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

Editors Michele Barone Emilio Borchi Andrea Gaddi Claude Leroy Larry Price Pier-Giorgio Rancoita Randal Ruchti I Astr...

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Editors

Michele Barone Emilio Borchi Andrea Gaddi Claude Leroy Larry Price Pier-Giorgio Rancoita Randal Ruchti

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Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications Pmi eedings ol the 9th ( onference

Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications Proceedings of the 9th Conference

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Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications Proceedings of the 9th Conference

Villa Olmo, Como, Italy

1 7 - 2 1 October 2005

Editors

Michele Barone Demokritos Athens, Greece

Emilio Borchi IN FN & University of Florence, Italy

Andrea Gaddi CERN, Switzerland

Claude Leroy Universite de Montreal, Canada

Larry Price ANL, USA

Pier-Giorgio Rancoita INFN-Milano, Italy

Randal Ruchti University of Notre Dame & US National Science Foundation, USA

\fc World Scientific N E W JERSEY • L O N D O N

• SINGAPORE • BEIJING

• SHANGHAI

• HONG KONG • T A I P E I • C H E N N A I

Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

ASTROPARTICLE, PARTICLE AND SPACE PHYSICS, DETECTORS AND MEDICAL PHYSICS APPLICATIONS Proceedings of the 9th Conference Copyright © 2006 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher.

ISBN 981-256-798-4

Printed in Singapore by Mainland Press

Organizing Committee: E. Borchi C. Leroy L. Price P.G. Rancoita R. Ruchti

INFN and University of Florence University of Montreal ANL INFN-Milano (chairman) University of Notre Dame/US National Science Foundation

Industrial Organizing Committee: M. Barone A. Gaddi

Demokritos Athens CERN Geneva

International Advisory Committee: D. Abbaneo S. Baccaro P. Binko A. Breskin A. Capone C. Fabjan K. Freudenreich S. Giani V. Hagopian E. Longo E. Nappi S. Pospisil P.L. Riboni T.J. Ruth D. Saltzberg J. Seguinot V. Sossi S. Volonte C. Waltham

CERN Geneva ENEA-Rome Observatoire de Geneve & SynSpace SA Weizmann Institute INFN and University of Romel CERN Geneva ETH-Zurich CERN Geneva Florida State University INFN and University of Romel INFN-Bari CTU Prague ETHZ-Zurich TRIUMF UCLA College de France, Paris University of British Columbia, Vancouver ESA Paris University of British Columbia, Vancouver

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Plenary Session Organizers: A. Capone (INFN & Univ. of Romel) S. Giani (CERN Geneva) E. Nappi (INFN-Bari) L. Price (ANL) R. Rutchi (Univ. of Notre Dame) D. Saltzberg (UCLA) J. Seguinot (College de France) V. Sossi (Univ. of British Columbia) S. Volonte (ESA)

Astroparticle Experiments Symposium on GEANT4 Applications Advanced Detectors and Particle Identification High Energy Physics experiments Accelerator and Data/Networking Developments Astroparticle Experiments Advanced Detectors and Particle Identification Radiotherapy and Medical Imaging Space experiment

Parallel Session Organizers: D. Abbaneo (CERN Geneva) S. Baccaro (ENEA, Rome) S. Giani (CERN Geneva) P. Binko (Observatoire de Geneve & SynSpace SA) D. Saltzberg (UCLA) A. Capone (INFN & Univ. of Romel) C. Fabjan (CERN Geneva) V. Hagopian (Florida State Univ.) C. Leroy (Univ. of Montreal) E. Nappi (INFN-Bari) J. Seguinot (College de France) S. Pospisil (CTU, Prague) L. Price (ANL) V. Sossi (Univ. of British Columbia) S. Volonte (ESA)

Secretariat: A. Cazzaniga N. Tansini

Centro Volta, Como Centro Volta, Como

Tracking Devices Radiation Damage Software Applications Software Applications Astroparticle Experiments Astroparticle Experiments Miniaturization of Particle Detectors Poster Session Calorimetry (both sessions) Advanced Detectors and Particle Identification Advanced Detectors and Particle Identification Medical Application Instrumentation High Energy Physics Experiments Radiotherapy and Medical Imaging Space experiment

PREFACE The exploration of the subnuclear world is done through increasingly complex experiments covering a wide range of energy and performed in a large variety of environments going from particle accelerators, underground detectors up to satellites and space laboratory. The achievement of these research programs calls for novel techniques, new materials and new instrumentation to be used in detectors, often of large scale. Therefore, particle physics is at the forefront of technological advance and also leads to many applications. Among these, medical applications have a particular importance due to health and social benefits they bring to the public. The International Conference on Advanced Technology and Particle Physics is held every two years. The conference returned to the "Centro di Cultura Scientifica A. Volta" for the 2005 Edition. The Conference welcomed about 300 participants. These participants were representing more than 140 institutions from about 30 countries. The conference allows a regular review of the advances made in all technological aspects of the experiments at various stages: preparation, data taking or upgrade. The open and flexible format of the Conference is conducive to fruitful exchanges amongst participants, shows the progresses made and gives research directions. The medical sessions gave an interesting example of merging advanced technologies: particle physics and numerical techniques. Industries specialized in advanced technologies were present at the Conference through a presentation of their products (organized by M. Barone and A. Gaddi) Plenary and parallel sessions covered Advanced Detectors and Particle Identification (organized by E. Nappi and J. Seguinot), Miniaturizing of Particle Detectors (organized by C. Fabjan), Astroparticle Experiments (organized by D. Saltzberg and A. Capone), Calorimetry (organized by C. Leroy), Software Applications (organized by S. Giani and P. Binko), High Energy Physics Experiments (organized by L. Price), Accelerator and Data/Networking Developments (organized by R. Ruchti), Medical Application Instrumentation (organized by S.Pospisil), Radiotherapy and Medical Imaging (organized by V. Sossi), Radiation Damage (organized by S. Baccaro), Space Experiments (organized by S. Volonte), and Tracking Devices (organized by D. Abbaneo). This edition of the conference was combined with a symposium on the applications of the GEANT4 simulation software. The symposium was organized by S. Giani. The poster session has always received special attention by the organizers of the conference. This session was organized this year by V. Hagopian. The posters were a great success. The number of posters has more than doubled to about 45 since the 8th IC ATPP Conference in 2003. Having their own rooms was conducive of numerous discussions by various interested parties. This year, there were several innovations in the poster presentations. First, one afternoon was designated as the Poster Parallel Session and all the presenters requested to be near their poster. Second, it was requested attaching a small picture of the presenter to every poster so that interested

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parties could recognize the person for possible discussions. Many of the presenters also left copies of their paper on the poster board, so interested parties could pick up a copy. Most of the posters were very elaborate with excellent graphics, printed on photographic type glossy paper. The posters were divided into three general categories as follows: 1. Astroparticle, Underground Experiments, Space Physics and Cosmic Rays; 2. High-Energy Physics Experiments, Trackers, Calorimetry, Software and Data Systems; 3. Advanced Detectors and Medical Physics Applications. Each poster submission was reviewed for relevance to the conference and accepted or rejected. The final poster papers accepted for publications have been included in the appropriate section of the Proceedings, as function of their topic. From the quality of the material of the posters, as well as the poster presentation, it is clear that a lot of effort went into the preparation. We would like to thank the poster presenters for their considerable effort that made the posters a truly important part of this meeting. We would like to thank the staff of the Centro A. Volta for the excellent support provided to the Conference organization at Villa Olmo. In particular, we would like to extend our appreciation and thanks to the Secretariat of Centro di Cultura Scientifica A. Volta for their help and efficiency with the organization of the Conference and its running. We are particularly indebted to Dr G. Castiglioni (President of Univercomo), who, by providing a sponsorship, allowed the Conference Organizers to largely reduce or waive the registration fees for the Ph.D. students. The organizers would like to thank the strong support of INFN and ESA which made the conference possible. Finally, we would like to thank the speakers for the high quality of their contributions and the participants for their enthusiasm in attending the Conference and contribution to the discussions.

Michele Barone Emilio Borchi Andrea Gaddi Claude Leroy Larry Price Pier-Giorgio Rancoita Randal Ruchti December 2005

CONTENTS

Preface

vii

Advanced, Miniaturized Detectors and Particle Identification A Novel Micromegas Detector for In-core Nuclear Reactor Neutron Flux Measurements The ALICE TPC The ATLAS RPC Test Stands Performances in High Magnetic Fields of Fine-mesh Photomultipliers for Fast Time-of-flight Detectors Recent Results on GridPix Detectors: An Integrated Micromegas Grid and a Micromegas Ageing Test RICH Detector at Jefferson Lab, Design, Performance and Physics Results Hybrid-Photon-Detectors in the LHCb RICH System Development of a Fast Transition Radiation and Tracking Detector for CBM at FAIR R&D on a Detector for Very High Momentum Charged Hadron Identification in ALICE The Design and Test of the ATLAS Diamond Beam Conditions Monitor The AMS02 Transition Radiation Detector (TRD) - A Gasfilled Detector for the International Space Station A New Automatic Microscope for High Speed Analysis of Nuclear Emulsions A Novel Type of Proximity Focusing RICH Counter with Multiple Refractive Index Aerogel Radiator A Subminiature Scintillation Detector for Catheter Operation . . . Analysis of Test-beam Data from a Prototype LHCb RICH Detector Single Crystal CVD Diamond Detectors for Hadron Physics Improvement of Particle Identification by Energy Loss in a Stack of Silicon Detectors Studies for a Fast RICH Properties of Scintillator Strip with Wavelength Shifting Fiber and Silicon Photomultiplier IX

1 3 10 21 26 31 36 42 47 54 60 66 71 77 82 87 92 98 103 108

X

Astroparticle Experiments The Underground Baradello Laboratory: Characterization of the Site and Early Results on Gamma-Ray and Neutron Spectrometry Measurements The Art of Detecting Neutrinos, 2 km Underground - The Three Phases of the Sudbury Neutrino Observatory Neutrino Physics at the South Pole - Recent results from the AMANDA Experiment First Results from GLAST-LAT Integrated Towers Cosmic Ray Data Taking and Montecarlo Comparison Background Analysis of Cuoricino in View of the Future Experiment CUORE Application of Nuclear Track Detectors in Astroparticle and Nuclear Physics Recent DAMA Results Double Beta Decay Measurement with COBRA Ultraviolet Diamond Photodetector Status of the KATRIN Experiment The RPC System for the OPERA Spectrometers Electromagnetic Shower Reconstruction in Emulsion Cloud Chamber A Low Background Micromegas Detector for the CAST Experiment Imaging Optical Adapters for Multi-Anode Photo Multipliers Detectors A 2D Stochatsic Montecarlo for the Solar Modulation of GCR: A Procedure to Fit Interplanetary Parameters Comparing to the Experimental Data Spatial Distribution of Energetic Heavy Ions near the Earth . . . . A Photon Tag Calibration Beam for the AGILE Satellite Observation Program of Isotope Composition in the Ultra Heavy Cosmic Rays High-Energy Cosmic Rays Investigated by Air-shower Measurements with KASCADE, KASCADE-Grande, and LOPES Scintillating Optical Fibers for Astroparticle Physics The T2K Experiment: Status and Instrumentation of the 280m Near Detector Results and Status of the PICASSO Experiment Data Acquisition System and Trigger Electronics for CACTUS . . . The Central Pixel of the MAGIC Telescope for Optical Observations Status report of ARDM Project: a New Direct Detection Experiment, Based on Liquid Argon, for the Search of Dark Matter

113

115 122 132 139 144 149 158 168 173 178 185 190 196 201

206 212 217 223 229 239 245 253 262 267 272

xi

Development of Multiple Photon Counting Coincidence (MPCC) Technique for Characterisation of Scintillators for Cryogenic Applications Results from Cuoricino Experiment and Prospects for CUORE . . . Timing Calibration of the NEMO Optical Sensors Design, Production, and First Results from the IceCube Digital Optica] Module

298

Calorimetry

305

Study of Electromagnetic Calorimeter Trigger-Primitive Performances for CMS Data Selection The CMS Electromagnetic Calorimeter The CMS ECAL Laser Monitoring System Pointing Calorimeter for Measuring KQL —> ifivv 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 D0 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 . . . . 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 .

277 282 292

307 312 318 323 328 333 338 343 348 353 358 363 369 374 379 389 395 401 406 411 416

XII

Inter-Calibration and Offline Compensation Studies of the ATLAS Calorimeter Using Beam Tests and Monte Carlo

421

G E A N T 4 and Software Applications

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Symposium on the Applications of the GEANT4 Simulation Software GEANT4 Applications for 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

427 429 437 444

High Energy Physics

531

Fast Shower Parametrisation for 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

533

451 462 470 475 480 485 490 495 500 506 511 519

539 546 551 556 565 576 590

xiii

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

Medical Applications a n d I n s t r u m e n t a t i o n s Spectral and Spatial Distribution of the Scattered Radiation of an Electron Radiotherapy Beam A Liquid Ionization Chamber as Monitor in Radiotherapy The Mammographic Head Demonstrator Developed in the Framework of the "IMF Project: First Imaging Tests Results Optimized Infrared Detectors for Infrared Synchrotron Radiation Microspectroscopy and Biomedical Imaging MATRIX: An Innovative Pixel Ionization Chamber for On-line Beam Monitoring in Hadrontherapy Development of a Dual Tracer PET Method for Imaging Dopaminergic Neuromodulation Rethinking Positron Emission Technology for Early Cancer Detection Positron Emission Tomography - Tracer Kinetic Modelling in Drug Development 3D-Reconstruction of Absorbed Dose Obtained from Gel-DosimeterLayers Accurate Determination of Radionuclidic Purity and Half-Life of Reactor Produced Lu-177g for Metabolic Radioimmunotherapy . . . Spatial Linearity Improvement for Discrete Scintillation Imagers . . High Resolution, High Sensitivity Detectors for Molecular Imaging of Small Animals and Tumor Detection Strip Ionization Chamber as Beam Monitor in the Proton Therapy Eye Treatment Low Dose, Low Energy 3D Image Guidance During Radiotherapy . Alpha Cyclotron Production Studies of the Alpha Emitter 2 n At/ 2 1 l 5 , Po for High-LET Metabolic Radiotherapy

600 605 616 621 626 637 643 648

653 655 661 666 671 677 682 692 697 705 710 715 720 725 732 742

XIV

Treatment Planning with IVIS Imaging and Monte Carlo Simulation Monte Carlo Simulations of a Human Phantom Radio-Pharmacokinetic Response on a Small Field of View Scintigraphic Device Applications of the Monte Carlo Code GEANT to Particle Beam Therapy Charge Sharing in Pixel Detectors for Spectroscopic Imaging . . . . Direct Thickness Calibration: Way to Radiographic Study of Soft Tissues A portable Pixel Detector Operating as an Active Nuclear Emulsion and Its Application for X-Ray and Neutron Tomography

747 752 758 768 773 779

Radiation Damage

785

Statistical Study of Radiation Hardness of CMS Silicon Sensors . . SIC PbWC-4 Crystals for the Electromagnetic Calorimeter of CMS Experiment MDT Chamber Ageing Test at ENEA Casaccia Neutron and Gamma Facilities Behavior of Thin Film Materials Under 7 Irradiation for Astronomical Optics Full Characterization of Non-Uniformly Irradiated Silicon Micro-Strip Sensors Beam Energy Monitor for 4-10 MeV Electron Accelerators Optical Link of the ATLAS Pixel Detector Ion Electron Emission Microscopy for SEE Studies An Analysis of the Expected Degradation of Silicon Detectors in the Future Ultra High Energy Facilities Investigation of VLSI Bipolar Transistors Irradiated with Electrons, Ions and Neutrons for Space Application Radiation-Hardness Studies of High OH~ Content Quartz Fibres Irradiated with 24 GeV Protons Development of Radiation Hard Silicon Detectors: The SMART Project Scintillator and Phosphor Materials: Latest Developments and Applications Crystals for High-Energy Physics Calorimeters in Extreme Environments Radiation Testing of GLAST LAT Tracker ASICS Magnetic Field and Radiation Tests of a Programmable Delay Line LCFI Charge Transfer Inefficiency Studies for CCD Vertex Detectors

787 792 797 802 807 812 817 822 827 832 838 843 850 857 864 871 876

XV

Photoluminescence and 7-Ray Irradiation of SrO-B203-P20s:Eu 2+ and SrMo0 4 :Eu 3 + Phosphors

883

Space Experiments

889

The Extreme Universe PAMELA Data Acuisition System AGILE MCAL, the Mini-Calorimeter Primary Helium CR Inside the Magnetosphere: A Transmission Function Study Lanthanum Halide Scintillators and Optical Fiber Readout for XRay/Gamma-Ray Astronomy and National Security Applications . The AGILE Silicon Tracker: Construction and Calibration Results . Ions Abundance Close to the Earth Surface: The Role of the Magnetosphere Design of a Silicon Transition Radiation Detector (SiTRD) for Accelerators and Space Applications The Origin of Helium-3 Isotope Enhancement in the Magnetosphere Observed by TSUBASA Satellite The Microscope Mission and Pre-Flight Performance Verification . . Performance of the Integrated Tracker Towers of the GLAST Large Area Telescope Performance of Neutron Detector and Bottom Trigger Scintillator of the Space Instrument PAMELA High Granularity Silicon Beam Monitors for Wide Range Multiplicity Beams Environmental Test Activity on the Flight Modules of the GLAST LAT Tracker The Time of Flight Detector and Trigger for the PAMELA Experiment in Space Fundamental Physics in ESA's Cosmic Vision Plan The AMS-02 Electronics System Launch in Orbit of the Space Telescope PAMELA and Ground Data Results CZT Detector Development for New Generation Hard-X/GammaRay Astronomical Instruments

891 898 904

Tracking Devices

1005

A Barrel IFR Instrumented with Limited Streamer Tubes for BaBar Experiment

909 917 922 928 935 940 945 952 957 962 968 973 979 989 994 999

XVI

A New Inner Layer Silicon Strip Detector for DO Calibration System for the Silicon Drift Detector of the ALICE Experiment Experience with the Resistive Plate Chamber in the BaBar Experiment Muon Identification and Reconstruction in the ATLAS Detector at theLHC Micrometric Position Monitoring Using Fiber Bragg Grating Sensors in Silicon Detectors Commissioning of the CMS Tracker Outer Barrel New Effects Observed in the BaBar Silicon Vertex Tracker: Interpretation and Estimate of Their Impact on the Future Performance of the Detector Tests of Substructures of the CMS Silicon Strip Tracker Test, Qualification and Electronics Integration of the ALICE Silicon Pixel Detector Modules Layout and Status of the CMS Silicon Tracker The CMS-Tracker Detector Controls System Optimization of the Readout Pad Geometry for a GEM-based Time Projection Chamber Tracking Strategy and Performance for the ATLAS High Level Triggers The LHCb Silicon Tracker The HI Silicon Tracker The LHCb Muon System Two- and Three-Dimensional Reconstruction and Analysis of the Straw Tubes Tomography in the BTeV Experiment Cryogenic Operation of Edge-Sensitive Silicon Microstrip Detectors The DEPFET Active Pixel Sensor as Vertex Detector for the ILC . Final Assembly and Intergration of the ATLAS Semiconductor Tracker and Transition Radiation Tracker 3-Dimensional Position Control for the AMS-02 Tracker with Infrared Laser Beams Development of a GEM-based High Resolution TPC for the International Linear Collider STAR Inner and Forward Tracking Upgrade

1012

1126 1131

List of participants

1137

1017 1022 1027 1032 1038

1044 1049 1054 1060 1067 1072 1077 1082 1087 1092 1098 1104 1109 1114 1120

Advanced, Miniaturized Detectors and Particle Identification Organizers: E. Nappi and J. Seguinot (Advanced Detectors and Particle Identification)

C. Fabjan (Miniaturizing of Particle Detectors)

S. Andriamonje J. Baechler M. Bianco M. Bonesini M. Chefdeville E. Cisbani J.M. Dickens C. Garabatos A. Gallas A. Gorisek J. Hattenbach M. leva P. Krizan L. Montani M. Patel M. Pomorski M. Regler

A Novel Micromegas Detector for In-core Nuclear Reactor Neutron Flux Measurements The ALICE TPC The ATLAS RPC Test Stands Performances in High Magnetic Fields of Fine-mesh Photomultipliers for Fast Time-of-fiight Detectors Recent Results on GridPix Detectors: an Integrated Micromegas Grid and a Micromegas Ageing Test RICH Detector at Jefferson Lab, Design, Performance and Physics Results Hybrid-Photon-Detectors in the LHCb RICH System Development of a Fast Transition Radiation and Tracking Detector for CBM at FAIR R&D on a Detector for Very High Momentum Charged Hadron Identification in ALICE The Design and Test of the ATLAS Diamond Beam Conditions Monitor The AMS02 Transition Radiation Detector (TRD) - A Gasfilled Detector for the International Space Station A New Automatic Microscope for High Speed Analysis of Nuclear Emulsions A Novel Type of Proximity Focusing RICH Counter with Multiple Refractive Index Aerogel Radiator A Subminiature Scintillation Detector for Catheter Operation Analysis of Test-beam Data from a Prototype LHCb RICH Detector Single Crystal CVD Diamond for Hadron Physics Improvement of Particle Identification by Energy Loss in a Stack of Silicon Detectors 1

2

F. Sozzi E. Tarkovsky

Studies for a Fast RICH Properties of Scintillators Strip with Wavelength Shifting Fiber and Silicon Photomultiplier

A NOVEL MICROMEGAS DETECTOR FOR IN-CORE NUCLEAR REACTOR NEUTRON FLUX MEASUREMENTS S. ANDRIAMONJE*, S. AUNE, A. GIGANON, I. GIOMATARIS, J. PANCIN, M. RIALLOT CEA Saclay DSM/DAPNIA

91191 Gif-sur-Yvette,

France

C. BLANDIN, S. BREAUD, B. GESLOT, C. JAMMES CEA/DEN/Cadarache,

13108 Saint-Paul Lez

Durance-France

Y. KADI, L. SARCHIAPONE CERNCH1211

Geneva-Switzerland

G. BAN, P. LABORIE, J.F. LECOLLEY, J.C. STECKMEYER, J. TILLIER IPC - CNRS/1N2P3 - ENS1CAEN - UCBN, 6 bd MarechalJuin,

14050 Caen, France

R. ROSA ENEA-Casaccia,

Via Anguillarese,

00060 Roma, Italy

G. ANDRIAMONJE IXL - Universite Bordeaux 1 F-33405 Talence Cedex, France

Future fast nuclear reactors designed for energy production and transmutation of nuclear wastes need new neutrons detectors able to measure the neutron flux over a large energy range from thermal energies to several MeV. A novel compact and very small detector, named Piccolo-Micromegas has been developed for this purpose. Description of the detector configuration especially dedicated to neutron detection inside nuclear reactor is given. The advantage of this detector over conventional neutron flux detectors and the results obtained with the first prototype are presented.

