEK, D. DUELLMANN, AND G. GOVI CERN, IT Department Geneva 23, CH-1211, Switzerland Y. SHAPIRO CERN, PH Department ATLAS Database Group Geneva 23, CH-1211, Switzerland Z. XIE Princeton University Princeton, New Jersey 08544 USA

The COmmon Relational Abstraction Layer (CORAL)1 is a C + + software system ,developed within the context of the LCG persistency framework, which provides vendor-neutral software access to relational databases with defined semantics. The SQL-free public interfaces ensure the encapsulation of all the differences that one may find among the various RDBMS flavours in terms of SQL syntax and data types. CORAL has been developed following a component architecture where the variouw RDBMS-specific implementations of the interfaces are loaded as plugin libraries at run-time whenever required. The system addresses the needs related to the deployment of relational data by providing hooks for client-side monitoring, database service indirection and application-level connection pooling.

1. Introduction CORAL is the set of software deliverables of the "Database Access and Distribution" work package of the POOL project 2 . The development of libraries for the vendor-independent database access, collectively referred to as the Relational Abstraction Layer (RAL), started within POOL in spring 2004. At that time the main requirement that needed to be met was the

'Corresponding author

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496 creation of an insulation layer that would allow the development of software components responsible for accessing data in RDBMS without the knowledge of the subtle differences among the various technology-specific solutions. Since then RAL was adopted by the relational components of POOL (File Catalog, Collections), the COOL (Conditions Database) project 3 , and several components of the software frameworks of the LHC experiments. In spring 2005 a formal review took place of the existing RAL API with feedback from the direct clients of RAL, within and outside POOL. During the review new emerging use cases have been considered that are related to the database deployment and distrubution issues. The outcome was the design of an improved version of RAL, which is now being developed and packaged independently of the rest of the POOL components under the new name CORAL (COmmon Relational Abstraction Layer).

2. Project scope The primary goal of CORAL is to provide functionality for accessing data in relational databases using a C + + API, free of SQL commands and types, shielding the user from the technology-specific APIs. The need for the maximal SQL insulation from the client software can be demonstrated simply showing the SQL statements for two technologies (Oracle 5 and MySQL 6 ), which are used mostly in HEP, in rather simple tasks: (1) Creation of a table • MySQL syntax: CREATE TABLE T_t (I BIGINT, X DOUBLE) • Oracle syntax: CREATE TABLE "T.t" (I NUMBER(20), X BINARYTDOUBLE) (2) Query fetching only the first rows of the result set • MySQL syntax: SELECT X FROM T.t ORDER BY I LIMIT 5 • Oracle syntax: SELECT * FROM (SELECT X FROM "T.t" ORDER BY I) WHERE ROWNUM < 6 The CORAL approach is to present an identical API for such cases, which allows the development of software components that can be used without any code modification or conditional constructs against multiple relational technologies.

497 CORAL is expected to be used by applications running on a gridenabled and distributed environment. It is therefore defining the necessary interfaces for database service indirection, multiple authentication mechanisms (certificate- and user/password pair-based), client-side connection pooling and client-side monitoring. CORAL is not a general purpose C + + connectivity library, such as ODBC, JDBC, the python or perl DBI. It is neither a system to accomodate all access patterns to relational data. Its scope is restricted to serve primarily the use cases relevant to the data handling and analysis of the LHC experiments. 3. Architectural Choices The architecture of CORAL is component-based. The overall system consists of a set of abstract interfaces and several implementation components which have no dependency on each other. Data and control flow only through the abstract interfaces in a user application. The technology insulation is achieved through the set of the abstract interfaces; a client component depends only on them. The functionality expressed by each interface is minimal but complete. The component architecture and the fact that the functionality expressed by each interface is minimal but complete, enable the parallel and independent development of the various CORAL components, as well as additional and user-provided implementations of the same interfaces. They also facilitate efficient unit and integration testing. The realization of the component architecture has been based on the plugin management and the component framework of the SEAL4 project. The latter allows the construction of context hierarchies onto which the various implementation components are hooked. In this configuration if a component requires the functionality of a particular interface, it simply queries its context tree for an available implementation. 4. Functionality of the public A P I A user may connect to a database schema by prividing a contact string which should have a format tech nology-protocol://hostna me: port-n u m ber/schema _na me The technology may take values such as oracle, rnysql, sqlite, etc. The protocol is optional and used only on embedded technologies such as SQLite 7 . It may take values such as file, http, etc.

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The loaded implementation of the IRelationalService interface parses the connection string and searches for available plugins providing the RDBMS functionality for the particular technology. No authentication credentials appear in the connection string. These are provided by the loaded implementation of the IAuthenticationService interface to the RDBMS plugin whenever an authentication is attempted. A user may enable client-side monitoring for a stated verbosity level. In this case the RDBMS plugin searches for a loaded implementation of the IMonitoringService interface and registers information such as begin and end of user sessions, signaling of transaction boundaries and the response times of the various SQL statements that are issued. Alternatively to the format specified above, a user may choose a free format which indicates a logical name for a database service. In this case a query to an implementation of the ILookupService interface has to be issued in order to obtain the corresponding connection string that corresponds to an actual service. The best way of doing so is to use an implementation of the IConnectionService interface which coordinates the control flow among the above components and provides application-level connection pooling. After having retrieved a valid handle to a database schema the CORAL API provides the user with a large subset of the functionality that is possible through generic SQL and RDBMS-specific connectivity APIs: • Schema definition and manipulation operations, such as creating, droping and redefining tables and views, as well as defining indices, keys and constraints. It is also possible to fully describe a given schema and its elements. • Manipulating data in tables such as inserting, modifying and deleting rows. It also allows operations to be executed in bunches, where only the input data change between subsequent iterations. This minimizes the total roundtrips to the database server and increases the overall performance of the application. Operations with BLOB types are facilitated through a Blob C++ type provided by CORAL. • Issuing queries involving one or more tables or views, allowing for row ordering, application of set operations, inclusion of sub-queries and the limiting the rows in the result set. Client-side row caching can be enabled to minimize the total roundtrips to the server. The CORAL interfaces have been designed such that the RDBMSspecific optimizations or standard "best" practices in RDBMS programming are handled internally by the implementation plugins.

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5. Implementation components Every CORAL release provides • three RDBMS-specific implementations for the interface set related to the functionality of accessing data in a relational database (Oracle, SQLite and MySQL), • two implementations of the interface set related to the retrieval of authentication credentials: one based on XML files and another on environment variables, • an implementation of the interface set responsible for peforming the necessary technology dispatching given a connection string and the available RDBMS-specific implementations found at run time, • an XML-based implementation of the interface set responsible for peforming logical to physical database service lookup operations, • a simple implementation of the interface set responsible for registering and reporting monitoring events, • an implementation of the interface set responsible for the client-side connection pooling and the overall system configuration. 6. Development and release procedures The development of CORAL is done incrementally based on individual component tags submitted by the developers. A collection of tags which is validated through a series of integration tests becomes a release candidate. Frequent internal releases are used for early validation by the experiment software integrators and for providing a reference release for incremental development for the CORAL developers. Every component provides the corresponding documentation. During the release procedure, the documentation fragments from all packages are automatically assembled to produce the CORAL User Guide and web documentation. References 1. 2. 3. 4. 5. 6. 7.

http://pool.cern.ch/coraJ/ http://pool.cern.ch/ http://lcgapp.cern.ch/project/CondDB/ http://seal.cern.ch/ http://www.oracle.com/ http://www.mysql.com/ http://www.sqlite.org/

G E A N T 4 SIMULATION OF T H E ATLAS M U O N SPECTROMETER

N. CHR. BENEKOS Max-Planck-Institut fr Physik, Fohringer Ring 6 80805 Muenchen, Germany E-mail: [email protected] D. REBUZZI INFN-Pavia Via A. Bassi, 6 27100 Pavia (Italy) E-mail: [email protected] The Large Hadron Collider (LHC) at CERN will offer an The ATLAS detector is designed to fully exploit the potential of LHC providing high accuracy measurements in terms of mass and type identification of particles emerging from the LHC proton-proton collisions. Given the complexity of the apparatus in this demanding environment, an Object-Oriented(OO) methodology is required to manage the full detector simulation. The GEANT4 simulation of the ATLAS Muon Spectrometer, implemented in a OO perspective, is an ambitious task since it deserves not only a very detailed description of the apparatus geometry, but also it covers issues connected to a fast but precise simulation and a correct implementation of the different detector responses. A report on the Muon Simulation activities during the last year is the content of the present paper.

1. Introduction The ATLAS detector 1 , currently being installed at CERN, is designed to provide precise measurements of 14 TeV proton-proton collisions at the Large Hadron Collider2, starting in 2007. The apparatus, one of the biggest and most complex ever designed, requires a detailed and flexible simulation to deal with questions related to design optimization and to issues raised by staging scenarios, enabling detailed physics studies which will lay the basis for the first physics discoveries. The reference simulation tool adopted by all the detector component applications is the GEANT4 4 , 5 package. It deals with the new physics environment, testing extensively the appara-

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tus prototypes and exploiting comparison with real data from the ATLAS Combined Test-Beam (CTB). The full simulation of the ATLAS detector consists of a collection of independent modules developed by the different teams of each subdetector (Inner Tracker, Calorimeters, Muon Spectrometer and Magnet System). Dynamic loading and organization in the form of plug-in modules make the implementation extremely simple and versatile. In this note we present a review of the activities of the simulation for the Muon System during the last year, starting from an analysis of the program design for the different components of the Spectrometer 7 , describing all the applications implemented (such as Detector Description 6 and and Digitization 8 ), with focus on the the test-beam physics and on the physics validation issues.

2. The ATLAS Muon Spectrometer Simulation The Muon Spectrometer of the ATLAS experiment is a large and complex system relying on several detector technologies. The simulation of muon events require a very careful description of these detectors, some of which provide a muon trigger (RPC,TGC) while the others make precision measurements of tracks (MDT,CSC). A detailed description of the passive materials, toroidal magnet systems and shield is also needed to account for Coulomb scattering and energy losses. Input for simulations comes from event generators after passing a particle filtering stage. The Muon Hit is the response of the detector following the interaction of a particle with its sensitive part (the gas in the CSC for example). Hits are labelled by a Simulation Identifier, SimID, a 32-bit integer in which the geometry information about the hit position is folded. The SimlDs are enough to access to all the geometrical information needed later on in the ATLAS offline full chain. Muon hits are stored in HitCollections, one for each Muon technology, where they are inserted in a random way (no sorting is performed at the simulation level). There are therefore four independent hit collections in the Muon Spectrometer, one for each technology, for each event. The hit information is combined with estimates of internal noise and subjected to a parametrization of the known response of the detectors to produce the simulated digital output, the Muon Digits, which are the input to the offline reconstruction programs. The digitization of GEANT4 hits in ATLAS Muon Spectrometer 8 is performed subdetector-by-subdetector.

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The Muon digit object carries two pieces of information: an offline compact identifier (OID) of a readout element and the simulated data coming from the same element (TDC, ADC counts on MDT tubes, charge on CSC strips, RPC time of flight, etc). The digits can be feed to the pattern recognition and track reconstruction algorithms as if they were real data. In addition of being capable of handling hits coming from a single bunch crossing, the Muon Digitization is also able to handle piled-up collisions. Hits from several bunch crossings are overlaid taking into account the global time of the hit which is defined as the GEANT4 hit time, plus the bunch crossing time with respect to the main crossing. 2.1. Muon Detector

Description

The Muon Detector Description 13 of the Muon Spectrometer, is provided by the GeoModel 6 ' 7 toolkit, a library of geometrical primitives that can be used to describe detector geometries. The main purpose of GeoModel, here used in its implementation for the Muon Spectrometer, MuonGeoModel (MGM), is to support a central store for a unique detector description information that can be accessed by many clients, the simulation, the digitization and reconstruction programs. All Muon System components and corresponding materials are described in full details using MGM mechanisms. Recentely, also the dead matter component of the Muon System (Toroids, Endcaps, Feet and Rails) has been included in MGM. The shielding description is ongoing. MGM copes witht the GEANT4 simulation via the Geo2G4 interface, which translates the MuonGeoModel description to GEANT4 by navigating the raw geometry graph. The ATLAS GeoModel project started in summer 2002. Since then the geometries of all ATLAS detector subsystems have been described using GeoModel mechanisms. 3. TestBeam Simulation Beam tests are an ideal proving ground for a detector simulation program as they provide a direct comparison with experimental data in a simplified detector setup. In 2004, a large scale system test has been performed in H8 experimental area at CERN. As extensively described in 9 , the H8 setup included twelve Monitored Drift Tube (MDT) chambers arranged to emulate one barrel tower (six barrel MDTs) and a wedge of one endcap octant (six endcap MDTs). Simulating the ATLAS Combined Test Beam permitted to test the reliability of the simulation suite and to validate the

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physics models employed. A 3D view of the simulated testbeam setup is shown in Fig 1.

\ iracKer Mag not

Figure 1. The ATLAS combined testbeam.

Using the data collected in the summer 2004 period, the barrel sagitta reconstruction was studied as a function of the incident muon momentum 12 , thus allowing to disentangle the intrinsic resolution contribution to its resolution, from the multiple scattering one. The results obtained have been compared to the GEANT4 simulation, resulting in an important validation step for the simulation code. For instance, an initial misagreement, between data and simulation, on the sagitta resolution, allowed to fix an RPC component material, which had been otherwise wrongly set in the simulation Detector Description. Among other parameters, data and simulation residuals has been compared, to check the digitization (and simulation) procedure, finding a good agreement, as shown in Figure 2. The Testbeam simulation used the same software as the full experiment, based on the same framework and implementation as for the ATLAS DataChallenges. 4. Conclusions The computing Data Challenges phase 2, ("DC2" n ), completely performed in the GEANT4 environment, proved the robustness and the stability 10 of the whole simulation apparatus on huge amount of generated event samples (single particle, B scans and Higgs) as well as on high statistics single particle, minimum-bias events , QCD-jets and physics event productions. The Muon Spectrometer simulation has been used successfully in the context of the ATLAS Data Challenges. It has also been validated in the context of

504 | Residual Comparison

\

J "'• 23 DATA I

•

SIMULATION

Residuals (mm)

Figure 2. Residuals for the data/simulation comparison.

CTB

Muon

barrel

chambers

(BIL/BML/BOL),

the experimental program of the ATLAS Testbeam, where analysis of the chamber performances relies crucially upon the simulation and the Detector Description model. 5. Acknowledgements The authors would like to thank all the core developers for the robust, versatile and complete code provided, the Muon subdetector people for their prompt implementation of any new proposed functionality, and Dr. Armin Nairz as ex-ATLAS Muon Simulation Coordinator for his excellent work and his important role in the success of this project. Sure to be incomplete in this review the authors would like to extend all their gratitude to everyone they might have forgotten. References 1. ATLAS Collaboration (1994), ATLAS Technical Proposal for a GeneralPurposes pp Experiment at the Large Hadron Collider at CERN, CERN/LHCC/94-43, CERN, Geneve. 2. D. Boussard et al., The Large Hadron Collider Conceptual Design, CERN/AC/95-05 (1995). 3. D. Boussard et al., ATLAS Muon Spectrometer Technical Design Report, CERN/LHCC/97-22, 1997. 4. The GEANT4 Collaboration, GEANT4 - A Simulation Toolkit, Nuclear Instruments and Methods in Physics Research, NIM A 506 (2003), 250303. 5. GEANT4 webpage, http://wwwasd. web. cern. ch/wwasd/geant4/geant4 • html 6. J. Boudreau, V. Tsulaia, The GeoModel Toolkit for Detector Description, Proceedings of CHEP04 Conference.

505 7. K. Assamagan et al., The Description of the ATLAS detector, Proceedings of CHEP04 Conference. 8. D. Rebuzzi, K. A. Assamagan, A. Di Simone, N. Van Eldik, Y. Hasegawa, GEANT4 Muon Digitization in the Athena Framework, in publication. 9. J.Bensinger et al., Muon Spectrometer Program in H8, http://cerutti.home.cern.ch/cerutti/h8-program2004.pdf. 10. D. Costanzo et al., Validation of the Geant^-Based Full Simulation Program for the ATLAS Detector: An Overview of Performance and Robustness, ATLAS Internal Note, ATL-SOFT-PUB-2005-002 (2005). 11. Gonzlez de la Hoz, et al., ATLAS Data Challenge 2 : A massive Monte Carlo production on the GRID, ATLAS Internal Note, ATL-SOFT-PUB2005-001 (2005). 12. G- Avolio, E. Meoni, A. Policicchio, F. Cerutti, S. Ventura, S. Rosati, D. Rebuzzi, Barrel Sagitta Resolution versus Momentum at 2004 B\8 Test Beam data and Comparison with GEANT4 Simulation, in publication. 13. ATLAS Muon Detector Description, http://muondoc.home.cern.ch/ muondoc/Software/DetectorDescription

TRACK E X T R A P O L A T I O N W I T H I N T R I N S I C NAVIGATION IN THE N E W ATLAS T R A C K I N G SCHEME

A. S A L Z B U R G E R * Institut

fur Experimentalphysik,

Universitat

Innsbruck

The transport of track parameters and their associated errors to surfaces within the detector is a very frequent process in the event reconstruction of high energy experiments. Track predictions are necessary for various pattern recognition algorithms and the transport of track parameter errors is essential for most of the track fitting techniques that, in general, rely on the first and second momentum of the underlying probability distribution. The realization of the newly developed track extrapolation engine for the ATLAS experiment and its specific property of a dedicated connective reconstruction geometry providing fast navigation and access to material and magnetic field information is presented in this paper.

1. Introduction The transport of track parameters (i.e. a track representation on a specific surface) and their associated covariances can be divided into two realms: the purely mathematical propagation of the track and its according errors depending on the underlying track model (in the following referred to as propagation) and the application of material effect updates according to the traversed material. The full process of propagation and material effects integration will be referred to as track extrapolation. The ATLAS experiment is one of the four experiments under construction to be operated with the Large Hadron Collider (LHC) at CERN, Geneva. Its design follows a classical scheme of particle collider detectors, consisting of a central inner tracker (embedded in a 2 Tesla solenoidal magnetic field), followed by electromagnetic and hadronic calorimeters, finally by an outer muon system that is realized as a stand alone tracking device with an extra toroidal magnetic field (peak intensity of up to 4 Tesla). However, the complex geometrical and magnetic field setup of the ATLAS *[email protected], CERN/PH-ATC

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detector puts stringent requirements on the reconstruction software, in particular for the extrapolation package: (a) extrapolation to surfaces of different types and arbitrary orientation (b) extrapolation within a highly non-homogeneous magnetic field (c) correct material effects integration during the extrapolation process (d) good performance in respect of cpu time consumption due to the high track density and event rates 2. Design of the Extrapolation Package The new track extrapolation engine has been fully integrated in the ATLAS C + + software framework ATHENA 1 and is embedded in the common ATLAS Event Data Model (EDM). Following the ATLAS software design principles, a rigorous separation between EDM objects and algorithms adhered to by the extrapolation software design. Each building block of the extrapolation package is implemented with an extra layer of an abstract interface carrying all public methods. a This allows flexibility and facilitates the exchange or introduction of newly developed algorithms.

Figure 1. Illustration of the involved algorithms in a sample extrapolation process. For the user only the highest level - consisting of input parameters, extrapolator engine and output parameters - is visible. The illustration shows also the three columns the extrapolation is built on: navigation, propagation, material effects integration

The extrapolation package is built on a three column structure, separating the conceptually different tasks, see Fig. 1: a T h i s enables in addition dynamical loading of libraries at runtime and therefore shortens the building and linking time and decreases the library sizes

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(a) propagation: realized through the IPropagator interface, see Sec. 3 (b) navigation: realized through the INavigator interface, see Sec. 4 (c) material effects integration: realized through the IMaterialEffectsUpdator interface, also described in more detail in Sec. 5 A central steering tool implemented under the IExtrapolator interface invokes these three processes by calling the dedicated sub-algorithms using only detector-technology independent representations of EDM objects. This structure along with a powerful configuration scheme is the key feature to make the extrapolation engine a universal tool for ATLAS. Its full power can be used on, for example, different layouts of the full ATLAS or any test-beam setup by exchanging the underlying reconstruction geometry only. In a similar way different track models can be taken into account by exchanging the propagation without compromising the navigation capabilities. In combination with the optimal choice of track fitting and reconstruction strategy, this extrapolation engine is aimed at achieving the highest possible accuracy in track reconstruction.

3. Propagation Four different propagation algorithms are currently implemented in the new track extrapolation package, following fully analytical or step-wise analytical track models. The different propagator algorithms extend the common IPropgator interface class and are designed to perform propagations to surfaces of all given types and orientations in the ATLAS reconstruction geometry: Propagations of both track parameters and associated errors according to an underlying linear or helical track model can be performed fully analytically with the so-called StraightLinePropagator respectively the HelixPropagator. Two propagation algorithms based on a Runge-Kutta integration algorithm are available in the new ATLAS extrapolation realm. They estimate the intersection of a track with a surface in a step-wise approach. A special flavor of a Runge-Kutta based propagator is the so called STEP-Propagator that corrects for material effects continuously during the propagation, as both energy loss and multiple scattering effects are embedded in the equations of motion through the magnetic field. This propagator is designed for the propagation through dense material, such as e.g. the propagation through the ATLAS calorimeter for matching of tracks determined in the

509 inner tracker with corresponding muon system tracks or simply for combined track fitting. A more detailed description of some of the propagator types can be found in 2 .

4. Navigation between Volumes and Layers The navigation and access to material and magnetic field information is based on a completely connective reconstruction geometry that is built by dedicated geometry builders by parsing the full ATLAS detector description. The reconstruction geometry is a simplification of the ATLAS detector geometry and combines the access to material information through confined material layers and magnetic field information by building the interface to the common magnetic field service. The simplification of the full detector geometry is vital for time performance; the number of describing volumes of the full detector geometry exceeds the number of volumes in the reconstruction geometry by five orders of magnitudes.

- SCT Barrel

Pixel SCT ^-""Vtft. v Gap Volume

Beam Pipe Volume J**^

Pixel Barrel

Figure 2. Left: Schematic illustration of the extrapolation process through three different volumes by means of navigating through their boundary surfaces. Right: Realization of the volume and layer structure for the ATLAS Silicon detectors in the central region.