1. Introduction Micromegas is a gaseous detector [1] that has been developed initially for tracking in high rate high-energy experiments. At present, Micromegas detector is used in many experiments [2,3] and due to its high performances is being " Corresponding author: sandriamoniefecea.fr

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employed for searching rare events such as the Solar axion CAST [4] and also for neutron detection [5,6]. The detection principle is simple: the gas volume is separated in two regions by a thin micromesh, the first one where the conversion and drift of the ionization electrons occur and the second one, 50-150 micron thick, where the amplification takes place. In the amplification region, a high field (40 to 70 kV/cm) is created by applying a voltage of a few hundred volts between the micromesh and the anode plane, which collects the charge produced by the avalanche process. The anode can be segmented into strips or pads. One of the main advantages of Micromegas is its robustness and its high resistance to radiations. These qualities have been exploited to develop a new detector to be used in nuclear reactor environment: operation with very high neutron flux, high gamma ray background and possibly at high temperature. In order to be placed inside an empty rod of a reactor (see Figure la) the detector needs to be compact, sealed and very small (3.5 cm x 3.5 cm x 3.5 cm), contrary to the usual Micromegas detector used in particle physics experiments hence the origin of the Piccolo-Micromegas name. After a short description of motivations, the full description of PiccoloMicromegas is given. The advantage of this detector over conventional neutron flux detectors and the results obtained with the first prototype at the CELINA 14 MeV neutron source facility at CEA-Cadarache are presented.

2. Motivations Fast nuclear reactors and Accelerator Driven Systems (ADS) are considered as an alternative for energy production and transmutation of nuclear wastes. One important step needed for approval of a demonstrator is the experimental validation of simulations. Of particular interest is the determination of the neutron spectrum (i.e neutron flux as a function of the neutron energy) for different configurations of the subcritical device. As well known, the neutron flux in ADS consists of neutrons produced via spallation reactions in the target and fissions from the multiplying blanket. Unfortunately the neutron spectra cannot be measured using only one type of detector. To cover the complete energy range of the produced neutrons, a new neutron detector concept based on Micromegas technology has been developed. For illustration the possible subcritical configuration based in TRIGA-ADS project which consists of coupling a 1 MW TRIGA reactor with a 140 MeV proton accelerator has been chosen [7]. The core has a cylindrical shape, the fuel

5

rods being arranged in seven concentric rings labeled A to G. The central rod and the rods named ring B are reserved for the spallation target and its cooling system. The measurement can be only achieved starting from ring C. The distribution of neutrons in three-dimensional space as a function of energy and time is simulated using innovative simulation codes (FLUKA and EA-MC [8,9] at CERN, and MCNP-4C [10] at ENEA/Casaccia). b)

a)

35 mm 1 0 ' 3 10" 2 10" 1 10°

101

102

103

104

105

106

10r

Neutron energy (eV) b)

Figure 1. a) View of the detector Piccolo-Micromegas placed inside the empty rod of TRIGA Neutron flux spectra at selected locations of the TRIGA-ADS reference configuration

An example of the neutron energy spectrum for different regions of the core for k src = 0.97 is given in Figure lb. The measurement of neutron flux spectra is a necessary step in order to characterize the neutronics of the system.

3. Description of Piccolo-Micromegas It has been demonstrated, from the results of CERN n_TOF experiments [11], that a Micromegas detector equipped with an appropriate solid neutron converter is well adapted for neutronic studies providing a fast response, a large dynamic energy range and a low sensitivity to gamma-ray background. This method has been extended to the measurement of neutron flux in a nuclear reactor core. Fissile elements such as 235U, 232Th are used simultaneously as neutron/charged particle converter in addition to B and recoil ions of the gas (Ar + iC4Hi0 quencher) filling the detector. The Piccolo-Micromegas detector shown in Figure 2a is designed to cope with severe constraints encountered in a nuclear reactor environment (high radiation, high temperature, neutron activation, sealed detector). For this reason

6

the detector structure, frames and sensitive elements have been carefully chosen and most of them are made out of stainless steel and ceramics. The detector consists of a drift electrode, a thin stainless steel cathode grid and four anode pads connected to fast amplifiers. The drift gap is 1 mm wide and the amplification gap 160 jim. In a second prototype, different amplification gaps were used in each compartment, in order to adapt the dynamic range of the collected charge (see paragraph 4.1). Because of the high reactor temperature and the high radiation yield, the electronics were placed outside and connected to the anode pads through special radiation hardened and low capacitance coaxial cables 10 meter long.

Figure 2. a) Schematic view of the Piccolo-Micromegas detector for neutron flux measurements inside a nuclear reactor, b) Variation of the amplitude of the signal of one PAD of PiccoloMicromegas detector versus High Voltage (HV) of the PAD for fixed Mesh Voltage at +110 V, the drift cathode in the ground and 1 bar Ar+iC4Hio.

The drift electrode is composed of four neutron/charged particle converters: • l0B for thermal and epithermal neutrons, • 235U for monitoring as the fission cross section of 235U from thermal energies up to several MeV is well known, • 232Th with a fission threshold of about 1 MeV is dedicated to high energy neutrons. The elastic scattering of neutrons with light atoms from the gas filling the detector (hydrogen) will produce nuclear recoils than could be detected with a typical threshold of several keV. The use of the Micromegas read-out was motivated by the following advantages:

7 •

a low sensitivity to gamma background has been demonstrated by many experiments [11,14] • a high radiation resistance [ 12] • a large dynamic energy range [11] allows for measuring simultaneously neutrons, alphas, fissions and recoil ions. • a very good robustness Using four converters with a unique detector will permit extracting practically on line a large range of the neutron flux spectrum in a specific position in the reactor. The large dynamic range of Piccolo-Micromegas will permit precise measurements and a detailed scanning of the flux into the whole reactor volume. The use of the pre-mixed gas of 4He + iC4Hi0 or other quencher like CH4 combined with the four neutron/charged particle converters before mentioned allows to obtain a large part of the neutron spectra of fast nuclear reactor and is well adapted to the requirements of the future ADS. At very high counting rate (>100 MHz) measurement will be performed on a current mode basis. At low counting rate, the fast response of the detector will allow counting one by one the incident particles using a low noise fast-preamplifier. It opens the way to measure the neutron flux at the peripheral part of the reactor and also in some cases when the full reactor power is not used. 4. Experiments and First results 4.1. Characteristics of Piccolo-Micromegas detector 55

Fe X-ray source and two types of gas mixtures (Ar-10%CO2 and Ar2%iC4Hio) have been used to determine the main characteristics of PiccoloMicromegas. The pressure of the sealed detector has been set to 1 bar. In order to adapt the dynamic range of the collected charges, two methods have been used: 1. Different amplification gaps in each compartment : this is ensured by the use of different thickness of anode pad. 2. To have more flexibility with the amplification gap, an identical amplification gap for the four pads (160 um) has been chosen. The drift electrode is grounded. The mesh and the four reading pads have been individually positively polarized. The signal from each pad is collected and shaped by a homemade fast amplifier. An oscilloscope-PC has been used for the data acquisition. The energy resolution of the detector at 5.9 keV 55Fe X-ray is about 20%. The gain of the detector as a function of the voltage applied on the different pads has been

8

measured (Figure 2b). The results show that a very low polarization of the pad is largely sufficient for the observation of the signal from fission fragments (several MeV compared to 5.9 keV energy deposited by the X rays of the 55Fe radioactive source).

4.2. Experiments and results with neutron source A first test with sealed prototype of the detector has been performed with 14 MeV neutrons given by the Cadarache CELINA facility. An example of the pulse obtained from the fission fragment emitted in the interaction of neutron s with 235U is shown in figure 3a). This figure shows clearly the contribution of the detected electrons (fast part of the pulse) and ions (slow part) with a total width of 150 nsec. A special routine has been developed in Matlab Code [13] to analyze and clean the data for possible spark and saturation. 1 a),:. 5" 0 £ -5

Pi

n c

1j

3

_ii

*m ¥^-

k r

- ioris

E -40

|

•200

200

Times (ns)

400

600

40

60

80

Amplitude (mV)

Figure 3. a) Example of the fission fragment pulse b) Amplitude spectrum of 235U fission fragments

An example of the spectrum of 235U fission fragment as a function of the amplitude of the pulse is reported in figure 3b). A full simulation of the PiccoloMicromegas detector placed inside the CELINA-Cadarache neutron source facility has been performed using FLUKA code [8]. The study has been centered on the particular case of 235U, because of the well-known fission cross section. For 50 |j,g of U the measured fission rate given in figure 3b) is in good agreement with the prediction (4 fissions/s). 5. Conclusion A new miniaturized Micromegas has been developed for the measurement of the neutron flux spectra in nuclear reactor. The first results presented here are obtained using a neutron source facility. The next step will be the measurement inside a nuclear reactor. Further improvements are necessary one of them is the

9 use of special connectors and cables, in order to operate the detector at high temperatures, up to 300°C. References 1. 2. 3. 4. 5. 6.

7. 8. 9.

10.

11. 12. 13. 14.

I. Giomataris et al, Nucl. Instr. and Meth. A 376 (1996) 29. I. Giomataris, Nucl. Instr. and Meth. A 419 (1998) 239. G. Charpak et al, Nucl. Instr. and Meth. A 478 (2002) 26. K.Zioutas et al, Phys. Rev. Letters, 94,121301, (2005) S. Andriamonje et al, Nucl. Instr. Meth. A 481 (2002) 36 S. Andriamonje et al, Advances in Neutron Scattering Instrumentation. Edited by Anderson, Ian S.; Guerard, Bruno. Proceedings of the SPIE, Volume 4785, pp. 214-225 (2002). The TRADE Working group, "TRiga Accelerator Driven Experiment (TRADE)" Feasibility Report. ENEA Report, March 2002. A. Fasso, et al., "Intermediate Energy Nuclear Data: Models and C", Proceedings of a Specialist's Meeting, Issy les Moulineaux (France) 1994 C. Rubbia, et al., "Conceptual Design of a Fast Neutron Operated High Power Energy Amplifier", CERN/AT/95-44 (EET) Geneva, September 29, 1995. Y. Kadi, et al., "The EA-MC Monte Carlo Code Package", in Proc. of the 5th Int. Meeting on Simulating Accelerator Radiation Environment — SARE-5: Models and Codes for Spallation Neutron Sources, Paris, France, July 17-18, 2000. J. F. Briesmeister, Editor ^"MCNP™, General Monte Carlo N-Particle Transport Code, Version 4C", Los Alamos National Laboratory report LA13709-M, March 2000). J. Pancin, et al., Nucl. Instr. and Meth. A 524 (2004) 102 G. Puill, et al., IEEE Trans. Nucl. Sci. NS-46 (6) (1999) 1894 http://www.mathtools.net/MATLAB/ S. Boyer, et al, Proceeding of GLOBAL 2005, Tsukuba, Japan, Oct 9-13, 2005

THE ALICE TPC JOACHIM BAECHLER European Organization for Nuclear Research, CERN, Department of Physics 1211 Geneva 23, Switzerland for the ALICE collaboration The TPC is the main tracking detector in the central region of the ALICE experiment. This paper describes the main components of the ALICE TPC and addresses the specific technological challenges. Furthermore, the results of comprehensive system tests are summarized.

1. Introduction ALICE [1] is one of the four experiments presently under construction for the Large Hadron Collider (LHC) at CERN in Geneva. ALICE is a general-purpose heavy-ion experiment designed to study the physics of strongly interacting nuclear matter and the quark-gluon plasma in nucleus-nucleus collisions. At a maximum cms energy of 5.5 TeV per nucleon in Pb-Pb collisions, up to 8000 charged particles per unit rapidity are produced within a central collision. Owing to the enormous particle multiplicity per event, very specific requirements are made on the performance of the detectors, the electronics and the data acquisition. The ALICE TPC, operated in a solenoid of 0.5 T, is the main detector of the ALICE experiment for tracking, momentum measurement and particle identification of charged particles. In conjunction with the Transition Radiation Detector (TRD), the Inner Tracking System (ITS), Cherenkov counters (HMPID) and Multi-gap Resistive Plate Chambers for time of flight (TOF), the TPC will provide identification of leptonic and hadronic particles in the momentum range from 0.5 to 10 GeV/c. 10

11 The specific requirements on the TPC for hadronic physics are: • Good two track separation • dE/dx resolution of better than 10% • Track matching with 85-95% efficiency, using inner and outer detectors. In combination with the ITS and the TRD, the TPC needs to satisfy the following criteria for electron physics: • • •

Track finding efficiency for tracks with p, > lGeV/c of > 90% . Momentum resolution for electrons of 2 % with a momentum of~4GeV/c. Rate capability for central collisions to be at least 200Hz.

1.1. TPC - Main Components The ALICE TPC is composed of a cylindrical gas volume (barrel), divided into two half volumes of equal size, separated by a 30um thick HV electrode to generate the drift field. It has an inner radius of about 80 cm and an outer radius of about 280 cm, with an overall length of 500 cm in the beam direction (Figures 1 and 2). At both ends of the barrel, conventional multi-wire proportional chambers with pad readout are mounted into end plates with 18 trapezoidal sectors each.

HVBtoctrode(i'''KV] field cage *^^

readout chamber

jtiSHBl

M&BSBt

Figure 1. Layout of the ALICE TPC showing the central electrode, the field cage and the end plates supporting the readout chambers.

12 The front-end (FE) electronic cards with an optical interface to the DAQ are connected to the backplane of the readout chambers via ~8cm long flat Kapton cables.

Figure 2. Photography of the ALICE TPC positioned in the clean room with readout chambers mounted, in preparation for electronics installation.

1.2. Field Cage To keep the structural material (mass) at a minimum, the field cage is composed of two outer cylinders (outer containment vessel, outer field cage vessel) and two inner cylinders (inner field cage vessel, inner containment vessel). The drift gas is confined in the outer and inner field cage vessel; the volume created between the containment- and field cage vessels is filled with C0 2 and serves as an electrical insulator. At the center of the of the field cage the central electrode of 30um thickness is strung and mechanically supported by the outer and inner field cage cylinders, to generate the drift field in the two half volumes. Two opposite axial potential degraders with potential strips create a uniform drift field on either side of the central electrode. A unique feature of this field cage is the method to remove the potential strips from the field cage surface and wind them instead around support rods held away from the surface. This avoids instabilities and discharges over the huge surface of the field cage cylinder walls.

13

In total, 18 such axial rods, in line with the 18 sectors of the endplate, support the potential strips over a length of 250 cm in either direction. This is shown in Figures 3 and 4.

Figures 3 and 4. TPC field cage prototype with the six support rods and potential strips (left), and the ALICE TPC field cage (right) with 18 support rods plus strips; the TPC is mounted vertically on one endplate with an opening for a readout chamber.

To ensure minimal multiple scattering and low secondary particle production, the material budget of the TPC field cage components is kept as low as possible. Furthermore, the oxygen content of the drift gas has to be maintained below 5ppm in order to assure good detector efficiency. Consequently, we use, wherever possible, only low-mass composite material providing sufficient mechanical rigidity and the required leak tightness. After assembly, the field cage was leak-tested with dummy chambers in the end plates. An oxygen level of < 5ppm was measured with one volume exchange of gas per day. It is a major challenge to guarantee the mechanical stability and mounting precision of 250um for the central electrode and the readout planes within the entire assembly. With detailed photogrammetry, before and after chamber installation, it was shown that the entire detector stayed within the required mechanical tolerances. 1.3. Readout Chambers The ALICE TPC readout [1] is based on conventional multi-wire proportional counters with cathode pad readout. The segmentation of the readout pad plane was chosen to optimize, in the high multiplicity environment of central Pb-Pb collisions, the momentum and dE/dx resolution. The radial dependence of the track density led to two different types of readout chambers and a radial segmentation of the readout plane. The active area varies radially from 84.1 to 132.1 cm and from 134.6 to 246.6 cm for the inner and outer chamber, respectively. The total active area is about 32.5 m2. The readout

14 chambers are made of standard wire planes, i.e. a grid of anode wires above the pad plane, followed by a cathode plane and gating wire plane facing the drift volume. To account for the different pad sizes chosen, the wire geometry is different for the inner and outer readout chambers. The general wire layout was tuned with Garfield (electrostatic simulation program [2]) to obtain the maximum charge coupling to the pads. As a consequence, the field wires between the anode wires were suppressed. The wire pad geometry was optimized [2] based on the specific event topology of the particle spectra and densities as they are expected to occur in central Pb-Pb collision. This optimization procedure led to three different pad sizes of 4 x 7.5 mm2 in the inner readout chambers and 6 x 10 mm2 and 6 x 15mm2 in the outer readout chambers. 1.4. Gas Mixture The requirements on momentum and position resolution in the high multiplicity environment, as well as on minimum interaction processes inside the detector, determined the choice of gas for the ALICE TPC. c

•5

100000

Ne-CO -N2 [90-10-5]

Ne-CO, [90-10] 10000

1200

1300

1400

1500 1600 A n o d e voltage (V)

Figure 4. Comparison of gas gain as a function of the anode wire voltage for a Ne-C0 2 and a NeC0 2 -N 2 gas mixture. The N2 admixture allows a stable operation with a higher gain.

The Ne-C0 2 (90%-10%) mixture provides low diffusion, low Z and large ion mobility. It was therefore chosen as the optimum gas mixture for our application. From further optimization work [3] it was found that a small addition of nitrogen (to improve quenching) to the Ne-C02 gas, results in higher operational stability of the readout chamber at the expense of a tolerable reduction in drift speed for constant E/p (Figure 4).

15

The physics results presented here were obtained with a Ne-C0 2 -N 2 (85.7-9.5-4.8) mixture. Unfortunately, the drift velocities in both the Ne-C0 2 and Ne-C02-N2 gas mixtures are not saturated at E-fields of 400 V/cm. Therefore, small variations in temperature and gas mixture have a significant bearing on the drift velocity and herewith produce distortions in the space coordinates. To limit these distortions to the intrinsic space resolution of the TPC of < lOOOum (limited by diffusion) the temperature gradient [4] needs to be smaller than 0.1K. These constraints require that the TPC be operated at high drift fields of > 400V/cm to achieve acceptable drift times of < 88us. Therefore, the HV on the central electrode is > 100 kV.

1.5. Front-End Electronics The front-end electronics [5] has to read out the charge detected by the 570132 pads from the cathode plane of the readout chambers. Each single pad is red out by a charge sensitive preamp/shaper, a 10-bit 10 MHz low power ADC, and an ASIC that contains a digital filter for tail cancellation and baseline subtraction as well as zero-suppression circuits and a multi event buffer. The charge induced on a single pad is amplified and integrated via a low input impedance amplifier. The continuously sensitive charge amplifier is followed by a semi-gaussian pulse shaper of second order (Figure 5). The output of the amplifier/shaper chip is fed into the ALICE TPC Read Out (ALTRO) chip containing 16 channels. Upon arrival of the first level trigger the data stream is stored in a memory. Once the second level trigger ("accept" or "reject") is received, the latest data stream will either be frozen in the data memory, until its complete readout has taken place, or discarded.