In the reconstruction geometry a volume is characterized by its confining boundary surfaces which in turn provide information of the attached volumes on both sides. Each boundary surfaces is uniquely oriented by its normal vector and can therefore serve as an intrinsic guide to the next volume along a track direction. Within a volume, the navigation is based on layers representing the simplified material description of the reconstruction geometry. Both boundary surfaces and layers extend the common surface

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representation and can therefore be naturally used together with the extrapolation engine. 5. Material Effects Integration The correct treatment of material interactions of the particle while traversing the detector to the given destination surface is crucial for the quality of the track reconstruction. Two different strategies are realized for the inclusion of material effects updates during the track extrapolation: a classical layer based approach and a volume based approach which is closely related with the development of the STEP-Propagator. In a layer based material update, the material has to be modelled as thin surfaces on which the update of the track parameters is done in a point-like correction to the curvature and an according smearing of the angular representation of the track. The volume based material update requires the inclusion of material interactions into the equations of motion as briefly described in Sec. 3. It is designed for propagation through dense material. Dedicated validation and calibration algorithms are embedded in the ATLAS track extrapolation package to facilitate the control of the inert material budget within the reconstruction geometry. 6. Conclusion A new extrapolation package has been deployed in the restructured ATLAS reconstruction software. It enhances the transport of track parameters and their associated errors to arbitrarily oriented surfaces of different types and within a highly non-homogenous magnetic field. The navigation involved in this task is given by a fully connective reconstruction geometry facilitating the access to material and magnetic field information. The extrapolation package is a vital component of the new ATLAS track reconstruction software and has been successfully used to reconstruct real data from the ATLAS Combined Testbeam in 20043. References 1. http://atlas.web.cern.ch/Atlas/GROUPS/SOFTWARE/00/architecture 2. Salzburger A., "The Track Extrapolation Package in the new ATLAS Tracking realm", Proc. of CHEP2004 3. Liebig W. et. al., "Test of the ATLAS Inner Detector reconstruction software using combined test beam data", Proc. of CHEP2004

APPLICATIONS OF GEANT4 FOR THE ESA SPACE PROGRAMME G.SANTIN Rhea System SA ,Louvain-La-Neuve, Belgium Currently based at the: Space Environment and Effects Analysis Section, European Space Keplerlaan 1, Noordwijk, 2200AG, The Netherlands

Agency

P.NIEMINEN, E.DALY Space Environment and Effects Analysis Section, European Space Keplerlaan 1, Noordwijk, 2200AG, The Netherlands

Agency

Knowledge of the potential impact of the radiation environment on evolving space borne devices rely on effective analysis tools for the understanding and the prediction of the basic effects of the particle environment on new technologies. In addition to experimental measurements, detailed numerical analyses are mandatory for the evaluation of the effects to detectors and components and, in human exploration initiatives, also to humans. The GEANT4 particle transport software is being used for a broad range of space applications. Current ESA-initiated activities in this area include calculation of radiation fluxes inside the ISS and for interplanetary missions, improved hadronic models, prediction of the radiation environment on other planets in presence of local atmospheres and magnetic fields, and studies of effects of radiation at the cellular and sub-cellular level.

1. Introduction Hostile environments such as energetic particles, hot and cold plasmas, meteoroids and space debris, or the effects of the residue of the atmosphere at very low orbital altitudes can severely limit space missions. Radiation shielding analysis is a crucial process in the spacecraft and space instrument development cycle to address the issues posed by the energetic particle environment, and simulations of the particle transport play a major role in spacecraft development, from design to operational phase and to post-flight analyses. In addition, particle transport studies are used for design, calibration and analysis of scientific detectors onboard space missions.

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512 1.1. GEANT4 and the space particle radiation environment GEANT4 is a toolkit for the simulation of particle transport in matter1 . It has been initially developed by a collaboration (CERN RD44) of more than 100 scientists from over 40 institutes in Europe, America and Japan, and is currently maintained and still upgraded by an international collaboration. While developed in the context of High Energy Physics (HEP) experiments, it is currently used also in nuclear physics, space applications, medical physics, astrophysics and radioprotection. The capability of handling particle transport in complex geometries, together with a good physics description make GEANT4 a good simulation tool for the description of the effects of radiation in space. In addition, open source approach, object orientation and high modularity ease the extendibility of the toolkit and interfacing to external modules in areas such as geometry modeling, visualization, post-processing, and analysis. 2. Selected GEANT4-based analyses for ESA missions The European Space Agency (ESA) has been a Member of the GEANT4 Collaboration since 1997 and has supported via its R&D programme the development of a number of space-specific GEANT4 tools, models and mission or instrument applications. These include the development of physics models of relevance to space applications and in the promotion of new engineering tools for the standard radiation analysis procedure for future European missions. The internal use of GEANT4 is also part of the standard support to mission design and a number of Space Science missions have benefited from GEANT4 analysis capabilities. We present here a selection of the applications of these models and tools to ESA and other missions. Some of the GEANT4-based tools developed to aid the engineering design and operational phases of ESA missions will also be outlined. 2.1. Detector response in optical systems The description of optical processes in GEANT4 has been a key advantage for the study of the AMS veto system. An analysis of the veto scintillating detectors and their optical signals has been performed by LIP Lisbon within an ESA funded project. An investigation of the influence of the scintillator surface properties has led to the development of detailed models for the simulation of

513 the internal reflection processes, which finally led to an agreement of the simulated behavior with the results of experimental ground tests. Optical simulations were also performed for the EUSO experiment by LIP Lisbon. In particular, the properties of the Fresnel lens were investigated; a detailed geometrical model of the lens profile led to the conclusion that a strong non-uniformity of the focusing power was caused by unexpected diffraction of optical photons in some areas of the lens.

2.2. Physics extensions in the low energy electromagnetic domain The Low-Energy extension of the electromagnetic processes is of interest for future space missions, with examples in the study of the elemental composition of planetary surfaces through X-ray fluorescence, in the new space borne X and gamma ray telescopes, with the possibility of polarized radiation analysis, or in the internal charge deposition analysis for the proof masses in the LISA mission. A significant example of the importance of these extensions is the case of the BepiColombo mission to Mercury: surface analysis will be performed with a spectrograph, which will detect the X-ray emission generated from the impact of solar protons on the materials on the planet. An ESA co-funded improvement in the Proton Induced X-ray Emission (PIXE) model by INFN Genova has enabled the description (with a spectacular agreement) of the X-ray fluorescence in ground experiments with protons impinging on several sample materials2' . The LISA mission aims at the detection of gravitational waves with a constellation mission composed of three spacecraft, with an arm length of 5 million kilometers and a required control of the relative position of the proof masses to the picometer level. Knowledge and control of the charging of the proof masses is of extreme importance. The internal charging of the masses by cosmic rays, solar particles, delta rays and displaced ions has been investigated by Imperial College, London3 both for LISA and its precursor mission LISA Pathfinder. For verification purposes, comparisons between simulations and experimental charging data from the NASA Gravity Probe B mission have also been made by IC. The low energy extensions of the GEANT4 electromagnetic packages have been essential to analyze these effects down to a lower energy of 250 eV. 2.3. Low energy grazing angle proton scattering The X-ray Multi Mirror mission (XMM) objectives include study of black holes and formation of galaxies through the observation of sources with three X-ray

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telescopes. An unexpected loss of Charge Transfer Efficiency of CCD's due to Displacement Damage was observed on the ACIS instrument on board the NASA Chandra mission, which has a similar configuration, just months before the launch of XMM. The cause of the damage was traced to radiation belt low-energy protons scattering off the X-ray mirrors. Surface scattering is the underlying physics process, for which an improved description has been obtained in a collaborative effort with participation from the GEANT4 developer collaboration and ESA experts. New physics was required for treating proton scattering at very small angles: in particular, the multiple scattering model has been improved and the Firsov model (for the description of grazing angle proton scattering from the surface of the X-ray mirrors) has been developed and added to the GEANT4 physics, giving a final prediction for the proton rate onto the XMM detectors which is in very good agreement with the observed rate in space4' 5' . The GEANT4-based study has led to a new operation planning with an extra beam shutter procedure during radiation belt passages, which has successfully protected the XMM X-ray detectors from damage by low energy protons. 2.4. Physics development and validation for the exploration initiative A recent increasing focus on robotic and human missions for exploration initiatives has led to new requirements for the particle transport simulations. The description of the radiation effects during long interplanetary journeys and a long permanence on other planets with scarce natural shielding brings the attention to planetary radiation models (e.g. scaling of solar particle event fluence to other planets, magnetic field description, surface maps, etc.), to physics models for hadronic and in particular nucleus-nucleus processes (for the description of the interactions of cosmic-ray high energy heavy ions), and to the description of biological effects (with macroscopic and microscopic models). The main goal of the DESIRE project is the accurate Monte Carlo calculations of the radiation fields and doses to astronauts inside the European Columbus module of the International Space Station. An extensive physics validation has been performed in the framework of the project (co-funded by ESA) with detailed comparisons the simulation of proton hadronic processes with experimental data6' . A complete geometry model of the International Space Station has also been developed, enabling the investigation of the dependence of the dose to astronauts on the directionality of the radiation impinging on the spacecraft.

515 Physics models for nucleus-nucleus collisions are the subject of recent and ongoing development within ESA funded projects. A recent addition to the GEANT4 hadronic package includes the implementation by QinetiQ7 of the abration/ablation model and new nuclear-nuclear cross-sections8' 9' . The evaluation of risks from human exposure to space radiation is also being addressed with the ESA funded REMSIM and GEANT4-DNA projects. The former has established preliminary strategies for manned missions to Moon and Mars, while the latter aims at the description of nano-scale damage effects of energetic particles at the cellular and DNA molecule level. Extensions of some of the electromagnetic processes in water down to energies of a few eV are being developed by the GEANT4-DNA collaboration. In addition, biological effect models are being reviewed and developed and a library of human phantoms is being added to the GEANT4 distribution10 . Simulations of particle transport on Mars in particular raise some specific issues, including the scaling of solar events and GCR fluxes at 1.5 AU, a broad range of altitudes on the Martian surface, seasonal, diurnal and local variations of atmospheric pressure, dust storms (for UV and X-rays), and surface backscattering (for which knowledge of the local geology is mandatory). In addition, local magnetic fields are present in the Martian southern hemisphere. The PLANETOCOSMICS GEANT4-based tool, by Bern University, enhances and generalizes the previous MAGNETOCOSMICS and ATMOCOSMICS11' tools, which were developed for the transport of particles in the Earth magnetosphere and atmosphere. The new tool has been extended with a description of the local magnetic field on Mars and of the dipole field on Mercury. Models for the planetary atmospheres are also included. Similar ESAfunded developments, with different Martian atmospheric models, are also ongoing in the frame of the RESTEC activity with Lisbon Institute of Physics. 2.5. Usability and interfaces ESA GEANT4-related R&D plans for the next years will focus strongly on making GEANT4 readily accessible to a variety of engineering applications and WWW-based radiation effects studies through the development of easy-to-use interfaces, advanced models, and auxiliary software. Particular attention is paid to the development and improvement of tools of relevance for the radiation environment in space to replace/supplement tools currently employed in ESA projects and missions12' . This has brought to the development of ready-to-use engineering tools utilizing the full GEANT4 physics and other capabilities but allowing rapid radiation analyses.

516 The Sector Shielding Analysis Tool (SSAT) performs ray tracing from a user-defined point within a GEANT4 geometry to determine, with very little computing resources, shielding levels and shielding distribution. The tool has recently been upgraded with the calculation of doses based on externally provided dose-depth curves. The Multi-Layered Shielding Simulation Software (MULASSIS)13' is a successful example of ready-to-use analysis tool with user-friendly interface. It performs fluence, Total Ionizing Dose (TID) and NIEL dose analysis in ID geometries made of several layers of arbitrary materials. A web interface to MULASSIS has been developed in SPENVIS14' . The recently developed Geant4 Radiation Analysis for Space (GRAS) tool15' addresses some of the issues left open, such as modularity of the analysis, flexibility of the design and complex 3D geometry models. Analysis types include at present TID, NIEL, NIEL, SEE, dose equivalent and equivalent dose16 17 , path length and charge deposit. A SPENVIS web interface is under development. 3. Conclusions We presented the application of GEANT4 to space missions in the context of the ESA programme. GEANT4-based applications and tools developed with ESA support have been applied for a wide range of studies including particle transport in planetary magnetic fields, instrument design, data analysis, shielding assessment and optimization. Synergies with medical activities have been useful to address critical areas of improvement, including interfaces and physics model extensions. Fruitful collaborations among ESA, academia and external institutes has led to the creation of Geant4-based applications and tools, which are now openly offered to the community. Further coordination and cooperation in basic research and related engineering developments are highly desirable. Acknowledgments This work was supported in part by the ESA Technology Research Programme, General Studies Programme, Portuguese Task Force, and the ESA Aurora Programme.

517 References 1. S.Agostinelli et al. [GEANT4 Collaboration], "GEANT4: a simulation toolkit", Nucl .Instrum. Meth. A506, 250 (2003). GEANT4 web site: http://cern.ch/geant4 2. S.Guatelli, A.Mantero, B.Mascialino, P.Nieminen, M.G.Pia, S.Saliceti, "GEANT4 atomic relaxation", Nuclear Science Symposium Conference Record, 2004 IEEE Volume 4, 16-22 Oct. 2004, 2178-2181 Vol. 4 3. T.Sumner, H.Araujo, D.Davidge, A.Howard, C.Lee, G.Rochester, D.Shaul, P.Wass, "Description of charging/discharging processes of the LISA sensors", Class.Quant.Grav.21:S597-S602 (2004) 4. R. Nartallo, E. Daly, H. Evans, P. Nieminen, F. Lei and P. Truscott, "Lowangle scattering of protons on the XMM-Newton optics and effects on the on-board CCD detectors", IEEE Trans. Nucl.Sci. 48, 1815-1821 (2001) 5. F.Lei, R.Nartallo, P.Nieminen, E.Daly, H.Evans, P.Truscott, "Update on the use of Geant4 for the Simulation of low-energy protons scattering off X-ray Mirrors at grazing incidence angles", IEEE Trans. Nucl. Sci. 51, Issue 6, Part 2, 3408-3412 (2004) 6. T.Ersmark, PCarlson, E.Daly, C.Fuglesang, I.Gudowska, B.Lund-Jensen, R.Nartallo, P.Nieminen, M.Pearce, G.Santin, N.Sobolevsky, "Status of the DESIRE project: GEANT4 physics validation studies and first results from Columbus/ISS radiation simulations," IEEE Trans. Nucl. Sci., 51, Issue: 4, 1378-1384 (2004). DESIRE web page: http://gluon.particle.kth.se/desire/ 7. P.R.Truscott, "Nuclear-Nuclear Interaction Models in GEANT4", QINETIQ/KI/ SPA CE/ SUM040821/1.1 (2004). 8. J. W. Wilson, RK Tripathi, FA Cucinotta, JK Shinn, FF Badavi, SY Chun, JW Norbury, CJ Zeitlin, L Heilbronn, J Miller, "NUCFRG2: An evaluation of the semiempirical nuclear fragmentation database," NASA Technical Paper 3533 (1995). 9. L.W.Townsend, J.W.Wilson, R.K.Tripathi, J.W.Norbury, F.F.Badavi, and F.Khan, "HZEFRG1, An energy-dependent semiempirical nuclear fragmentation model," NASA Technical Paper 3310 (1993). 10. P. Nieminen and M.G. Pia, "II progetto GEANT4-DNA", AIRO Journal, March 2001 (in Italian) 11. L. Desorgher, E.O. Fluckiger, M.R. Moser, R. Butikofer, "GEANT4 simulation of the propagation opf cosmic rays through the Earth atmosphere", published in *Tsukuba 2003, Cosmic Ray* 4277-4280 (2003). For information about the MagnetoCosmics tool, see the URL: http://reat.space.qinetiq.com/septimess/magcos 12. G. Santin et al., "New GEANT4 Based Simulation Tools for Space Radiation Shielding and Effects Analysis", Nuclear Physics B (Proc. Suppl.) 125, 69-74 (2003)

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13. F.Lei, P.RTruscott, C.S.Dyer, B.Quaghebeur, D.Heynderickx, P.Nieminen, H.Evans, and E.Daly "MULASSIS: A GEANT4 Based MultiLayered Shielding Simulation Tool", Proceedings of IEEE Nuclear and Space Radiation Effets (NSREC02), Phoenix (USA), 20-24 July 2002. 14. D.Heynderickx, B.Quaghebeur,, E.Speelman, E.Daly, "Space Environment Information System (SPENVIS): A WWW interface to models of the space environment and its effects", AIAA-2000-0371 (2000). SPENVIS web-site: http://www.spenvis.oma.be/spenvis/ 15. G.Santin, V.Ivanchenko, H.Evans, P.Nieminen and E.Daly "GRAS: A General Purpose 3D Modular Simulation Tool for the Space Environment Effects Analysis", accepted for publication on IEEE Trans. Nucl. Sci. (2005). 16. "1990 Recommendations of the International Commission on Radiological Protection", Annals of the ICRP, Vol. 21, No. 1-3 (1991). 17. "Relative Biological Effectiveness (RBE), Quality Factor (Q), and Radiation Weighting Factor (wR)", Annals of the ICRP Vol. 33, No. 4 (2003).

DETECTOR SIMULATION IN HIGH ENERGY PHYSICS J.P. WELLISCH Private scholar, 1279 Avenue de Jura Sergy, F-01630, France The geant4 simulation tool-kit has been developed as a simulation tool-kit for particle physics, and an ever growing list of laboratories and institutes are using geant4 as baseline for their simulation needs. In addition, there are a growing number of high energy physics experiments that use the technology provided by this program in the preparation of the detector and the interpretation of the data. We will give a brief history of detector simulation in high energy physics as it relates to geant4, and summarize the activities in the field. We will also analyze the reasons for the current successes of the geant4 tool-kit with a penchant towards hadronic physics simulation.

1. History of Geant4 At the time of writing, the effort that resulted in the current Geant4[l] releases is close to 12 years old. The first prototypes for large parts of the code were developed not inside the geant4 collaboration, but as part of a CERN R&D project, RD44[2], This project was originally conceived as a technology R&D project, with the goal to study modern software development techniques, software processes, and technologies (C++), to eventually construct the next CERN version of the GEANT[3] simulation program. As such it was the first successful attempt to replace a major CERN software package for the next generation of high energy physics experiments. At the end of the RD44 phase, a Memorandum of Understanding was written to give the program a future R&D and deployment path. CERN management decided that they were not interested to be host laboratory for this effort in future. The name GEANT was abandoned, being a well known trademark of CERN, and was replaced by the name under which the program is known today - Geant4. Already during the RD44 phase a sizable number of requirements were collected. The main source of this was high energy physics and the preparation of LHC detector simulation, but a variety of requirements were also collected from other areas like heavy ion physics, CP violation physics, cosmic ray simulation, medical applications, and space science and engineering. In order to meet such requirements, a component approach was chosen for the physics 519

520

implementations, which in turn allowed the group to build in a large degree of flexibility and functionality. 2. Geant4: Components and use-cases The basic considerations that drove the overall software design, in particular for the physics, was the need for extensibility. An open and extensible architecture was used. Implementation frameworks, in particular for the case of hadronic physics[4], were used to create abstractions of the responsibilities in the problem domain, and RD44 invited collaboration with all experts. The abstract interfaces defined in the implementation frameworks then allowed to de-couple individual efforts, and enabled independent development by physicists with deep expertise in the field of modeling hadronic interactions. 2.1. Overall architecture

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Figure 1: Package diagram showing the architecture of Geant4 version 7.0.

The package diagram of the Geant4 architecture is shown in fig.l at the example of the Geant4 7.0 release. The architecture[5] is centered around an implementation framework for physics processes, that generalizes all physics implementations in Geant4, and provides the necessary top level abstractions. It implements the interaction with the technical categories concerning event generation, events, runs, visualization, etc. This interaction is through its being

521 used by the tracking package, the heart of the Geant4 simulation infrastructure. The physics framework also provides the information on geometrical quantities and detector properties through its usage of the track package. This apparently simple architecture allows to decouple any physics implementation from the simulation infrastructure part of the system. Note that even the transportation process is funneled through the process interfaces. 2.2. Electromagnetic Physics Packages

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Figure 2: Package diagram of electromagnetic physics in Geant4 version 7.0.

Figure 2 shows the packages in electromagnetic physics for Geant4 version 7.0[6]. The Standard package provides the complete basic functionality needed for HEP detector simulation, while the Muons and X-rays packages provide this physics for muon interactions and X-ray generation and interactions. The Optical package provides for optical photon transport and interactions of optical photons with bulk matter and optical surfaces. The High Energy package contains extensions to the Standard package, describing processes that gain in relevance as the TeV energy scale is approached. The Parameterizations package allows for shower parameterizations, and the Low Energy package contains an incomplete set of processes alternative to those provided in Standard, with a focus on low energy particle generation. 2.3. Hadronic Physics Packages Hadronic physics in Geant4 was constructed around a set of implementation frameworks, as an aggregation hierarchy that is five levels deep. Today it constitutes more than half of the Geant4 source code.

522

Figure 3 shows the top-level package diagram. The Processes package implements the interfaces of the hadronic physics framework, the Hadronic management package, which implements the physics framework, and provides additional framework functionality for biasing, material selection, exception handling, white-boards, multithreading, etc. It delegates any further action that directly relates with physics. The Stopping package provides a set of processes for hadrons coming to a rest through energy loss. The Inclusive Cross-section package provides the cross-section data sets that are used at tracking time to determine step sizes. The final state modeling package is a subsystem that is illustrated in figure 4 and discussed later. They all make use of a class utility package that contains classes describing for example nuclear properties, but also technical codes like exception classes, whiteboards, monitoring or templatemeta-programs for type-list based subsystem configuration at compile-time.

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Figure 3: Package diagram of hadronic physics top-level architecture in Geant4 version 7.0.

Figure 4 shows the package structure of Hasfinalstate modeling subsystem. This level contains all physics implementations. In the case of hadronic physics, the strategy was to name the packages after the physical concept used to model the final state generation, hence the Binary Cascade package implements the binary cascade concept[7] the chips package implements chiial invariant phase-space decay[8] and Imaginary R-matrix implements the imaginary part of the R-matrix through resonant and non-resonant 2-particle scattering[7]. A more detailed description of these packages is beyond the scope of this paper, and can be found in the Geant4 physics manual available from the Geant4 web site. Note also that iheparton string[9] and de-excitation packages[10] are subsystems in their own right, containing themselves 12 packages in total.

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Given the complexity of physics modeling in the hadronic subsystem, a set of pre-packaged "educated guess" simulation engines were provided[l 1]. They can be found at the physics lists web site. The use cases currently included in this are high energy physics calorimetry excluding linear collider "tracking" calorimetry, high energy physics trackers, the "average" particle physics detector, low energy dosimetry applications with neutrons, low energy nucleon penetration shielding, linear collider neutron fluxes, high energy penetration shielding, medical and military neutron applications, low energy dosimetric applications, high energy production targets (ex. pion fluxes from 400 GeV protons on Carbon or Beryllium), medium energy production targets (ex. fast particle fluxes from 15-50 GeV protons on light targets), LHC neutron fluxes, air shower applications, and low background experiments. These pre-packaged and versioned physics list were elementary pre-condition for both wide user validation of geant4 hadronic physics and its supportability.

3. Geant4: Predictive power of associated physics modeling Another key to the successes that Geant4 sees in the field of High Energy Physics was an extensive verification program. The psychological need

524

addressed was to convince people that the models in particular in hadronic physics have strong predictive power at the microscopic level. The verification effort performed by the Geant4 hadronic working group is grouped into several sections concerning thin target comparisons for final state generators, inclusive cross-section testing, verification of model components, code comparisons, complete application tests, and robustness. 3.1. Thin target comparisons for final state generators The thin target comparison suite contains at the time of writing close to 2500 individual plots. Figure 5 shows two of these as illustration of the method and the predictive power of quark gluon string model[12]. It compares predictions for negative pion production in K+ Gold interactions at 100 GeV kaon energy to experimental data[13]. We show rapidity and transverse momentum distributions. We see good agreement between the model prediction and experimental data. QGS model is the currently most suitable high energy generator for high energy physics calorimeter simulation. Whltmore et.al, Zeitschrift f. Physik C62, 1 99 (1 9 9 4 ) 1.6 1.4

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525 3.2. Cross-section comparisons As an illustration of this type of comparison, we show in figure 6 the comparison of the proton reaction cross-section prediction of the Wellisch-Axen systematics[14] with experimental cross-section values for various targets[14]. We find good agreement.

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526 3.3. Verification of model components This part of the verification suite is aimed at demonstrating the individual model components are correct, and not tuned away from their physical values. This includes among others verification of nuclear masses, density distributions, particle width, and total reaction cross-section predictions by cascade codes. In figure 7 we show the prediction of the A4* production cross-sections in protonproton scattering, which is part of the description of the imaginary part of the Rmatrix in binary cascade or QMD codes. The data points are taken from the CERN/HERA collections[15], and scaled with the appropriate Clebsch-Gordon coefficients. The line is the result of the Monte Carlo simulation component.

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Figure 7: Comparison of the prediction of the A " production cross-sections in proton-proton scattering. This is part of the description of the imaginary part of the R-matrix in binary cascade or QMD codes. The data points (solid symbols) are taken from the CERN/HERA collections. They are scaled with the corresponding Clebsch-Gordon coefficient.

3.4. Code comparisons Code comparisons are generally considered the least effective comparison technique, and are resorted to only in cases where no data exist for a quantity of relevance to a simulation problem. Preconditions for such a comparison are that the codes compared use different modelling techniques to arrive at the prediction, and that they have not been tuned on the same experimental data for other quantities. If these conditions are fulfilled, code comparisons can be very useful. We give in figure 8 as an example the result of a code comparison

527 between the RADLIST[16] code and the Geant4 photon evaporation and internal conversion codes. For the reader to judge if anything can be concluded.

Radiation CEK CE CEL CE CE CE CEK CE CEL CEM+ CEK CE 7 7 y y

RADLIST (BNL) G«ant4 Energy (keV) Intensity (lOOdks) Energy (keV) Intensity (lOOdks) 7.301 71.00 (6.0) 7.301 70.55 (1.88) 12.899 10.00 (0.70) 13.567 7.40 (0.6) 5.95 (0.54) 13.562 13.687 0.35 (0.13) 14.315 0.85 (0.21) 14.405 0.45 (0.19) 114949 1.83 (0.14) 114.949 1.95 (0.31) 120.497 5.70 (0.53) 121.215 0.19 (0.020) 121.968 0.03 (0.005) 129.361 1.30 (0.16) 129.362 1.25 (0.25) 134.910 0.25 (0.11) 14.413 9.16 (0.15) 14.413 10.05 (0.71) 122.061 85.60 (0.17) 122.061 86.05 (2.07) 136.474 10.68 (0.08) 136.474 10.05 (0.71) 692.410 0.15 (0.01) 692.030 0.15 (0.09)

Figure 8: Code comparison between the RADLIST code and the photon evaporation and internal conversion code used in Geant4.

3.5. Complete application tests Complete application tests are introduced to second the validation efforts in the experiments. Complete application tests can only be done in close collaboration with the test-beam communities. In figure 9 and 10 we show the result of such an analysis using a modern physics simulation engine, QGSP 2.9 and PACK 4.0. The ATLAS hadronic end-cap calorimeter pion test-beam measurement[17] is simulated. The end-cap is a flat-plate liquid argon sampling calorimeter with an EST readout structure. The absorber material is Copper. It has 4 longitudinal compartments (readout layers). For details of the detector set-up we refer to the original publication of the ATLAS community[18]. We see excellent agreement for energy resolution and signal ratio of electrons to pions. We also see good agreement in the shower shape comparison for all longitudinal compartments of the calorimeter and all pion energies. It is this kind of comparison that must constitute the final acceptance test of a simulation engine for physics simulation.

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529

4. Geant4: Deployment in High Energy Physics The deployment of Geant4 in high energy physics was largely based on initiatives of individual working groups. The first experiment to adopt Geant4 as infrastructure for their simulation program was BaBar[19]. Geant4 was used there starting from 1999, a time where Geant4 was not really fit for the job, but by now more than 1010 high quality BaBar events have been simulated for physics analysis. In the years to follow new high energy physics experiments like AMS[20] and PANDA[21] adopted Geant4 for their simulation needs, and the international linear collider community became very active. The HARP[22] experiment was approved also with the argument that the resulting data will be useful to verify final state generators in hadronic simulation engines, and has itself adopted Geant4 for their simulation program. For LHC[23], from 1999 onwards, with the advent of "educated guess physics lists", and a set of agreements of the hadronic working group of geant4 with individual detector groups, a very solid validation program based on test-beam comparison was put together. Much later this was harnessed into LCG[24] validation meetings, and Fabiola Gianotti did a good job in keeping the effort alive. In parallel, during the year 2000, it became clear that for the purpose of LHC hadronic event simulation test-beam studies were not sufficient to address the validation of a simulation program in full. With the kind of integral energy flows and complexities involved in full detector simulation at LHC, robustness of the physics codes, but also of geometry, transportation and in particular magnetic field transport is a major issue. An agreement was found between the hadronic working group of Geant4 and the CMS [25] simulation management that CMS would focus their validation efforts mainly on the robustness aspect, while ATLAS[26] would focus mainly on the test-beam. CMS and LHCb[27] have begun to look into aspects of radiation shielding. Today three of four LHC experiments have adopted Geant4 as their primary simulation infrastructure, and have their simulation programs in routine use. The work for the physics TDR of CMS is based on Geant4 simulated data with the QGSP physics simulation engine. The number of full LHC events simulated today approaches 109. 5. Conclusions After 11 years of consistent effort, Geant4 today comes with the most complete physics modeling abilities. It is the de-facto standard detector simulation tool in High Energy Physics.

530

References 1. S. Agostinelli et al, Nucl. Instrum. Meth. A 506, 250 (2003). 2. S. Giani et al, Geant4: CERN/LHCC 98-44, 1998. 3. R. Brun, F. Bruyant, A.C. McPherson, M. Maire, P. Zanarini, CERN Data Handling Division, DD/EE/84-1, 1987. 4. J.P. Wellisch, Comput. Phys. Commun. 140, 65 (2001). 5. S. Agostinelli et al, Nucl. Instrum. Meth. A 506, 250 (2003). 6. M. Maire et al, Geant4 electromagnetic physics, Prepared for Monte Carlo 2000, Lisbon, Portugal, 23-26 Oct 2000 7. G. Folger, V.N. Ivanchenko and J.P. Wellisch, The European Physical Journal A, Vol. 21 No. 3,407 (2004). 8. P.V. Degtyarenko, M.V. Kossov and H.P. Wellisch, Eur. Phys. J. A 9 (2000) 421; Eur. Phys. J. A 9 (2000) 411; Eur. Phys. J. A 8 (2000) 217. 9. G. Folger, J.P. Wellisch, „Parton string models in Geant4", Prepared for Monte Carlo 2005, Chattanooga, TN, USA, 18.-22. Apr 2005. 10. V. Lara, J.P. Wellisch, Prepared for "Computing in high energy and nuclear physics", Padova, Italy, 7.-11. Feb 2000, pp52-55. 11. J.P. Wellisch, "Hadronic Simulation Engines for Geant4", Prepared for Monte Carlo 2005, Chattanooga, TN, USA, 18.-22. Apr 2005. 12. A.B. Kaidalov, K. A.Ter-Martirosyan, Phys. Lett. B117 247 (1982); Sov. J. Nucl. Phys. 39 (1984) 979. 13. J.J. Whitmore et al, Zeitschrift fur Physics C 62, 199 (1994). 14. H.P. Wellisch and D. Axen, Phys. Rev. C 54, 1329 (1996). 15. CERN High Energy Analysis Group records, preprint denominations CERN-HERA-YY. 16. T.W. Burrows, "The Program RADLST", Brookhaven National Laboratory Report BNL-NSC-52142, February 29, (1988). 17. ATLAS LARG unit, ATLAS Liquid Argon Calorimeter technical design report, CERN/LHCC 96-41, Dec 1996. 18. B. Dowler et al, Nucl. Instrum. Meth. A 482 (2002) 94. 19. B. Aubert et al, "The BaBar detector", Nucl. Instr. Meth. A 479, 1 (2002) 20. AMS collaboration, "AMS: Alpha magnetic spectrometer on the international space station", Nucl. Instrum. Meth. A 535, 134. 21. The Panda Collaboration, "Technical progress report for PANDA", from http://www.epl.rub.de/~panda/db/papersDB/PC19-050217_panda_tpr.pdf 22. The HARP Collaboration, CERN-SPS/2003-027, SPSC-P-325. 23. LHC Conceptual Design Report, CERN/AC/95-05. 24. F. Gianotti et al., CERN-LCGAPP-2004-10. 25. The CMS Collaboration: The Compact Muon Solenoid Technical Proposal, CERN/LHCC 9438, LHCC/P1 1994. 26. ATLAS Collaboration, Technical Design Report, CERN/LHCC/99-01. 27. LHCb collaboration, LHCb Technical Proposal, CERN/LHCC 984.

High Energy Physics Organizers: L. Price (High Energy Physics)

R. Ruchti (Accelerator and Data/Networking Developments)

E. Barberio M. Barone P. Casado A. Di Simone A. Di Ciaccio R. Gardner M. Krammer K. Lang W. Liebig P. Paolucci Fr. Pastore J. Puerta-Pelayo

R. J. J. A.

Schmidt Smyrski Ulbricht Ventura

Fast Shower Parametrisation for the Electromagnetic Cascades Technical Aspects of the Manufacture of 4 Grease Pad Assemblies for the CMS Experiment Implementation and Performance of a Tau Lepton Selection within the ATLAS Trigger System at the LHC ATLAS Detector Simulation: Status and Outlook The ATLAS Experiment at CERN: Two Years Before the Start of the Data Taking Progress in Grid Computing for Particle Physics Design, Production and Quality of the Sensors for the Silicon Strip Tracker of CMS Running MINOS ATLAS Inner Detector Results from the 2004 Combined Test Beam Data The CMS Muon System Performances of the ATLAS Level-1 Muon Trigger Processor in the Barrel Certification, Installation and Commissioning of Muon Drift Tube Chambers for the CMS Central Muon Detector The LHC Collider: Status and Outlook to Operation PANDA: a Detector for Research with Antiprotons Test of QED with the Reaction e+e~ -» 77(7) Muon Reconstruction and Identification for the Event Filter of the ATLAS Experiment 531

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FAST SHOWER PARAMETERISATION FOR ELECTROMAGNETIC CASCADES

J. WENG CERN & University of Karlsruhe Switzerland / Germany E-mail: Joanna. [email protected] E. BARBERIO AND A. WAUGH Univerity of Melbourne, University of Sydney Australia E-mail: [email protected], [email protected]

We developed a package for parameterizing electromagnetic showers in the context of Geant4. The algorithm is based on previous work done for the GFLASH package, but has been adapted to the new simulation context and modified to better reproduce the performance of the LHC calorimeters. The algorithm uses a probability density function (PDF) of the shower profile for electrons/positrons with an energy above 800 MeV. This PDF includes shower shape and energy fluctuations. A mixture of full simulation and fast parameterisation is also possible. This new package, included in Geant 4.7, leads to a significant gain in simulation time for LHC events, with minimal sacrifice of accuracy. This algorithm has been integrated within the ATLAS and CMS simulation framework and compared to full simulation.

1. I n t r o d u c t i o n T h e ATLAS and CMS detector simulation is based on t h e G E A N T 4 package 3 . G E A N T 4 simulates detector effects on physics events using a detailed microscopic description of the interactions between particles and m a t t e r . It is very accurate, but it can be very time consuming. In particular the simulation of electromagnetic cascades in calorimeters is expected t o account for a considerable amount of the total simulation time. Since it increases almost linearly with the energy absorbed in the detector, at LHC energies of 14 TeV, fully simulating one event, using individual particle tracking, may take several minutes. W h e n a large number of simulated events are needed for physics analyses, a full simulation approach is not viable .

533

534 In the past, within ATLAS and CMS, stand-alone fast simulations (6 and 5 ) were developed as an alternative to full simulation, however, this approach is not sufficiently accurate for many analyses; for example, studying pathological events that may look like a discovery. The running of past HEP experiments, CDF, HI, Zeus proved the importance of an intermediate level of simulation, faster than full simulation, but integrated in the same framework so that it could provide the same kind of output as full simulation on which the full reconstruction chain could be run. A set of equations, derived from the HI and Zeus parameterisation, can be used to parameterize the electromagnetic shower development in different calorimeters 1>2. These equations were implemented in the GEANT3 (GFLASH) framework. In this paper we present the implementation of an electromagnetic shower parameterisation inside the Geant4 3 framework. Shower parameterisations based on this approach have also been integrated into the simulation framework of ATLAS and CMS to speed up the simulation of the ATLAS liquid argon sampling and the CMS homogenous lead tungstate electromagnetic calorimeter. 1.1. Theoretical

background

The spatial energy distribution of electromagnetic showers is given by three probability density functions (PDF), dE(r) = Ef(t)dtf{r)drf{<)>)dct>,

(1)

describing the longitudinal, radial, and azimuthal energy distributions. Here t denotes the longitudinal shower depth in units of radiation length, r measures the radial distance from the shower axis in Moliere units, and (j) is the azimuthal angle. In , it is assumed that the energy is distributed uniformly: /() = 1/27T. The mean longitudinal profile of a shower is described by : {

E

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T(a)

[Z)

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535

energy, the material of the calorimeter and the direction of the incident particle, as a function of the shower energy. The inhomogeneous material of a sampling calorimeter influences the shower shapes. The shower maximum occurs earlier than in an homogeneous calorimeter with the same effective material properties. Hence, ln(T) and ln(a) parameterize also as a function of the sampling frequency and the value of e/MIP averaged over the whole shower. 2. Use cases of the new shower parameterisation 2.1. Shower parameterisation

in Geant 4

The new implementation of the GFLASH parameterisation has been integrated into Geant4 and is available from version 4.7.0 onwards. The following table shows the performance of the shower parameterisation package in a simple test setup: a block of lead tungstate, the material the CMS calorimeter consists of. The parameters needed for the parameterisation are calculated on the fly for the considered material. The speed-up is significant and reaches 100 times and more for electrons above 50 GeV.

Table 1. Speed-up of single electrons in a pure lead tungstate cube in Geant4 version 4.7.0)

Electron Energy

Time/event full Geant 4

Time/event parameterisation

Speed-up factor

1 GeV 5 GeV 10 GeV 50 GeV 100 GeV 500 GeV 1000 GeV

0.10 0.46 0.92 4.60 9.37 46.50 91.75

0.006 0.009 0.013 0.045 0.080 0.312 0.566

16 48 67 102 117 149 162

In general, a somewhat worse agreement with Geant4 is observed than with GEANT3 - especially for the radial profiles. This is one the hand due to technical differences (ex. shower start point), on the other hand the parameterisation parameters obtained from GEANT3 do not describe Geant4 shower profiles with equal accuracy. Here a retuning may be necessary. Typical longitudinal and radial profile are shown in Figure 1.

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Figure 1. Longitudinal (left) and radial shower profile (right) for 50 GeV photons in pure lead tungstanate (PbWOi). The points correspond to the parameterisation (radial profile retuned), the histogram to the full Geant4 simulation.

2.2. Shower parameterisation

in CMS

The concept of parameterized showers has been included into the CMS full detector simulation OSCAR 4 . It is used to parameterize electrons and positrons in the barrel and endcap electromagnetic calorimeter. The radial profiles were retuned. Comparisons between the GFLASH based and the full OSCAR simulation of the energy depositions in the central crystal, the 3x3 and 5x5 crystal matrices show good agreement to the > 1% level, as illustrated in Fig. 2. The transverse and longitudinal shower profiles are also well modeled to within 1-3%. The shower parameterisations allow a significant time performance gain in the simulation, with speed increases in the range of a factor of 3-10. The gain for a simulation including the full detector geometry depends on the event type, the particle energy and the detector r\ region. For instance, a single electron or photon with an energy of 100 GeV in the ECAL barrel is simulated 10 times faster using parameterisation. For a large extra dimensions full signal event, pp —> j+G, with a single photon above 1000 GeV, the gain in speed is around a factor of 4. 2.3. Shower parameterisation

in

ATLAS

The new implementation of the shower parameterisation has been integrated into the ATLAS detector simulation. The longitudinal profiles show acceptable agreement and needed only small adjustments. The radial profile is derived by fitting the core and the tail of the radial distribution, once the longitudinal parameters are fixed. The number of spots neces-

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sary to correctly reproduce the radial fluctuations is very large. For the time being, an alternative function is used, that well describes the core and tail region of the radial profile. As described elsewhere, the ATLAS electromagnetic calorimeter has a 'accordion' geometry; for this reason, it was not initially clear if the procedure developed for a standard sampling calorimeter would be applicable. However, if the proper sampling frequency and e/MIP are used, the parameterisation described above reproduces the longitudinal shape of electromagnetic showers over a wide range of energies (0.5-100 GeV). The radial profile is fitted using a two parameter function which takes into account the dependence on energy, E, and shower depth, T. After first fitting for E, radial parameters are derived as a function of r . 3. Conclusion We implemented an electro-magnetic shower parameterization inside the simulation framework of Geant4. Experiment specific versions based on this implementation has been intergrated in the detector simulation of CMS and ATLAS. After tuning of the longitudinal and radial shower profiles the shower model yields good agreement with the Geant4 particle tracking and has proven to significantly speed-up the simulation in CMS and ATLAS. References 1. G.Grindhammer, M.Rudowicz and S.Peters, NIM A290(1990)469 G.Grindhammer and S.Peters,hep-ex/0001020

538 2. J. del Peso and E. Ros, NIM A306(1991)485-499 3. Geant4 see http://wwwinfo.cern.ch/asd/geant4/geant4.html and documentation therein. 4. OSCAR: CMS Simulation Package see http://cmsdoc.cern.ch/oscar 5. FAMOS: CMS Fast Simulation Package see http://cmsdoc.cern.ch/famos 6. E. Richter-Was, D. Froidevaux, L. Poggioli, ATL-PHYS-98-131

TECHNICAL ASPECTS OF THE MANUFACTURE OF 4 GREASE PAD ASSEMBLIES FOR THE CMS EXPERIMENT M. BARONE Institute of Nuclear Physics "NCSR "Demokritos "-Athens-Greece e-mail: [email protected] P. L. RIBONI ETHZ-Zurich-Switzerland;

emaihpierluigi. riboni@cern. ch C. SPITAS

Technical University of Crete, Dept of Production Engineering Greece; e-mail: [email protected]

and

Management-

V. SPITAS Technical University of Crete, Laboratory of Applied e-mail: vspitas@central. ntua.gr

Mechanics-Greece

The CMS detector is a worldwide endeavour gathering an extended number of contributors and putting together many parts coming from different countries. In this context CERN operates like a sort of 'general coordinator' and is obliged to put a special emphasis on strict criteria of quality, calculations, control certificates and full traceability of materials and operations, which are all of paramount importance to the successful integration and function of the parts. This paper discusses the technicalities related for the selection of machining parameters to optimally balance geometrical/ dimensional accuracy of the produced parts and their cost-effectiveness. These are the components of the grease pad assemblies that support and allow the precise positioning and alignment of the CERN CMS apparatus. The work, an in-kind Greek contribution, mainly consists of machining several large steel plates to various forms, including large pockets and inclined surfaces.

1. Introduction CERN's current important project is the Large Hadron Collider, LHC, which will enter in operation by the middle of 2007. LHC is a super-conducting particle accelerator of 27 km in circumference being installed in an existing underground tunnel, previously devoted to another accelerator, the LEP. One of the 4 experiments to be performed at the LHC is the CMS (Compact Muon Solenoid). 539

540

This experiment is built and funded by an international collaboration of highenergy physics institutes from thirty-six countries and by CERN. The CMS experiment consists of a massive magnet equipped with several dedicated particle detectors. The magnet system consists of a 4 Tesla solenoid Superconducting Coil, having a free bore of 6 m and a length of 13 m, enclosed in a Return Yoke comprising the Barrel Yoke and the two End cap Yokes. The Return Yoke is designed as a regular twelve-sided structure. The important dimensions of the completed detector are: a length of 21.6 m, an outer diameter of 14.8 m, and a total mass of 12'500 tonnes. On both extremities of the Yoke is located one Hadronic Forward Calorimeter (HF). It is supported by a mechanical lifting system and centered on the beam pipe. This paper covers the manufacture of mechanical components of 4 grease pads for the YE3 disc. 2. Organization of the work The CMS experiment is financed by its supporting collaboration through mutually agreed procedures. A CERN team was asked to organize and specify the production of the grease pads for the Endcap discs, the Greece representative of the CMS collaboration was in charge of the tendering, contract and follow-up of the manufacture and supply by the firm Metallo in Athens, as in-kind contribution. Consultants from the Technical University of Crete have played an important role from the tendering till to the acceptance of the goods. The supply was covering the following points: 1) The Engineering Report (ER) including la) The bill of material, lb) Definition of machining procedures, lc) Definition of control procedures, Id) The Quality Assurance Manual, 2) Base materials certificates, 3) Machining, 4) Dimensional and conformity measurements, 5) Protection against rust and conservation, 6) Packaging, 7) Transport and delivery to Point 5 Cessy, France. In this context CERN operates like a "general contractor" therefore is obliged to make controls to get quality and the full traceability of the materials and operations which are all of primary importance to the successful integration and function of the parts. 3. Grease pads' description Each pad is fitted with two movable cylindrical pots inside cavities of appropriate shape. The pots are free to slide on a mating stainless steel plate and pressurised grease contained by means of special gaskets keeps the surface friction coefficient to a minimum and prevents damage to the sliding surfaces.

541

Additionally, to compensate for a 1.23° inclination of the cavern floor at point 15, where the final detector assembly is to take place, a further set of angular plates per pad is foreseen, so that each final grease pad assembly is a superposition of no less than 7 plates of specialised shapes, as shown in Fig. 1. The total weight per pad is about 2.5 tonnes. The detailed work description is contained in specification CMS-SpF-4181 [1]. 4. Fabrication process The proper function of the grease pads can be severely impaired by geometrical deviations caused by the fabrication process, which chiefly involves material removal over large surfaces by milling. Such deviations can be the result of 1) residual stresses 2) static deflections and/ or 3) dynamical deflections. Residual stresses can in turn either la) be induced by raw material manufacture, meaning that they are present in the hot rolled stock delivered from the mill and cause bending when material is removed in an asymmetrical manner, or lb) be induced by heat and plastic work generated locally during cutting. Residual stresses can be measured by the hole-drilling method either by use of strain-gauges [2] or laser speckle interferometry [3], by reflective photoelasticity [4] or x-ray diffraction [5], Stresses of type (lb) mostly affect thin and intricate geometries, and are furthermore not normally a problem unless a poor selection of machining parameters is made, resulting in excessive heating of the workpiece. If the machining parameters are carefully monitored and the chip is checked for visual signs of excessive heating (shape, length and colour), then such residual stresses have a negligible effect. Statical deflections (2) are a direct result of the cutting force itself, and tend to deform the work-piece by compression, bending and/ or torsion. Depending on the magnitude of this force and the combined stiffness of the work-piece, fixture and machine tool, such deflections can range from quite insignificant to catastrophic. Large work-piece overhangs, thin-sectioned beams and knife-edges are all associated with low rigidity and are at a greater risk of causing significant static deflections. It is not always possible to increase the stiffness without conflicting with other significant operational and/ or manufacturing requirements, so, when deflections must be closely controlled, the cutting force must be kept in check by suitably selecting the cutting parameters. Finally, dynamical deflections (3) are produced by the same mechanism as static deflections. The cutting forces consist of a fixed and a periodically variable vector component, the former producing static deflections and the latter dynamical deflections, tool chattering and noise. Dynamical deflections in particular tend to produce defects that are in most cases excessive roughness or waviness and are therefore less critical to dimensional accuracy, though they do have a marked effect on surface integrity and tool life. Compared to static

542

deflections, however, vibration is quite easier to spot, due to the noise and characteristic tool failures that come with it.