Figure 5. Schematic of the ALICE TPC readout electronics

16 The ALTRO chip contains 16 times the TSA 1001 ADC from ST Microelectronics in the form of 8 ADC IP blocks. This ADC has differential inputs and can work up to a clock frequency of 20 MHz with a resolution of 10 bits, while consuming only 12 mW. After digitization a Baseline Correction Unit removes systematic perturbations of the baseline by subtracting a pattern stored in memory. Between triggers, a self-calibration feature can correct any baseline shifts due to temperature shifts or low frequency perturbations. The tail cancellation filter is a three-stage IIR digital filter that removes the long complex tail of the detector signal, thus narrowing the charge clusters to improve identification. It also removes the undershoot that typically distorts the signal amplitude when pile-up occurs. A second correction of the baseline is performed based on the moving average of the samples that fall within a certain acceptance window. The algorithm removes non-systematic perturbations of the baseline. Zero suppression finally removes all the data that is below a certain threshold, except for a specified number of pre- and post-samples around each peak.

1.6. TPC Test Facility We have set up a comprehensive test facility in which every individual component of the ALICE TPC can be tested, either separately or jointly with other parts of the detector system. A cosmic ray scintillator array allowed to trigger on single particles or on cosmic showers. Figure 6 shows the signals generated on a single pad by a cosmic shower event and the effect of the ALTRO filter on the baseline. Since the local occupancy is similar to that in global central Pb-Pb collisions a detailed study and optimization of the baseline filters could be performed. In spring 2004, a final "integral test" was performed in the PS-T10 test beam facility at CERN. An inner readout chamber (IROC) with the final electronics - DAQ chain was operated using the newly proposed gas mixture. The purpose of this test was to check the performance of the detector in combination with the entire readout chain, including the readout software. Thanks to the smooth and stable operation of the complete system, we were able to extract from the data the Bethe-B loch-curve in a momentum range from 1.0 up to 7GeV/c.

17 f —— MwtXaiw

t

i,

[

gbtys,

^IkAw

^ W-Wil^J lyyutyft,Vv* Figure 6: High-multiplicity cosmic ray event in the prototype TPC. Signal on a single pad, before ALTRO action (upper), and after ALTRO action (lower).

Because of the high performance DAQ system with a data throughput of 100 MB/s it was possible to take events even without zero suppression allowing the study of the original signal characteristics with sufficient statistics. In total we have accumulated about 6'000 non-zero suppressed events for each momentum setting.

30<£

SO

lOO

120 140 •-arm (<=m)

Figure 7: Space point resolution in the pad plane (x-y) as a function of drift length.

18

500

30t

\)

Figure 8: Space point resolution in the z direction (drift direction) as a function of drift distance.

Furthermore, we have successfully tested the ALTRO baseline restoration algorithm and could compare online-corrected data with uncorrected ones and hence prove the reliability of the online data reduction. The space resolution in the pad plane (x-y) and the drift direction (z) is shown in Figures 7 and 8, respectively.

Figure 9. dE/dx beam data: Triangles correspond to measurements with the three-component mixture Ne-C02-N2. The solid line is the mean energy loss curve (for Ar-CH4 (90/10) [1]), superimposed on the data.

19 The results of the dE/dx measurements obtained in 2004 with the new three-component gas mixture Ne-C02-N2 (85.7-9.5-4.8) during the test beam run at CERN are shown in Figure 9.

Conclusions The ALICE TPC was designed to cope with the extreme instantaneous particle densities produced in heavy-ion collisions at the LHC. Although conventional in its concept, this detector system incorporates innovative and state of the art technologies, from the mechanical structure to the readout and data processing chain. Carefully optimized components and extensive test programs performed in parallel with production are to ensure that the TPC can be operated in a stable and safe mode. Integral leak tests with argon, for example, have demonstrated the excellent gas tightness of the entire field cage system allowing the running of the TPC with less than 5ppm oxygen at nominal gas flow. On the other hand, extensive surveys of the TPC after chamber installation showed mechanical tolerances of the field cage elements being below 200um with respect to the readout chambers. The integral tests performed in 2004 with the TPC system demonstrated that the complete electronics and DAQ chain, in combination with an Inner Readout Chamber, work according to specification [6]. Furthermore, the dE/dx curve taken over an extensive momentum range was derived from the newly proposed three-component gas mixture Ne-C02-N2. The dE/dx resolution was found to be better than 9% for the inner readout chamber, the position resolution in the pad- and drift direction confirmed the design values. The comparison of the measured energy loss [6] with the predictions from the models developed by Bichsel [7] is still underway. The understanding thereof could potentially lead to an improvement of the particle identification capability of the TPC.

20

References 1. ALICE Collaboration, ALICE TDR 7, CERN/LHCC 2000-001 2. R. Veenhof, "Calculations for the ALICE TPC Read-Out", http://rjd.home.cern.ch/rjd/Alice/ 3. C. Garabatos, "The ALICE TPC" , Proc. 10th International Vienna Conference on Instrumentation, Vienna, Austria, NIM in physics research, Section A, Accelerators, Spectrometers, detectors and associated equipment 531 (l):197-200. 4. T. C. Meyer (ALICE collaboration) 2004, Proc. Of the 42nd Workshop of the INFN ELOISATRON Project, Innovative Detectors For Super-Colliders, Erice, Italy, 28 Sept-4 Oct 2003. 5. L. Musa et al., The ALICE TPC Front End Electronics, Proc. of the IEEE Nuclear Science Symposium, October 2003, Portland 6. P. Christiansen et al.,''Performance Test of the ALICE TPC IROC Module at the PS", ALICE-INT-2005-029. 7. H. Bichsel, "Particle identification at STAR-TPC with ionization measurements", prepared for 8th International Conference on Advanced Technology and Particle Physics (ICATPP 2003): Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications, Como, Italy, 6-10 Oct. 2003.

THE ATLAS RPC TEST STANDS M. B I A N C O * on behalf of t h e A T L A S R P C g r o u p t The trigger system for the Barrel Muon Spectrometer of the ATLAS experiment at LHC will be realized with 1116 Resistive Plate Chambers1 (RPC) units for a total surface of about 4000 m 2 . The difficulty in accessing the apparatus after the installation has been completed demands a rigorous quality control (Quality Assurance) for each detector. To achieve this task three different cosmic ray test stands have been built in Lecce, Naples, and Rome "Tor Vergata" University and INFN Laboratories. For each R P C unit, the three test stands certify its components (gas volumes, strip panels, high and low voltage connection etc..) and provide measurements of efficiency, dark current, cluster size, and in general all parameters that characterize the detector.

1. Introduction The ATLAS Muon Spectrometer 2 will use Resistive Plate Counter (RPC) detectors in the barrel region to trigger on muons and to identify the bunchcrossing. It is a complex system composed of 1116 units, which span different sizes and configurations covering a total surface of about 4000 m 2 ,distributed in the middle and outer layer of the muon spectrometer. The ATLAS RPCs units are assembled in two layers, each one made up two gas volume and up to four planes of read-out strips. The detector work in saturated avalanche mode and use a ternary gas mixture (C2H2F4/C4Hio/SF6 : 94.7%/5%/0.3%) All RPC module of the ATLAS Muon Spectrometer are assembled in a facility located at Lecce * C o n t a c t a u t h o r , E-mail: [email protected] — I N F N Sezione di Lecce, via per Arnesano, 73100 Lecce, Italy. t G . A i e l l i c , M. Alviggi '", M. B i a n c o " ' , P. C a m a r r i " ' , V. Canale ' " , R. C a r d a r e l l i c ' , G. Carlino , G. C h i o d i n i " , R. de Asmundis , M. Delia P i e t r a ' , D. Delia Volpe ' , C. Delle F r a t t e " , A. Di Ciaccio*' , A. Di S i m o n e ° ' , E. Gorini "' , F . G r a n c a g n o l o " , P. Iengo , B. L i b e r t i " , S. Patricelli ' , R. Perrino , M. P r i m a v e r a , R. Santonico ' , G. Sekhniaidze , A. Salomon ", E. Solfaroli , S. S p a g n o l o " ' , G. F . Tassielli "' , "I.N.F.N. sez.Lecce, Lecce, d Italy, bI.N.F.N. sez.Napoli, Napoli, Italy, "I.N.F.N. sez.Roma,Tor VERGATA, Italy, Dipartimento di Fisica dell'Universita degli Studi di Lecce, Lecce, Italy, 'Dipartimento di Fisica dell'Universita degli Studi di Napoli "Federico II", Napoli, Italy, ! Dipartimento di Fisica dell'Universita di Roma2 "Tor Vergata, Home, Jtaiy.

21

22

INFN Laboratories 3 . The difficulty in accessing the apparatus after the installation on site, requires a rigorous quality control (Quality Assurance Test) for each assembled units. 2. The Cosmic Rays Test Stands In order to perform the Quality Assurance Test three different cosmic ray test stands have been built in Lecce4, Naples 5 , and Rome "Tor Vergata" Physics Departments and INFN Laboratories. The mechanical structure of the three test stands is similar (Fig.[l]) and each apparatus consists of several subsystems: gas distribution, high and low voltage distributions, trigger logic, read-out and data acquisition (for a total of about 4000 electronic channels per station).

Figure 1. Overview of the ATLAS RPC Test Stand in Lecce

The Naples test stand uses scintillators as trigger detectors and two drift chambers for tracking, while the others two setups use Resistive Plate Counters both for triggering and tracking purposes. The three cosmic ray test stands allow full control of gas composition, high voltages and front-end threshold voltage settings, for each unit to be tested. Read out is based on custom made 64-channel receiver modules (developed and built at INFNRome "Tor Vergata") and 96-channel latch VME modules (developed at and built INFN-Lecce), while the data acquisition and the detector control system arebased on use National Instruments software6. Naples teststand started the test RPC in October 2002 and finished

23 its work in December 2004; it has certified all the Barrel Middle Layer chambers (Large BML an Small BMS)for a total of 546 RPC units. Lecce teststand fully operative since August 2004; has certified 300 RPC units, (mainly Barrel Outer Layer Small chambers BOS), it is going to finish the test in October 2005. Rome Tor Vergata test stand started the test in June 2005 and has tested 62 units (Barrel Outer Layer Large chambers BOL) out of 192, and it should finish its work at the end of the year. 3. Quality Assurance Tests The following measurements are performed in Lecce for each gas volume and strip panel after a group of RPC units (up to 8) has been installed on the stand: Current-Voltage (I-V) Curve : for each RPC unit, gas volume the dependence of dark current (gap current) on the applied high voltage together with the current absorbed by power supply is measured. The gap current can be attributed to the sum of two contributions: the ohmic current through the bakelite, spacers and frame, and the current due to avalanche generated inside the gas volume. The gap current measurements are fitted with the equation: I = V/Rsuik + he%

(1)

where RBuik is the ohmic resistance, Jo is the average primary cluster charge produced per unit time and Vb can be correlated on the effective Townsend coefficient of the gas mixture. Efficiency Curve: the efficiency of the single readout panel and gas volume is measured (at different front-end voltage thresholds) as a function of the high voltage; the data are fitted with the function:

W =

%5EE-

W

1 + 81 Aio% where £o is the panel plateau efficiency at infinite high voltage, V50 is the value of the voltage where the efficiency is 50% of eo and A^% is the high voltage difference between the of 90% and 10% plateau efficiency. From this behaviours the detector working poin can be set for each gas volume. Noise Measurement :for every strip panel the free running counting rate versus the high voltage is measured at different front-end voltage thresholds. Gas Volume "Tomography": for each gas volume a plot of the bidimensional efficiency is made to verify the absence of local inefficiencies.

24

Most of the test time is taken for this purpose for Napoli an Lecce test sequence where this test is done. The precision of this method is determined by the tracking extrapolation, about 400/wn for the Naples setup, 7-8 mm for Lecce setup. Gas Tightness:a leak test is performed to ensure gas volume tightness for every gas gap. The differential pressure at the gas volume entrance is measured for about two hours, after having put in overpressure the gap and hermetically closed the volume. Quality Assurance Results During the test each single components, of the detector is carefully checked. The average of read-out plane efficiency, at working point is above 97% and the average of the gas volume efficiency is above 98% (In figure [2] the distribution of the plane and gap efficiency for the all the BOS chambers tested in Lecce are shown). Entrt«* MOM

RMS

iwt «.*»! 441117

140

1

IE

80

1

lllyL

60

r

40

r

t20

Meoral

isa

100

ISP

20

89

032

0.84

036

0.9B

0.97

0.98

0.99

Figure 2. Distribution of plane and gap efficiency separated for large (BOSA, BOSB, BOSC) and small (BOSD, BOSD*, BOSE) type.

The average working point cluster size, at the is RJ 1.7, while and the average single counting rate is at maximum 2Hz/cm2 as requested from the experiment. The main contributions that determine the rejection of a chamber component are: (1) Local inefficiency present in the gas volumes. (2) Front-end electronics of dead channels. (3) To high gas volume dark current at working point. (4) Gas leak test failure. Table 1 show the number of occurrencies for each failure and its relative fraction, for all the BOS chambers tested in Lecce. All the components

25

which do not satisfy the requirements are removed from the units and are replaced. The Gas Volume entry, is cumulative of the gas volume rejected for local inefficiency and high dark current. Given the large number of components present in each detector, (4 gas volumes and 8 strip panels for a typical chamber) and the stringent request on Quality Assurance Test, the chambers rejction rate is about 20%. Table 1. ATLAS RPC Failure rate for the BOS chambers tested in Lecce. Failure tipology

Number of failures

N. of components tested

% of

failures

E t a Strip panel

49

864

5.7%

Phi Strip panel

40

864

4.6%

Gas Volume ( Local ineff. and dark current test)

34

864

4.0%

Gas Leak

11

432

2.5%

HV Connectors

5

864

0.6%

iGap Shunt Resistor

3

864

0.3%

Conclusion More than 900 RPC units have been tested and accepted up to now. The remaining will be tested in time for the installation schedule. All the chambers accepted are sent to CERN to be integrated with tracking chambers and to be installed on the apparatus. All the data collected during this test phase, will be the starting point to setup the ATLAS Muon trigger detectors in the barrel region, and an accurate analysis of this data will be used to improve the Resistive Plate Chambers detector knowledge. References 1. R. Santonico and R.Cardarelli, Nucl. Instr. and Meth 187, 377 (1981) 2. ATLAS collab., ATLAS Muon Spectrometer Technical Design Report, CERNLHCC 97-22, ATLAS TDR 10 (1997). 3. R. Perrino, F. Grancagnolo, G.Fiore, R. Gerardi and A. Miccoli, Nucl. Instr. and Meth A 508 , 116 (2003) 4. G. Chiodini et al., ATLAS RPC Cosmic ray Teststand ad INFN Lecce proceeding of XXIV physic in Collision-Boston, arXv:physics/0409075 (2004). 5. M. Alviggi et
P E R F O R M A N C E S I N HIGH M A G N E T I C FIELDS OF FINE-MESH PHOTOMULTIPLIERS FOR FAST TIME-OF-FLIGHT D E T E C T O R S

M. BONESINI 1 , F. STRATI \ G. BACCAGLIONI 2 , G. VOLPINI 2 , G. CECCHET 3 , A. DEBAR! 3 , R. NARDO' 3 , M. ROSSELLA 3 , S.DUSSONI 4 , F. GATTI 4 , R. VALLE 4 1

3

INFN - Sezione di Milano, Dipartimento di Fisica G. Occhialini 2 INFN - Sezione di Milano, Laboratorio LASA INFN - Sezione di Pavia, Dipartimento di Fisica Nucleare e Teorica ^ INFN - Sezione di Genova, Dipartimento di Fisica

The performances of 1 in., 1.5 in. and 2 in. fine-mesh Hamamatsu photomultipliers have been measured in magnetic fields (up to 1.2 T) to determine gain and timing properties. Rate capabilities have also been studied.

1. Introduction In TOF detectors based on scintillators, a simple parametrization of the timing resolution at is given by x : (T

=

'* V

sci + <Jpmt + °~pl l

^

2

+ elec

/i\

()

where: a8Ci accounts for the scintillator response, apmt for the photomultiplier's (PMT's) jitter, api for the path length fluctuations and aeiec for the jitter of the electronic readout system. Npe is the average number of photoelectrons (p.e.). The dominating factors for the timing resolution at are A^e and the counter dimensions (mainly its length L). Below 100 ps, contributions such as apmt become increasingly important. Additional problems may be given by the operation inside high magnetic fields (0(1T)) or with high incoming particle rates.

26

27

2. The experimental setup Fine-mesh PMTs show good gain and timing properties in high magnetic fields, up to 1-1.5 T. While they can drive very high single pulse current without loosing peak voltage linearity (up to 35 V on a 50 fi load); to avoid dynode damage and pulse amplitude reduction (loss of linearity) a maximum average anode current (typically 100 fiA for R5924 PMTs) must not be exceeded. Systematic studies have been done for 1" (R5505), 1.5" (R7761) and 2" (R5924) Hamamatsu fine-mesh PMTs (see table 1 for details), using a iron yoke resistive dipole magnet at LAS A (INFN Milano), with magnetic fields up to 1.2 T and an open gap of 12 cm. A fast light Table 1. Main properties of Hamamatsu fine-mesh PMTs. Tube diameter No. stages Q.E. at peak Gain (B=0 T) typ. Gain ( B = l T) typ. Risetime (ns) TTS (ns) max (Ia) (mA)

R5505 1" 15 .23 5 x 10 5 1.8 x 10 4 1.5 0.35 0.010

R7761 1.5" 19 .23 1.0 x 10 7 1.5 x 10 5 2.1 0.35 0.010

R5924 2" 19 .22 1.0 X 10 7 2.0 X 10 5 2.5 0.44 0.100

pulse from a PLP-10 Hamamatsu laser (~ 405nm, 60 ps FWHM pulse width, max repetition rate 100 MHz) was sent to the PMT's photocathode via a multimode CERAM OPTEC UV 100/125 optical fiber (with a measured dispersion of < 15 ps/m, see 2 ) . At the end of the fiber a small plexiglass prism, inserted in a black plastic cover in front of the PMT window, allowed illumination at the center of the photocathode. The laser spot was focussed into the optical fiber (aligned by a micrometric x-y-z flexure system) by a set of lenses, after removable absorbitive neutral density filters, to give light signals of different intensities. Tests were done usually with a signal corresponding to about 300 photoelectrons: a typical value for a minimum ionizing particle (MIP) crossing a scintillator l"-2" thick, 6-10 cm wide, at a distance of 1-2 m. The optical power was periodically monitored with an OPHIR PD-2A laser powermeter. The number of photoelectrons (Npe) was estimated via absolute gain measurement and was affected by a large error, due to the lack of a clear SER peak in fine-mesh PMTs. This number was cross-checked with the powermeter measurements. For gain measurements the PMT signal was acquired in average mode by a Tektronix TDS 3032 digital scope (300 MHz bandwidth, 2.5 Gs/s sam-

28

pling rate) triggered by the laser output sync, that had a maximum jitter of 15 ps as respect to the delivered light pulse. In addition, the signal was sent after a passive T divider to an EG-G Ortec 570 shaper (shaping time ~ lfis, gain ~ 200) followed by an Ortec TRUMP-8k Multichannel Analyzer installed on a PC. In part of the timing measurements, the same MCA chain was used with an ORTEC 566 TAC and a CF 8000 Ortec discriminator, while in the other the PMT signal was split 50% to an ADC line and 50% to a TDC line. On the TDC line the signal, after being discriminated by a CAEN V895 module, was sent to give the TDC STOP to a CAEN V480 TDC module (25 ps/ch). The laser sync out signal was fanned out by a Phillips Research 711 discriminator to give both the START of the TDC and the gate for the used QADC (CAEN V465, 0.1 pC/ch). The jitter in the readout electronics was expected to be less than 30 ps. Acquisition, in VME standard, was made by a VXI-MXI 2 National Instrument controller.

3. R e s u l t s The PMTs were inserted in the central region of the test magnet, where the field had a uniformity better than ~ 0.1%, using plexiglass supports to incline them up to 60° as respect to the field lines. Environment light was accurately masked, to reduce noise. Measures were done to see gain reduction, timing resolution and rate capability as a function of magnetic field (B) and relative orientation angle (9). Due to the effect of the magnetic field on the accelerated electrons inside the PMTs we can expect a reduction of gain as B increases and also a marked dependence of the relative gain as a function of 6. Fine-mesh PMTs are well behaving up to a critical orientation Bc depending on the photocathode size: typically 0C = 30° - 4 5 ° . Some results (for a standard HV value of +2000 V) are shown in figure 1. Figure 2 shows the PMT response (P.H. in mV) as a function of the laser shot repetition rate R (simulating an increasing particle rate), in different conditions of the external magnetic field B. These results can be easily understood, remembering that the rate capability of fine-mesh PMT is limited by the maximum allowable anode current Iain addition, the pulsed cycle of an accelerator was simulated giving laser shots in a 1 ms window (with a repetition rate up to 1.5 MHz) followed by a 20 ms interspill. As expected, no difference was found as respect to the operation mode with a continuos laser pulsing at the same frequency. Timing characteristics of fine-mesh PMTs show a weak dependence on field strength and direction, in spite of the large reduction in gain (up to

29

B(T)

B(T)

Figure 1. Behaviour of P M T gain in magnetic field B for typical fine-mesh 1", 1.5", 2" PMTs at 6 = 0° (left) and a typical 2" P M T at various inclinations 9 (right). In ordinate the gain ratio at B and B = 0 T is reported.