CMSSYBXM0076

Figure 1. Sub-assembly of a grease pad manufactured by firm Metallo under contract no. CA 1305972 for CERN, identifying plates CMSSYBXM0071 and CMSSYBXM0076

Though vibration problems can be solved solely on empirical basis most of the time, a machining set-up that is prone to static deflections is also bound to suffer from excessive vibration. Therefore remedies towards decreasing static deflections, i.e. reducing the cutting forces or enhancing stiffness, will often improve the dynamical response as well, though additional measures may have to be taken. In relation to the above, and given the high performance demands placed on the grease pads, this paper presents in the form of an engineering analysis some key technical considerations that have gone into the manufacture by the Hellenic firm Metallo of 4 such assemblies under CERN contract no. CA 1305972. Table 1

Goals: flatness of large plates according to cost-effective machining form tolerances Means: maximum depths of cut & maximum low deflections feeds ...hence high cutting forces ...hence high deflections & residual stress/ strain fields ...hence accuracy of form may be compromised The analysis treats the matter of geometrical and dimensional accuracy, as is influenced by the aforementioned factors, and calculates the effect of machining

543

parameters on statical deflections and form accuracy with the aim to produce tangible manufacturing guidelines. Only the most critical processes and factors, as identified by the analysis, are isolated and analysed, whereas typical procedures and work-piece geometries are not considered. This analysis is triggered by the need to establish cost-effective manufacturing procedures and parameters without compromising the tolerance specifications, as is shown in Tab. 1 5. Analysis a) Deflections and tolerances An explanation of the modelling and calculus used in the FEA analysis can be found in [8]. 'initial selection of tool/ machining parameters for max. productivity

verification with machine power readout to check validity of estimate

calculation of material removal rate

calculation of deflections using FEA

estimation of cutting forces based on specific power

trial cut

instruct machine shop and produce first prototype

Figure 2. Analysis flow chart

The main stage in the analysis is to evaluate the effect of the calculated deflections on the geometrical integrity of the work-pieces. Machining an elastically deformed piece means that material is removed in a different way than anticipated. This can affect both geometrical and dimensional tolerances. Main problems to consider are 1) localisation of defects, 2) large cumulative deflections, 3) large dynamical cutting force components (vibration). Engineering materials can present random deviations from their assumed properties, which affect the forces involved in machining. As a rule, better quality materials are associated with smaller standard deviations, but the stochastic nature of the problem means that extra allowances must be made to ensure that no parts are compromised. Keeping maximum errors one order of

544

magnitude smaller than the official tolerances is a safe rule of thumb for prototype production. Based on the above analysis, the employed methodology is as shown in Fig. 2. 6. Results and discussion Plate CMSSYBXM0071 This is the plate that houses the grease pots. Its general geometry and way of fixturing on the machine table is depicted in Fig. 3. Due to machine limitations, more rigid fixturing alternatives [9] were not deemed practical. The long side slots were identified as potential problem areas and an analysis was called for to determine if the initial cost-effective choice of milling parameters was causing excessive deflections. In particular, two distinct cases were considered: case 1- loading at the edge of a small cantilever beam feature (localised deflection?) case 2- midpoint loading (large overall deflection?)

Figure 3. Basic work-piece orientation for plate CMSSYBXM0071

Figure 4. FEA results for plate CMSSYBXM0071

The FEA (Fig. 4) established that case 1 produced the most unfavourable conditions (almost 3 times the displacement of case 2), but at under 3% of the tolerance limit specified for plate CMSSYBXM0071, the choice of rnacliining parameters, proposed for cost effectiveness, was deemed safe. 7. Conclusion In this paper an applied methodology was presented to estimate the effect of tool and machining parameter selection on the cutting forces and resulting workpiece deflection. This methodology has been applied on the manufacture of 4 grease pad assemblies for the CMS detector and a selected example of the analysis regarding parts CMSSYBXM0071 has been given.

545 The results suggest that factors such as part fixturing/ orientation along with the tool selection and machining parameters have a significant effect on the induced deflections. Non-rigid features, knife-edges and large part overhangs are all high-risk cases that deserve special analysis, especially if cost-effective production parameters are to be chosen. If the initial choice is unsatisfactory, then this process of analysis- parameter selection can evolve as an iterative cycle until a satisfactory solution is reached. Benefits that have been observed from the application of this process are: • Less of the usual trial-and-error (without excessive safety margins) • Better-controlled quality • Workshop savings in material and man-hour resources This paper focuses purely on the deflection issues associated with machining parameter selection. Other important aspects involved in the selection, such as controlling maximum roughness, chip evacuation, tool life etc lie outside the scope of this paper and have not been mentioned. Acknowledgements Most of this work was done within the framework of the CERN order no. CA 1305972, placed on firm Metallo. Permission by CERN and Metallo to present pertinent photographs of the grease pad assembly is kindly acknowledged. Permission by AIAS Engineering to publish 3-D part models and FEA results used in the project is also acknowledged. References 1. CERN specification CMS-SpF-4181 2. ASTM Standard E837-01, ASTM (2001) 3. Diaz F. U., Kaufmann G. H., Galizzi G. E., 33, p.p. 39-48, (2005) 4. Blum A. E., 13, p.p. 96-101 (1977) 5. Hilley M. E., Ed., Residual stress measurement by x-ray diffraction, SAE J784a, SAE, Warrendale, PA, p.p. 21-24 (1971) 6. Stirnmimann J., Kirchheim A., New cutting force dynamometers for highprecision machining, Industrial Tooling Conference, Southampton (1997) 7. ASM Handbook Vol. 16 'Machining', ASM International (1995) 8. Zienkiewicz O. C , Taylor R. L., The Finite Element Method, McGraw-Hill (1989) 9. W. E. Boyes, Handbook of jig and fixture design, 2nd Edition, SME (1989) 10. SECO Main Catalogue and Technical Guide 'Milling Cutters and Inserts', SECO

IMPLEMENTATION A N D P E R F O R M A N C E OF A TAU L E P T O N SELECTION W I T H I N T H E ATLAS T R I G G E R SYSTEM AT T H E LHC

A. dos Anjos, S. Armstrong, J.T. Baines, C.P. Bee, M. Biglietti, A. Bogaerts, M. Bosman, B. Caron, P. Casado, G. Cataldi, D. Cavalli, G. Comune, P. Conde, G. Crone, D. Damazio, A. De Santo, M. Diaz Gomez, A. Di Mattia, N. Ellis, D. Emeliyanov, B. Epp, S. Falciano, H. Garitaonandia, S. George, V. Ghete, R. Goncalo, J. Haller, S. Kabana, A. Khomich, G. Kilvington, J. Kirk, N. Konstantinidis, A. Kootz, A.J.. Lankford, A. Lowe, L. Luminari, T. Maeno, J. Masik, C. Meessen, A.G. Mello, R. Moore, P. Morettini, A. Negri, N. Nikitin, A. Nisati, C. Osuna, C. Padilla, N. Panikashvili, F. Parodi, E. Pasqualucci, V. Perez-Reale, J.L. Pinfold, P. Pinto, Z. Qian, S. Resconi, S. Rosati, C. Sanchez, C. Santamarina, D.A. Scannicchio, C. Schiavi, E. Segura, J M. de Seixas, S. Sivoklokov, A. Sobreira, R. Soluk, E. Stefanidis, S. Sushkov, M. Sutton, S. Tapprogge, S. Tarem, E. Thomas, F. Touchard, G. Usai, B. Venda Pinto, A. Ventura, V. Vercesi, T. Wengler, P. Werner, S.J. Wheeler, F.J. Wickens, W. Wiedenmann, M. Wielers, H. Zobernig THE ATLAS HIGH LEVEL TRIGGER GROUP * presented by P. CASADO + IFAE, Edifici Cn, Bellaterra, E-08193, Spain

The ATLAS experiment at the Large Hadron Collider (LHC) has an interaction rate of up to 109 Hz. The trigger must efficiently select interesting events while rejecting the large amount of background. The First Level trigger will reduce this rate to around 0(75 kHz ). Subsequently, the High Level Trigger (HLT), comprising the Second Level trigger and the Event Filter, will reduce this rate by a factor of O(103). Triggering on taus is important for Higgs and SUSY searches at the LHC. In this paper tau trigger selections are presented based on a lepton trigger if the tau decays leptonically or via a dedicated tau hadron trigger if the tau disintegrates semileptonically. We present signal efficiency with the electron trigger using the data sample A —> TT —• e hadron, and rate studies obtained from the dijet sample. 'http://atlas.web.cern.ch/atlas/groups/daqtrig/hlt/authorlists/como2005.pdf tWork supported by the Institut de Fisica d'Altes Energies (IFAE) and Universitat Autonoma de Barcelona

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1. Introduction The Large Hadron Collider(LHC) is expected to start data taking in 2007 at CERN (European Organisation for Nuclear Research). Proton-proton collisions will be produced at a center of mass energy of 14 TeV and design luminosity of 10 3 4 cm~ 2 s - 1 . ATLAS is a multipurpose detector which will have to cover a wide variety of aspects of high energy physics phenomenology : from discovering new physical phenomena to performing precision measurements. In order to achieve this task, the detector is composed of different tracking subdetectors, a solenoid, electromagnetic (e.m.) and hadronic calorimeters, muon subdetectors and a toroid, as well as the Trigger and Data AcQuisition systems (TDAQ) 1 . 2. The ATLAS Trigger system (1) The ATLAS Trigger system is organised in three levels. The first level (LVL1) is hardware-based and has to reduce the input rate from 40 MHz to 75 kHz in less than 2.5 /xs. The result of the LVL1 selection contains information about the type of trigger and position of the possible particle candidates that cause the event to be accepted. After a positive LVL1 decision the data are transferred from electronic pipeline memories to distributed buffers in so called Read-Out Systems (ROSs). The second (LVL2) and third (Event Filter -EF-) levels are software based systems running on linux PC farms. They are referred collectively as High Level Trigger (HLT). They use full granularity data of all detectors and can also combine information from different detectors. The LVL2 accesses a few percent of the total detector data in a Region of Interest (Rol) provided by the LVL1 result by direct network request to the ROSs. This reduces the event rate to about 2 kHz. After a positive LVL2 decision the complete event is assembled and made available to the EF, which has access to the full detector data and the latest calibration and alignment information. Events accepted by the HLT are stored on tape for further analysis. The goal is to achieve an average decision time of 10 ms and 1 s for LVL2 and EF, respectively, although the system could easily scale to accommodate larger execution times if needed. (2) The HLT selection software 2 has been designed with Object Oriented methodology and is implemented in C + + . It can run in the LVL2, EF and offline environments. This is possible because the differences between the environments are hidden by well defined interfaces and the same framework

548

(Athena 3 ) is used. The HLT selection software is subdivided into four main sub-packages: (a) The Steering organizes the processing of the HLT algorithms, (b) The Event Data Model (EDM) covers all data entities in the event, (c) The Data Manager provides the means for accessing the event data during the trigger processing, (d) The HLT Algorithms can either reconstruct the event or check hypothesis about previously calculated quantities. (3) The data access of the HLT algorithms is a fundamental part in the design of the HLT architecture. The HLT algorithm requests data in a Rol using the offline transient data store (Athena Storegate 4 ) as interface and the Region of Interest package, a software component which returns Storegate pointers to the data inside the specified detector geometrical region. These data are in form of C + + objects, convenient for the algorithm (e.g. calorimeter cells which hold energy and position), contrary to the data coming from the detector electronics which is in raw format (i.e. a binary file with the read-out channel information). 3. Tau selection The selection of taus is important at the LHC for the discovery of new particles. Taus appear in decay modes with significant branching fractions, for example in charged Higgs, in A —> TT (A being the pseudo-scalar Higgs from the Minimal Supersymmetric Standard Model). In this paper we focus on A —> TT -+ e hadron which represents 25% of the A —» TT production. The tau can decay via two modes: leptonically, with one electron or muon in the final state (we only consider here the electron decay); or semileptonically, with hadrons in the final state. In both cases the HLT selection procedure follows the Region of Interest mechanism. The data request is restricted to a region with different size for the various trigger. In the case of the tau decaying into an electron, one can use the single isolated electron trigger of ATLAS. At LVL1 calorimeter information is used with a segmentation in trigger towers of 0.1x0.1 in A77 x A. A cluster is defined with the two most energetic pairs of e.m. towers (adjacent in eta or phi) from the central 2x2 core of the 4x4 window. An isolation cut is applied in the outer region of the 4x4 region and in the hadronic calorimeter. At LVL2, the energy and position measurements obtained at LVL1 are refined; the leakage into the hadronic calorimeter is evaluated and variables related to the shower shape in the e.m. calorimeter are used to perform preliminary particle identification. If a candidate is found to be

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Figure 1. E.m. radius (left) and isolation fraction (right) for taus decaying hadronically (black) and jets (red).

consistent with an electron, track reconstruction is performed in the Inner Detector (ID); cluster to track association is done using (77, (j>) matching criteria, achieving further rejection against fake candidates; in case the matching was successful, the ET/PT ratio between the transverse energy measured in the e.m. calorimeter and the transverse momentum of the corresponding ID track is evaluated for particle identification. If the objects under analysis fulfill the required signatures, the event and its LVL2 result are passed to the EF, where more precise clustering algorithms can be performed, and where information on the complete event is available, along with more precise calibrations and alignment constants. In the case of tau decaying into hadrons one has to use the genuine tau trigger, used in conjunction with missing ET at pr « 35 GeV, and standalone for pr « 60 GeV. At LVL1 the cluster is defined from the e.m. towers as in the electron case and in addition, the inner core of the hadronic calorimeter is used. The isolation region is defined from the outer part of the 4x4 window of the e.m. and the hadronic calorimeters. ET of the cluster can be higher than for the electron trigger while the applied isolation cut is looser. The selection of LVL2 and EF has as a guidance the offline. In this case two of the variables with highest rejection power are depicted in Fig. 1. The rejection factor obtained using the whole selection is 200 for an efficiency of 50%. At LVL2 we are using previously considered variables and new quantities imported from the offline. The list is the following: (1) Total energy (ET) calculated in an inner and outer region for the e.m. and the hadronic calorimeters. (2) The e.m. radius: energy weighted radius of the cluster calculated in the e.m. calorimeter. The distribution of this variable for the offline is shown in Fig. 1 (left). (3) The width in the energy deposition: energy weighted standard deviation in 77. (4) The isolation fraction: fraction of energy deposited in a region of

550

Figure 2. Efficiency of the electron trigger as a function of the PT of the electron (red) and as a function of the PT of the tau (black).

0.1 < R < 0.2. T h e distribution of this variable for signal and background is also shown in Fig. 1 (right). 4.

Results

T h e sample A -> TT -^ ehadron at low luminosity with electronic noise and pile u p has been used to calculate efficiencies of the isolated electron trigger as a function of p r of the electron and of pr of the r , normalizing with respect to events having an offline reconstructed electron. This is shown in Fig 2. For the electron case, there is the expected sharp rise between 20 and 30 GeV and the maximum value is « 85 %. These results confirm those obtained in the collaboration for electrons a t py = 25 GeV and extend t h e m to a broader region in pr- For the taus, the slope is much less sharp, the trigger reaches an efficiency plateau for pr > 70 GeV. In the lower pr region the electron isolated trigger on taus complements with the hadron t a u plus missing ET trigger. Finally, the HLT rates of two relevant triggers for these studies are estimated using the dijet sample with electronic noise a n d pile up a t low luminosity: for the electron isolated trigger the value is s»32 Hz and for the t a u and missing ET trigger the result is « 5 Hz. References 1. ATLAS High-Level Trigger, Data Acquisition and Controls, Technical Design Report, ATLAS TDR-016. ATLAS HLT/DAQ/ DCS Group. 2. "Architecture of the ATLAS online physics-selection software at LHC", Proceedings of the 8th ICATPP Conference on Astroparticle, Particle, Space Physics, Detectors and Medical Applications, 255-259 (2003). 3. http://atlas.web.cern.ch/Atlas/GROUPS/SOFTWARE/00/architecture 4. The StoreGate: a Data Model for the Atlas Software Architecture, Proceedings CHEP03, La Jolla, California, March 24-28, 2003.

ATLAS D E T E C T O R SIMULATION: STATUS A N D OUTLOOK

A. RIMOLDI University

of Pavia and INFN,

Italy

A. D E L L ' A C Q U A , A. DI S I M O N E , M. G A L L A S A N D A. N A I R Z CERN,

Switzerland

J. B O U D R E A U A N D V. T S U L A I A University

of Pittsburgh,

USA

D. C O S T A N Z O University

of Sheffield,

UK

The simulation program for the ATLAS experiment at CERN is currently in a full operational mode and integrated into the ATLAS's common analysis framework, ATHENA. The robustness of the application was proved during the Data Challenge 2 (DC2). The simulation software, based on GEANT4, has been interfaced within ATHENA and to GEANT4 itself using the LCG dictionaries and Python scripting. The Python interface has added the flexibility, modularity and interactivity that the simulation tool needs to manage, in a common way, the different simulation setups: full ATLAS detector, test beams and cosmic ray studies. The comparison with real data has been possible in the context of the ATLAS Combined Test Beam (2004) and ongoing cosmic ray studies.

1. The Geant4 ATLAS detector simulation Starting from 2004, the GEANT4 simulation framework has replaced the previous GEANT3-based simulation program l as the only officially supported software for the ATLAS detector simulation. It consists of a fullfeatured, object-oriented simulation suite (G4ATLAS), integrated in the common framework for the ATLAS offline software (ATHENA) 2 . The implementation of G4ATLAS has been driven by key concepts like dynamic loading and action-on-demand. The resulting software is thus highlyconfigurable and flexible, and can therefore accommodate all the require-

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ments the simulation of a complex experiment such as ATLAS calls for. The user can choose at run time which kind simulation to run (whether full detector, test beam setup or cosmic simulation). However, since the underlying simulation code is always the same, consistency is ensured throughout all the different applications. 1.1. Geometry

description

The geometrical description of the ATLAS detector has been decoupled from the simulation framework. A dedicated set of classes (GeoModel) provides the necessary volume and material description. The GeoModel description is automatically translated into a GEANT4-compatible geometry at run time. On the other hand, being GeoModel completely independent from GEANT4, the same description can be used also by reconstruction applications, thus ensuring total agreement between the geometries used for simulation and reconstruction. The GeoModel description has been optimized to handle the large number of volumes (about 106) involved in the description of the ATLAS experiment: the number of volumes itself is kept to the minimum and, whenever possible, volume parameterization is used to improve the performance. 1.2. Job

control

In the ATHENA framework, jobs are configured and controlled using the python scripting language: python commands are grouped in job Options files, which contain all the information about the job to run (e.g. which algorithms to run, what input files to use, the data to be written in the output file, etc.). The user has thus all the advantages from a full featured programming language, such as flexibility and ease-of-use, and can use the existing bindings to interact directly with the underlying C + + code. GEANT4, on the other hand, has no native interface to python and the configuration of a simulation job is normally achieved using specific commands, which can be grouped into macro files. For the simulation of a complex experiment such as ATLAS, the number of macro files involved increases rapidly, leading to maintainability issues. Moreover, the macro files mechanism does not integrate with ATHENA's python job configuration, resulting in an unacceptable lack of flexibility. To solve this problem, a python layer has been developed (PyG4Atlas) which allows, using bindings to the underlying C + + G4ATLAS code, to fully configure a simulation job by means of python commands. Integration in the ATHENA frame-

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work is thus also achieved for what concerns job configuration, while the efforts requested to the users to interact with the simulation are kept to a minimum. 2. Examples of applications 2.1. Combined

test

beam

A full slice of the ATLAS detector (inner tracker, calorimetry, muon spectrometer) has been tested in 2004 on CERN's H8 beam line. The simulation infrastructure is successfully managing all the different aspects of such a complex experimental setup. Simulation of the subdetectors must be possible in both combined and stand-alone modes, while beams with different particles, energies and profiles must be simulated; extra detectors, such as scintillators, must be properly handled, together with some extra dead material used for material studies. All the above mentioned simulation parameters are indeed fully accessible from the python interface for the advanced user. Moreover, a database is provided associating the different configurations to a run number: a user only needs to specify in the jobOptions the run number, and the simulation infrastructure will automatically setup the corresponding set of parameters. 2.2. Cosmic

simulation

G4ATLAS provides full support for cosmic simulation allowing the user to choose a specific setup, consisting of the ATLAS detector, the ATLAS cavern, the rock overburden and the surface buildings. The geometrical description of the different elements is provided by GeoModel, and primary cosmic generation is done via a specific generator (CosmicGenerator) providing single muons at gound level. A scoring layer surrounds the ATLAS detector: particles crossing it will be saved to an external file, which can then be used to start a new simulation without having to propagate particles through the rock overburden.

3. Performance Being the simulation used for massive productions on the GRID, its performance in terms of CPU time per event and memory usage at run time needs to be constantly monitored. They are indeed measured at each nightly build of the ATLAS offline software, by a set of dedicated jobs using different sin-

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gle particle samples. More detailed tests, including full physical events, are performed at each ATHENA release.

Table 1. Average CPU time per event for single particle samples. Average disk size per event is reported as well. # simulated events

CPU time per event (kSI2K)

Event size (KB)

Single muons Pt = 200GeV, |»7| < 6.0

1000

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1000

1.97

3.2

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1000

63.13

34.25

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1000

17.43

33.29

Single electrons P t = lOOGeV, |JJ| < 6.0

1000

604.52

Pt = 50GeV, |TJ| < 2.5

1000

82.8

14.43

Pt = 5GeV, \t)\ < 6.0

1000

34.32

56.54

Table 1 shows the timing results obtained for ATHENA release 10.5.0 using single particle events. Concerning memory usage at run time, it is measured at different steps of any job startup (ATHENA initialization, GeoModel initialization and G4ATLAS startup), as well as during the event processing. In particular, at each nightly build, plots are automatically produced showing the memory usage as a function of the number of processed events, in order to easily spot any major software problem such as memory leaks. Figure 1 shows the results of such a measurement done using single muons in ATHENA release 10.5.0. A summary of the tests done in the past years can be found in Ref. 3. Concerning physics performance, the 2004 combined test beam has been an important benchmark to evaluate the reliability of the simulation, and to compare real data with simulation. For all the subdetectors, analysis has shown very good agreement between the simulated response and the one measured at the test beam. Moreover, reconstruction of full physical events provided results at least as good as the ones obtained with the previous GEANT3-based simulation.

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Conclusions

T h e simulation software of t h e ATLAS detector, implemented using t h e G E A N T 4 toolkit, is presently a robust and fully featured application suite, integrated in t h e ATLAS offline software. T h e needed flexibility, configurability and ease-of-use are achieved by means of python scripts, which allow t h e users to have full control on t h e simulation job. T h e simulation suite is being successfully used for massive production since 2004, and its performance is continuously monitored using a u t o m a t e d nightly tests, as well as more detailed measurements at each software release.

References 1. ATL-SOFT-2003-013, "Strategy for the transition from Geant3 to Geant4 in ATLAS", by D. Barberis, G. Polesello, A. Rimoldi. 2. The A T H E N A framework: http://atlas.web.cern.ch/Atlas/GROUPS/SOFTWARE/00/architecture/General/ Documentation/AthenaDeveloperGuide-8.0.0-draft.pdf

3. ATL-SOFT-PUB-2005-004, "Validation of the GEANT4-Based Full Simulation Program for the ATLAS Detector" by D. Costanzo, A. Dell'Acqua, M. Gallas, A.Nairz, N. Benekos, A. Rimoldi, J. Boudreau, V. Tsulaia,

THE ATLAS EXPERIMENT AT CERN: TWO YEARS BEFORE THE START OF THE DATA TAKING ANNA DI CIACCIO On behalf of the ATLAS Collaboration University of Roma Tor Vergata and INFN Tor Vergata, via della ricerca scientiflca, 00133 Roma, Italy The ATLAS experiment at the Large Hadron Collider, under construction at CERN, is the biggest high energy-physics detector ever built on an accelerator. It will be one of the two general purpose experiments installed at the LHC. The ATLAS detector is now approaching the end of the construction and it is entering the installation and commissioning phase. The status of the different sub-detectors will be described.

1. Introduction ATLAS [1] is a general purpose detector which is designed to exploit the full discovery potential of the Large Hadron Collider (LHC) that is being built at CERN, Geneva. The LHC [2] is a proton-proton accelerator which will reach an energy in the center of mass of Vs = 14 TeV, giving the possibility to investigate a wide range of physics up to masses of several TeV. The most prominent issue is the search for the origin of the spontaneous symmetry breaking mechanism in the electroweak sector of the Standard Model. One of the possible manifestation of such a mechanism could be the existence of a SM Higgs bosons (H) or a family of Higgs particles (H*, h, H and A) when considering the Minimal Supersymmetric extension of the Standard Model (MSSM). The search for supersymmetric particles and of other final-state physics signals beyond the Standard Model (heavier W and Z bosons, extra-dimensions, quark compositeness, etc) are possible thanks to a detector that has been designed to provide as many signature as possible using electrons, gamma, muons, jets and missing trasverse energy as well as b-quark tagging. The actual plan foresees the start of the LHC operation and of the detector data taking in summer 2007.

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2. The overall detector concept The ATLAS detector, shown in Fig. 1, has been designed to have a large pseudo-rapidity coverage up to |n| < 5. It has a diameter of 24 m and a length of 44 m. The total weight is 7000 tons. The basic design criteria are the following: • an efficient inner tracking system for precise lepton momentum measurements and b-quark tagging; • a very good electromagnetic calorimeter complemented by a full coverage hadronic calorimeter for jet and missing trasverse energy measurement; • a stand-alone muon system for precision momentum measurement up to the highest luminosity and with low p T muon trigger capability. Muon Detectors

Barrel Toroid

Electromagnetic Calorimeters

Inner Detector

Hadronic Calorimeters

Shielding

Figure 1. A view of the ATLAS experiment.

3. The magnetic system The magnet configuration is based on a super-conductive solenoid with a field of 2 Tesla in the inner detector cavity and three super-conducting air-core toroids: a long barrel consisting of eight independent coils arranged with an eight-fold

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symmetry outside the barrel calorimeter and two end-cap magnets. They generate a large field volume and a strong bending power within an open structure. Multiple scattering effects are therefore minimal and a high resolution and a high acceptance muon momentum measurement is achieved with three muon stations of high precision tracking chambers. The total bending power, integrated between the first and the last muon chambers, increases from about 3 Tm at n =0 to about 8 Tm at n =2.8. The whole magnet system represents a cold mass of 700 tons and a total weight of 1400 tons. The inner solenoid coil is integrated into the vacuum vessel of the barrel LAr calorimeter cryostat, thus eliminating the material and space of independent vessel walls. Recently the assembly of the eight coils of the barrel toroid has been completed and shortly after the barrel calorimeter system, that a year ago has been transferred into the pit, has been inserted in the central position at z=0.

Figure 2. A photography of the ATLAS barrel spectrometer assembled with its eight coils by the end of September 2005. At the other end of the hall, the barrel LAr and Tile calorimeter system is visible with in front the solenoid coils and cryostat.

4. The inner tracker The ATLAS inner detector uses three sub-detectors for tracking of charged particles from r = 5 cm to r = 107 cm inside a solenoid magnet of 2 T. The

559 innermost detector is a high resolution silicon pixel detector. It provides precise 3D tracking information close to the interaction point allowing secondary vertex reconstruction and hence b identification. It is followed by the SCT, a large area tracking device based on silicon strip detectors. The TRT, based on straw tubes, provides continuous tracking and improves electron identification due to its ability to detect transition radiation. In the barrel region the high precision detector layers are arranged in concentric cylinders around the beam axis, in the end-cap the detectors are mounted on disks perpendicular to the beam axis. The pixel layers are segmented in R and z- The SCT uses small angle stereo strips to measure both coordinates. The TRT is made of straw tubes parallel to the beam direction. 4.1.1. The pixel vertex detector The ATLAS silicon pixel detector consists of more than 1700 modules for a total sensitive area of 1.7 m2 and over 80 million pixel cells organized in 3 layers in the barrel and 3 disks for each of the end-caps. The basic unit is a silicon sensor with an active area of 60.8 mm x 16.4 mm, divided in pixels of 50um x 400um size. The pixels are read out by 16 front-end chips arranged in two rows and bump bonded to the sensors providing pulse height information by a time-over-threshold technique. In summer 2005 a corrosion of some of the cooling pipe of the staves has been found. It is caused by the presence of nickel in the Aluminum pipe endings (needed to braze the fittings) and by the exposure to water without a proper drying procedure. The galvanic currents has produced corrosion in 15% of the pipes that are now leaky. The problem has produced a six months delay in the pixel detector installation that is now foreseen only by February 2007.

4.1.2. The Semi conductive tracker The Semiconductor Tracker will be located between the innermost pixel and the outer Transition Radiation Tracker at radii between 25 and 50 cm. It consists of a central barrel section (4 barrel layers) and two end-caps (9 end-cap disks on each side) allowing for charge particle tracking up to a pseudorapidity of ± 2.5. The total number of silicon microstrip detector modules that will compose the SCT is 4088 with about 6 million read-out channels.

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The radiation environment requires the detectors and electronics to be radiation hard up to a total fluencies of 2 x 1014 1 MeV neutrons/cm2 for the anticipated 10 years of operation. In order to limit the anti-annealing of the silicon detectors, the SCT will operate at an ambient temperature of -7° C. An efficient and accurate track and vertex reconstruction requires an SCT detector efficiency of at least 95%, as well as a resolution of 16|xm in Rep and 500 pan in z direction. The four barrel layers (Figure 3) have been recently assembled together and put on the thermal enclosure, the preparation of the disks for the end-caps is proceeding according to the schedule.

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4.1.3. The Transition radiation tracker The Transition Detector Radiation (TRT) consists of 36 layers of 4 mm diameter straw tubes with a resolution of ~200um, interspaced with a radiator to produce transition radiation from electrons. There are two thresholds for recording hits,

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the high threshold being used to detect TR photons. The TRT provides separation between hadrons and electrons. The barrel TRT is organized in three rings, each containing 32 modules made of axially aligned straws, and 2 endcaps made each of 2 set of independent eight plane wheels with radially oriented straws. The barrel assembly has been completed by September 2005 and fully tested. Figure 4 shows a cosmic muon traversing the barrel TRT. The end-cap rings will be both completed and transported to CERN by December 2005. In the next months the silicon tracker will be inserted into the barrel TRT and they will be tested together. The SCT end-caps will be inserted into the TDR end-caps during the spring 2006.

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5. The electromagnetic and hadronic calorimeter The calorimeter provides precision measurements for energy and direction for electrons, photons, hadronic jets and missing transverse momentum. In addition it provides a fast trigger up to the highest luminosities. It consists of an inner barrel cylinder and two end-caps cryostats, using the radiation resistant Liquid Argon (LAr) technology for the electromagnetic calorimeter, which are surrounded over their full length by a hadronic scintillator tile calorimeter. The barrel electromagnetic LAr calorimeter consists of accordion shaped lead absorbers, separated by 2 mm (LAr gap) interleaved with read-out electrodes. At n=0 the thickness is 26 XO. The barrel e.m. calorimeter has a highly granular first sampling (integrated pre-shower detector) which is preceded by a separated coarser pre-sampler to preserve the energy and direction after the cryostat and the coil material. The measured performance of the barrel and end-caps e.m. calorimeters at various n positions from 0 up to 2.7 can be described with a 10% /VE sampling term and a constant term of 0.3%. Wt^mr

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Figure 5. A cosmic ray muon traversing the tile calorimeter.

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The hadronic scintillator tile calorimeter is based on a sampling technique with plastic scintillator plates (tiles) embedded in the iron absorber and read-out by wavelength shifting fibers. The active calorimeter depth at n= 0 is about 9.5 Xai,s, followed by about 1.4 X^ of passive iron in the support structure and integrated with the solenoid flux return yoke. The hadronic calorimeter uses scintillator tiles in the barrel and liquid argon in the end-caps (n > 1.5); its resolution can be described with a 50% /VE sampling term and a constant term of 3%. Forward calorimeters cover the regions 3 < n < 5 with a resolution better than 100% /VE± 10%.

6. The muon chambers The muon system is based on the magnetic deflection of the muon tracks in the large superconducting air core toroid magnets, instrumented with trigger and high precision tracking chambers. The momentum resolution is ApT/pT = 2xl0~2 at 100 GeV and ApT/pr =10"x at 1000 GeV. In the barrel region tracks are measured in three cylindrical layers (stations); in the transition and end-cap regions the stations are installed vertically. Over most of the pseudorapidity range (|r|| < 2) a precision measurement of the muon tracks in the principal bending direction of the magnetic field is provided by Monitored Drift Tubes (MDTs). At larger pseudorapidity range (2.0 < |n| < 2.7) Cathode Strip Chambers (CSCs) are used to stand higher rate and background conditions. The MDTs provides a single wire resolution of 80 um when operated at high gas pressure (3 bars). The trigger chambers covers the pseudorapidity range between 2.0 < |r|| < 2.7. Resistive Plate Chambers (RPCs) are used in the barrel region at |n| < 1.05 and Thin Gap Chambers (TGCs) in the forward region (1.05< |T)| < 2.4). The trigger chambers serves several purposes: a trigger with a well defined muon p? discrimination, bunch cross identification and the second coordinate measurement in the non-bending plane with a precision of a less of 1 cm. Most of the muon chambers have been produced and they are going to be completed at CERN with electronics and service cables. For the barrel the assembly of the stations MDT-RPC (in the middle and outer layers) is progressing in the BB5 hall at CERN and for the end-caps the first TGC-MDT sector (EIL4) has been assembled. Moreover 56 barrel muon stations have already been installed inside the coils of the spectrometer. By the end of the year the installation of all the stations

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(BMS) in the middle layer of the small sectors of the spectrometer will be completed and of a part of the outer stations (BOS). The completion of the muon barrel stations installation is foreseen by the first half of 2006. 7. Conclusions A relevant part of the ATLAS detector construction is finished. The installation of the detector and all related services underground is advancing at a good speed. Recently major milestones passed are the successfully completion of the installation of the eight toroidal coils of the barrel spectrometer and the insertion of the barrel calorimeter at z=0. Commissioning activities have started with cosmic rays at full detector level in surface and soon will start the data taking with cosmic rays in situ for the calorimeter and a part of the barrel muon chambers (sector 13). The ATLAS experiment is on track to start the data taking in 2007 for the start of the operation of the Large Hadron Collider.

References 1. 2.

ATLAS Collaboration, ATLAS Technical Proposal, CERN/LHCC/94-43, 1994 The LHC project at CERN, LHC-project-report-36 (1996).

PROGRESS IN GRID COMPUTING FOR PARTICLE PHYSICS ROBERT GARDNER Enrico Fermi and Computation Institutes, University of Chicago Chicago, Illinois 60637, USA We discuss progress in building and exploiting distributed Grid computing infrastructures with an emphasis on capabilities required for the next generation of particle physics experiments at the LHC. Associated with a persistent and reliable Grid infrastructure linking compute, storage and network resources together, experiments, organized as Virtual Organizations (VOs), are building information architectures that allow discovery, access, and manipulation of datasets and algorithms. We use the ATLAS experiment as a context for describing the scale and complexity of the processing tasks associated with large scale data analysis relevant to the LHC.

1. Introduction 1.1. Overview In this article we take a brief look at recent progress in what has now become a large, worldwide effort in building distributed computing infrastructure capable of meeting the demands of the experiments built as part of the Large Hadron Collider Project fl] at CERN [2]. The LHC experiments, which are leading drivers of distributed computing technology, are acquiring lessons using this infrastructure that will provide for a large-scale, distributed, secure and reliable computing platform for 0(1000) physicists that will require the ability to discover and use many types of computing services for their processing tasks. The efforts are also benefiting existing high energy and nuclear physics experiments (some of which have been busy converting their legacy production systems to open Grid software, protocols and standards) as well as applications from other science domains. After reviewing the essentials of the computing requirements which set the scale for the challenge at hand, we consider various approaches to joining together distributed resources, review a few of the results and general lessons learned to date, and examine approaches for moving forward. 565

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1.2. The Pressing Needfor Grid Infrastructure in Particle Physics The LHC opens a new regime in particle physics made accessible for the first time by a combination of high luminosity and high proton-proton center-of-mass energy. The ATLAS collaboration and detector is one of four experiments [3-5] at the LHC that will make discoveries designed to elucidate the nature of matter and the interactions of the fundamental particles. The initial focus of the large, general-purpose detector will be to probe the mechanism of electroweak symmetry breaking by discovering new particles (such as the "Higgs" particle) and measuring their properties. Further studies will look for the presence of supersymmetric partners to the Standard Model groupings of quarks, leptons and gauge bosons. The datasets created during ATLAS runs will be larger than any previous particle physics effort, with multiple-Petabytes of data produced each year. These datasets will hold event collections corresponding to and event rate of 25 MHz (each raw trigger composed of about 2 megabytes of raw electronic data which is subsequently processed into tracks, jets, showers, leptons, vertices and the like). During the lifetime of the experiment the scale will grow from 10's to 100's of Petabytes (1015 bytes) per year requiring Petaflops of processing capacity and high bandwidth networks to push data to those resources. To access and process this enormous data archive, a distributed computing model has been adopted [6], which requires at least two-thirds of the compute resources be located outside the CERN laboratory. The model is intrinsically distributed, which is (largely) a departure from generations of previous experiments. The reasons for this boil down essentially to the availability of large scale computing resources outside of CERN and the rapidly advancing capacity of the networks joining them. The picture is of a hierarchy of computing centers, or "Tiers", which present at various levels data processing capabilities appropriate to the center and role of that center. 1.3. The Pressing Need for New Information Architectures Along with the very large scale of the data and compute resources comes the need to create an information system capable of providing effective management of the data and processing algorithms and tasks involved. A typical high level process flow, from theory to data analysis and back again, is depicted in Figure 1.

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Figure 1 High level process flow in high energy particle physics, view from the perspective of a process management framework.

Viewed from a process management perspective, there is relatively little complicated in the traditional "chain" of processing operations that take raw data from the detector to the physicist's desktop where subsequent, fine grained analysis happens (calibration, examination of cuts, comparison with theoretical models, Monte Carlo simulations, fitting). However, the problem complexity grows enormously when one maps the set of processes onto a physically distributed set of resources at the scales required here. Decomposing what is involved here, we look at some typical capabilities expected by the LHC physicist and the resulting patterns that emerge which suggest and provide guidance to building an information architecture to go along with the core computing infrastructure. An individual physicist-user or working group will wish to apply algorithms to a particular selection of input data in a given execution environment. The input datasets are discovered from a global archive typically via queries to metadata catalog services, which collect and store assigned name/value pair attributes for a number of experimental conditions and event trigger requirements. These queries will involve dialogs to determine the appropriateness of the returned datasets. For example the physicist may wish to place restrictions on the dataset type (raw detector data, reconstructed event data, analysis object data, or result objects from previous analyses) or event-level attributes (describing attributes of the collision such as the presence of large transverse momentum electrons). Candidate datasets may also be inspected for their provenance information - what was the specific set of abstract and concrete transformations that generated the dataset? Which specific software releases and calibration constants were used during the reconstruction? The physicist may wish to recursively inspect the dataset at many levels - to examine individual provenance of event fragments corresponding to different

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algorithms employed during the processing. The dataset may be described virtually, with pieces described but not yet materialized or once materialized and now deleted. A full provenance chain describing intermediate processing steps and dataset dependencies may need to be examined for appropriateness to a given analysis. Decisions on whether to proceed may be governed by the availability of the dataset, or whether significant re-processing may be required to deliver it to the algorithms. Dataset properties relating to splitting may be examined to determine if a distributed computation is possible. Transformation metadata may be used to provide estimates of resource requirements, information which the physicist may also weigh in deciding whether to proceed with a given computation. The dataset and application configurations are submitted to a workload management service, which itself will depend on a number of other services providing, for example, dataset catalog services, a basic security infrastructure, information services containing attributes of distributed compute and storage elements, authorization and accounting services, job monitoring, data provenance services, etc. Work requests can be minimal, low-time latency executions, which satisfy requirements for interactive analysis, or long-lived processes requiring large scale resources and which may take weeks or longer to satisfy. The physicist-user monitors overall task progress and job processes which in turn may interact with dataset and replica catalog services during processing. The results are gathered together and passed back to the owner of the request who may then perform additional processing steps. A framework for describing the state of process threads, faults, and recovery procedures is needed. These are being developed within the individual VO communities by teams of software developers. 2. Grid Architectures, Principles and Deployments There are now many successful Grid projects worldwide that encompass a variety of architectures, deployment approaches, and targeted application domains. For example, European efforts include the LHC Computing Grid Project (LCG) [7] project, the European Data Grid (EDG) and its follow-on EGEE (Enabling Grids for E-Science) [8], and DataTAG [9], which focused on transatlantic grid testbeds and high performance networks. NorduGrid [10] links computational centers in Scandinavia to deliver production services for highenergy physics applications. The U.S.-based Grid3 project [11], which began operations in Fall 2003 and completed by Spring 2005, extended these and other

569 efforts by relying on immediate application requirements to drive architectural principles and decisions. 2.1. Physical and Service Architectures, and Principles Projects develop a distributed architecture for linking resources taking into account several factors, including recognized emerging standards, such as being developed in the Global Grid Forum, lessons from previous projects, constraints on site policies of the resource providers, and naturally, the realistic use cases from VOs. We briefly touch on the physical, service, and principles guiding decisions in implementation and technology choices. 2.1.1. Typical Site Architecture Some abstractions for compute and storage resources have entered into common use as they describe a canonical computing center, as show in Figure 2. A Compute Element (or CE) describes a typical Linux farm in which execution nodes (which can be high-end servers or white box commodity) are connected via Gigabit Ethernet switches, sometimes completely internally with no visibility or connection to the public internet, and a small number of "gatekeeper" nodes which provide a hosting environment for the Grid services which communicate to external, higher level Grid services. Optionally within a site's administrative domain there can be a Storage Element (or SE) which provides a disk-based (tertiary, backend tape-based systems are provided by larger facilities) storage for caching of datasets. These usually consist of a set of network attached servers or commodity disks configured as RAID volumes. Both CE and SE elements are made known to the external environment through an information service, which may a local database or a publishing service to a higher level server which aggregates information from several such servers, providing an information index service.

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CE: Compute Element

£E: Storage Element

Figure 2 A common abstraction for computing and storage resources as viewed from the point of view of various, higher level services which may be Grid specific (such as a discovery service) or VO-specific, such as user authentication and authorization database.

The CE is defined in terms of its platform (operating system and processor type), local batch queuing system (such as PBS or the Condor scheduler), the environment a job expects to find once it begins execution, including pointers to various pre-installed utilities, common software installation areas, and characteristics of the local storage on each execution node. The Unix account structure is setup by the facility administrator to map to various supported VOs according to roles, policies, and agreements between the resource provider and the accessing VO. There are a number of systems that have been developed for this purpose, including the Virtual Organization Management System (VOMS) [12], the Grid User Management System (GUMS) [13], only to name a couple. 2.1.2. Typical Software and Service Architectures Very quickly, one arrives at a layered view of the software and service infrastructure that must be in place in order to provide users access to the underlying resources. This is show schematically in Figure 3.

571 Applications, Portals and VO Services Wor&oed and Laeof KepftcQ

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Figure 3 Simple middleware architecture diagram showing services that span sites (hosting CE and SE elements) in a Grid.

Usually what is called for is a simple architecture that links execution and storage sites and provides services for virtual organization management, workload and replica management, monitoring and accounting services, all provisioned through middleware. There are leading providers of middleware that the LHC experiments are using. The EGEE project, which is delivering technology to the LCG project, has a large scale software engineering effort to produce a set of packages, collectively known as "gLite". The main components of gLite span a set of service groups including security services (providing a service to establish the authenticity of users, as well as auditing and authorization), information and monitoring services that provide remote services used by either the VO and Grid operations centers, data services which host replica and metadata catalogs, and job management services which provide package installation, job provenance, policy enforcement and workload management. The EGEE gLite software utilizes at its core the Virtual Data Toolkit (VDT) [14]. The VDT is also used in the U.S.-based Open Science Grid project. The VDT provides services from the Globus Toolkit [15], Condor [16], GriPhyN, and PPDG, as well as components from other providers. VDT allows grid facility administrators to configure their sites easily with simple and welldefined interfaces to existing facility configurations, information service providers, and storage elements. Additional services such as Replica Location Service (RLS) [17], Storage Resource Manager (SRM) [18], and dCache [19] are deployed at sites according to requirements from individual VOs.

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2.1.3. Policy and Interoperability Policy issues affecting use of the resources come into play through sharing agreements between VOs and priorities and optimizations made within a VO. Sites comprised of CE's and SE's are typically associated with a VO, and some sites may be providing resources to the wider Grid environment on behalf of several VOs (such as a large national computing facility or laboratory). Thus a policy and accounting infrastructure is required to establish fair share policies and bartering among VOs. The reality today is that there are multiple, core Grid infrastructures. This has come about from a number of factors, including a natural process of trial and error by one project, and the need to take independent decisions and approaches by another. But by and large the underlying protocols are the same or identical, and it helps that between OSG and LCG/EGEE a common core layer VDT is shared between the two. However, several differences exist in specifics of how a site's architecture is organized, which services are expected to be running, and what their interfaces are. Thus, efforts at Grid interoperability are focused on providing compatibility between the infrastructures, abstracting these differences away from the end-user physicist. There are several approaches here, ranging from publishing the essential details of the resource (as expressed in an agreed to, common schema) to a resource broker that the VO's workload management system (or end-user client tools) sends jobs. 2.2. Existing Deployments and Projects 2.2.1. The LCG/EGEE Grid The LCG project is organized by CERN to provide a computing environment for the four LHC experiments. The project receives deliverables for middleware technology, infrastructure, and operational support from the EU-funded EGEE project which is providing a multi-science core foundation for Europe. The aforementioned re-engineered middleware set of packages, gLite, is provided by EGEE. The deployed infrastructure consists, as of July 2005, of 140 sites dispersed in 34 countries, aggregating more than 15,000 processors. The experiences gained while supporting data challenges by LHCb and ATLAS include ramping to a capacity of >3500 concurrent jobs, with ATLAS averaging 8,000 jobs per day at peak of operations. Over 1 million SI2K-years of CPU time were consumed, resulting in 400 TB of data produced, transported, and archived in its storage facilities.