11nch PMT- rate effects

1.5 Inch PMT -rate affects

2 inch PMT- rale effects

• B=0T, 2000 V a B= t>r. 2I0OV a B~0T. 1900 V ° B= Of, 1750V

r B= l.O5T,2O00\ . B*I.05T.2IOO\ • B= I.OS T. 1900 \ t B=1.05T,175O\

i 0

'

o

. U

*t

'* ; > •••

rate R (KHz)

f

rateR(KHi)

Figure 2. Rate capability of typical 1" , 1.5" (at 9 - 0°) and 2" (at 6 = 15°) fine-mesh P M T , as a function of magnetic field B and HV (measured P.H. in mV versus rate in KHz). The light signal corresponds to ~ 300 p.e.

a factor 100). Only at high fields ~ IT the multiphotons timing resolution &TDWC seems to begin to be affected, as shown in figure 3 for a typical 2" fine-mesh PMT. Measurements of the time resolution vs photon statistics confirmed that the timing resolution (JTDWC follows a 1/\ 'Npe law with a

30 2.0 inch PMT Timing studies

2 inch PMT • Timing atudtoa

1.5 Inch PMT - rate affe«U

' llOp.e.B=ffT D *

o*

0' 20' 30'

8

' 400 p.e. B=0T ' H00p.e. B=0T

8

B(T)

B(T)

rale R (KHz)

Figure 3. Transit time ratio at magnetic field B and B=0 T (a), timing resolution &TDWC in 25 ps units as a function of the magnetic field B (b) for a typical 2" Hamamatsu R.5924 fine-mesh PMT. Timing resolution OTDWC as function of rate R (at B=0 T) for a typical 1.5" Hamamatsu R7761 fine-mesh PMT (c).

term added in quadrature to account for the electronic noise of the system. The behaviour of the multiphoton time resolution OTDWC was also studied as a function of the laser repetition rate R for Npe in the range ~ 100 —1000 p.e. No deterioration was found going from 1 KHz up to 1 MHz, at fixed values of the magnetic field up to 1 T, as shown in figure 3. 4. Conclusions Fine-mesh PMTs show good timing properties even in magnetic fields up to 1 T. Rate capabilities are driven by the mean flowing anode current, giving a good gain linearity and timing resolution if the maximum allowable value is not exceeded (typically 100 fJ,A for a 2" PMT). Gain behaviour becomes to be problematic only if the inclination of the PMT axis as respect to the B field exceed a critical value 6C ~ 30° — 45° depending on the PMT size. References 1. W.B. Atwood, Lecture at SLAC Summer Institute, 1980, p. 287; M. Bonesini,"A Review of Recent Techniques of Tof Detectors", 8th ICATPP Conference, Villaolmo, Conf. Proceedings, p. 455, 2003. 2. M. Bonesini et al., IEEE Trans. Nucl. Science 50 (2003) 541

RECENT RESULTS ON GRIDPIX DETECTORS: A N INTEGRATED MICROMEGAS GRID AND A MICROMEGAS AGEING TEST M. CHEFDEVILLE1, A. AARTS, H. VAN DER GRAAF, S. VAN DER PUTTEN NIKHEF, PO BOX 41882 Amsterdam, 1009 DB, The Netherlands A new gas-filled detector combining a Micromegas with a CMOS pixel chip has been recently tested. A procedure to integrate the Micromegas grid onto silicon wafers ('wafer post processing') has been developed. We aim to eventually integrate the grid on top of wafers of CMOS pixel chips. The first part of this contribution describes an application in vertex detection (GOSSIP). Then tests of the first detector prototype of a grid integrated on a bare silicon wafer are shown. Finally an ageing test of a Micromegas chamber is presented. After verifying the chambers' proportionality at a very high dose rates, the device was irradiated until ageing became apparent.

1. Introduction: Micromegas, GridPix, GOSSIP and InGrid The Micromegas chamber [1] is a parallel plate gaseous detector that uses a grid placed 50 um above an anode plane. This grid separates the drift space where the conversion takes place from the amplification gap. After the passage of an ionizing particle through the drift space, primary electrons drift towards the grid, travel through the holes and initiate an avalanche in the amplification gap. A pixel-readout CMOS ASIC to be placed below the grid as a direct anode fully employs the granularity offered by the Micromegas. As the grid hole pitch matches the pixel pitch, each grid hole is equipped with its own electronic chain and thus behaves as an independent detector. In such 'GridPix' detectors, the projection of the track onto the pixel plane can be recorded. By distributing a clock signal over the pixel matrix and recording the arrival time of avalanches, the drift time could also be measured, realizing a complete three dimensional tracker. Recently we have combined a Micromegas grid with the Medipix2 pixel CMOS chip [2]. In a small drift volume o f ! 4 x ! 4 x I 4 mm3, primary electrons from

' Author Email address: chefdevi @ nikhef.nl Extended author list: J. Timmermans (NIKHEF), Jan Visschers (NIKHEF), i. Schmitz (University of Twente, The Netherlands)

31

32

cosmic muon tracks were observed (fig. 1). The single electron detection efficiency was derived to be better than 90 %.

Fig. 1: Cosmic tracks in He-iC4Hio

This 'GridPix' readout can be applied to gas-filled detectors in general, but the application in TPCs (Time Projection Chamber), in u-TPCs and in TRDs (Transition Radiation Detector) may boost their performance since individual 3D information of all primary electron clusters would become available. With a drift gap of only 1 mm, 'GridPix' can be used as a vertex detector in intense radiation environments (Gas On Slimmed SI Pixels: GOSSIP). Whereas in semiconductor vertex detectors the signal is generated by means of electronhole pairs in the depletion layer, in GOSSIP electron-ion pairs are created in the drift volume. With adequate amplification a single primary electron is sufficient for track detection. In semiconductor, the signal is amplified by low noise amplifiers outside the sensor, while in GOSSIP the signal is pre-amplified in the sensing medium. Since the input pad area of the pixels can be rather small (below 15 um2), the source capacity at the preamp input can be as small as 20 fF. Given the pulse height distribution, the fast signal development [1] and a gain of 103, the low-noise amplifier parameters in the GOSSIP pixel chip can be chosen such that the power consumption can be as low as 1 uW per pixel. The gas flow can be used as cooling. The standard CMOS pixel chip can be thinned to 30 um, reducing the detector mass. The combination of these thin Si foils with gas opens a new window in light detector construction. Furthermore, in GOSSIP, there is no bias current. 2. InGrid By means of wafer post processing, a Micromegas grid can be integrated onto the chip (InGrid). With this technique no manual mounting of the grid is necessary and the detector is compact and mass producible. The pillars on which

33

the grid rests, can be made so small that no area is lost for detection. Furthermore, an accurate control of the geometrical parameters (grid thickness, gap height, alignment) is possible. A description of the process can be found in [3]. On a 100 mm diameter silicon wafer, 19 geometrically different InGrid fields are patterned (fig. 2). These fields have a diameter of 15 mm and are 50 um high.

Fig. 2: Ingrid wafer (19

fields)

Fig. 3: Grid supported by pillars

The supporting structures are pillar type (fig. 3) and wall type. In the latter, each grid hole has its own gas-filled cell. Some results previously obtained with wall supported Micromegas can be found in [4]. 3. InGrid tests The InGrid wafer was tested in a chamber in which a drift volume was formed by placing a cathode foil 10 mm above the wafer. The grid was connected to a low-noise charge sensitive preamplifier [5] with an integrating time constant of 1 us. The charge signal from the grid is completed within 50 ns, so the output signal is proportional to the charge signal from the grid [2]. The drift field was kept at 1 kV.cm" . The electronic response was calibrated by injecting precisely known charge pulses into the pre-amplifier. The gas gain versus the grid potential was measured by means of 5.9 keV 55Fe quanta in a mixture of Ar/Isobutane 80/20 (fig. 4). The precise exponential relation is expected given the supporting structure height constancy of InGrid. The energy resolution is deduced from the spectrum of fig. 5. The asymmetry of the photo peak of 55Fe quanta is clearly visible. The source emits quanta of 5.90 and 6.49 keV with a ratio of 9:1. Due to differences in absorption, we left this ratio free in the fit to the pulse height spectrum. The width (RMS) of the 5.90 keV peak is 6.5 %. At a given grid voltage, the gain from pillar supported field was observed to be 30 times higher than the wall supported one.

34

4. Ageing test In the most common ageing effect, deposits are formed on the chamber electrodes. Assuming that this effect is proportional to the integrated charge density, GOSSIP should withstand a large irradiation dose due to its large ratio of the anode surface and gas volume, and due to the expected low gain (~ 103). In a first test, the ageing of a Micromegas chamber was measured using a 90/10 Ar/Methane mixture. The chamber was built from standard materials, including Kapton tape, G10, Viton O-rings, PVC insulated wires, and epoxy (Araldite). Gain in Argon + 20% ICjH, ' •''' / r.

3.931

/' 3 -5.897 0.3340E-01

PI P2

. _^—-

^

: -

.—-' """" 360

380

400

420

440

Fig. 4: Gain in Ar/iC 4 Hi 0 80/20 K

Fe spectrum in Argon + 20% iC,H,

O a

§5000 u 4000 3000 2000 1000 0

0

10

20

30

40

50 60 70 output pulse (mV)

Fig. 5: 55 Fe spectrum in Ar/iGjHio 80/20

The Micromegas grid, mounted in a frame, was placed onto a polished aluminum anode block. By means of a carefully calibrated charge preamplifier the charge signals from the grid could be measured, and the currents through the anode, the grid and the cathode could be registered by measuring the voltage drop over serial resistors. With this set-up, irradiated with quanta from an 55Fe source, we certified the absolute relation between the gas gain, current and counting rate, and dark currents were below the level of 0.1 nA

35

We then irradiated the chamber with 8 keV X-ray quanta from a Cu target, the target itself being irradiated with quanta from an X-ray tube. We varied the current through the tube and observed that the chamber current changed proportionally up to a current density of 0.5 uA/mm2 (reached with the maximum power of the X-ray tube, combined with a gas gain of 11 k). We then covered half of the grid surface with Pb, shielding this part of the detector from irradiation. During a period of two months, we noticed a slow but regular decrease of the anode current. After collecting 0.3 Coulomb/mm2 at the irradiated area of the anode block, we re-calibrated the chamber: no change in the 55Fe pulse height spectrum was observed at the non-irradiated region of the anode. At the irradiated area, the gain was reduced by a factor - 4 . A clean white deposit containing Si, C and O was found on the anode. The Micromegas remained clean while de cathode foil showed a deposit containing similar elements as the anode [6]. 5. Conclusions and future plans With a GridPix chamber, diffusion of primary electrons may become the only spatial resolution limit. This may determine the maximum drift length. Grid integrated readout chips can be made (InGrid). The good energy resolution suggests the study of InGrids with a smaller multiplication gap. This would result in faster charge signals with a smaller spread in single electron response. A gain of about 103 required for sufficient single electron efficiency in GOSSIP could be reached. This would be beneficial against ageing and HV stability. A first ageing test indicated that the processes in Micromegas chambers and wire chambers are similar.

References 1. I. Giomataris et al., Nucl. Instr. and Methods A376, (1996) 29. 2. M. Campbell et al., Nucl. Instr. and Methods A540, (2005) 295. 3. M. Chefdeville et al., An electron multiplying 'Micromegas' grid made in silicon wafer post-processing technology, accepted for publication in Nucl. Instr. and Methods. 4. D. Bernard et al., Nucl. Instr. and Methods A481, (2002) 144. 5. E Bakker and P. Rewiersma., Honeycomb Strip Amplifier, NIKHEF report ETR 94-01, 1994. 6. A.A.A. Aarts et al., A high rate, high dose aging test of a Micromegas chamber, paper in preparation.

RICH D E T E C T O R AT J E F F E R S O N LAB, D E S I G N , P E R F O R M A N C E A N D PHYSICS RESULTS

E. CISBANI? S. COLILLI, F. CUSANNO, S. FRULLANI, R. FRANTONI, F. GARIBALDI, F. GIULIANI, M. GRICIA, M. LUCENTINI, M.L. MAGLIOZZI, L. PIERANGELI, F. SANTAVENERE, P. VENERONI Istituto Superiors di Sanita and INFN Roma, gruppo collegato Sanita G.M. URCIUOLI INFN Roma M. IODICE INFN Roma III G. DE CATALDO, R. DE LEO, L. LAGAMBA, S. MARRONE, E. NAPPI, V. PATICCHIO University of Bari and INFN Bari R. FEUERBACH, D. HIGINBOTHAM, J. LEROSE, B. KROSS, R. MICHAELS, Y. QIANG, B. REITZ, J. SEGAL, B. WOJTSEKHOWSKI, C. ZORN Thomas Jefferson National Accelerator Facility A. ACHA, P. MARKOWITZ Florida International

University

C.C. CHANG, H. BREUER University of Maryland

* luppercasecorresponding Author

36

37 Since 2004 the hadron spectrometer of Hall A at Jefferson Lab is equipped with a proximity focusing RICH. This detector is capable of identify kaon from pion and proton with a angular separation starting from 6 sigma at 2 GeV/c. The RICH design is conceptually similar to the ALICE HMPID RICH; it uses a C 6 F14 liquid radiator and a 300 nm layer of Csl deposited on the cathode pad plane of an asymeetric MWPC. The RICH has operated for the Hypernuclear Spectroscopy Experiment E94-107, which took data in the last two years. Design details and performance along with first physics results from the hypernuclear experiment are shortly presented.

1. Introduction A proximity focusing RICH has been designed, built and operated in the first hypernuclear spectroscopy experiment (E94-107) performed in Hall A at the Jefferson Laboratory in 2004 and 2005. The Hall A is dedicated to high resolution and high luminosity electron scattering experiments, served by a continuous electron beam with energy up to 6 GeV. It is equipped with two identical, Quadrupole-QuadruploDipole-Quadrupole High Resolution Spectrometers 1 that accept particle momenta up to 4 GeV/c. The spectrometers are dedicated to the electron and hadron detection, having slightly different detectors packages. The E04-107 experiment has performed the first systematic study on 9 Be(e,e'K+)9 Li A , 12 C(e,e'K+)12 B A and 16 0(e,e'K+)9 N A hypernuclei. The A hyperon represents an important probe in the nuclear medium due to the fact that it is not blocked by the Pauli principle and has a relatively weak coupling with the nuclear core, being reasonably well described by the shell models. Moreover, the A — N interaction represents an important aspect of the Neutron Start 3 . The lepton probe offers some advantages respect to the hadron probes in hypernuclear physics: • presence of strong spin transitions (spin terms are relevant in A — N potential) • generally higher energy resolution Moreover the electron probe permits the study of complementary (Z-l) hypernuclei. On the other side, the cross section are quite small resepct to the hadronic ones, forcing to investigate phase space with high background, which therefore represents a challenging aspect of such experiment. In fact, for a clean kaon identification, the required 7r rejection factor in E94-107 has been estimated of one order of magnitude larger than standard Hall A PID system based on time of flight and two aerogel Cerenkov

38

counters (with n — 1.015 and n = 1.055) in the hadron arm.

Figure 1. Schematic view of the proximity focusing RICH

2. P r o x i m i t y focusing RICH layout Figure 1 presents the conceptual layout of the Hall A RICH, which is based nf the ALICE HMPID design 4 . The Cerenkov photons are produced by over threshold particles in the liquid freon C6F14 6 with refractive index of 1.28 and thickness of 1.5 cm. Then they refract on a quartz window of 0.5 cm and diverges in a 10 cm proximity gap. Eventually they are converted to electrons by a thin layer of Csl (300 nm) deposited on 3 pad planes that represent the cathode of a Multiwire Proportional Chamber. The induced charge of the electric avalanche is readout by a sample and hold analogically multiplexed front-end electronics 5 . Spatial information is obtained from the segmented pads, whose size is 8 x 8.4 mm 2 . The relevant parameters of the RICH are in table 1.

39 Table 1. Radiator Photon converter Position Detector F E Electronics

Relevant parameters of the Hall A RICH

15 mm thick Liquid Freon (C6F14, n=1.28) 300 nm Csl film coated on Pad Planes 1940 X 403 mm 2 - Multi Wire/Pad Proportional Chamber filled with Methane at STP, H V = 1050 4- 1100 V 11520 analog chs, multiplexed S&H

3. Evaporation System and Quantum Efficiency Measurement One of the most important aspect of the RICH is the deposition of the proper layer of Csl on the pad plane. This is obtained by means of the evaporation of a pure Csl powder in a high vacuum chamber (few 1 0 - 7 mbar). The Rome group has therefore built and installed at JLab an evaportation chamber for large area pad (up to about 650 x 650 mm 2 ). The desired vacuum is obtained in less than 24 hours by three vacuum pumps (scroll, molecular and cryogenic). The Csl powder is heated at 500 °C by four crucibles evaporates toward the pad plane. A quantum efficiency (QE) online measurement system 7 has been integrated into the evaporation chamber. An UV light generated by a deuterium lamp is directed on the evaporated pad passing through one of three wavelength filters (160, 180 and 220 nm). A small wire chamber at 2 mm from the pad collects the photoemitted electrons, whose current (few nA) is measured by a picoamperometer. A mirror can divert the UV beam to a reference PMT. The wire chamber over PMT currents ratio times the known reference PMT QE provides a measure of the deposited Csl QE; a typical measure is reported in figure 2. 4. RICH performance and physics results The RICH has operated during the E94-107 experiment providing a clean kaon identification as shown in figure 3.The mean number of photoelectrons measured on a sample of pions is slightly more than 13 with an angular resolution (a) of 5 mrad which corresponds to a kaon-pion separation of 6
40 QE SCAN of PAD OLD-SX at ^=160 nm (date 170505)

0.95

,„***—«*"*

Figure 2. Typical QE distribution using time of flight and aerogels only) for the smallest detectable peaks. The obtained excitation energy spectra have been compared to some theoretical models, showing: • a good agreement for the 1 2 BA, even afor the cross section of the ground state. • an overall agreement on the 1 6 NA, with larger discrepancy in the relative strenghts of the most excited multiples • substantial disagreemnt with the 9LiA hypernuclear level.

5. Conclusion a n d Perspectives The RICH build and successfully operated in the E94-107 experiment, is now available in Hall A for a effective PID of hadrons up to 2-2.5 GeV/c. Extension of the momentum range to about 4 GeV/c could be done with

41

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relatively small effort, replacing the present freon with one with smaller index of refraction (e.g. with C5F 12 ) and increasing the proximity gap of few centimeters. Some of the upcoming Hall A experiments and proposals are considering the advantage using the RICH. References 1. 2. 3. 4. 5. 6. 7.

J. Alcorn et al., Nucl. Instr. and Meth. A522, 294 (2004) F. Garibaldi et al. Jefferson Lab Proposal E94-107 J. Shaffner-Bielich et al. arXiv astroph/0005490, Alice Collaboration Alice TRD 1 CERN/LHCC98-19, (1998) Gassiplex Chip Paper TBD F. Garibaldi et al. Nucl. Instr. and Meth. A502, 117 (2003) F. Cusanno et al. Nucl. Instr. and Meth. A502, 251 (2003)

H Y B R I D - P H O T O N - D E T E C T O R S IN T H E LHCB RICH SYSTEM

J. M. D I C K E N S * Cavendish Laboratory, Cambridge, CBS OHE, UK E-mail: [email protected]

The LHCb experiment will use two Ring Imaging Cherenkov (RICH) detectors for particle identification. The RICH detectors will use three radiators to produce Cherenkov light and Hybrid Photon Detectors (HPDs) for single photon detection. Photoelectrons, produced by incident photons at the photocathode of the HPD, are accelerated, demagnified and cross-focussed by electric fields onto a silicon sensor, which is bump-bonded to a pixel chip. A number of pre-series HPDs have been built and tested, operating at the LHC bunch crossing rate of 40MHz. The quantum efficiencies of the HPDs as a function of wavelength have been measured and dark counts and aging tests have been performed. The tests show that the HPDs meet all the requirements of the LHCb experiment.