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Of major importance were lessons provided by the exercises which included dealing with site-level stability (and the varying degree of systems expertise between the contributing sites), and a job efficiency of > 90% under controlled conditions. 2.2.2. The Open Science Grid The Open Science Grid is a persistent, shared, multi-virtual organization (VO) Grid infrastructure capable of providing production level services for large-scale computational and data-intensive science applications. The OSG Consortium receives contributions from several projects, including the GriPhyN virtual data research project [20], the Particle Physics Data Grid, PPDG [21], and the International Virtual Data Grid Laboratory, iVDGL [22], as well as the U.S. ATLAS and U.S. CMS Software and Computing Projects of the LHC. The OSG extends the capabilities of the previous project Grid3 but still follows a simple two-tier approach, in which each resource (compute, storage, application, site, user) is logically associated with a VO. At each site, a core set of grid middleware services with VO-specific configuration and additions are installed, with registration to a VO-level set of services such as index servers and grid certificate databases. Where appropriate, VO-level services were combined into top-layer services at the OSG Grid Operations Center, which provided monitoring applications, display clients, and verification tasks and an aggregate view of the collective OSG resource and performance. Approximately 30 sites comprised of national laboratories and university-based centers contributed approximately 14,000 processors to the environment, providing resources to more than 35 VOs. 3. Performance and Lessons from ATLAS We briefly summarize here some results from the past two years of data challenge exercises taken by the ATLAS experiment. ATLAS used LCG, NorduGrid, and Grid3, and designed a production system capable of reaching all three. A major exercise was launched in June 2004 and consisted of phases covering large scale production of physics datasets (order 10M events) with a chain of production steps including Pythia event generation, GEANT4-based simulation, digitization and pileup. Almost 45 million CPU days (in SI2K-CPU) were used to execute over 100K jobs that contained nearly 8 million events and about 30 TB of data. The three Grids contributed with almost an equal amount of work each. The lessons drawn consisted of the following:

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1. Protect grid services that are vulnerable in a multi-VO environment. 2. The most likely single point of failure is data staging. 3. An infrastructure to establish policy is needed to distinguish user roles within a VO. 4. Delay binding of jobs to resources until the last possible moment in order to optimize utilization. 5. Robustness and reliability are required to keep operating costs low. 4. Approaches Moving Forward There are two significant courses being undertaken by Grid infrastructure providers and VO application framework developers. The first major key is to develop an infrastructure that will provision and protect servers on the "edge" of the compute resource, essentially providing a container for a VO manager to drop in a service. This approach facilitates the complex workflows and methods that may be specific to the application and not general enough to be a common middleware service. An example is a distributed data management system, in which a VO has specific requirements on the functionality of reliable file transfers, local space management, and catalog structure. A second major thrust is to deploy a fully services oriented architecture using, for example, the web services resource framework (WSRF) that intends to hide much of the complexity and details of using the resource behind a standard interface, simplifying access and discovery of datasets and other services made available by resource providers. Acknowledgments This work was supported in part by a grant of the National Science Foundation, NSF ITR-0122557. The author gratefully acknowledges useful discussions with J. Shank, R. Jones, D. Barberis, L. Robertson, R. Pordes, I. Legrand, M. Lamana, E. Laure and M. Mambelli. References 1. 2. 3.

The Large Hadron Collider Project at CERN; Available from: http;//lhcnew-homepage.web.cern.ch/lhc-new-homepage/. CERN, the European Laboratory for Particle Physics; Available from: http://cern.ch/public/. The ALICE Experiment at CERN; Available from: http://alice.web.cem.ch/Alice/AliceNew/.

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4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15.

16.

17.

18.

19. 20. 21. 22.

CMS: Compact Muon Solenoid at CERN; Available from: http://cmsinfo.eern.ch/Welcome.html/. The LHCb Experiment at CERN; Available from: http://lhcb.web.cern.ch/lhcb/. Report of the Steering Group of the LHC Computing Review (2001); Available from: http://lhc-computing-review-publie.web.cern.ch/lhccomputing-review-public/Public/Report final.PDF. The LHC Computing Grid Project (LCG); Available from: http ://lcg. web. cem.ch/LCG/. EGEE: Enabling Grids for E-Science in Europe; Available from: http://public.eu-egee.org/. Research and Technological Development for a Data TransAtlantic Grid; Available from: http://datatag.web.cem.ch/datatag/proiect.html. NorduGrid: Nordic Testbedfor Wide Area Computing and Data Handling; Available from: http://www.nordugrid.org/. The Grid2003 Project, The Grid2003 Production Grid: Principles and Practice, in High Performance Distributed Computing (2004). EU DataGrid Java Security Working Group, VOMS Architecture vl.l (2003); Available from: http://grid-auth.infn.it/docs/VOMS-vl 1 .pdf. G. Carcassi, Grid User Management System; Available from: http://prid.racf.bnJ.gov/GUMS/index.html. The Virtual Data Toolkit (VDT); Available from: http://www.lscgroup.phvs.uwm.edu/vdt/. I. Foster, Kesselman, C, Globus: A Metacomputing Infrastructure Toolkit, International Journal of Supercomputer Applications 11 (2), 115-129 (1998). M.J. Litzkow, M. Livny and M.W. Mutka, Condor - A Hunter of Idle Workstations, 8th International Conference on Distributed Computing Systems 104-111(1988). A. Chervenak, E. Deelman, et al., Giggle: A Framework for Constructing Scalable Replica Location Services, in SC'02: High Performance Networking and Computing (2002). B. Sim, A. Shoshani, J. Gu, ed. Storage Resource Managers: Essential Components for the Grid. Resource Management for Grid Computing (2003). The dCache Project; Available from: http://www.dcache.org/. P. Avery, I. Foster, Towards Petascale Virtual Data Grids (GriPhyN Project); Available from: http://www.griphvn.org/. Particle Physics Data Grid; Available from: http://www.ppd g.org/. P. Avery , I. Foster, R. Gardner, H. Newman, A. Szalay, An International Virtual-Data Grid Laboratory for Data Intensive Science, Technical Report: GriPhyN-2001-2 (2001. http://www.griphvn.orgA.

D E S I G N , P R O D U C T I O N A N D QUALITY OF T H E SENSORS FOR T H E SILICON STRIP T R A C K E R OF CMS

M. K R A M M E R Institute

for High Energy

Physics of the Austrian Academy of Nikolsdorfergasse 18, Vienna, Austria E-mail: Manfred. KrammerQhephy. oeaw. ac. at On behalf of the CMS Silicon Tracker Collaboration

Sciences,

The construction of the Silicon Strip Tracker of CMS requires about 25000 large area silicon sensors. By the time of the conference the delivery of these sensors was almost completed. The paper reviews the design of the sensors including the considerations to achieve the required radiation hardness. A comprehensive quality assurance program was developed by CMS to monitor the quality of the sensors produced by two companies. After explaining the basic elements of the quality assurance program an overview of the results is shown. During the production several problems were identified and eventually solved in cooperation with the manufacturing companies.

1. The CMS Experiment At CERN, the European Laboratory for Particle Physics in Switzerland, the large proton-proton collider LHC is under construction. The LHC will produce collisions at a centre of mass energy of 14 TeV. The startup of the LHC collider is foreseen for summer 2007. To exploit this machine the CMS experiment, a multipurpose experiment, was designed and is under assembly at CERN. The acronym CMS stands for 'Compact Muon Solenoid' and indicates one of the key features of this experiment, the precise measurement of muons. The CMS detector will consist of several shells of different detector elements. Particles created in the high energy collisions in the very center of the detector will first traverse the 'Inner Tracker' a system of silicon sensors designed to detect charged particles. Outside of the Inner Tracker is the electromagnetic calorimeter, a detector consisting of about 80000 scintillating crystals, whose purpose is to absorb electrons and photons and to measure their energy. Further outside is a shell of hadronic calorimeters

576

577 measuring the energy of hadrons, e.g. pions, protons, etc. Just outside the hadronic calorimeter is a superconductive solenoid magnet which will create a magnetic field of 4 Tesla. All charged particles traversing this field are deflected with a bending radius which depends upon their charge and momentum. The outside shell of CMS is formed by the muon chamber system, a gas detector system designed to measure the tracks of through going muons. Combined with the muon tracks measured in the Inner Tracker the momentum of the muons can be determined with high precision. The complete CMS detector will be 21.6 m long, with a diameter of 15 m and with an overall weight of 12500 tons.

2. The Layout of the CMS Inner Tracker The CMS Inner Tracker : is divided into substructures: The pixel detector very close to the interaction point, which is not further explained in this paper, and the strip tracker consisting of the inner barrel detector (TIB), the inner discs (TID), the outer barrel (TOB) and the two end cap detector systems (TEC). The overall length of the Inner Tracker will be 5.4 m with a diameter of 2.4 m. The total weight will be about 3 tons. The temperature inside the tracker will be adjusted such that the maximum temperature of the silicon sensors will not exceed —10°C. The total electrical power dissipation is expected to be about 45 kW. Figure 1 shows a cut through a quarter of the Inner Tracker. The solid lines in this sketch represent the silicon sensor modules. The modules of the TIB are mounted on four and the TOB modules on six concentric shells. The supporting structures for the TID and TEC are discs, two times three discs for the TID and two times nine discs for the TEC. The basic construction element of the Silicon Strip Tracker is a module (see figure 2). The supporting frame of a module is made of carbon fiber or graphite. Glued onto the frame is a Kapton layer to isolate the frame from the silicon and to provide the electrical connection to the silicon backplane. A ceramic multilayer hybrid holds the readout chips and the auxiliary chips. A glass pitch adapter is mounted between the hybrid and the first silicon sensor to match the different pitches of the chips input pads and the sensor strips. Wire bond connections between the individual channels of the readout chips and the pitch adapter, between the pitch adapter and the first sensor and between the two sensors make the electrical connections. The modules of the TIB, the TID and the four inner rings of TEC will consist of only one silicon sensor, whereas the modules of the TOB and

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The CMS Inner Tracker Layout.

the three outer rings of TEC will hold two sensors. All barrel modules are rectangular. The disc modules have a wedge shape in order to form rings. Figure 2 shows a final module of the second ring of the TEC. The first two layers in TIB and TOB, the first two rings in TID and rings 1, 2, 5 in TEC are instrumented with double-sided modules. These are made of two independent single-sided modules, mounted back to back and rotated by 100 mrad with respect to each other. Table 1 lists some numbers illustrating the overall dimension of this project. Table 1. Some key numbers from the Silicon Strip Tracker construction Area of active silicon Number of silicon sensors Different sensor designs Number of modules Number of strips Number of electronics channels Number of readout chips Number of wire bonds

« 200 m 2 24,244 15 15,232 RJ 9,600,000 ss 9,600,000 « 75,000 ss 25,000,000

3. The Design of the Silicon Sensors To build such a large silicon system, the influence of the design of the silicon sensors on the mass production was studied first. In close collaboration with industry the basic parameters of the sensors were defined to reach maximum

579

Figure 2. A ring 2 module for the TEC mounted on a transport plate

efficiency for the mass production. The result was a rather simple design compared to previous projects in high energy physics 2 . In this respect it was decided to use only single sided sensors. A second criterion driving the design was to optimize the use of the wafer area as the silicon sensor prize is determined by the number of wafers rather than the actual sensor surface. The 15 different sensor types were designed to make maximum use of the area available on 6 inch wafers as most of the manufacturers use 6 inch fabrication lines for the sensor production. Concerning the silicon material standard high resitivity n-type silicon wafers are used. The potential advantage of oxygenated material with respect to radiation hardness was neglected in favor of the safety of using well understood material. The proton-proton collisions inside the LHC detectors will create an enormous radiation background which will damage every detector material. The innermost layers of the silicon strip detector will receive an estimated fluence of 1.6xl0 14 neutrons c m - 2 (1 MeV neutron equivalent) during 10 years of LHC operation. The fluence is gradually lower for sensors mounted further away from the interaction point. Silicon is inherently radiation hard compared to other detector technologies. However the silicon sensors will be nevertheless modified and damaged. The two most important macroscopic

580

effects of the irradiation are the increase of the leakage current which is linear with fluence, and the conversion of the n-type bulk into p-type with ever increasing carrier concentration. A consequence of the second effect is the need for increasing bias voltage after the type inversion which eventually leads to detector breakdown. For the longtime operation of the sensors in the Inner Tracker, it is therefore essential that the sensors can sustain very high bias voltages up to 400 - 500 V. An appropriate choice of the resistivity of the original silicon wafers is important to influence the evolution of the depletion voltage. CMS has decided to use different material for the inner part of the tracker compared to the outer part: the inner four barrel layers (TIB), the inner discs (TID) and the four innermost rings of the forward discs (TEC) are fabricated using silicon of relatively low resistivity (1.25 - 3.25 kfi cm) with a thickness of 320 ^m. The outer six barrel layers (TOB) and the three outermost rings of the end-cap discs (TEC) will be made of silicon with high resistivity (3.5 - 7.5 kfl cm), using wafers of 500 fim thickness. The low resitivity sensors for the inner part of the detector will require a high bias voltage at startup (up to 330 V), however the type inversion will occur at a higher fluence and hence later in time. At the end of the foreseen lifetime of 10 years, the sensors which are exposed to the highest fluence will.require a bias voltage which is not much higher than the one at startup. For the sensors in the outer part the choice of high resistivity material allows to use thicker wafers with still manageable high depletion voltage. Another choice related to the radiation hardness was to use silicon wafers with a lattice orientation of (100). CMS studies have shown that this material has advantages compared to material with a lattice orientation of (111). The interstrip capacitance remained nearly unchanged after irradiation for sensors produced on (100) material, but is increased significantly if (111) material was used 3 . In order to cover the barrel layers and the end-cap discs with silicon sensors 15 different designs are needed: four different rectangular designs for the barrel and 11 different wedge shaped designs for the inner discs and the endcaps. Table 2 and 3 show an overview of the different sensor types, their dimensions, the strip pitches and the number of sensors needed for the particular type. The principal design parameters are identical for all designs. Figure 3 shows a corner of a wedge shaped sensor. The sensors have p + strips on n-type bulk material. The strip pitch varies from 80 /um up to 205 fxm. The ratio strip width to strip pitch is 0.25 for all types. The strips are

581 AC coupled and the coupling isolation is formed by two thin layers of Si02 and Si3N4. The thickness of the aluminum readout lines is required to be at minimum 1.2 /im. An array of polysilicon resistors (1.5 ± 0.5 MQ) is implemented to connect the individual strips to a common p + bias ring. To enable the sensors to be operated at a bias voltage of up to 500 V the aluminum readout strips are wider than the p + strip implants (4 /um to 8 fim metal overhang). For each strip two aluminum pads are foreseen for bonding. The DC pad is in direct contact with the p + strip implant for testing. The active sensor area is surrounded by the p + bias ring and one floating p + guard ring. Metal field plates on top of the guard ring extend also beyond the guard ring p + implant. An n + implant is realized along the sensor edges. Outside of the sensor area a set of standardized test structures, identical for all 15 wafer types, is implemented which is used for the CMS quality control. Two companies have produced silicon sensors for the CMS tracker: Hamamatsu Photonics K.K. (Hamamatsu-City, Japan) is producing the large part of sensors (> 25000 sensors, including spares). The second company ST Microelectronics (Catania, Italy) has produced about 2700 sensors for the outer part (TOB and rings 5 to 7 of TEC).

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582 Table 2. Inner Barrel 320 fim thick sensors and Outer Barrel 500 /wn thick sensors, geometrical dimensions and multiplicities Type IB1 IB2 OBI OB2

Length [mm] 63.3 63.3 96.4 96.4

Height [mm] 119.0 119.0 94.4 94.4

Pitch [/jm] 80 120 122 183

Strips 768 512 768 512

multipl. 1536 1188 3360 7056

Table 3. Geometrical dimensions and multiplicities for 320 jim thick (W1-W4) and 500 ftm thick (W5a-W7b) wedge sensors for TID and TEC: W l has two different versions for TID and TEC, whereas the TID shares identical W2 and W3 sensors with the TEC. Type W l TEC W l TID W2 W3 W4 W5a W5b W6a W6b W7a W7b

Length 1 [mm] 64.6 63.6 112.2 64.9 59.7 98.9 112.5 86.1 97.5 74.0 82.9

Length 2 [mm] 87.9 93.8 112.2 83.0 73.2 112.3 122.8 97.4 107.5 82.9 90.8

Height [mm] 87.2 112.9 90.2 112.7 117.2 84.0 66.0 99.0 87.8 109.8 90.8

Pitch [/im] 81-112 80.5-119 113-143 123-158 113-139 126-142 143-156 163-185 185-205 140-156 156-172

Strips 768 768 768 512 512 768 768 512 512 512 512

multipl. 288 288 864 880 1008 1440 1440 1008 1008 1440 1440

4. Sensor Quality Assurance To ensure the quality of more than 25000 sensors the CMS sensor group has developed a comprehensive procedure to qualify the sensors 4 . This quality assurance program contains four parts: • Tests performed at the companies prior to shipment. The contract lists a set of measurements and acceptance criteria. These measurements are for example tests of the leakage current, the determination of the depletion voltage and a check for pinholes and broken or shorted aluminum lines. Only sensors passing these tests are delivered to CMS. • Quality Control on sensors. This is an exhaustive test of the sensors. The total sensor leakage current and the total sensor capacitance is scanned as a function of the bias voltage and, at a bias voltage of 400 V, several parameters for each strip are measured in a strip scan.

583 • Process Control on test structures. Using the specially designed set of test structures more than 10 parameters are measured. These parameters allow to monitor the production process and provide data which cannot be measured on the sensors themselves. • Irradiation Tests. Using a neutron and a proton irradiation facility test structures and sensors are checked for the required radiation hardness. The goal of the CMS quality assurance procedure is to deliver 99% good sensors to the module assembly centers where the sensors, the readout hybrids etc. are glued to mechanical frames. A failure of a sensor at this stage will cause significant additional costs due to the lost material and working time. Furthermore the tests have to make sure that the sensors will be sufficiently radiation hard for the expected 10 years of operation in CMS. In the following the Quality Control, the Process Control and the Irradiation Tests are explained in more details. 4.1. CMS Quality

Control

The Quality Control (QTC) is done in five CMS centers: Karlsruhe (D), Perugia (I), Pisa (I), Rochester (USA) and Vienna (A). It consists of an optical inspection and an electrical characterization. During the optical inspection the sensor is checked by eye for obvious damages and the edges of the sensor are scanned using a microscope to check for defects introduced during cutting. The electrical characterization is done using fully automatic systems which measure the total leakage current and the detector capacitance as a function of the bias voltage up to 550 V and subsequently measure on each strip the values of the single strip leakage current, the polysilicon resistor, the coupling capacitance and the leakage current of the dielectric. This strip scan is done with the sensor kept at a bias voltage of 400 V. This measurement takes several hours and is therefore only done on a sample of sensors. 4.2. CMS Process

Control 5

The Process Control (PQC) is done in three CMS centers: Florence (I), Strasbourg (F) and Vienna (A). A set of standard test structures, identical for all 15 sensor designs and for both producers, is used to perform ten measurements in an automated way: C(V) on a MOS structure, C(V) on

584

a diode, interstrip resistance, I(V) on a mini-sensor, interstrip capacitance, I(V) on a gate controlled diode, resistance of the polysilicon resistors, resistance of the aluminum lines and resistance of the p + implants, coupling capacitances, and the breakdown voltage of the decoupling capacitor. The results of these measurements are compared to the specifications and are used to detect possible changes in the production process of the companies. 4.3. Irradiation

Tests

The irradiation tests for a small number of sensors and test structures are done by two laboratories: a neutron irradiation facility in Lovain-la-Neuve (B) and a proton irradiation facility in Karlsruhe (D). The irradiations are performed up to the fluence expected in CMS for the particular sensor type at a temperature of —10°C . After irradiation and a short period at room temperature to allow for beneficial annealing the structures are remeasured at —10°C. The tests before and after irradiation consist of the measurement of the leakage current, the determination of the depletion voltage and the measurement of several single strip parameters: the values of the strip capacitance, the interstrip resistance, the polysilicon resistance and the coupling capacitance. 5. Status of Sensor Production and Quality Summary The status of the production and sensor delivery at the end of September 2005 is the following. In total 28600 sensors have been ordered from HPK and STM. This number includes spare sensors, but also the sensors used for the various stages of prototyping. From these ordered sensors 90% are already delivered. Until end of September 2005 in total 8673 sensors and 4144 test structures have been tested by the CMS groups. Remember that the test procedure foresees only sample tests after a 100% test at the companies. From the delivered and already tested sensors we have qualified 25555 sensors for the use in CMS. These sensors are subsequently shipped to the assembly centers and a good fraction is already mounted into modules. The final shipment of sensors is expected to arrive at CERN in November 2005. The results shown in this paper can therefore be considered as almost final. The first parameter measured during the QTC procedure, and probably the one which provides the most important information on the quality of the sensors, is the total leakage current. Figure 4 shows the total leakage currents measured at a bias voltage of 450 V. The figure shows the results

585

measured by the QTC centers on samples of HPK sensors (left plot) and STM sensors (right plot). Only accepted sensors within the specification of maximum 10 fiA are plotted. The average value measured is about 580 nA for HPK sensors and about 1870 nA for STM sensors.

Total Leakage Current at 450V for accepted HPK Sensors Entries: 1252 »- H Mean: 576.4

Total Leakage Current at 450V for accepted STM Sensors Entries: 2203 Mean: 1865-48

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Leakage currents at 450 V for HPK and STM sensors

The second very important parameter for the CMS sensors is the depletion voltage. Figure 5 shows the depletion voltage measured by the QTC centers on the same sample of sensors (HPK sensors left plot, STM sensors right plot). The specifications allow a depletion voltage range of roughly 100 V to 300 V, slightly different for the inner layer and the outer layer sensors. Both companies use up essentially the full range.

Full Depletion Voltage for accepted HPK Sensors Entries: 1252 Mean: 174.7

FuH Depletion Voltage |V]

Figure 5.

Full Depletion Voltage for accepted STM Sensors Entries 2203 Mean: 172.90

Full Depletion Voftage [V|

Depletion voltage for HPK and STM sensors

Figure 6 shows per sensor the distribution of the number of strips outside

586

the specification. The plots in this figure show the combined results from both the companies data and the QTC data measured on samples. A strip is outside the specifications if one of the following defects is present: high single strip leakage current, polyresistor value outside the specification, aluminum short or open, coupling capacitance outside specification or short (pinhole). In the case of the HPK sensors on average one strip is bad in ten sensors while for the STM sensors each sensor has on average 1.9 bad strips. The specification allows a maximum of 1% bad strips on a sensor (the sensors have either 512 or 768 strips). The sensors accepted from both companies fulfill this requirement. In the case of HPK the percentage of bad strips on a sensor is 0.02% and for STM sensors this value is 0.3%.

Number of Bad StriDs for accepted STM Sensors Entries: 2681 Mean: 1.91

Number of Bad Strips for all accepted HPK Sensors Entries: 17778 Mean: 0.1

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Number of bad strips per sensor

The measurements done for the so-called Process Control provided additional information on the quality of the sensors in many details. Most of the measurements explained in section 4.2. confirm a very stable production within the specifications and with time. Examples can be seen in figure 7 for the measured poly resistor values (STM sensors) and in figure 8 for the measured breakdown voltage of the coupling capacitance (HPK sensors). The specification of the breakdown voltage requires a minimum value of 100 V. This measurement is a destructive measurement and is therefore a good example for the usefulness of the test structures as such a measurement cannot be performed on the real sensors. Both figures show the parameter as a function of time. These parameters as most others are very stable. We have however also detected some non-conformities while checking

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the measurements of the Process Control. The following figures shows two examples. Figure 9 shows the calculated specific resistivity of the thin sensors from HPK deduced from the measurement of the depletion voltage of a test diode. The specification for this parameter follows the radiation resistance requirements as explained in section 3. For the sensors of the inner layers the specific resistivity should not exceed 3.25 kf2 cm. For a production period in 2003 HPK has used a wafer material with too high specific resistivity. As a consequence part of these sensors had to be rejected

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Figure 10 shows the flatband voltage measured on a MOS test device for STM wafers. The flatband voltage is an indicator for the oxide quality. In an irradiation experiment we have determined the tolerable maximum value for the flatband voltage of this test stucture to be 10 V. For some batches this value was exceeded by far. This defect is particularly dangerous as the non-irradiated sensors behave perfectly. After irradiation a large increase of the interstrip capacitance would have occurred causing the sensors to fail after a few years of LHC operation. Sensors from these batches were clearly rejected. 6. Conclusions The delivery of the sensors for the Silicon Strip Tracker of CMS is almost completed. The sensors and the associated test structures were tested and qualified by CMS. The quality assurance procedure developed by CMS is of an unprecedented scale and did involve eight institutes. In total 16 electrical parameters have been measured on a total of 13000 structures as of end of September 2005. This procedure has worked very efficiently and we have identified several problems which were eventually solved together with the companies. The quality of the sensors accepted for CMS is excellent and fulfills all the

589 Flatband Voltaga vs. Data of M«a«urem«nt for STM |

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Figure 10. Measured flatband voltage as a function of the production period (STM) requirements for the long time operation of the Strip Tracker inside the CMS experiment. Only a very small p a r t of the results could be shown in this paper. As a final indicator for the quality of the sensors accepted for CMS the percentage of good strips can b e quoted to b e 99.94%. Acknowledgments T h e results presented in this paper were collected by a large number of competent colleagues from the CMS sensor working group over a period of several years. I would like t o acknowledge their effort and t h a n k t h e m for this work. References CMS Tracker TDR, CERN/LHCC 98-6 CMS TDR 5 (1998) and CMS TDR Addendum, LHCC 2000-016 (2000) Semiconductor Tracking Detectors in e+e- Vertex Detectors, M. Krammer, Nucl. Instr. and Methods A 379 (1996) 384. Investigation of design parameters for radiation hard silicon microstrip detectors, S. Braibant et al., Nucl. Instrum. and Meth. A 485 (2002) 343. The Silicon Sensors for the Compact Muon Solenoid Tracker - Design and Qualification Procedure, J.-L. Agram et al., Nucl. Instrum. and Meth. A 517 (2004) 77. Process control strategy of the silicon sensors production for the CMS tracker, T. Bergauer et al., Nucl. Instrum. and Meth. A 494 (2002) 218.