1. The LHCb Experiment The Large Hadron Collider Beauty (LHCb) experiment 1 is a single-arm forward spectrometer designed for the study of CP violation and rare Bmeson decays at the Large Hadron Collider (LHC) at CERN. The main physics goals are to constrain Standard Model CP violation, through precision measurements of the Cabibbo-Kobayashi-Maskawa (CKM) unitarity triangle, and to search for new physics in rare B meson decays. 1.1. RICH

detectors

Particle identification is essential for LHCb, to distinguish between topologically similar decays. The LHCb experiment has two Ring Imaging Cherenkov (RICH) detectors 2 , one upstream of the magnet (RICH1) and the other downstream (RICH2). The RICH detectors are designed to separate pions and kaons, in the momentum range 2 - 100 GeV/c. The RICH *on behalf of the LHCb RICH collaboration

42

43

radiators are chosen to give 7r/K separation over the required momentum range. RICH1 (~2 - 60 GeV/c) uses aerogel and C 4 Fi 0 gas while RICH2 (20 - 100 GeV/c) uses CF 4 gas. Cherenkov photons, produced in a cone, are reflected and focussed by a combination of flat and spherical mirror segments onto the photon detector plane. The photons must be detected over a total area of 2.8 m 2 , in the wavelength range 200 to 600 nm. The photon detectors must operate at the LHC bunch crossing frequency (40MHz) and have granularity better than 2.5x2.5 mm. Pixel Hybrid Photon Detectors (HPDs), described in the next section, will be used in a close-packed array. A total of 484 HPDs are required to equip the two RICH detectors. 2. Hybrid P h o t o n Detectors The pixel HPD has been developed for use in the LHCb RICH detectors 3 and a schematic diagram is shown in figure 1. The HPD consists of a ~80 mm diameter vacuum tube with a quartz entrance window. A pixelated silicon anode, bump-bonded to a pixel binary readout chip 4 , is enclosed in the vacuum envelope of the HPD. The inside surface of the quartz entrance window is coated with an S20 multi-alkali photocathode which produces photoelectrons with a typical peak quantum efficiency of ~25%. The photoelectrons are accelerated to 20 keV, cross-focussed and demagnined by a factor of ~5 using electric fields. The silicon anode has 8192 pixels, each bump-bonded to a single channel of the readout chip. The pixels are logically ORed in groups of 8, giving 1024 effective 500 /xm x500 /um pixels. This translates to an effective granularity of 2.5 mm x2.5 mm at the entrance window, after demagnification. 2 . 1 . HPD

Characteristics

Six pre-series HPDs have been fabricated and have undergone extensive laboratory tests. All 8192 pixels were read out separately. The tests and their results are described in the following sections and summarised in table 1. The pre-series tubes all meet the LHCb specifications. 2.1.1. Quantum efficiency The quantum efficiencies (QEs) of the tubes were measured using a Xenon arc lamp and monochromator, relative to a photodiode of known QE. The results of the QE measurements are shown in figure 2. The pre-series tubes

44 3f f$*#i effsr Ce-ssmie eerrttr

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Wivo LenQlh (nm)

Figure 1. Schematic of t h e H P D , showing an e x a m p l e of a p h o t o electron trajectory. Table 1.

Figure 2. Q u a n t u m efficiency as a function of wavelength for t h e nine pre-series H P D s .

Results from laboratory tests of pre-series HPDs.

Item

Specification

Results

Pixel Respone

> 95%

> 99%

Minimum threshold

< 2000e-

Typically 1200e~

Noise

< 250e"

Typically 160e~

Leakage current

Typically ljiA at 80V bias

< l^A

Ion feedback rate

Max. 1 0

-2

relative to signal

2

0.03 - 3.0 kHz/cm 2

Dark count rates

< 5 kHz/cm

Photoelectron detection efficiency

Typically 85%

78 - 89%

all have QEs above the specification and are typically >20% in the wavelength range 250 - 450 nm. 2.1.2. Pixel response Pixel operation was checked using a pulsed LED. The majority of tubes had >99% working pixels, well above the 95% specification. One tube was found to have a noisy pixel and another a bad internal wire bond, resulting in a lost connection to one column of 256 pixels. This tube was still found to have a working pixel fraction of 94.8%. The leakage current of the silicon sensor for the majority of tubes was found to be <1 fiA, which is within the specification of typically 1 fiA, set to limit heating and noise. 2.1.3. Minimum threshold and noise levels The discrimination threshold and noise levels were determined using an input test pulse of varying amplitude. The average global thresholds were

45

found to be typically 1200e~ and the noise levels typically 160e~. The ~100e _ pixel-to-pixel spread can be reduced by ~40% using an additional 3 bit adjustment to the threshold of each pixel. 2.1.4. Ion feedback Ion feedback occurs when a photoelectron ionizes a gas molecule in the HPD volume. The positive ion is accelerated back to the photocathode where it creates secondary photoelectrons, which strike the pixel chip with a delay of typically a few hundred nanoseconds. Figure 3 shows the ion feedback rate, measured as a function of the delay between the primary photoelectron and the secondary cluster of photoelectrons. The typical rate of ion feedback is 10~ 3 , much less than the specification of < 1 0 - 2 .

HPD 4 1

300

400 Gat» delay [ns]

Figure 3. Rate of ion feedback events as function of delay with respect to the primary photoelectron

Figure 4. A typical backpulse spectrum of signals of photoelectrons hitting the silicon.

2.1.5. Photoelectron detection efficiency The photoelectron detection efficiency is defined as the probability of a photoelectron, which is converted at the photocathode, being detected as a photon hit by the binary readout electronics. The main causes of inefficiency are small signals below threshold from charge-sharing, late discrimination of the signal due to time walk, and backscattering of photoelectrons at the silicon surface. When a photoelectron lands on the silicon it creates a cloud of charge (typically ~5000e~). The total number of photoelectrons striking the pixel chip can be calculated from measurements of the current in the chip supply line used to bias the silicon sensor. Figure 4 shows a typical so-called "backpulse" spectrum, which has been fitted with a Poisson

46

convolved with the probability density function of a single photoelectron response. The detection efficiency is calculated as the ratio of the number of photoelectrons read out through the binary electronics to the total number of estimated true hits measured from the analogue backpulse distribution. The detection efficiency of the six pre-series tubes was found to be 84 89%, depending on the threshold. 2.1.6. Dark Counts The main source of dark counts is thermionic emission from the photocathode. The rate therefore depends predominately on the photocathode sensitivity to longer wavelengths. The rate of hits observed with no light source, averaged over a large number of events, was found to be in the range 0.03 - 3.0 kHz/cm 2 . All tubes satisfy the requirement of <5 kHz/cm 2 . 2.2. Beam

tests

Six tubes were tested in a 10 GeV/c n/e beam, using a prototype RICH detector at the CERN PS facility. A complete pre-series electronics chain was used to read out the data at the full LHCb rate. The tubes performed as expected, in agreement with the results of the laboratory tests 5 . 3. Conclusions Extensive laboratory and beam tests of pre-series HPDs have shown that they meet the requirements for the LHCb RICH detectors. Full series production of 550 HPDs is now underway. References 1. S.Amato et al., LHCb Technical Proposal, CERN-LHCC-98-4, 1998. 2. The LHCb Collaboration, LHCb Technical Design Report: Reoptimized detector design and performance, CERN-LHCC-2003-030, 2003. 3. M. Alemi et al., First operation of a Hybrid Photon Detector prototype with electrostatic cross-focussing and integrated silicon pixel readout, Nucl. Instr. and Meth. A 449-48, 2000. 4. R. Dinapoli et al, An analog front-end in standard 0.25tim CMOS for silicon pixel detectors in ALICE and LHCb, Proceedings of the 6th Workshop on Electronics for LHC Experiments, CERN-LHCC-2000-041, 2000. 5. M.Patel et al. Analysis of Test-beam Data from a prototype LHCb Rich Detector, These proceedings.

DEVELOPMENT OF A FAST TRANSITION RADIATION AND TRACKING DETECTOR FOR CBM AT FAIR CHILO GARABATOS FOR THE CBM COLLABORATION GSI, Planckstr. I 64291 Darmstadt, Germany

A transition radiation and tracking detector is being developed for the CBM heavy-ion experiment at the future facility FAIR (GSI, Darmstadt). An approach of such a detector based on pad-readout multi-wire proportional chambers is being studied, in particular with respect to the gain stability of these devices under particle rates of up to 105 Hz/cm2. Results of a first test beam campaign and of first measurements with an X-ray tube show that, for thin prototype detectors, such rates produce a moderate decrease (typically 5%) in pulse-height due to space-charge build-up, and also a slight degradation (5%) of the spatial resolution. Future activities aimed at improving and better quantifying the performance of this detector will be discussed.

1. Introduction The international accelerator Facility for Antiprotons and Ions Research (FAIR) [1] is being designed at GSI and will provide particle beams for research in various physics fields. In particular, the heavy-ion experiment Compressed Baryonic Matter (CBM) [2] will record nucleus-nucleus collisions at beam energies between 10 and 35 AGeV and up to 107 interactions per second, for exploring the QCD phase diagram at high net baryon densities and moderate temperatures. The detector, shown in Fig. 1, is composed of a magnetic spectrometer with several stations of silicon detectors, a RICH for electron identification, an array of TRD stations for electron identification and particle tracking, a TOF wall for hadron identification, and finally an electromagnetic calorimeter. The TRD is composed of 3 stations, each containing 3-4 layers in its present configuration. The total detector area ranges from 486 to 649 m . It is expected to provide, at a maximum particle load of 105 Hz/cm2, a n rejection factor of < 10"2 at 90% electron efficiency, and a position resolution of 300 um. Each layer consists of a radiator and a gaseous detector. Several options are considered at present for the implementation of the gaseous detectors: multi-wire proportional

47

48

chambers (MWPC), straw tubes, and GEMs. In this paper we will concentrate on R&D carried out with MWPCs for such a TRD, for which the space-charge effect under high particle rates is the main concern. In section 2 we describe first attempts at optimizing the rate capability of wire chambers. Simulation results showing the n rejection capability for different configurations are described in section 3. Further measurement possibilities with an X-ray generator will be illustrated in section 4. 2. Rate capability of MWPC First prototypes of pad-readout wire chambers produced by groups from Dubna, Bucharest and Darmstadt were tested with high intensity proton beams at the GSI SIS [3]. In particular, several prototypes with short anode-to-cathode distance (3 mm) and small anode pitch (2-4 mm) were examined. Shortening the inter-electrode distance and the anode pitch decreases the total charge to be amplified at each anode wire, thus reducing the space-charge effect around it. The pads have an area of 7.5 x 80 mm2, not optimized for the CBM conditions, where a 100 mm2 pad area is foreseen in the regions with highest track densities. Figure 1 shows the position resolution of these chambers as a function of the incoming particle rate, which reaches 100 kHz/cm2. A slight degradation of the resolution is observed at higher rates. However, possible pile-up of signals from several tracks induced on the same readout pads may contribute to this deterioration. The trigger scheme used in the experiment, where the coincidence between two 4 x 4 cm2 scintillators defines an event, did not allow for pastfuture protection. Thus, several tracks may, at high rates, hit the same pad. A first attempt at studying the rate capability dependence on gas composition and gain was performed with the data obtained during this test beam campaign. The signal drop at the highest measured rate (100 kHz/cm2) relative to low rate, for two different C0 2 concentrations in xenon, is plotted as a function of the total charge per track (the gain times the primary electron yield) in Fig. 2 [4]. Although the error bars are rather large and there are very few data points, the figure suggests a higher degradation of the counting rate capability of this particular chamber at higher gains. On the other hand, since xenon ion mobility is 4 times lower than for C0 2 + , one would expect stronger space-charge in xenon-rich mixtures. However, the exact ion composition and the magnitude of the electric fields through which ions move may complicate the picture.

49 400 E 3.

300

200

100

120 Rate (kHz/cm2) Figure 1. Average position resolution of two 6 mm thick chambers, with 2 and 4 mm anode pitch, as a function of the incoming particle rate. The resolution was extracted assuming that both detectors show equal performance.

3- 10

I

-i

9 8 7 6 5

-

1

i

|

r-

•

Xe-C0 2 [85-15]

•

Xe-C0 2 [70-30]

i

*^

i

1

«> 11

2> 4

(>

3

11

-

2 1 n

i

20

.

i

i

40

60

.

i

80

.

i

100 120 Pulse-height

Figure 2. Total signal drop at the highest measured rates as a function of the measured pulse-height at low rates, for two different xenon mixtures in a chamber with 6 mm gas gap.

In any case, a handle reflecting the charge-dependent nature of the spacecharge effect is identified. A more systematic and precise set of measurements

50

on this issue will allow us to optimize the gas choice and the gas gain against efficiency and rejection power at high rates. For the next test beam in February 2006, the experimental set up will be improved with a segmented scintillator trigger with past-future recording, chamber pad planes with area 100 mm2, and faster readout electronics. 3. Results of simulations Simulations have been carried out [5] to study the pion rejection as a function of the radiator configuration, number of detector layers, and gas composition. The chamber gap is always 6 mm. Figure 3 illustrates the pion efficiency dependence on number of layers and number of foils per radiator.

g 10 >> o c a

2it

1'

o Q.

10"'

10"2

10" 3 8

9

10

11

12

No. of layers Figure 3. Pion efficiency at 90 % electron efficiency as a function of number of detector layers, for different radiator thickness with foil parameters di = 15 \xm and d2 = 200 urn; p = 2 GeV/c.

The target value of 1 % pion efficiency is well within reach and allows for optimization of the material budget. The relatively thin gas gap of the MWPC makes it important to maximize the gas density. As shown in Fig. 4, and increase from 70% to 90% xenon contents results in a factor 3 improvement in pion efficiency. Again, this handle is to be optimized against rate capability and chamber stability. A preliminary choice of 3 TRD stations with 3 layers each has been made for further simulations.

51

§]o.16

-

»

•

o |

—*— 12 layers - 1&200/90

0.14

!T

g 0.12 CL

0.1 0.08 0.06 0.04 70

75

80

85

90

95

Xe content [%] Figure 4. Pion efficiency at 90 % electron efficiency as a function of number of detector layers, for different radiator thickness with foil parameters di = 15 nm and 62 = 200 nm; p = 2 GeV/c.

4. X-ray measurements With an X-ray tube with Copper target it is possible to make precision measurements of counting rate capability of wire chambers as a function of various parameters. After proper estimation of the size of the collimated X-ray beam and of beam intensity, the gain dependence on rate can be easily measured, and for different gases and geometries. As an example, Fig. 5 shows the gain for different voltages and gas mixtures as a function of flux of 8 keV X-rays. The gain drop is clearly visible and can be quantified by fitting to the data the formula given by Mathieson [6]: \n(GIG0)IG = KJ? where G is the gain at a given rate R, and G0 is the gain at zero rate. The parameter K thus characterizes both gas mixture and chamber geometry. As an example, lines of constant gain drop as a function of rate are drawn in the figures. These set of measurements will be extended to xenon-based mixtures and may, in addition, cast light on the abundance of different ion species in the avalanche and contribute to improving the simulations.

52 Ar-CO, [90-10] JUUUU

1%'

2%

' \ 5 % ' \ 1 0 % gain drop

50000

moon

JNjV ^ ^ T * * * ^ ^ :

2000

100

10

1000

10000 Rate (kHz/cm 2 )

Ar-CO, [70-30] •i

. 1% ' . 2 %

' . 5 % ' \ 1 0 % gain drop

50000

A

A

'***•

10000

10

100

1000

10000 Rate (kHz/cm 2 )

Figure 5. Gain drop measured for different initial gains as a function of X-ray rate. A fit to Mathieson's formula yields a parameter K which allows one to estimate the gain drop for different working conditions (dashed lines) and different gas mixtures.

5. Conclusions He have shown first steps towards a fast and large TRD for use at the high rate experiment CBM (FAIR), based on multi-wire proportional chambers. A rate capability of 100 kHz/cm, with no significant degradation in the required performance, with thin chambers with short anode pitch and a xenon-based gas mixture, seems within reach. The required pion efficiency of 1 % is shown to be achievable for various configurations by suitable simulation tools.

53

The results obtained so far indicate a number of parameters which will be tuned with further accurate measurements and simulations. For this second iteration, a reasonable staring-point scenario (9 TRD layers, high xenon contents, moderate gain, etc.) can now be configured. Acknowledgments Technical help from A. Radu and H. Junk is acknowledged, as well as all the colleagues who contributed to the results presented in this paper. Work supported by EU project JRA4 and INTAS grant number 03-54-4169. References 1. 2. 3. 4. 5. 6.

http://www, gsi.de/fair/index e.html Compressed Baryonic Matter Experiment - Technical Status Report, http://www.gsi.de/onTEAM/dokumente/public/DOC-2005-Feb-447.html GSI Scientific Report 2004, page 362. M. Petris, private communication. GSI Scientific Report 2004, page 11. E. Mathieson, Nucl. Instrum. Meth. Phys. Res. 249 (1986) 413.

R&D ON A DETECTOR FOR VERY HIGH MOMENTUM CHARGED HADRON IDENTIFICATION IN ALICE A. GALLAS INFN.Sez. Bari Bari, 70121, Italy

on behalf of the ALICE/HMPID

group.

The latest theoretical and experimental results from experiments at RHIC suggest investigating a physics domain in heavy ion collisions for pt higher than the one planned to be covered at present by the Particle Identification (PID) system of the ALICE experiment. We present here a possible upgrade of the High Momentum Particle Identification Detector (HMPID) based on the idea of the Threshold Imaging Cherenkov (TIC) detector operated for the first time by the NA44 experiment.

1. Introduction The ALICE HMPID detector, at present under construction, will identify charged hadrons of p T up to 5 GeV/c. In the following we present the principle of a detector able to identify hadrons in the range 10 to 30 GeV/c and that fits in the present ALICE layout. The available space and particle multiplicity are actually the main constraints, making practically impossible the use of a conventional RICH detector with a gaseous radiator as required to cover the momentum range of interest. The detector layout we present here is an evolution of the TIC detector already built and operated by the NA44 collaboration [1] almost 10 years ago. The TIC principle allows a compact layout, combining the easy particle identification of a Cherenkov threshold counter with the possibility of identifying the Cherenkov signal track-by-track exploiting some of the angular information supplied by the more sophisticated RICH counters. Further more, since the TIC images only photons it is suitable for experiments with several particles in the detector acceptance.

54

55

2. Detector Layout The Threshold imaging Cherenkov detector principle is based on the 2-D localization of the Cherenkov photons emitted when particles move through it with a speed exceeding c/n, thus allowing them to be separated from slower particles ('below threshold'). The TIC detector operated in the NA44 experiments was used to reject pions and isobutane was chosen as radiator medium due to its easy operation. However the optical properties of the materials employed (isobutane as radiator medium, methane as MWPC gas and fused silica window to separate the two) lead to the rather poor yield of <6> photons for 1 m radiator, still enough to achieve a rejection factor of about 100 [1]. The performance results of the HADES RICH [2] show that the replacement of isobutane with C4Fi0, which has the same refractive index, and of fused silica with CaF2 would give a much higher photon yield of about <20> photons for a 50 cm radiator. It is therefore conceivable to design a detector having an improved PID capability, able not only to reject pions but to identify charged hadrons on a track-by-track basis and over a broad momentum range, still compact such to fit in the present ALICE layout, where about 2 m are available at the bottom of the solenoid magnet, see Figure 1.

HMPID

Figure 1 Schematic cross section of the ALICE experiment. At the bottom we see the space for the new particle detector.

56

It should be noticed that the production of C4F10 has been discontinued; a replacement is therefore required in order to actually operate the detector. In the following we assume the use of C4F10 since its properties are very well known, this ease the description of the principle of the detector. The detector (Figure 2) consists of two radiators separated by a thin CaF2 window. Assuming C4Fi0 and CF4 Table 1 shows the identification capability. Above ~15 GeV/c only "weak"" identification is possible, i.e. mesons vs. baryons identification. However, this would allow the measurement of important observables in heavy ion physics like the antiprotons/protons or mesons/baryons ratios. pad chamber Csl

C a F , window

Figure 2. Schematic view of the two stages threshold imaging Cherenkov detector.

The photons are reflected by UV mirrors placed at 45° relative to the radiator axis, and enter the photon detector which is located on the sides of the second radiator vessel. Table 1. Particle identification capabilities of the proposed detector.

Momentun GeV/c 3
16
C4F10

CF4

PID

l

0

7t

l

1

n

l

1

l

0

71 K

0

0

Proton

1

1

1

0

71, K Proton

57

3. Photon detector The simplest choice for the photon detector is a MWPC with a pad cathode coated with a thin Csl layer, as in the present ALICE HMPID [3]. We are considering the possibility to build a windowless detector, i.e. running pure CF4 in the MWPC. This would lead to a simple construction, reasonable cost and a photon yield as high as <40> photons detected for a 50 cm radiator [4] due the very high transparency of CF4 up to wavelengths as high as 110 nm. However, the stable operation of a MWPC with pure CF4 in combination with Csl has to be proven and can turn out to be impossible. Aging of the anode wires are also a concern, although the expected dose in ALICE (< 1 mC/cm for 30 years of LHC operation) is well below the critical values reported in [5]. Finally scintillation of CF4 can spoil the performance of the detector but this effect has not been reported in [4]. A possible way to overcome the above problems would be a more classical design, i.e. a window to separate the radiator from the a MWPC filled with methane but this would lead to a drastic decrease of the photon yield due to the cut-off of methane in the UV region and has to be carefully evaluated by both simulation and experiment. It should be stressed that due to the very similar Cherenkov angle of C4F)0 and CF4 (~3 and 1.8 degrees) the photon spots from the two radiator will somehow overlap; it is therefore of the utmost importance to maximize the number of detected photons as well as to optimize the ratio between the lengths of the two radiators in order to separate and identify the two signals. A more sophisticated solution is the use of a multi-GEM detector with a Csl layer deposited on the first GEM foil, as proposed in [4] and [6]. The use of a GEM could lead to a more stable detector operation, in particular in a very high rate and multiplicity environment. A solid state vacuum based photon detector with a semi-transparent photocathode sensitive to visible light (like MCP) is also very attractive but further developments are required in order to allow for operation in a strong magnetic field. The capability to detect light in the visible part of the spectrum would also open the possibility to use aerogel (n =1.01) as radiator medium, greatly increasing the potential of this proposal, see Table 2.