R U N N I N G MINOS

KAROL LANG* The University of Texas at Austin, Department of Physics, 1 University Station CI 600, Austin, TX 78712-0264, USA E-mail: [email protected]

MINOS is a long baseline experiment at Fermilab to study neutrino oscillations. The experiment started its operations in February 2005 and is now recording beam neutrino interactions in its two magnetized toroidal tracking calorimeters. The distance between the two detectors and the energy of the neutrino beam produced by protons from the Main Injector cover a range of the "atmospheric" A m 2 . The MINOS far detector has been active since 2003 and registers interactions of cosmic ray muons and neutrinos. We briefly describe experimental details, our experience running MINOS, give a glimpse of beam neutrino data, and present preliminary results with atmospheric neutrinos.

1. Introduction The long baseline MINOS experiment at Fermilab was proposed 10 years ago l'2 when the world of particle physics was puzzled by "the solar neutrino deficit" and "the atmospheric neutrino anomaly". The planning, the design, and the construction of MINOS turned out to be challenging largely due to the complexity of the civil construction of the neutrino beam line. The experiment, comprising two detectors and a beam line, was fully commissioned in early 2005 and is now running. During the construction period of MINOS several experiments improved our understanding of the physics of the atmospheric and solar neutrinos. Results from Super-Kamiokande, 3 ' 4 SNO, 5 MACRO, 6 and Soudan-2 7 have now firmly established the existence of neutrino oscillations. Recently, the first long baseline accelerator and reactor experiments, K2K 8 and KamLAND, 9 provided further evidence for neutrino mixing and a non-zero mass *On behalf of the MINOS Collaboration.

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Figure 1. Monte Carlo studies of the MINOS sensitivity for an integrated flux of 7.4 x 10 2 0 protons on target. Points with error bars in the plot on the left show the ratio of the expected oscillated measured energy spectrum in the far detector for a hypothetical A m 2 = 0.0025 eV 2 and sin 2 20 = 1.0 to the no-oscillation prediction. The plot on the right shows the resulting regions of sensitivity of the oscillation parameters. The effects of the hypothesis of neutrino decay and decoherence are also shown in the figure on the left.

of neutrinos. These fundamentally new phenomena extend the horizons of physics. Only further and more precise studies will help to better understand them. MINOS, a next generation accelerator long baseline neutrino experiment is expected to make strides in these explorations. 2. The Physics Program of MINOS MINOS is conducting a two-detector long baseline study of neutrino oscillations. To cover an atmospheric range of Am 2 and sin22# neutrino mixing, MINOS employs two magnetized toroidal tracking calorimeters 735 km apart and a neutrino beam which uses the Main Injector at Fermilab. The main physics goals of MINOS are: (i) Decisive observation of the v^ -» vT transition and verification of its dominating role in the atmospheric Am 2 region. (ii) Measurements of Am 2 and sin22# mixing parameters with about 10% level of accuracy. (iii) Measurement (or improvement of existing limits) of the v^ ->• ve , the Up -> Sterile transitions, or test more exotic hypotheses for neutrino disappearance, such as neutrino decay, extra dimensions, or decoherence, all for the atmospheric Am 2 . (iv) Test the CPT symmetry in the atmospheric neutrino interactions through independent detection of the sign of the muon charge in the charged current events.

592

Figure 1 illustrates the main MINOS technique to study neutrino oscillations and the sensitivity for the integrated number of protons on target of 7.4 x 1020 (which possibly corresponds to about 3-year running time). The mixing parameters will be determined by measuring the disappearance of the charged current events, as projected from the near detector to the far detector. 3. The experimental setup 3.1. The NuMI beam line The NuMI beam line, schematically depicted in Figure 2, is about 1 km long and declines toward the Soudan Mine in Minnesota at a 58 mrad angle. It uses 120 GeV protons extracted from the Main Injector. Presently, the 8GeV Booster provides up to 6 proton batches into the Main Injector. Most of the time the first batch is used for the production of anti-protons for Tevatron, so a typical 5-batch proton intensity in NuMI is about 2.2 x 10 13 protons per pulse (ppp) (a 6-batch intensity of 2.7 x 10 13 ppp has recently been achieved). The primary beam from the Main Injector is delivered Absorber

Hadron Monitor

Muon Monitors

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Figure 2. A schematic not-to-scale view of the NuMI neutrino beam line with the main focusing and beam monitoring elements.

in a 8.67 fis single-turn extraction every 2-3 s onto a graphite target of 6.4 x 15 x 940 mm 3 segmented longitudinally into 48 fins. The target is water cooled and designed to withstand up to 4 x l 0 1 3 ppp. Early problems in the cooling system of the target have been successfully resolved and the beam line is operating well. Particles produced in the target are focused by two magnetic "horns", which generate strong focusing toroidal magnetic field when pulsed with about 200 kA peak current. The sign and momentum selected secondaries enter then the 675m-long and 2m-diameter decay pipe. The two-horn sys-

593

tern yields a wide-band neutrino beam. The central energy of the beam is determined by the relative position of the two horns and the target, which select a particular focusing momentum band for pions and kaons produced in the target. The target can be moved remotely in a matter of minutes, while changing the position of the second horn requires several weeks of effort and has not yet been attempted. Presently, the horns are separated by about 10 m and the downstream end of the target is inserted into the upstream horn. By shifting the target upstream different neutrino spectra are achieved. As a commissioning step as well as a part of the beam

Figure 3. Left: Preliminary charged current energy spectra in the near detector registered for the main three beam configurations. Right: Intensity spectra from three beam muon monitors taken for the "LE-10" beam setting described in the text.

systematics studies program, MINOS has exercised several target and horncurrent settings: the "LE" setting was with the upstream target-end 35 cm in front of the first horn and horns pulsed with 200 kA, the "LE-10" setting had the target 10 cm more upstream and the horns pulsed with 185 kA, the "PME" setting had the target 90 cm further upstream and the horns pulsed with 200 kA, and the "PHE" setting had the target 150 cm yet further upstream and the horns pulsed with 200 kA. Also, for calibration and testing purposes MINOS took data with the "LE-10" configuration and the horns pulsed with 170, 185 or 200 kA. The resulting preliminary energy spectra of charged current events in the near detector for the main three configurations are shown in Figure 3. At the end of the decay volume the beam line is instrumented with ionization chambers to monitor the secondary beam. 10 The hadron monitoring station is located just upstream of the beam absorber. The three muon monitoring stations are located in three excavated enclosures down-

594

stream of the beam absorber with 0 m, 12 m, and 30 m rock thickness to the absorber. The pointing accuracy of the muon stations allows to align the neutrino beam direction to about 25 /iradians in one spill, as illustrated in Figure 3. The physics reach of MINOS directly depends on the integrated neutrino flux so improvements in the proton intensity are paramount to the success of the experiment. The currently operating beam is expected to deliver about 1 x 1020 protons on target in 2005. Higher beam intensities are needed and future scenarios for increasing intensity provide for a gradual increase to 6 x 10 20 protons on target per year.

3.2. The near and far

detectors

The 1 kton, "near" detector operates 1,040 m downstream of the target, or 300 m downstream of the end of the decay pipe, and about 100 m underground. The near detector determines the neutrino beam characteristics such as the energy spectrum, the rate of neutral and charged current events, and the flavor composition of the beam. While the entire detector can be used for beam monitoring only the central region within a 25 cm radius around the beam axis will be used for the neutrino oscillation analysis. The larger mass, 5.4 kton, "far" detector is placed in the Soudan Mine 735 km away from the target. The role of this detector is to measure the rate and energy of the v^ neutral current and charged current interactions and identify statistically a fraction of i/e or anti-neutrinos. Both detectors have an identical segmentation, but different front-end readout optimized for cost, a low event-rate in the far detector and multiple events per spill in the near detector. The far detector consists of 484 scintillator planes assembled in two super-modules, each an 8 m-diameter octagonal toroid. The near detector is a 3.8 m by 4.8 m octagonal toroid. Each 2.54 cm-thick steel plane supports scintillator strips which are 1cmthick and 4.1 cm-wide and grouped into 20- or 28-strip wide light-tight modules. The scintillator strips were extruded using polystyrene doped with 1% of PPO and 0.03% of POPOP. Each strip is co-extruded with a reflective outer layer, except along a 1.4 mm-wide and 2.0 mm-deep groove for embedding a light-collecting 1.2 mm-diameter wavelength-shifting fiber. In the far detector, fibers from strips spaced apart by about 1 m in a scintillator plane are grouped into sets of 8 and at each end are coupled to one pixel of a 16-anode photomultiplier (PMT), n so that each PMT reads out 128 fibers. In the near detector fibers are read out individually from

595 one end by 64-anode PMTs. 12 In the far detector the front-end electronics is based on the VA chip from IDE Inc., and in the near detector it is based on the modified QIE chip designed at Fermilab. The data acquisition reads out effectively all the hits in the detectors during a NuMI spill. The far detector has been in full operation since August 2003 and since that time has been used as an observatory for the atmospheric neutrinos. The near detector was fully commissioned at the end of 2004 and is now collecting hundreds of thousands of beam neutrino interactions. 3.3. The calibration

detector

One of the technical challenges before MINOS is to determine the energy scale between the two distant detectors located deep underground. It will be necessary to determine the relative (inter-detector) energy scale to within 2% and the absolute scale to 5% at each location. An important component of understanding the detector response to beam and cosmic ray muons, hadrons, and electrons was a special MINOS calibration detector (CalDet) which was exposed to a test beam at CERN in 2001-2003. Measurements by CalDet provide an energy and topology response function for MINOS and allow the transfer of the energy scale. CalDet events can also be used to tune the Monte Carlo simulations. CalDet was a non-magnetized mini-MINOS detector. The l x l m 2 transverse size and 60 planes contained hadronic and electromagnetic showers with energy of several GeV. The readout fibers were lengthened to approximate a large size of the far and near detectors. CalDet has verified the MINOS calibration scheme and the expected detector response. Figure 4 shows the hadronic and electromagnetic preliminary energy resolutions measured by CalDet and expected to characterize both larger de-

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596

4. B e a m neutrinos NuMI beam is the most intense high energy neutrino beam ever constructed. Even at the low-energy setting there are multiple interactions per spill in the near detector. The front-end electronics readout, the event topology, and the timing of recorded scintillator signals allow to reconstruct most of events without biases. Figure 5 shows the scatter plot of the transverse coordinates of vertices of charged current interactions recorded in the target section of the near detector and illustrates the statistical strength of the MINOS near detector. The figure also shows a preliminary distribution of time of events in the far detector relative to the spill time. These charged current events were sampled from the full data set following a blind analysis procedure. As expected, the events clearly cluster within a ~10 /is spill window.

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Figure 5. Preliminary results from the near and far detector. The left plot shows the scatter plot of the reconstructed transverse coordinates of vertices of charged current interactions. The density of the events clearly shows the cross-sectional shape of the detector, the central coil hole and two different sizes of scintillator planes. The events were recorded for the three main beam energy settings shown in Figure 3. The right plot shows the timing distribution relative to the spill. The reconstructed and identified charged current cluster within the central 10 /is reflecting the duration of the spill. The identified cosmic ray muon events are shown by a dashed histogram.

5. Atmospheric neutrinos in the far detector The far detector is located 713 m, or 2,070 meters water-equivalent, underground and can naturally serve as a cosmic ray observatory. This is the first large underground detector which is magnetized and thus capable of measuring momenta and the sign of the cosmic ray muons, permitting to distinguish between neutrino and anti-neutrino interactions.

597 MINOS detectors are optimized for interactions of beam neutrinos originating from Fermilab which exhibit a mostly horizontal flow of energy. The detector lacks hermeticity for interactions of particles entering from the side, from above or beneath it. A cosmic ray veto shield surrounds the top portion of the detector to mitigate this shortcoming. 5.1. Upward

muons

A separation of downward-going from upward-going muons can be achieved by requiring that muons traverse a minimum of 20 planes or 2 m, have wellreconstructed trajectories within the fiducial volume of the detector, and their scintillator signals have good time records in the crossed planes. Figure 6 shows preliminary distribution of a signed ratio 1//3 of the speed of light to the velocity of well-measured muons. Downward muons cluster around l/{3 « + 1 , while upward muons, resulting from neutrino interactions in the rock beneath the detector, have 1//3 « — 1. 30

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-0.8 -0.6 -0.4 -0.2 cosQ Figure 6. Preliminary results for upward muons. The plot on the left shows the distribution of the inverse speed of light, 1//?, of cosmic muons reconstructed in the far detector for 464 days of running (160 days with the reversed magnetic field). 91 events near 1//3 = — 1 are identified as upward-going. The plot on the right shows the distributions of events as a function of the cosine of their angle on incidence with respect to the zenith. The histograms show the expected distributions for no-oscillations and for Am§ 3 = 0.0024eV 2 and sin 2 20 2 3 = 1-0.

5.2. Contained

vertex

events

Charged current atmospheric neutrino interactions with a vertex contained within the fiducial volume of the far detector are required to have a well

598

reconstructed track and no associated hits in the cosmic ray veto. The track direction is determined from timing in the scintillator planes. In addition, the events are imposed charge-weighted topological cuts determined from Monte Carlo studies. Using data recorded in the 18 month period from August 2003 to February 2005, which correspond to a live-time of 418 days or an exposure of fiducial 4.54 kiloton-years, 107 events were identified. Among these, 69 are fully contained and 38 are partially contained (i.e., have muon which exits the detector). For no neutrino oscillations 127 ± 13 are expected, while 96 ± 10 are expected for A m | 3 = 0.0024eV2 and sin 2 20 23 = 1-0. Through timing, the events can be further classified to upward or downward-going. Finally, 54 events with cleanly reconstructed tracks have the sign of the muon well measured which allows to attribute the interactions to neutrinos or anti-neutrinos. Figure 7 shows the charge-separated up-down distributions of contained events for muon neutrinos and muon anti-neutrinos. This is a low statistics measurement compatible with previous experiments with atmospheric neutrinos, but for the first time showing separate zenith angle distributions for neutrinos and anti-neutrinos. A full analysis and results are described in an article submitted for publication. 13

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6. Outlook At the time of this writing MINOS has integrated about 6.5 x 1019 protons on target or about 2 million events in the near detector. The events in the near detector are being used for tuning the Monte Carlo modeling of both the neutrino interactions and the detector response. A significant effort is

599 currently directed toward the optimization and improvements of the event reconstruction and the energy calibration of b o t h detectors. For the longer term, the precision with which MINOS will measure neutrino oscillation parameters strongly depends on the neutrino flux, thus the collaboration is involved in the R&D program to improve the proton b e a m intensity. For the far detector, MINOS has adopted the blind analysis protocol which at present precludes statistical assessment of the events. However, the collaboration plans for an internal "unblinding" at the end of 2005 to reassess the o p t i m u m of the beam setting. First MINOS publication with results on b e a m neutrino oscillations are expected in 2006. This work was supported in p a r t by grant DE-FG03-93ER40757 of the US Department Energy. References 1. The MINOS Collaboration includes research groups from Argonne National Laboratory, University of Athens, Benedictine University, Brookhaven National Laboratory, California Institute of Technology, University of Cambridge, College de France, Fermi National Accelerator Laboratory, Harvard University, Illinois Institute of Technology, Indiana University, ITEP Moscow, Lebedev Institute, Lawrence Livermore National Laboratory, University College London, University of Minnesota, University of Minnesota-Duluth, University of Oxford, University of Pittsburgh, IHEP Protvino, Rutherford Appleton Laboratory, University of Campinas, University of Sao Paolo, University of South Carolina Stanford University, University of Sussex, Texas A&M University, University of Texas at Austin, Tufts University, Western Washington University, College of William and Mary, and Universisty of Wisconsin. 2. Fermilab Proposal P875, February, 1995; The MINOS Detectors, Technical Design Report, Fermilab, NuMI-L-337, October, 1998. 3. Y. Ashie et al., Phys. Rev. D 7 1 , 112005 (2005). 4. S. Fukuda et al., Phys. Lett. B539, 179 (2002). 5. Q. R. Ahmad et al, Phys. Rev. Lett. 89, 011301 (2002). Q. R. Ahmad et al., Phys. Rev. Lett. 89, 011302 (2002). 6. M. Ambrosio et al., Eur. Phys. J. C36, 323 (2004). 7. M. C. Sanchez et al., Phys. Rev. D68, 113004 (2003). W. W. M. Allison et al., Phys. Rev. D72, 052005 (2005). 8. E. Aliu et al, Phys. Rev. Lett. 94, 081802 (2005). 9. T. Araki et al, Phys. Rev. Lett. 94, 081801 (2005). 10. D. Indurthy et al., it AIP Conf. Proc. 732, 332 (2004). 11. K. Lang et al., Nucl. Instrum. Meth. A545, 852 (2005). 12. N. Tagg et al., Nucl. Instrum. Meth. A539, 668 (2005). 13. P. Adamson et ah, First Observations of Separated Atmospheric Muon Neutrino and Muon Anti-neutrino Events in the MINOS Detector, to be submitted to Phys. Rev. D.

ATLAS I N N E R D E T E C T O R RESULTS F R O M T H E 2004 C O M B I N E D T E S T B E A M DATA

W. LIEBIG and R. PETTI* CERN, 1211 Geneve 23, Switzerland E-mail: Wolfgang. [email protected], [email protected]

The ATLAS Inner Detector has taken part in the 2004 Combined Test Beam programme and has made efficient use of this unique opportunity to exercise the hardware and software integration before commissioning the full ATLAS detector. Data from runs with e, it, /j. and 7 beams have been used to determine the detector performance (noise, occupancy, stability etc.), alignment corrections and reconstruction performance of the Inner Detector and its contribution to combined particle reconstruction with the calorimeters and muon spectrometer. Preliminary results are presented and compared to test beam simulation, allowing feed-back and improvements to the ATLAS simulation long before the start of LHC.

1. Introduction The ATLAS experiment 2 is a general-purpose detector for the Large Hadron Collider at CERN and consists of an inner tracking detector, calorimeters and muon spectrometer. As final part of a 10-year long detector development and testing programme, the ATLAS collaboration has operated modules from all detector systems in a Combined Test Beam. It has been arranged to resemble as close as possible a slice of ATLAS as it will be seen by particles originating from the interaction point of the LHC beams at polar angles of 25° and 90°. This paper describes the Inner Detector (ID) part of the Combined Test Beam and first results obtained from the data. The ID group has gained valuable experience from integrating the hardware and software in this small-scale version of the ATLAS experiment, from studying the performance of the sub-systems and of the entire ID and from the combined particle reconstruction using also the calorimeters and muon chambers. The comparison of the real data to a dedicated test beam simulation allows to tune the detector simulation and digitisation. * On behalf of the Inner Detector Combined Testbeam working group and of the ATLAS Collaboration. See ref. 1 and 2 for a detailed list of authors.

600

601

Figure 1. ATLAS Combined Test Beam setup as described by the Geant4 simulation.

1.1. Setup and

Data-Taking

The test beam setup consisted of six pixel detector modules arranged in three tracking layers, eight silicon strip detector modules (semiconductor tracker, SCT) in four layers and 6 transition-radiation tracker modules (TRT). The pixel and SCT modules were placed in a magnetic field of 1.4 T while the TRT - unlike the full ATLAS - was outside the field due to space constraints. Charged tracks were bent in the direction which is covered by the precision co-ordinate measurements (50 fxm pixel pitch and 70/jm average strip pitch). The second co-ordinate is determined by measurements from both pixel and SCT (using a 40 mrad stereo angle between strips from back-to-back modules). A track then passes though an average of 40 TRT straw tubes measuring its distance to a central wire from the drift-time information. The TRT thus provides only measurements in the direction of magnetic deflection. The visualisation from the detector simulation in Fig. 1 shows how the ID is then followed by the calorimeters and muon chambers. The ID has taken data in full combined mode in Oct-Nov 2004. It has collected data from runs with /x ± , TT*, mixed e±/n±, p and 7 beams at energies ranging from 1 to 350 GeV. For most runs data were taken with and without magnetic field. A total of 22 million events have been validated for the analysis of detector performance.

602

1.2. Event Reconstruction

and

Simulation

The software chain to reconstruct test beam events is in most cases identical to what will be used for real ATLAS data. Nevertheless many components had to be newly developed mainly due to the need to deal with real data and due to the commissioning of an entirely new, modular reconstruction chain in the ATLAS software framework. The offline software commences by decoding the raw data stream followed by the clustering and drift circle creation, the pattern recognition and tracking and finally the output to data formats suitable for analysis. Geant4 has been used to simulate the test beam, obtaining the geometry information from a detector description common to reconstruction and simulation. The output hits from the simulation are transformed to raw measurements by the digitisation code which simulates the charge drift and diffusion as well as signal read-out.

2. Calibration and Alignment The reconstruction of real data requires the software to deal with alignment and calibration corrections, and dead/noisy channel information. Once determined for a denned interval of validity, they are stored in the conditions database and provided to the detector description and reconstruction code. Dead/noisy channels have been monitored for all three detector technologies and are masked in the reconstruction. The pixel and SCT module positions are aligned in the two directions orthogonal to the beam using an iterative procedure which minimises the residuals of straight line tracks on a small set of selected reference runs without magnetic field. The residuals obtained after alignment are displayed in Fig. 2 (left) for all six pixel modules combined. The resulting sigma is 16 fim, which is slightly smaller than the simulation with a sigma of 17.4 /mx. For the SCT, residuals with a sigma of 11.2/un in data and 12.2 fan in simulation have been found. For the TRT, the calibration and alignment are performed in a common procedure which determines the drift velocity in the tubes, the drift time offset and the wire position from the extrapolation of tracks reconstructed in silicon detectors. While the intrinsic precision of the overall ID alignment is close to 1 (j,m, studies on the stability of the alignment as a function of run indicate residual variations of ~ lOfim for the silicon detectors, and larger for the TRT and for the runs with field on. The alignment strategies foreseen for ATLAS are now being exercised on the test beam data so that they can be validated providing at the same time a better understanding of alignment uncertainties at the test beam.

603

Figure 2. Residuals obtained on data with magnetic field off are shown for the pixel detector in the r coordinate (left) and for the TRT (right).