58 Table 2. Particle identification capabilities of the detector using areogel.

Momentun GeV/c Kp<3 3
Aerogel

5
16
CF„

PID

0

0

71

1

0

1

0

0

0

7t K K

C4F10

1

1

0

0

0

0

K

0

0

Proton

1

0

K

0

0

Proton

1

1

1

0

71, K Proton

71 K

4. Summary and outlook A possible detector to extend the PID capability of the ALICE experiment at LHC for charged hadrons up to 30 GeV/c has been presented. Based on the idea of the TIC detector successfully operated by the NA44 collaboration, this evolution combines the simplicity of a threshold counter with the photon position information of the more sophisticated RICH technique, resulting in the capability to track-by-track identification in the high multiplicity environment characteristic of the heavy ion experiments. The proposed design is compact and well suited for applications at colliders and could make use of present available technologies although it would greatly benefit from further developments in the photon detector field. Simulation studies are underway to optimize the detector layout in order to match the physics requirements and assess its full performance potential. We also plan to test the behavior of MWPC operated with CF4in the presence of Csl. References 1. A. Braem et al., Nuclear Instruments and Methods A 409,426-431 (1998). 2. K. Zeitelhack et al., Nuclear Instruments and Methods A 433, 201-206 (1999)

59

3. Alice Collaboration, HMPID TDR, CERN/LHCC 98-19. F. Piuz et al., Nuclear Instruments and Methods A 433, 222 (1999). 4. A. Kozlov et al. Nuclear Instruments and Methods A 523 345-354 (2004). 5. J. Wise et al. J. Appl. Phys. 74 No 9 5327-5340 (1993). 6. Thomas Meinschad et al., Nuclear Instruments and Methods A 535, 324-329 (2004).

T H E D E S I G N A N D T E S T OF T H E ATLAS D I A M O N D BEAM CONDITIONS MONITOR

A. GORISEK * t CERN, Switzerland E-mail: [email protected]

Diamond's radiation hardness, low capacitance and charge collection time make it an ideal choice for very high speed environments. Because of these properties the ATLAS experiment has chosen to use diamond for its Beam Conditions Monitor (BCM) the innermost part of the ATLAS detector which will begin being installed in the Spring 2006. Results from final prototype ATLAS-BCM modules and the first production modules are presented. These use polycrystalline Chemical Vapor Deposition (pCVD) diamond sensor material, and a 2 ns shaping time readout electronics and show the advantages of diamond in a very fast signal environment.

1. I n t r o d u c t i o n In ATLAS a dedicated system, ATLAS Beam Conditions Monitor (BCM),

is being developed which will detect anomalous beam events that would cause background (for example beam gas interactions) to the normal events of colliding protons or even more drastically cause a danger to ATLAS spectrometer (for example several proton bunches hitting the beam collimator). The radiation hard BCM detectors will be placed at ±z on both sides of the interaction point along the beam-pipe. Common to both types of background mentioned is that they mostly happen well up or downstream of the interaction point. That would give them a unique signature of causing two signals spaced in time for At=2z/c, the second signal being simultaneous with regular interaction events which in LHC happen every 25ns. Optimal z position for distinguishing between these two classes of events would thus be At = 12.5 ns or 2z=12.5ns c=3.75m and z-1.875m. More ambitiously the ATLAS BCM would be used also for interaction rate measurement and luminosity estimate *, where it would be used as a *on behalf of ATLAS BCM Group ton leave from J. Stefan Institute, Slovenia

60

61 complementary system with ATLAS main luminosity monitor LUCID 2 . 2. Design The design requirements for ATLAS BCM sensors and electronics are determined by the hostile radiation environment and by the rate of interactions in LHC. The detectors have to meet stringent requirements to withstand the radiation field up to 10 15 /cm 2 particles, mostly pions, in 10 years of operation. The proton bunch interactions are spaced for 25 ns, thus for distinguishing the real interaction events and background events the detectors have to be able to cope with rates up to « 100 MHz. This makes hard requirements on the timing properties of the BCM detectors, namely fast rise time ( « Ins), narrow width ( « 3ns) and fast baseline restoration ( « 10ns). Unlikely event of a single proton hitting the TAS collimator would result in showers with particle fluxes at ATLAS BCM sensors of about l c m - 1 3 . The fluxes caused by interactions of proton bunches will be somehow smaller, thus expecting on average less than 1 minimum ionizing particle crossing the BCM detector per interaction. For being able to estimate the luminosity as well as for early detection of incidents it is important to be able to measure single minimum ionizing particles. ATLAS BCM will be mounted inside the ATLAS Pixel detector Support Tube (PST) with inner most part of diamond sensors at r = 55 mm (ss 20 mm from the beam pipe) and at ±z = 184mm. On each side of the interaction point there will be four detectors at tp = 0°, 90°, 180° and 270° mounted at 45° towards the beam-pipe.

Figure 1. pCVD diamond sensor as used in ATLAS BCM (left). Top view of one of ATLAS BCM modules with electronics compartment sealed for electric shielding and detector compartment still open (right). Two pCVD diamond sensors mounted on top of each other are visible. The rear part of the detector box houses two stage fast amplifier.

62

2.1. pCVD

Diamond

Sensors

The sensors used in ATLAS BCM were developed by RD42 collaboration and produced in Element Six Ltd. 5 . They are lcraxlcm poly-crystalline carbon vapor deposition diamonds of thickness approximately 500yum. Both sides of sensors have 8x8 mm 2 proprietary metal contacts made at Ohio State University (see Fig. 1). The sensors were tested up to 2.2xl0 1 5 p/cm 2 fluence with signal degradation of only 15% 6 . Timing properties of the signal formed are determined by high bias voltage of ±1000V (2V//xm electric field) and short charge collection times in pCVD material. Another very important property of diamond sensors is very low leakage current (of order of nA in our typical sensor at ±1000 V), which allows a simpler detector design without active cooling. In the detector box two sensors are glued back to back, with outer contacts at high voltage and signal taken from the middle (see 1). 4

2.2.

Electronics

Figure 2.

The rear part of the detector box houses two stage fast amplifier.

Front end electronics is mounted in the far side of the detector box to decrease the radiation field by «30%. The signal is transmitted to the two stage RF amplifier developed by Fotec 7 which is based on standard components, 500 MHz Agilent MGA-62563 GaAs MMIC low noise amplifier for the first stage and Mini Circuits 1GHz Gali 52 InGaP HBT amplifier for the second stage (see Fig. 2). The two amplifiers were tested for radiation hardness. The most crucial first stage Agilent amplifier was irradiated with a combined fluence of 5xl0 1 4 n/cm 2 and 5xl0 1 4 p/cm 2 . After irradiation signals from a detector show loss of gain of «20% with no significant increase in noise. Further processing of the signals will be done after « 14 m of coaxial

63

cable at a region of ATLAS spectrometer where a radiation tolerant electronics can be used. A suitable candidate is NINO, a time-over-threshold ASIC developed by MIC at CERN 8 . 3. R e s u l t s Measurements presented were obtained in various stages of development of prototype detectors and the final detector boxes. They were tested with low energy protons (125 MeV and 200 MeV) at MGH Boston, high energy pions in SPS at CERN and in a on-the-bench setup with electrons from 90 Sr source. The low energy protons caused signals equivalent to 2.3 minimumionizing-particle (MIP) signals. Performance of single diamond detector was compared to performance of the detector equipped with two diamond sensors (see Fig. 3). The double detectors show double signal while the increase in noise is only 30%. It was also confirmed that inclining the detectors for 45° increases the signal by a factor of \/2 without having any effect on noise. 400 300 200 100

0

40

SO U [mV]

Figure 3. Comparison of amplitude distribution obtained with detector with double diamond sensor (left) and single diamond sensor (right). In the left plot a peak with half of the signal is clearly visible, corresponding to events where particle went only trough one of the sensors.

In the high energy pion beam the detector response to a single MIPs was studied. Typical signals have signal and noise distributions that give « 7.5 for most probable value of signal to noise distribution. Here signal to noise distribution was obtained by dividing the amplitude values by the RMS of the fluctuation of the baseline, where no signal was expected. It was also observed that limiting the signal to 200 MHz from original 500 MHz results in better signal to noise values which increases for sa 20% (see Fig. 4). This was confirmed by applying 1st order filters offline to the data taken at full bandwidth (see Fig. 6). Typical timing resolution can be estimated from the time difference distribution for simultaneous events for two different detectors (see Fig. 5). The width of such distribution is about 1 ns which

64

is more than sufficient and doesn't change for more than 10% with limiting the bandwidth to 200 MHz. 160

-_ :

A

-*i"l"T'T"

120

801

400 :

40 16

20 U [mV]

OE-

2

4 U [mV]

Figure 4. Distribution of pulse amplitudes with most probable value of 2.72 mV (left) and distribution of baseline fluctuation, noise, with RMS=0.286 mV (right), with SNRf»9.5. The data were taken at bandwidth of 200 MHz.

Figure 5. Distribution of time difference of two different diamond detectors in SPS beam with RMS of m 1.1 ns.

| signals | 0.004 0.003 0.002 0.001

^-*-—*

\ y*^^

x r

0 0

.....................

200 400 600 800 1000

HV(V)

0

200 400 600 800 1000

HV(V)

0

200 400 600 800 1000

HV(V)

Figure 6. Amplitude (left), noise (middle) and signal to noise ratio as a function of 1st order low pass filter frequency applied to original signal recorded at 1GHz .

Further measurements were done using a 37 MBq 90 Sr source. A setup was developed which will be used for the reception tests of the final detectors. Typical distribution of signals and noise obtained at limited bandwidth to 200 MHz is shown in Fig. 7.

65 MPV= 0.002784±0.000004

Sigma= 0.0003281 ±0.0000001

4000

0.002

0.004

0.006

0.008 0.01 amplitude [V]

3.003 -0.002 -0.001

0.001

0.002 0.003 noise [V]

Figure 7. Signal distribution with most probably value of 2.8 mV (left) and noise peak with a = 0.3 mV (right). 200 MHz bandwidth limit was applied. 4.

Conclusion

Detectors built from p C V D diamond sensors and fast R F current amplifiers were developed and tested for ATLAS Beam Conditions Monitor. T h e final detectors are to be installed in ATLAS in 2006.

References 1. S. Ask, Simulation of Luminosity Monitoring in ATLAS, CERN-ATL-COMLUM-2005-002, CERN, September 2005. 2. The ATLAS Luminosity Measurement Letter of Intent, CERN/LHCC/2004010, CERN, March 2004. 3. M. Huhtinen, Possible consequences of LHC beam losses for CMS, talk at LHC Machine Protection WG: 24 October 2003, private communication, [email protected]. 4. CERN RD-42 Collaboration: CVD Diamond Radiation Detector Development, http://rd42.web.cern.ch/RD42/. 5. Element Six Ltd., King's Ride Park, Ascot, Berkshire SL5 8BP, UK 6. W. Adam et al., The development of diamond tracking detectors for the LHC, Nucl. Instr. Meth. A, Vol. 514, November 2003, p. 79-86. 7. FOTEC, Viktor Kaplan Str. 2, A-2700 Wr. Neustadt, Austria 8. F. Anghinolfi, et al., Nucl. Instr. and Meth. A533 (2004).

T H E AMS02 T R A N S I T I O N RADIATION D E T E C T O R (TRD) - A GASFILLED D E T E C T O R FOR T H E I N T E R N A T I O N A L SPACE STATION

J. HATTENBACH /. Physikalisches Institut IB, Sommerfeldstr. 14, Turm 28, 52074 Aachen. E-mail: hattenbach@physik. rwth- aachen. de

On behalf of the AMS02-collaboration. The AMS02-Experiment (Alpha Magnetic Spectrometer) is a particle detector designed to operate on board the International Space Station (ISS) for at least three years [1]. The physics goals of this experiment is to perform a precise and longterm measurement of the primary cosmic radiation and search for exotic particle candidates, such as SUSY-WIMPS - the most promising candidates for Cold Dark Matter. One of the subsystems of AMS02 is a gasfilled Transition Radiation Detector (TRD) which will be used for particle separation at high energy ranges.

1. Introduction The AMS02-TRD is used to separate light from heavy particles eg. positrons from protons. To perform precise positron spectroscopy at an energy range up to 300 GeV, a proton rejection factor of > 106 is needed due to the flux-ratio of protons to positrons of 104 in the primary cosmic radiation. This rejection factor will be accomplished by a combination of an electromagnetic calorimeter and the TRD. 2. The T R D The TRD comprises 20 layers of a radiator fleece-material, in which the transition radiation is produced, combined with gasfilled proportional counters. The 20 layers are held and positioned by an octagon-shaped supportstructure, which provides a mechanical accuracy of 100 fim. The gas-supply system consists of two separate vessels for the two components of the counter-gas. They contain 46 kg Xenon at 100 bar and 4 kg CO2 at 65 bar, which will be mixed in a separate mixing vessel at a ratio of 80% to 20%. 66

67

The gas flow in the TRD-volume is kept atll/h [2]. The TRD is divided into 41 separate gas circuits of 8 modules each. For an overview of the TRD readout electronics, which is divided into a front-end part, located directly at the readout side of the modules, and a crate-electronics part, see [3], [4]. 2.1. Straw-tube

chambers

The TRD consists of 328 straw-modules, which comprise 16 straw-tubes each. Polycarbonate endpieces on both sides serve as gas manifolds and readout devices. Mechanical stability is provided by transversa] and longitudinal carbon-fiber-composite stiffeners. A single straw-tube has an inner diameter of 6mm, while the tube wall is only 72 jj,m thick.

Figure 1. A prototype straw-tube module.

CJXXJXXtXtXXtXXi) 6 longitudinal stiffeners

Strips across every 10 cm

Module lengths between 86 cm and 201 cm

Figure 2. Cross-section of a straw-tube module.

To form one tube, two layers of kapton material of 25 (j,m thickness are glued by polyurethane. A very thin layer of aluminium provides electrical conductivity, while a carbon-layer finishes the wall on both sides. The anode wire (diameter: 30 fim) in the center of each straw is made of goldplated tungsten. Since in space vacuum is on the outside of the tube walls, gastightness is an important issue. Even in the absence of any macroscopic leak, gas-losses by diffusion are inevitable. Therefore, leak-rates of every single straw-module used in the TRD have been measured, using pure CO2 [5]. Taking into account the amount of CO2 and the gas tightness of the flight straw modules, the TRD could be operated for more than 20 years on the ISS.

68

To accomplish the goal of proton rejection, gasgain inhomogenities for the TRD should be kept below 3%. Measurements with test-modules indicate that such an increase in gasgain is produced by a mechanical wire displacement of 250 um [6]. To verify that the inhomogenities due to wire displacements are well below this threshold, all 328 TRD modules have been tested by measuring gasgain at 10 positions along the chamber, for all 16 straws. The modules were mounted on a granite table which has a surface precision of < 5 \im. The primary ionisation was induced by a Fe 55 radioactive source. The results show a mean RMS of gasgain for all flight modules of 0.6%, which corresponds to a mechanical wire displacement of 100/OTI [6].

RMS [%]

(a)

(b)

Figure 3. (a): The TRD-support-structure with 17 layers of straw-modules integrated, (b): Measured gasgain RMS of all modules, after production (blue) and after insertion into the support-structure (red).

2.2. Support

structure

The support structure of the TRD has a conical-octagon shape and consists of CFC-skinned aluminium-honeycomb walls with 30 mm thickness (see fig. 3a). The inner 12 layers are oriented perpendicular to the upper 4 and lower 4 layers, so the TRD can provide 3D-tracking information of a traversing particle. To verify the mechanical precision of the support structure, the vertical slit heights of all 1312 slits have been measured. Results show that the deviation of some of the slit heights is more than 100 /zm, which makes shimming of the modules inserted at these positions necessary. After inserting the modules and shimming the critical slits, gasgain measurements

69

were performed with each module in the same way as after production. There is no significant increase in gasgain RMS after integration on the modules into the support structure (see fig. 3b). 3. Performance and Space-Qualification Space qualification tests include vibration tests to simulate a space shuttle launch and thermal vacuum tests, as well as electromagnetic interference and corona tests. Gasgain and gastightness of the straw modules has been monitored during these tests. 3.1. Vibration

test

For the vibration test, a test jigg with 4 modules with a maximum length of 659 mm was used. The test was performed on a vibration table, test cycles include a sine sweep (0.5 g) from 10 to 2000 Hz to determine the first eigenfrequency of the structure, a random spectrum with a mean acceleration of 6.8 g to simulate launch or landing with a space shuttle, and a second sine sweep to verify the first eigenfrequency has not changed after performing the random spectrum. The test results show no such change [7]. The measured first eigenfrequency of a module (length: 40 cm) is f0 = 128.5 Hz. A Comparison of this result with a FEC a coupled load modal analysis show good areement. 3.2. Thermal-Vacuum

test

For the thermal-vacuum test, the same 4-layer test jigg has been inserted into a thermal-vacuum tank at the MPE b . During 70 hours, 9 thermocycles have been performed, with temperatures ranging from —40°C to +60°C. The test jigg was kept in vacuum at 1 0 - 6 mbar. After thermal cycling, the 4 modules of the jigg were tested again for gastightness and gasgain, with no significant changes in both values [7]. 3.3. Beamtest

at

CERN

A 20-layer TRD prototype was tested in the X7 SPS-beam at CERN in 2000, using 3 • 106 recorded events of Elecrons, Muons, Pions and Protons. The proton rejection factor, determined by a neural network analysis, is well above the desired 102 for energies up to 300 GeV [8]. "Finite Elements Calculation b Max-Planck-Institut fur Extraterrestrische Physik, Munchen, Germany

70 S10

4

AMS TRD Prototype

•

X7 Beamtest

o

@ 90 % Electron Efficiency

(0

u_ c o •510 3 o "5T cc c o 2102 Q.

combined NN

f f

f

±

min. required p+ R e j e c t i o n

50

100

150

200 250 EBeam/GeV

Figure 4. AMS02 TRD proton rejection, from [8]. 4. Conclusions The AMS02-TRD has passed all critical qualification tests. Integration of the 328 straw-tube modules into the support structure has been finished, and further integration of the detector is ongoing. A 20-layer prototype has performed as expected in a test beam. Pull integration of the AMS02experiment is scheduled to be finished in 2007. References 1. The AMS-Collaboration. AMS on ISS: Construction of a particle physics detector for the International Space Startion. Nucl. Instr. and Meth. A (to be published). 2. A. Bartoloni, 9th Pisa Meeting On Advanced Detectors: Frontier Detectors For Frontier Physics (2003), La Biodola, Italy. 3. Th. Kirn, Th. Siedenburg, Nucl. Instr. and Meth. A 535 (2004), 165-170 4. Th. Kirn et al., Frascati Physics Series Vol. XXV (2001), 161-172 5. Th. Kirn, Nucl. Instr. and Meth. A 522 (2004), 69-72 6. J. Hattenbach, Diploma Thesis at Aachen Physics I institute (2004). 7. S. Fopp, Dissertation at Aachen Physics I institute Sz 8540 (2004). 8. Th. Siedenburg, Performance of the AMS-02 Transition Radiation Detector. Nucl. Instr. and Meth. A (submitted) (2005).

A N E W A U T O M A T I C MICROSCOPE FOR HIGH S P E E D ANALYSIS OF N U C L E A R EMULSIONS

M. IEVA INFN, 70126 Bari, ITALY E-mail: Michela.Ieva@ba. infn. it C. P I S T I L L O Universita

& INFN E-mail:

Napoli, 80126 Napoli, [email protected]

ITALY

Nuclear emulsion plates record tracks of charged particles with sub-micron accuracy. The development of fully automatized scanning systems during the last two decades has made possible the use of large mass nuclear emulsion devices. The European Scanning System (ESS), developed in the framework of an R&D project for the Opera experiment, is able to scan emulsions with a 20 cm2/h speed. This speed is one order of magnitude faster than in past systems. We have measured for this system a tracking efficiency above 85% over the angular range [0,600] mrad for minimum ionizing particle crossing 43 micron thick emulsion layers. Correspondingly we have measured an instrumental background of the order of 1 track/cm2 over the above mentioned angular range.

1. Introduction The production techniques and employment of nuclear emulsions1 have been developed, mainly, during the second part of the last century, but still nowadays, this kind of detector is largely used in the nuclear and particles physics thanks to very high spatial accuracy (< 1/xm) that is possible to obtain. Obviously the final use and especially the information extraction techniques have been improved keeping up with hardware and technological progress. The employment of nuclear emulsion in large scale experiments, has been made possible thanks to the development of fully automatized scanning system. OPERA 2 will play a fundamental role in the environment of long baseline experiments to observe an oscillation signal. The conclusive test to conferme uM —• vr oscillation will be the appearance of vT in the pure CNGS CERN i>n beam. To be sensitive in the parameter range suggested

71

72

by atmospheric neutrino experiments, the OPERA detector is located in the INFN Gran Sasso Underground Laboratories at ~ 730fcm from beam source. The r leptons are identified through the direct observation of their different decay topology in electron, muons and single charged hadron. This is possible using the Emulsion Cloud Chamber(ECC), a sandwich of dense passive material (Pb) plates, acting as target, and the emulsion sheets that offer needed high precision tracking. 2. The European emulsion Scanning System The European Scanning System (ESS) have been developed in a collaboration involving the European groups belonging to the OPERA experiment. The R&D started in 2001, when the first microscope prototype was assembled using the custom developed hardware available at that time. After several upgrades the system has reached its designed performance in 2004. Unlike the scanning systems developed in the past, the ESS is exclusively based on the use of commercial hardware components some developed in collaboration with specialized companies. This provide the flexibility needed to upgrade the system following the technological progress. Indeed, during the three past years, thanks to the use of CMOS camera and parallel processing techniques, the scanning speed of system was increased from ~ 1.0cm2/h to ~ 20cm2/h. The ESS design, in fact, had to take into account the request of high scanning speed (waited rate of v^ daily interactions is ~ 30 this means ~ 2000 emulsion sheets to be scanned each day) while keeping the extremely good accuracy provided by nuclear emulsions. To reach this goal, the ESS hardware has been conceived with some distinctive features. 2.1. Hardware

set up

One of the ESS system presently operating is shown in figure 1. A motor driven scanning table for horizontal (XY) movements and a granite arm for vertical (Z) displacement, are fixed to high quality table, which provides a virtually rigid and vibration-free working surface, holding the components in a fixed position. High-performance mechanics are employed for both horizontal and vertical movements with sub-micron accuracy and very small settling time. Stages are equipped with nano-stepping motors3, and linear encoders feeda

Vexta, 5PH stepper.

73

Figure 1. View of Bari (Italy) OPERA emulsion lab. microscope. Stage dimension out of the table base (1 x d x h): ~ 35cm x ~ 40cm x ~ 50cm (d = ~ 80cm with the granite arm)

back b The use of micro-stepping units allow the extremely (~ 0.1/xm) precise position control. The movements in horizontal plane allow measuring of different zones of a sheet, while changing the focal depth of the microscope, make possible to see different layers. The optics equipment consists of an illumination and lens system located below the stage table, a revolver with several objectives having different magnification power and a CMOS camera 0 , both placed on the tip of a granite arm. The field of view achievable is 395x315 fim and the camera sensor with mega-pixel resolution yields about 0.3x0.3 /xm of emulsion surface grabbed per pixel. Movements and working of microscope are completely controlled by a work station, double processor xeon 3.0 GHz bus pcix, equipped with both motion controller11 and vision processor6 boards. The high camera framerate, up to 500 frames/s at full resolution, and the use of the powerful image processor allow the image handling at the requested speed. The emulsion support is provided with a vacuum system to avoid unwanted sheets movements during the data taking.

b

MS-8-ZSS-LS010 at 8" x 8" for horizontal axes e MT-85 2"-ZSS42-l-Lie4 for vertical movements. c Mikrotron MC1310 (1.3 MPixel). d National Instruments, Flex Motion pci7344. e Matrox Odyssey X PRO.

74

2.2. Mechanics

system

calibration

In order to estimate the best accuracy achievable with the microscope mechanics, manual precision measurements f of single grain position have been realized using a mouse pointer square to cut a zone of ~ 2/im2 around grain. Coordinates (X,Y) of pointed grain are obtained calculating the corresponding position of gray levels center of gravity. Figure 2 shows results of coordinates measure moving grain position.

I

y precision down

I

h y Nent = 61 Mean s 220.5 RMS s 0.3143 Constant a 14.46 Mean = 220.S Sigma = 0.2726

1412-

I 216

Figure 2.

216

217

216

219

220

221

222

Repeated measure of grain coordinates moving stage position.

As shown, the resolution is almost <JX,Y — 0.2 /j,m and the same measure for Z axis produces az — 0.7 \im. Resolution on XY axes essentially depends on the camera resolution, while Z precision comes from microscope focal depth, which is almost 2 fim. The resulting tracks angular resolution can be described by the formula:

°{tix,y) =
(1) (2)

w h e r e i9 is t r a c k s l o p e .

f

For high precision measurements have been used a Hitachi KP-F110 with a CCD 1 Megapixel (1024x 1024) at 30 frame/s camera and an OB.CFI PLAN.ACR.50XOIL AN 0.9 oil immersion objective.

75

3. Emulsion analysis and ESS performances Like carefully described in ref.3, the emulsion analysis is realized moving the focal plane in order to span over the emulsion thickness meanwhile the vision processor is grabbing tomographic images at different layers. The first step of analysis consist of recognition of positions of the black grains leaved by the charged particles passage. Then, the particles trajectory are reconstructed, in each single emulsion plate (mikrotracks), by combining grains from images at different levels and recognizing their geometrical alignment. The last step is the reconstruction of trajectory through the entire volume of a brick. In order to track completely the charged particles it needs to calculate the roto-translations parameters between following emulsion sheets and then search the aligned sequences of single-plate tracks (basetracks). The evaluation of tracking efficiency is based on the reconstruction of tracks passing through a sequence of emulsion sheets: the fraction of found holes (basetracks leaking) during the track reconstruction gives an estimate of tracking inefficiency. Several tests have been performed at CERN PS using high intensity pions beams. The measured tracking efficiency as a function of track angle is shown in figure 3.

•

* 4 •

I

11 i

••

t i 1]

;

K

r t

*

L "H [

°!{rad)

Figure 3. Tracking efficiency as a function of track slope. The efficiency ranges from 94% (vertical tracks) to 85% (at ~ 400 mrad). The instrumental background, due to random alignments of fog grains, is evaluated to be of the order of 1 track/cm2. The ESS performances fulfills the OPERA requirements. About 20 ESS

76 with t h e same scanning performances have been assembled into the several laboratories belonging to the O P E R A collaboration. References 1. W.H. Barkas, Nuclear Research Emulsion, Academic Press Inc. (London) Ltd. (1963). Powell, Power and Perkins, The Study of Elementary Particles by the Photographic Method, Pergamon Press (1959). 2. The OPERA Collaboration, An appearance experiment to search for Vp, —> vr oscillation in the CNGS beam, CERN/SPSC 2000-028. 3. N. Armenise et al, High-speed particle tracking in nuclear emulsion by last generation automatic microscopes, acceted for publication on Nucl. Instr. Meth. A(2005).

A NOVEL T Y P E OF P R O X I M I T Y FOCUSING R I C H C O U N T E R W I T H MULTIPLE R E F R A C T I V E I N D E X AEROGEL R A D I A T O R

P. K R I Z A N Faculty

of Mathematics and Physics, University of Ljubljana J. Stefan Institute, Ljubljana, Slovenia E-mail: [email protected]

and

(FOR THE BELLE AEROGEL RICH GROUP) A proximity focusing ring imaging Cherenkov detector, with the radiator consisting of two or more aerogel layers of different refractive indices, has been tested in 1-4 GeV/c pion beams at KEK. Essentially, a multiple refractive index aerogel radiator allows for an increase in Cherenkov photon yield on account of the increase in overall radiator thickness, while avoiding the simultaneous degradation in single photon angular resolution associated with the increased uncertainty of the emission point. With the refractive index of consecutive layers suitably increasing in the downstream direction, one may achieve overlapping of the Cherenkov rings from a single charged particle. The impact of this improvement on the 7r/K separation at the upgraded Belle detector is discussed.

1. RICH with multiple refractive index radiator Proximity focusing ring imaging Cherenkov (RICH) counters with nongaseous radiators have the advantage of smaller size and are thus well suited as particle identification devices within more complex particle spectrometers with tight space constraints. Such a detector with aerogel as radiator is envisaged for the upgrade * of the particle identification system of the Belle spectrometer at the KEKB collider 2 ' 3 . In the present Belle detector, a threshold-type Cherenkov detector (ACC) 4 enables a good n/K separation. However, ACC does not provide sufficient separation for high momentum particles around 4 GeV/c in the forward end-cap region, and a proximity focusing RICH with aerogel as radiator is being studied as a candidate for the detector upgrade. Different aerogel radiators as well as different photon detectors have been investigated in search of an optimal counter, which would provide the required 4-5 a pion-kaon separation in the 1-4 GeV/c momentum range 5 . 77

78

Good velocity resolution of a charged particle track requires good resolution of the Cherenkov angle of a single photon and many detected photons. The number of photons may be increased by increasing the radiator thickness, but the price to pay is an increased uncertainty of the emission point, i.e. single photon angular resolution. The key issue in the performance of a proximity focusing RICH counter is to improve the Cherenkov angle resolution per track <7track = o-g/^Np.e,. As can be seen from Fig. 1, the optimal thickness should be around 20 mm, which was also verified in previous investigations 5 . However, this limitation can be overcome in a proximity focusing RICH by using a non-homogenuous radiator. By judiciously choosing the refractive indices of consecutive aerogel radiator layers, one may achieve overlapping of the corresponding Cherenkov rings on the photon detector (Fig. 2). This represents a sort of focusing of the photons within the radiator, and eliminates or at least considerably reduces the spread due to emission point uncertainty. Another possibility is the opposite, i.e. to obtain separate rings from separate aerogel layers in which the emission point uncertainty is given by the thickness of the corresponding layer. One may also try combinations of the two main schemes. Note that such a tuning of refractive index for individual layers is only possible with aerogel, which may be produced with any desired refractive index in the range 1.006-1.06.The dual radiator combination can readily be extended to employ more than two aerogel radiators. In this case, the indices of aerogels should gradually increase from the upstream to the downstream layer. While the principle and the beam test measurements were presented in some detail in 6 , the present report reviews the main features of the focusing configuration of radiators.

2. Measurements and results The tests of the technique were carried out at the KEK-PS 7r2 and T l beam lines, where pions with momenta up to 4 GeV/c are available. The experimental set-up was basically the same as in the previous beam test 5 . The counter is composed of one or more layers of aerogel radiator and a photon-detection plane, parallel to the radiator face at a distance of 20 cm. Multi-anode PMTs (Hamamatsu H8500) were used as photo-detectors. A total of 16 PMTs were positioned in a 4 x 4 array and aligned at a 52.5 mm pitch. The surface of each PMT is divided into 64 (8 x 8) channels with 6.0 mm x 6.0 mm pixel size. As radiators aerogel samples with refractive

79

n=1.05\-4S LS

12

— n=1.05 A -}0 —

n=1.05 A »20

I 9 8 7

6

0

~Id

20

30

40

JO

Thickness [mm] Figure 1. The variation of the figure of merit, <Je/y/Np.e., with radiator thickness in the case of a single radiator, proximity focusing RICH, for three different transmission lengths at 400 nm (A in mm).

Figure 2. Proximity focusing RICH with a nonhomogeneous aerogel radiator in the focusing configuration.

indices between 1.01 and 1.05 were used; the transmission length of the samples at 400 nm (A) was around 30 — 40 mm. In Fig. 3, we compare the data for two 4 cm thick radiators; one with aerogel tiles of equal refractive index (n = 1.046), the other with the focusing arrangement (nupstr = 1.046, n^nsir = 1-056). The improvement is clearly visible. The single photon resolution a$ = 14.3 mrad for the dual radiator is considerably smaller than the corresponding value for the single

80 sooo

.

1095. / 116

XVndf PI P2 P3 P4 PS

A

\ 1 1 \

72S9

X*/ndf PI P2 P3

6000

'

0.3074 0.1428E-01 74.49 834.4

P4 P5

4000

2467. / 116 i 5495. U 0.2965 \ 0.2O72E-O1 85.32 J \ 1 1 796.0

4000

nf= 10.27 2000

nb= 1.05

1 J

0

0.1

0.2

itf= 10.64

I

nb= 1.36

\

2000

1

0.3

i

0.4

0.5

ejmd)

0

0.1

0.2

0.3

0.4

O.S

ejrad)

Figure 3. The accumulated distribution of Cherenkov photon hits depending on the corresponding Cherenkov angle for a 4cm homogeneous radiator (right) and a focusing configuration with n-upstr = 1.046, n^nstr = 1.056 (left).

radiator (ag — 20.7 mrad), while the number of detected photons is the same in both cases. In order to evaluate the performance of a multiple radiator RICH in the focusing combination, we tested a dual radiator configuration with indices of 1.046 and 1.051, and a triple radiator configuration with indices of 1.046, 1.051 and 1.056. The thickness of each radiator was 10 mm, such that the dual radiator is 20 mm thick, and the triple radiator is 30 mm thick. The performance of these set-ups is compared with the single radiator combination with n — 1.046 and radiator thickness of 10 mm, 20 mm and 30 mm. Figure 4 shows ag, Np.e. and crtrack distributions against the thickness of the radiators. Np.e. increases as the radiator becomes thicker, and there is no significant difference of Np.e. between single and multiple radiator combinations. On the other hand, ag of the single radiator RICH increases considerably as the radiator gets thicker, but ag for the multiple radiator case increases only slightly. As a result, crtrack is improved by introducing the multiple radiator combination, and the triple radiator with 30 mm gives the best <7track of 4.5 mrad. 3. Conclusion We have studied dual and multiple radiator combinations of the proximity focusing aerogel RICH detector. We confirmed that the number of detected photons could be increased compared to the single radiator RICH without deteriorating the single photon angular resolution. In the triple radiator configuration we have achieved a Cherenkov angle resolution per track of ctrack = 4.5 mrad, which corresponds to a 5.1cr K/ir separation at 4 GeV/c.

81

20

D single radiator • multi-radiator

L (a) •

D

15

•

:

•

• i

10 15 10 5

20 30 Thickness [mm]

: (b)

40

D single radiator • multi-radiator

f

B

'"

•

i

i D I

I

I

I

10

i

20 30 Thickness [mm]

40

• single radiator • multi-radiator

(C)

• 10

•

n

20 30 Thickness [mm]

40

Figure 4. Single photon resolution (a), number of detected photons (b) and single track resolution (c) for the focusing multiple radiator RICH combinations. The multiple radiator with a 20 mm thick radiator is a dual radiator, while that with 30 mm is a triple radiator.

References 1. K. Abe et al. (edited by S. Hashimoto, M. Hazumi, J. Haba, J. W. Flanagan and Y. Ohnishi), "Letter of Intent for KEK Super B Factory", KEK report 2004-04, http://belle.kek.jp/superb/. 2. A. Abashian et al. (Belle Collaboration), Nucl. Instr. and Meth. A 4 7 9 (2002) 117. 3. S. Kurokawa and E. Kikutani, Nucl. Instr. and Meth., A499, 1 (2003), and other papers included in this Volume. 4. T. Iijima et a l , Nucl. Instr. and Meth. A 4 5 3 (2000) 321. 5. T. Matsumoto et al., Nucl. Instr. and Meth. A521 (2004) 367. 6. T. Iijima, S. Korpar et al., Nucl. Instr. and Meth. A548 (2005) 383.

A SUBMINIATURE SCINTILLATION DETECTOR FOR CATHETER OPERATION R.SCAFE *, L.MONTANI. N.BURGIO, G.IURLARO, A.SANTAGATA, C.CIAVOLA Casaccia Research Centre, ENEA 00060 S.Maria di Galeria, Rome, Italy G.ALONGE Portici Research Centre, ENEA Via Vecchio Macello, 80055 Portici, Naples, Italy

The feasibility of a subminiature scintillation detector to be inserted in a catheter for lesion localization in nuclear medicine SPECT has been studied. Measurements on a simple laboratory setup have been performed and compared with Monte Carlo results. Further simulations, at 30keV and 140keV, concerning a configuration reproducing severe clinical conditions have shown poor lesion detectability. Several factors affecting the response have to be investigated to improve the capability of lesion localization characterizing such detector.

1. Introduction The need for radiation detectors able to localize vascular lesions has been recently introduced [1-3]. This work is aimed to study the feasibility of a subminiature scintillation detector to be inserted in a few millimeters catheter for nuclear medicine diagnostic and laparoscopic surgery. The final goal is to make a small gamma-ray detector, based on a scintillation crystal readout by an APD, having cross-dimensions suitable for insertion into few-millimeters catheters [4], To this aim the capability of such a device to localize a small uptaking lesion in a homogeneous background has been investigated. This work reports both preliminary experimental results and Monte Carlo simulations. 2. Equipment and Method Measurements have been performed using a laboratory setup (Figure 1) including a CsI(Tl) crystal (by Hilger Crystals, UK) and a Si-APD mod.S866455 (by Hamamatsu Photonics, JP) [5] coupled to a low-noise conventional * Corresponding author: [email protected]; Ph: +3906-3048-3434; Fax: +3906-3048-3989

82

83

Figure 1 The laboratory setup. Left: the detector assembly with the whitened crystal coupled to the APD on a BNC plug. Right: the detector in a black box coupled to the preamplifier and inserted into the Lucite blocks. Two source's trajectories are depicted.

electronics described elsewhere [6]. Crystal size is 2.85x2.85x5.00mm3 and APD active area is 5.0x5.0mm2. Despite its large overall dimension and dead zone surface (74%) the S8664-55 APD model has been chosen because of its thin ceramic package and small optical window thickness (0.45mm). Our APD choice has been driven by the confidence that in the near future new models, having the same advantages but smaller active areas and reduced dead zones, will be available. In order to improve light collection, the crystal has been wrapped in a white carbon polymeric film on its five blind surfaces. Two 57Co lmm diameter spherical sources, emitting at 122keV (89.0%) and 136keV (11.0%), well simulating low-energy nuclear medicine conditions, have been used for lesion and background representation (0.18MBq and 1.52MBq activities respectively). The background source has been positioned at three distances from the detector along its axis. Lucite blocks produce a scattering effect simulating the tissues diffusion (Figure 1, right). Count rates have been measured, at room temperature, varying: (a) the Lesion to Detector Distance (LDD), (b) the curve radius of lesion-source trajectory and (c) the geometry of the background irradiation. Monte Carlo Nuclear Particles (MCNP) transport simulations [7] have been carried out at 57Co energies to reproduce the experimental setup. The energy deposited in the CsI(Tl) crystal at photoelectric window (±10%), that is proportional to the detector signal, has been evaluated at several LDD along different trajectories. To asses the limits of the technique, further simulations have been performed at 30keV and 140keV photon energies. The responses to different radionuclides in conditions reproducing a very severe clinical situation have been foreseen. The geometry (Figure 2) consisted of a duct positioned at the center of a homogeneous cylindrical background-source (40cm diameter,

84

30cm height) adjacent to a homogeneous lesion-source (0.5cm diameter, 0.2cm height). Crystals with different sizes, summarized in Table I, have been simulated at several LDD inside a Perspex catheter 0.1mm thick. Water

Water

Aluminium coaling

Table I Dimension of crystals used for simulations

Pcrspcx' catheter

Crystal Cr\ilal

CI C2 C3 C4

Diameter (mm) 2 3 1 1

Height (mm) 2 1.5 2 4

Figure 2 Sketch of simulation setup

3. Results and Discussion The absolute value of count rate gradient measured approaching the lesion source to the detector along a rectilinear trajectory shows a sharp peak in all the considered background conditions (Figure 3, left). These results indicate the detector capability of localizing the source used for lesion representation. 57Co pulse height spectra acquired at the smallest LDD are shown in Figure 3 (right) for different background conditions and give a 13.5% (FWHM) energy resolution value.

la o.

800

600400 200

O

-8-6-4-2 0 2 4 6 400 100 200 300 Lesion to Detector Distance (cm) Pulse height (a.u.) Figure 3 Left: absolute value of count rate gradient obtained approaching the source to the detector in different background conditions. Right: pulse height spectra acquired at the smallest LDD in different background conditions.

85

0

2 4 6 8 10 Lesion to Detector Distance (cm)

Figure 4 Comparison between experimental and simulated count rates as a function of LDD at maximum background.