3. Particle Reconstruction 3.1. Momentum

Measurement

The momentum has been measured for different beam energies and tracking strategies as well as for tracks with and without TRT. The results from using only pixel and SCT which are fully contained in the magnetic field reproduce the beam energy very accurately. Including the TRT improves the momentum precision due to the larger lever arm, but also introduces some biases on the reconstructed momentum. The detailed description of the field and the corresponding silicon alignment are being finalised. 3.2. Material

Description

and Multiple

Scattering

The multiple scattering corrections in the reconstruction have been probed by parameterising the residuals between track segments from different detector parts as a function of momentum. A rather good agreement between data and simulation has been found for pion beams as can be seen from Fig. 3. Also the increased multiple scattering due to additional material layers, which were inserted during part of the running to emulate the ID services at low polar angles, is well described by simulation. 3.3. Particle

Identification

with the

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The transition radiation from electron tracks in the TRT is detected by a signal discrimination operating at two different levels. A large measured fraction of high-level to low-level hits on a track can therefore be used to distinguish e from IT. Such a rejection curve is displayed in Fig. 3 for the lowenergy data at 2 GeV showing a much improved agreement with simulation compared to previous stand-alone test beam data after adjusting the highlevel hit threshold in the simulation.

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3.4. Combined

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Tracks from the ID have been matched with hits in the calorimeters. Full correlation between track and shower coordinates is visible after geometric corrections were applied. This technique is employed in the reconstruction of photon conversions inside the ID and for calibrating the e — ir separation. High-energy muon beams form another part of the Combined Test Beam data. They were injected at low settings for the bending fields, such that they could be tracked through the whole apparatus over a length of about 40 m. These muon tracks have been reconstructed successfully by making use of the new software chain. 4. Conclusions ATLAS has learnt a lot from the hardware and software integration work done for the Combined Test Beam and from the detailed analysis of detector performance. In particular, we benefit from the chance to study the combined performance well before the full ATLAS detector takes data. The test beam also serves as a benchmark to test and develop new algorithms. Acknowledgments We would like to thank the members of the ATLAS collaboration and in particular all those involved with the combined test beam. References 1. G. Gorfine, Tracking Performance of the ATLAS Pixel Detector in the 2004 Combined Test Beam, proceedings of Pixel 2005, Bonn. Submitted to NIM A. 2. The ATLAS Collab. ATLAS Detector and Physics Performance: Technical Design Report, CERN/LHCC/00-14. May 1999.

THE CMS MUON SYSTEM PIERLU1GIPAOLUCCI* Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Complesso Universitario di Monte Sant 'Angelo Napoli, 80126, Italy The Compact Muon Solenoid Muon system, designed to trigger and identify muons, is based on three different technologies; drift tube (DT) in the barrel region, cathode strip chamber (CSC) in the endcap and resistive plate chambers (RPC) in both the barrel and endcap. The three subsystems, the local muon trigger and the global muon trigger will be described here giving some results obtained during the quality control tests. The chambers installation is now underway; the strategy adopted for the commissioning of the muon system will be described in this paper. 1.

Introduction The Compact Muon Solenoid [1] is one of the four experiments of the Large

Hadron Collider [2] at CERN. CMS is a complex apparatus consisting of several subdetectors in order to identify particles and measure their energies and momentum.

Figure 1. The Compact Muon Solenoid detector. * On behalf of CMS muon collaboration

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A principle feature of CMS is its strong magnetic field of 4 Tesla, which is produced by a superconducting coil. The coil volume houses the tracker and the electromagnetic and hadrons calorimeters. An iron return yoke surrounds the coil and houses the muon chambers as is shown in figure 1. Since the very high collision rate and the low cross section of the "new physics" events a very important part of the experiment is the trigger and data acquisition system [3] [4]. The experiment is currently being assembled and tested in a surface hall SX5 and it will be lowered into the cavern during the 2006. 2. The Muon Spectrometer At LHC, electrons and muons will play a fundamental role in the studies of any physics sector; from the Higgs search, where the H —* 1111 are the so called "golden channels", to the "new physics" and from the electroweak precision measurements to the b and t physics. For these reasons the CMS muon system [5] has been designed and built to have excellent trigger, identification and reconstruction performances. The LHC physics signatures and their separation from the large expected background, the muon trigger rates and the trigger limits dictated by the DAQ and trigger systems, impose several requirements to be fulfilled by the muon system: • Muon trigger: excellent trigger performances on single and multi-muons events and an unambiguous identification of the bunch crossing is obtained by combining fast dedicated trigger detectors, Resistive Plate Chambers, with detectors having precise spatial resolution, Drift Tube and Cathode Strip Chambers. • Redundancy, in both trigger and reconstruction, is obtained using three technologies, combined in order to have two independent muon systems in the whole angular region. • CMS has been designed with a magnetic field of 4 Tesla and the possibility to measure muons twice, in the tracker and in the muon spectrometer in order to perform very good results in the muon momentum and charge measurements in the whole t| region and from few GeV up to a TeV. • Muon identification: to achieve a very high efficiency (> 95%) up to r) = 2.4 are required at least 16 interaction lengths of material and a very sophisticated set of algorithms able to use the three sub-detectors data. • Robustness: The detectors must be capable to work with a strong magnetic field and high radiation and interaction background [5].

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The muon spectrometer consists of 250 DTs, 540 CSCs and 912 RPCs chambers to be built and installed for the 2006. It cover an angular region delimited by 0 < | r) | < 2.4 and is divided in two major regions: barrel and endcap (figure 2). The barrel region (0 < | n | < 1.2) is composed by 5 wheels, each divided in 12 sectors with 4 iron gaps. The muon stations consist of one DT chamber and two RPC chambers joint together and are placed in the gaps of the iron return yoke plates. In this region the magnetic field is low and almost uniform and the expected muon rate is about 1 Hz/cm2 compared with a neutron induced background of 1-10 Hz/cm2. The endcap region (0.9 < | n | < 2.4) has a strong and not uniform magnetic field (up to 3.5 T) with a y and neutron induced background rate of about 1 KHz/cm2. The two endcaps, made of 3 iron disks and 4 layers divided in 2 or 3 stations (ME1/1, ME1/2, ME 1/3, ME2/1, ME2/2, ME3/1, ME3/2, ME4/1 and ME4/2) of CSC and RPC chambers. Drift T u b e s

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3. The CMS Muon Trigger system For the nominal LHC design luminosity of 1034 cm~V', an average of 17 inelastic events occurs every 25 ns. This input rate of 109 interactions per second must be reduced by a factor of at least 107 to 100 Hz, the maximum rate that can be achieved by the DAQ. The CMS trigger system [3] is based on two levels, at first level (LI) all data are stored for 3.2 fxs, after which no more than 100 KHz (50 KHz for the

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Figure 3. The Muon trigger schema from the chamber trigger to the Global Muon Trigger.

The LI muon trigger [6] uses all three muon detectors DT, CSC and RPC, each with its own trigger logic as shown in figure 3. DT and CSC first process the local information of every chamber generating local triggers then muons from different chambers are collected by the Track Finder which combines them to form a muon track with an assigned transverse momentum value. Up to four best (highest p T and quality) muon candidates from each system are selected and sent to the Global Muon Trigger. In the case of RPC hits from all the stations are collected (PACT) [7] and if they are aligned along a possible muon track, a p T value is assigned and the information is sent to the Muon Sorter. It select the four highest p T muon from the barrel and four from the endcaps and sends them to the Global Muon Trigger (GMT). Finally transverse momentum thresholds are applied by the GMT for all the trigger conditions. 4. Drift Tube chamber The barrel muon system consists of 4 concentric shells of drift tube chambers (MBl, MB2, MB3 and MB4). Each chamber, except MB4, is made of 3 independent units, called superlayer (SL), and a thick honeycomb plate glued together (fig.4). Each superlayer is composed by 4 layers of drift tubes, with all wires parallel. The wires of odd layers inside a SL are staggered by half-cell width with respect to the even layers. The two external SLs measure the muon trajectory in the CMS bending plane (r,<j>) while the third SL measure the z

609

coordinate. The drift tube is 43 x 13 mm2 and is composed by three electrodes: a wire, two I-beams (cathodes) and two strips.

Figure 4. The drift tube chamber is made by 3 superlayers and a thick honeycomb plate glued together.

The time needed by the primary electrons to reach the anode is measured and the spatial position of the track is calculated from the time measurement and the known time-to-distance relationship. To resolve the left-right ambiguity of a single cell it is necessary to use the staggered layers and reconstruct the track position in at least three of the four layers. A spatial resolution of 100 um in the (r,<(>) coordinate and of 150 (am in the (r,z) coordinate has been measured in the test beam [8]. 4.1. DT local trigger system The DT front-end electronics, called Bunch and Track Identifier (BTI), forms track segment from coincidences of at least three aligned hits in four layer of one DT superlayer, using the mean-timer method, obtaining a resolution of 1.4 mm and 60 mrad. The positions and directions of the segments are sent to the Track Correlator (TRACO), which attempts to combine segments from the two SL of the same chamber and produce <)> coordinate with a resolution of 10 mrad. The Trigger Server collects the two best <j) segments and the 6 segments and selects, at every bunch crossing, two segments with highest quality and p-r. The Track Finder identify the best tracks per sector (30 sectors in (> | and 12 in r\) assigning the (p, r|, p T and quality. 5. Resistive Plate Chamber RPCs has been chosen in both the barrel and endcap as dedicated trigger detectors. Because of their fast response and good time resolution (a < 1.5 ns) they guarantee a precise bunch crossing assignment of the muon tracks.

610

The barrel muon stations are equipped with two RPC layers for the innermost stations (RB1 and RB2) and one layer for the outer stations (RB3 and RB4) for a total number of 6 layers per sectors. The endcap are instrumented with one RPC layer per station for a total of 4 layers. -HV: Fiy-np

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An RPC gap is made by two parallel bakelite plates (1-2 1010 flxm) placed at a distance of 2 mm and filled with a gas mixture of 96% C2H2F4, 3.5% i-C4H10 and 0.5% of SF6. High voltage is applied to the outer graphite coated surface of the bakelite plates in order to have an electric field inside the gas gap able to generate a charge avalanche along the track of a ionizing particle. The avalanche induces a signal on the aluminium strips placed outside the gap and isolated from the graphite. An RPC chamber (fig.5) consists of two double-gaps (forward and backward), made by 2 gaps with a read-out strips plane in the middle. Every chamber is equipped with 10, 12 or 18 front-end electronic boards [9] each connected to 16 readout strips. 5.1. Production, test and results RPC barrel chambers are assembled at the HiTec and General Tecnica factories and in the Bari and Sofia laboratories. Then are characterized with a cosmic rays at the station sites of Bari, Pavia and Sofia. The endcap chambers are assembled and tested at CERN. Tests done during the production phase are: gas and cooling tightness, dark current versus high voltage and strip noise rate at the working point (9.5 KV). The chamber is accepted if the dark current distribution at 9.5 KV has a mean value lower than 5 uA and a standard deviation below 1 uA and if the noise rate is less than 5 Hz/cm2. During the cosmic tests the chambers are characterized by

611

the efficiency, dark current and noise rate curves as function of HV working point.

Figure 6. The endcap chamber efficiency distribution at 9.5 KV for a subset of chambers.

The average efficiency measured for the RPC chambers is about 97% that become the 99% using the tracking capabilities of the cosmic tower (fig 6.) [10][11]. The accepted chambers are sent at ISR where the chamber stability is studied for a month, after that RPCs are ready to be joint at the DTs 6. Cathode Strip Chamber The endcaps are instrumented with CSC chambers [12] providing precise time and position measurements. Each chamber is trapezoidal in shape and consists of six gas gaps, each equipped with a plane of radial cathode strips (3.15-16 mm) and a plane of anode wires (spacing 2.5-3.16 mm, diameter 30-50 ixm) running almost perpendicular to the strips (fig.7). w ^ w w Cathode •

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The gas ionization and subsequent electron avalanche produces a charge on the anode wire and an image charge on a group of cathode strips. Thus each

612 chamber measures up to six space coordinates (r, <> | , z). The wire signal gives a fast information with a spatial resolution of about 1 mm while a precise spatial measurement (a,A 100-240 urn) is achieved using the center of gravity of the charge distribution induced on the cathode strips [13]. All CSCs, except those in the station ME 1/3 are overlapped in § to avoid gaps in the muon acceptance. 6.1. CSC local trigger system The CSC local trigger uses both the cathode and anode data to found the local charged tracks (LCT). The two trigger views, based on different electronics boards and different algorithms, are able to find up to two local tracks per chamber during any bunch crossing. The two projection are then combined in a 3-D LCT by a timing coincidence. The data of each endcap sector (60°) are collected by the Track Finder that identify the best 3 muons assigning to them <> |, T], PT and a quality bit. 7. Muon system installation and commissioning The installation and commissioning of the muon system is going on since the 2004 when the first CSC chambers have been installed. At present every muon sub-systems is working on the installation and commissioning of the system following its own test procedures. CSC: every installed chamber is commissioned with a gas leak test, an HV and electronics test made with a portable DAQ in which cosmics (fig. 8) are taken in a self-triggering mode. After that a long term electronic test is done keeping the low voltage on for a long period. Up to now about the 96% of the CSC chambers have been installed and 50% of those have been commissioned.

Figure 8. A cosmic ray observed with CSC chambers during the commissioning.

613

DT-RPC: after having been coupled with the RPC at the ISR, the chambers are sent at SX5 where are installed in the iron. At that point every chamber is commissioned testing the gas and the cooling connections, the high voltage and the electronics integrity, looking at dead channels and measuring the noise rate. More tests are under development in both the DT and RPC groups. 20 sectors of the wheels +1 and +2 have been already installed, corresponding to about the 30% of the whole system. About 50% of the endcap RPC chambers (reduced system) have been produced and they will be installed from October 2005. The commissioning will follow the same barrel procedures [14]. 8. Expected muon system performances A full detector simulation has given the following results for the transverse momentum resolution of muon tracks (fig.9): • Muon system stand-alone ^ 8-15% ApT/pT at 10 GeV V 16-53% ApT/pT at 1000 GeV • Muon system with tracker matching S 0.8-1.5% ApT/pT at 10 GeV S 5-13% ApT/pT at 1000 GeV For low p T the resolution measured in the muon system is limited by multiple scattering in the iron yoke. For high p T the resolution is limited by the muon chambers resolution, even when the tracker is taken into account. 10';;'

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614 The efficiency of the reconstructed muon tracks is almost flat in r\ up to | r] | ~ 2.1 as is shown in figure 9, for W —* uv events [15]. The Global Muon Trigger combines 3 independent triggers having a very high efficiency over the full r) angular coverage. The efficiency of the LI muon trigger to identify a single muon as a function of the generated r| is shown in figure 9.

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615

In figure 11 is shown the single muon trigger rates of the regional triggers and of the GMT as a function of the p T threshold applied (at a luminosity of 1034 c r n V ) . A single muon rate of 8 KHz is obtained with a p T threshold of 25 GeV (efficiency 90%). The dimuon trigger rate is 2.8 KHz with a p T threshold of 8.5 GeV. 9. Conclusion The construction of the three muon detectors is close to the end (summer 2006) and about 96% of the CSC and 30% of the DT-RPC chambers have been already installed at SX5. The commissioning of the system is going on showing a very good results. In the first part of the 2006 a very important test, called cosmic challenge, will be done using a fraction of the whole CMS detectors (2 or 3 sectors for the muon system) in order to verify the estimated global performances of the muon system described in this paper. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

CMS collaboration, Technical Design Report, CERN/LHCC 94-38 (1994). The LHC project at CERN, LHC-project-report-36 (1996). CMS collaboration, The Trigger and Data Acquisition project, Volume I CERN/LHCC 2000-038 (2000). CMS collaboration, The Trigger and Data Acquisition project, Volume II CERN/LHCC 02-026 (2002). CMS collaboration, The Muon project, CERN/LHCC 97-32 (1997). H. Sakulin and A. Taurok, "The LI Global Muon Trigger for the CMS experiment", CMS-CR-2003-04 (2003). M. Andlinger et al., "Pattern Comparator Trigger for the muon system of the CMS experiment", CERN-PPE-94-22. C. Albajar et al., Nucl. Instr. Methods A 525 (2004) 465. M. Abbrescia, et al., "New developments on front-end electronics for the CMS Resistive Plate Chamber" Nucl. Instr. Methods A 456 (2000) 143-14. M. Abbrescia, et al., Nucl. Instr. Methods A 508 (2003) 137-141. M. Abbrescia, et al., "The cosmics rays quality tests procedures for the CMS barrel resistive plate chambers", Nucl. Instr. Methods A 533 (2004) 208-213. G. Charpac, et al., Nucl. Instr. Methods 167(1979) 455. D. Acosta , et al., Nucl. Instr. Methods A 494 (2002) 504-508. A. Sharma, CMS RPC Endcap Status Report, CERN-OPEN-2004-041. I. Belotelov, N. Neumeister, "Performance of the CMS offline reconstruction software", CMS AN 2005/010 (2005;.

P E R F O R M A N C E S OF T H E ATLAS LEVEL-1 M U O N T R I G G E R PROCESSOR IN T H E B A R R E L

V. BOCCI, G. CHIODI.G. CIAPETTI, D. DE PEDIS, A. DI GIROLAMO, A. DI MATTIA, E. GENNARI.C. LUCI, A. NISATI, E. PASQUALUCCI, FR. PASTORE, E. PETROLO, F. SPILA, R. VARI, S. VENEZIANO, L. ZANELLO Un. of Rome "La Sapienza" and I.N.F.N. Rome, Italy G. AIELLI, R. CARDARELLI, A. DI CIACCIO, A. DI SIMONE, L. DI STANTE, A. SALAMON, R. SANTONICO Un. of Rome "Tor Vergata" and I.N.F.N. Rome 2, Italy A. ALOISIO, M. ALVIGGI, V. CANALE, G. CARLINO, F. CONVENTI, R. DE ASMUNDIS, M. DELLA PIETRA, D. DELLA VOLPE, P. IENGO, V. IZZO, A. MIGLIACCIO, S. PATRICELLI, G. SEKHNIAIDZE Un. of Napoli "Federico II" and I.N.F.N. Napoli, Italy E. BRAMBILLA, G. CATALDI, E. GORINI, F. GRANCAGNOLO, R. PERRINO, M. PRIMAVERA, S. SPAGNOLO Un. of Lecce and I.N.F.N. Lecce, Italy V. APRODU, D. BARTOS, S. BUDA, S. CONSTANTIN, M. DOGARU, C. MAGUREANU, M. PECTU, L. PRODAN, A. RUSU, C. UROSEVITEANU NIPNE-HH Bucarest,

Romania

The ATLAS level-1 muon trigger will select events with high transverse momentum and tag them to the correct machine bunch-crossing number with high efficiency. Three stations of dedicated fast detectors provide a coarse PT measurement, with tracking capability on bending and non-bending projections. In the Barrel region, hits from doublets of Resistive Plate Chambers are processed by custom ASIC, the Coincidence Matrices, which performs almost all the functionalities required by the trigger algorithm and the readout. In this paper we present the performance of the level-1 trigger system studied on a cosmic test stand at CERN, concerning studies on expected trigger rates and efficiencies.

616

617

1. Barrel Level-1 Muon Trigger overview TGC2 —

Figure 1. The ATLAS Level-1 Muon Barrel Trigger schema.

The ATLAS muon trigger system 1 provides the correct association of the muon candidates with the corresponding bunch-crossing and classifies tracks based on their transverse momentum. Rejection of cavern background is assured by fine time resolution and required independent coincidence on bending (rj) and not bending ((p) views in dedicated systems . In the Barrel, three layers of Resistive Plate Chambers (RPC) are used, tightly integrated with the MDT precision chambers in the muon spectrometer. Two of them, located inside the air-core toroids, provide the Low-px trigger selection (px >6 GeV/c) required by B-physics studies, while an outer station allows to select muons above 20 GeV/c. A detector layer is composed by two RPC gaps, each read out by two layers of orthogonal pick-up strips. A schema of the trigger system is sketched in Fig.l. The trigger logic identifies candidate muon tracks coming from the interaction vertex within a px range. Coincidence of hits is required within geometrical roads, calculated on the basis of the expected curvature of the track, assuring a 90% trigger efficiency at the imposed nominal px threshold. The geometrical roads are projected from the central layer, called pivot, on to the coincidence layers, pointing to the interaction vertex. The High-px trigger is made more robust by requiring the confirmation of a Low-px trigger. The baseline trigger configuration requires a 3/4 majority coincidence for the Low-p T system and a 1/2 for the High-px • Different criteria, allowed by the programmability of all the trigger parameters, can be imposed in order to control the trigger rate under different conditions

618 of luminosity, background and detector performance. The logic for the Level-1 trigger relies on custom ASICs, called Coincidence Matrix (CMA) 2 , which apply time and space coincidences on hits from the pivot and the coincidence layers and provide the trigger result and the readout of the RPC strips with a time resolution of 1/8 of a 25 ns bunch crossing period. The granularity is given by the region of interest (Rol), A<j> x ATJ = 0.1x0.1. The trigger and readout information from two (j) and two 77 CMA's are combined in a PAD, the on-detector trigger electronics processing unit. The algorithm of a tower is performed by connecting a Low-px and a High-px PAD. The latest collects all the trigger and readout data and sends them, via a single optical link, to the off-detector electronic system using a time multiplexing approach, which gives highest priority and fixed latency to trigger data. The Receiver and Sector Logic (RX/SL) board 3 combines data from up to eight PAD boards, generating the trigger signal of one of the 64 trigger sectors of ATLAS. When a level-1 accept signal is generated, the readout data are made accessible, through dedicated readout buffers, to the higher level triggers. Stringent requirements must be fulfilled by the CMA trigger processor ASIC. All the trigger part of the electronics is organized in pipelines, running at the bunch crossing frequency, and the readout buffers contained in the chip allow for a level-1 latency of maximum 2.5 [is. The CMA processor generates the output trigger patterns with a latency of a few bunch crossing periods. After digitization, RPC signals are aligned in time with programmable delay lines and shaped, adding dead-time if necessary, in order to perform the coincidence. The time alignment of signals from different layers and stations within the same bunch crossing window is required, compensating delays due to time of flight of muons, signal propagation along the RPC strips and the cabling. The CMA chip allows a time calibration with 16 channels granularity and the readout windows is optimized to perform bunch crossing identification with high efficiency and preserve the readout information.

2. Studies of performances of a trigger tower The whole trigger slice has been extensively tested with 25 ns beams to ensure the integrity of readout and trigger chains 4 . The CMA chip underwent a second and final redesign to cope with extended input delay lines of up to about 200 ns 5 . We used some pre-production samples to perform additional tests with real RPC signals at the CERN BB5 site with cosmic

619 Table 1. Summary of the trigger tower acceptance measurements (Low-px , 3/4 majority) with different R P C working conditions. The expected value from the toy MC is 47% at full detector efficiency. R P C HV (V) R P C F E threshold (V) Ll-trigger acceptance ±0.5 (%)

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rays a . A tower with up to three RPC stations has been equipped with the trigger electronics, in order to set up one trigger tower on each half-chamber side. We used Barrel Medium Small (BMS) chambers, corresponding to the inner chambers of a small sector of ATLAS, and reproduced the complete level-1 trigger slice of ATLAS. A reference trigger signal has been provided by two additional RPC chambers, situated at the top and bottom of the tower. To set up the system, we programmed readout windows with 8 BC's length and used the raw hits time distributions from each RPC layer to extract the calibration constants needed for time alignment. These data were very useful to validate calibration procedures. For validating the trigger logic and the coincidence procedure, a simple toy Monte Carlo (MC) has been generated reproducing the geometry of the tower and the cosmic rays angular distribution at the ground (oc cos 2 0). This model identifies a muon tracks with 100% efficiency, digitizing chamber hits with a cluster size equal to 1. Imposing the Low-px3/4 coincidence in extended full geometrical roads, a 47% expected trigger acceptance is obtained. We measured the same quantity, convoluted with the RPC efficiencies, for detectors at different high voltage values and different front-end thresholds15. Results listed in Tab. 1 are coherent with the expected value and show the good robustness of the trigger system. Using dedicated counters on the trigger processors, we checked the consistency of cosmics trigger r