Measured and simulated count rates are reported in Figure 4 as a function of LDD in the case of maximum considered background. An acceptable agreement is observed apart from the points at small LDD where a 30% discrepancy is present, probably due to electronic dead time and mechanical positioning uncertainties. Simulations of clinical conditions have led to the results shown in Figure 5 where count rates per total source-activity unit are reported as a function of LDD for background to lesion concentration ratios of 1:2, 1:5 and 1:10. The plots refer to the cases of 30keV (left) and 140keV (right) gamma-ray energy detected with a 1mm diameter, 4mm height crystal. Simulations with the other considered crystal dimensions (Table I) have given similar results, demonstrating a difficult lesion visibility even at very large uptaking ratios in all considered configurations. In conclusion, measurements performed in a simple laboratory setup indicated a detector response enhancement when approaching a source which 4.80-1

E,=140 keV

1.120-

4.78-

S > 1.115-

4.76

H 1.110 -Si. 105

A

/-"O

i ii

4.74 4.724.704.68 -

2

-

1

0

1

2

Lesion to Detector Distance (cm)

—•— Uptake ratio 1:10 » - Uptake ratio 1:5 * Uptake ratio 1:2

?-"-''"*'

* * -

2

-

1

0

1

2

Lesion to Detector Distance (cm)

Figure 5 Simulated count rate per total activity unit vs. LDD at 30keV (left) and 140keV (right). Data corresponding to different uptaking ratios are plotted together.

simulates a small lesion even in presence of background, suggesting that the method could be used for lesion localization. However, first simulations of clinical conditions have demonstrated a very poor lesion visibility even at high

86

lesion-to-background ratios and at very low energy (30keV). Nevertheless, we believe that the detector assembly can be significantly improved introducing a directional shield to reduce the background radiation reaching the detector. Moreover a number of specific clinical conditions have to be investigated, less severe than the one considered in this work. As an example the case of lung lesions detection could be studied which presents the advantages of a reduced background contribution, due to the low organ density and to its proximity to the body surface. Further studies will be performed to foresee the detector response in such conditions, also investigating different detector geometries. Acknowledgment This work has been supported by Italian National Council for New Technologies, Energy and the Environment (ENEA) under Project P466. References 1. J.R.Davies, J.H.Rudd, P.L.Weissberg: "Molecular and Metabolic Imaging of Atherosclerosis"; J Nucl Med, Vol.45-11 pp. 1898-1907 (2004) 2. T.Mukai, R.Nohara, M.Ogawa, S.Ishino, N.Kambara, K.Kataoka, T.Kanoi,K.Saito, H.Motomura, J.Konishi, H.Saji: "A catheter-based radiation detector for endovascular detection of atheromatous plaques"; Eur J Nucl Med Mol Imaging, Vol.31 pp.1299-1303 (2004) 3. J.Narula, H.W.Strauss: "Imaging of unstable atherosclerotic lesions"; Eur J Nucl Med Mol Imaging, Vol.32 pp. 1-5 (2005) 4. R.Scafe, D.Della Sala, T.Fasolino, G.Alonge: "Rivelatore a Scintillazione Sub-miniaturizzato", Italian Patent Application, RM2005A000204, (2005) 5. Hamamatsu Photonics K.K.: "Si APD S8664 series data sheet"; KAPD1012E02, Sep.2003 DN, Japan 6. R.Scafe, G.Iurlaro, L.Montani, T.Fasolino, G.Alonge, D.Della Sala: "Silicon avalanche photodiodes evaluation for low-energy single-photon scintillation readout", Eurosensors XVIII, Digest of technical papers, ISBN 88-7621-282-5, Palombi Editore, Rome, Italy, (2004) 7. J.F.Briesmeister (Editor): "MCNP 4C - A general Monte Carlo N Particles transport code", Los Alamos National Laboratory Report, LA-13709-M (2000)

ANALYSIS OF T E S T - B E A M DATA F R O M A P R O T O T Y P E LHCB RICH D E T E C T O R

M.PATEL * CERN, Geneva, Switzerland E-mail: [email protected]

A prototype of the LHCb Ring Imaging Cherenkov detector has been constructed. The prototype module contained six pre-production pixel Hybrid Photon Detectors which were read out at the full LHC speed of 40 MHz using a prototype on-detector electronics readout system. The photon detectors were mounted in a gas vessel and Cherenkov rings were observed from an N2 gas radiator using electron and pion beams. The photoelectron yields and Cherenkov angle resolutions observed are in line with expectations.

1. Introduction The LHCb experiment 1 will make high precision studies of CP violation and other rare phenomena in B meson decays. Particle identification is essential for this physics programme and will be provided by two Ring Imaging CHerenkov (RICH) detectors 2 . Three different radiator materials: silica aerogel, and the fluorocarbon gases C4F10 and CF4, will be used to produce Cherenkov light from charged particles traversing the detectors. The resulting single photons must be detected for wavelengths in the range 200 to 600 nm, over a total active area of 2.6 m 2 . The photon detectors are required to have a granularity of 2.5mmx2.5mm, a time resolution better than the LHC bunch crossing rate of 25 ns and an active-to-total area ratio of ~70%. Photon detector prototypes that satisfy these constraints have been developed and subjected to extensive laboratory and beam tests. Before launching the production of ~550 of these detectors, a beam test was performed with the first six pre-production photon detectors and prototypes of the on-detector electronics system. In the following sections, the photon detectors, the readout system and prototype RICH detector used are briefly described, before presentation of the test-beam data. *On behalf of the LHCb RICH Collaboration.

87

88

2. The Hybrid Photon Detector The photon detector adopted for the LHCb RICH detectors is the novel pixel Hybrid Photon Detector (HPD). The HPD consists of a pixelated silicon detector anode assembly which is encapsulated in a vacuum envelope (Fig 1). The silicon sensor chip of the anode assembly is divided into 1024 effective pixels of 500x500^m, with each pixel bump-bonded to one channel of a LHCb PIX1 binary readout chip 4 . The HPD tube has a 7mm-thick quartz entrance window with an S20 multialkali photocathode deposited on the inside. The typical quantum efficiency is ~25% at 270nm. Photoelectrons (p.e.) emitted from the photocathode are accelerated onto the anode assembly by a 20 kV cross-focussing electron optics system. This results in ~ 5000e - in the silicon. A number of early HPD prototypes have been subject to a programme of laboratory and beam tests and all devices have fulfilled the requirements of LHCb 3 . SI pixel array (1024 dements) Ceramic carrier V L I C

Solder *umf bonds

!

PCI

M

/JMIfA

Binary -l-ntrnnl

Figure 1. The Pixel HPD.

Figure 2. The RICH readout electronics.

3. The Readout Electronics The test-beam readout scheme is shown in Fig 2. The first stage is the binary front-end pixel chip, LHCbPIXl, which is encapsulated inside the HPD vacuum envelope. This chip discriminates single p.e. hits, resulting in binary data which are then read out on receipt of a triggered event. A pair of HPDs are connected via a kapton cable to the Level-0 readout board, mounted on-detector. The Level-0 board interfaces the HPD pixel chips to the trigger, timing and control systems, and receives and distributes the local low-voltage power to the chip. On a receipt of a Level-0 trigger accept, the data from a pair of pixel chips are multiplexed out at a 40 MHz rate. The Gigabit-Ethernet protocol is used to drive the data through optical fibres to the off-detector (Level-1) optical receiver. The receiver card was mounted onto an S-link to PCI bridge (FLIC) 5 . Triggered events

89

were buffered on the FLIC until being read out to a PC through the PCI interface. Test-beam data were driven through the readout system at full LHCb speed with the pixel chip running at 40 MHz and the optical links channelling the data to the receiver at the full 1.6 GHz rate. 4. The Prototype RICH Detector Beam tests were performed at the CERN-PS in the T9 facility which provided 10 GeV/c negative pions and electrons. The Cherenkov detector used is shown schematically in Fig 3. A light-tight vessel contained the N2 gas used as a Cherenkov radiator. Beam particles entered the vessel through a thin aluminum foil and impinged on the centre of a parabolic mirror which was inclined at an angle of 13.4° to the mirror axis. The mirror had a focal length f= 1016 mm, a diameter of 200 mm and a reflectivity of more than 90% in the range 225-450 nm. The detector plane was fixed at a distance of 1047 mm from the mirror centre with dimensions chosen to ensure that saturated (P—l) electron rings could be fully contained within a single HPD. The HPDs were isolated from the Cherenkov gas by a 5mm thick quartz plate through which the Cherenkov photons entered the detector housing.

Figure 3. Schematic of the prototype RICH detector.

Figure 4. Ring image on an from a pion beam and N 2 ra di a tor.

H PD

5. Photoelectron Yields and Cherenkov Angle Resolution A ring image obtained on the pixel chip from a 10 GeV/c pion beam with the N2 radiator is shown in Fig 4. The image is integrated over ~ 30k events. Very few hits are seen outside of the ring indicating the low level of noise.

90

The Poisson distribution of the number of measured p.e.'s is modified by three main effects. Firstly, due to charge sharing effects at the pixel boundaries, a single p.e. may give rise to two pixel hits. Conversely, two p.e.'s can give a hit in the same pixel. Each of these effects was determined to be at the ~ 2.5% level. Secondly, in the test-beam, the 25 ns clock and beam trigger were asynchronous. If a track arrived late in time with respect to the clock, the hit signal may be assigned to a clock tick that is not subsequently strobed and the data may be lost. In contrast to the situation in the final RICH detector, the detection efficiency is therefore reduced by a timing efficiency factor of ~0.9. Thirdly, the efficiency of the HPD, defined as the probability for the detection of a p.e. after it has been converted at the photocathode, is typically 85% from laboratory measurement. The number of measured p.e.'s per event was fitted with a Poisson distribution, modified for the above effects.The data and the fit are shown for a typical HPD in Fig 5. A summary for all HPDs is given in Table 1. The uncertainty in the expectation is dominated by measurements of the quantum efficiency of the HPDs and is at the 10% level. The data are generally in good agreement with the expectation. The discrepancy for HPD 1 originates from a known timing problem. The low number of p.e.'s observed in HPD 6 is currently under investigation.

10

12

14

16

18

Figure 5. The number of p.e.'s observed on a single HPD from pion data (points). The fit to these data is also shown (solid line).

Cherenkov angle (mrad)

Figure 6. The Cherenkov angle distribution from data (black) and Monte Carlo (red) from a pion run.

The Cherenkov angle distribution observed on a single HPD from a pion run is shown in Fig 6. The distribution from a GEANT-4 based Monte Carlo (MC) simulation that includes the contributions from chromatic, pixelisation, emission point, beam divergence and charge sharing effects is also shown. A Gaussian fit to the data gives a width, a, of 1.1 mrad compared to 1.0 mrad from the MC. The data and MC agree within the systematic

91 errors of ~ 10%, which are currently under study. The MC allows the width to be broken down into the contributions from the effects listed in Table 2. Here the pixelisation error dominates, in contrast to the situation in the final RICH detectors which have different geometrical configurations.

HPD

Mfit)

^(theory)

Ratio

1

7.8

6.0

1.3

2

11.5

11.2

1.0

3

8.8

8.9

1.0

4

9.7

10.7

0.9

5

10.1

10.0

1.0

6

8.5

11.4

0.8

Table 1.

a{8c)l Chromatic

0.4

Pixel

0.8

Emission point

0.3

Beam divergence

0.2

Charge sharing

0.3

MC Total

1.0

DATA

1.1

mrad

Table 2.

6. Conclusions A test of a prototype LHCb RICH detector has been performed, employing six pre-production pixel Hybrid Photon Detectors with prototypes of their readout electronics. The data were read out at the full LHC speed of 40 MHz with a prototype on-detector readout system, using optical fibre transmission at 1.6 GHz. Using lOGeV/c electron and pion beams from the CERN-PS T9 facility and an N2 gas radiator, Cherenkov rings were observed with low background. Resolutions and photoelectron yields were in line with expectations. The HPD performance meets the requirements of LHCb, and full production of ~550 HPDs is now ongoing. References 1. S.Amato et al, LHCb Technical Proposal, CERN-LHCC-98-4, CERN, 1998 2. The LHCb Collaboration, LHCb Technical Design Report: Reoptimized detector design and performance, CERN-LHCC-2003-030, 2003 3. M.Moritz et al, Performance Study of New Pixel Hybrid Photon Detector Prototypes for the LHCb RICH Counters, Nucl.Instrum.Meth.A442,164-170,2000 4. R. Dinapoli et al, An analog front-end in standard 0.25/im CMOS for silicon pixel detectors in ALICE and LHCb, Proceedings of the 6th Workshop on Electronics for LHC Experiments, CERN/LHCC/2000-041, 2000. 5. A.Guirao et al.,Readout of high speed S-LINK data via a buffered PCI card, Conference Proceedings, Frascati, PCaPAC 2002

SINGLE CRYSTAL CVD DIAMOND DETECTORS FOR HADRON PHYSICS MICHAL POMORSKI*, E. BERDERMANN, M. CIOBANU, B. E. FISHER, A. MARTYMYIANOV, P. MORITZ, M. REBISZ, B. VOSS Gesellschaft fur Schwerionenforschung (GSI), Planckstr. 1 Darmstadt, 64291 .Germany J. MORSE, M. SALOME European Synchrotron Facility (ESRF) Grenoble, 38043 CEDEX, 6 rue Jules Horowitz, France We report on characterization of several single crystals CVD diamond detectors tested for the use in heavy-ion spectrometry and minimum ionizing particles time of flight measurements. An energy resolution of 16 keV (FWHM) is achieved using a 24l Am asource, which is comparable to the energy resolution of silicon detectors. The collected charge maps, measured using 12C micro beams confirm the excellent homogeneity of the novel material. Using low impedance broadband electronics and a transient-current technique the mobility and saturation drift velocity of electrons and holes are estimated.

1. Introduction Due to its remarkable physical properties [1-3] diamond is one of the most promising wide band-gap materials for particle detection in present and future hadron physics experiments, where radiation hardness and speed are the crucial requirements. Recent progress in the growth of ultra pure Single-Crystal CVD diamond [4] has opened perspectives to fast and high-resolution radiation sensors based on this novel type of artificial diamond 2.

Detector Preparation and Experimental setup

The samples characterized in this work are single crystal diamond films made by Chemical Vapor Deposition (CVD) at Element Six, Ascot, UK. Cleaning and oxidation in boiling mixture of H2SO4/KNO3 is performed before electrodes in "sandwich" configuration are applied by sputtering. Different metallic layers have been tested consisting of: Ti(20nm):Pt(30nm):Au(100nm),

92

93

Cr(50nm):Au(100nm) and Al(lOOnm). No differences in IV characteristics or other detector properties are found so far for various metallization. Vacuum 10° mbar

.

^^^_n

Air

SC-DD

•

: '"'Am •'erhMI

»D> I

IT QND j

.•/- HV E [V/|im]

GND

Fig. 1. (left) IE characteristics for several SC diamond samples. Unexpected huge spread between samples and low break down field is found for most of the tested samples; (right) Mounting of the samples and geometry of irradiation.

Before detector irradiation the I-V characteristics is probed in order to determine the safe range of detector HV bias. The current-voltage curves of various samples, normalized to the contact surface are shown in Fig. 1 (left). For most of the samples, unexpected huge spread and low break-down fields have been observed. Optical microscopy and AFM studies (not presented here) of the samples surfaces show damaged regions probably due to polishing processes after diamond growth. Such undefined state of the interlayer diamond-metal can be the reason for the observed behavior. Further work is ongoing indenting to improve post-growth surface preparation and thus the electrode metallization. The diamond detector is clamped between two printed circuit boards (Fig. 1, right). This mounting provides electrical contact and a particles collimation as well. A 24lAm a-source is placed above the collecting electrode. Impinging a particles of 5.468 MeV have a short range of-12 urn in diamond, thus, for the presented irradiation geometry, the choice of HV polarity enables mainly holes drift at positive bias, and electrons drift at negative potential [5]. 3. Charge Collection Efficiency (CCE) and Energy resolution Using a classical nuclear electronics chain, cross-calibrated by silicon detector and spectroscopic pulser [6], the collected-charge characteristics is measured. Results for 4 different samples are displayed in Fig. 2 (left). Blue curves correspond to the electrons collection efficiency and red data to the

94

efficiency of holes. In all cases we observe saturation of collected charge to ~68.7 fC at relatively low bias. BDS14 E-0.8 V/um 241Am a 1.6x10*

holes

1.4x10*

electrons

1.2x10*

5.486 MeV a particle injection

w c g

•

1.0x10'

:

8.0x10*

|

6.0x10

• 0.00

0.25

0.50

0.7S

1.00

— holes electrons 1.25

1.50

4.0x10" 2.0x10'

'

.J

66

E|V/pm]

y

& collected charge [fC| 69

I

•

i ,

;

69

Fig. 2 (left) Collected charge characteristics of electrons (blue), and holes (red) obtained from four SC diamond detectors, (right) The a-spectra recorded for holes and electrons drift, respectively. All satellite lines are distinguished. The energy resolution (FWHM) for holes drifts is AE/E ~ 0.003.

Good energy resolution of monoenergetic a-particles impinging randomly an area of a few square millimeters, is an indicator of the diamond bulk's homogeneity. The spectrum presented in Fig. 2 (left) is an evidence for excellent homogeneity of this diamond crystal, as well as for long term stability (time of measurements >12h). An energy resolution of AED (FWHM) - 17keV is extracted from a Gaussian fit of the 24lAm main peak, which is comparable to the value of AESiD= 14keV obtained with a silicon detector under same experimental conditions. In addition having such excellent energy resolution and detection stability we can assume charge collection efficiency close to 100% at plateau of curves presented in Fig.2 (left). Thus estimation of average energy needed for eh pair creation is possible, which amounts to -12.8 eV/e-h. 4. Timing Properties - Time of Flight Technique We are using the intrinsic ToF technique [6-8], in order to probe the timing properties of SC diamond detectors. The electric field is applied along the <100> crystal direction. The average (over 500 single shots) current signals, recorded with a DBA-II broad band voltage amplifier of 50Q impedance [7] and a digital scope of 3GHz bandwidth, are presented in Fig. 3 for holes- (red) and electron drift (blue). The FWHM of these signals marks the transition time t,r of the charge carriers cloud between the moment of its generation near one contact and its arrival to the opposite electrode. The flat top of the signals indicate a uniform internal electric field (no space charge and negligible trapping). Thus the drift velocity v^ of carriers is constant at fixed bias and can be easily calculated from the simple relation Vi,=Aliu, where d is the sample thickness. The

95 dependence of the drift velocity from the applied external electric field E for six tested samples is displayed in Fig. 3 (right). - 2.0x10''

E=1.23W|im

E=1.23V/|im

< 1.0x10*

«

E=0.12 V/|im 2.0x10* 3.0x10*0

1.0x10* 2.0x10

3.0x10

104

time [s]

10*

E[v/xrl Fig.3 (left) An example of broad-band current signals (average from 500 single shots) for holes(rcd) and electrons (blue) drift at increasing electric field. The oscillation on top is due to a weak 50il mismatching, (right) Charge carriers drift velocity as a function of electric field for 6 tested SC diamond samples.

Since the transport process at very high electric field is difficult to be theoretically described, the data are fitted by the empirical equation [6-10]: 'dr

(E) =

l+-*-£

(1) Mo Where u 0 - low field mobility, vsal-saturation velocity. Excelent reproducibility of the drift velocity of holes is found for various samples, in contrast to the electrons, where spread of experimental points affects significantly the precision of the fit. Thus, the extrapolated parameters are given for an interval. Table 1 shows the timing parameters, extracted for several tested samples. Table 1. Measured cfective mobilities (u*rr) and drift velocities (Vdr) at typical detector operation range 0.6 < E < 2 [V/u,m], as well as low filed mobilities (u«) and saturation velocities (v s „) extrapolated from fits of the experimental data.

electrons holes

Measured Hen [cm2/Vs] vdr Inm/ns] 1000-450 50-90 1400-550 70-110

Extrapolated vsat [cm/s] Ho [cmVVsl -1300-3000 1.3-1.9*107 ~1.4*107 -2400

96

5. Mapping with heavy-ion micro beams In order to study spatial homogeneity, a SC CVD diamond detector has been irradiated with l2C2* ions of a kinetic energy E = 9.5 A MeV at the micro beam facility of GSI. The irradiated area was 60x93 [urn2] and the sample was scanned in a lum raster. Classical charge sensitive nuclear electronics was used for the signal readout. The obtained 3D distribution of the detector's pulse heights is presented in Fig. 4 (left); no regions of lower (±2%) CCE in scanned area are observed. In Fig. 4 (right) the calibrated ID spectrum is shown, where the left peak is the pulser line used to control the stability of the electronics, the intermediate one the l2C2+ ion distribution and the right one due to an unavoidable beam contamination (~ 0.6%) with 18 0 +3 ions having the same A/Q. The unexpected oxygen peak has been used for energy calibration of the spectrum. The obtained energy resolution of the 12C peak is AE/E = 0.01.

200

Energy [MeV] Fig.4 (left) Pulse height map of a SC CVD diamond detector measured with

12

C ions of E = 9.5

AMeV using nuclear spectroscopy read-out. The irradiated area was 60 x 93 [urn2], (right) The Corresponding calibrated ID spectrum.

The obtained map and the good energy resolution of the 12C peak confirm the excellent homogeneity of single crystal diamond, which was already indicated during the irradiation tests with ct-particles. 6. Summary and Outlook The study conducted here gives a clear evidence that SC CVD diamond is a competitive material for silicon pin diodes, frequently used in hadron physics experiments. Most of the tested samples show stable operation, negligible charge

97 carriers trapping and material homogeneity. The energy resolution found for alpha particles of E a = 5.5 MeV and 12C ions of E = 9.5 AMeV is close to the values obtained from silicon detectors. The timing properties of diamond are superior to silicon. Fast and comparable drift velocity for holes and electrons is measured, which results in the absence of slow signal components. However, the low e-h pair creation compared to silicon, is a drawback for MIP- and weaklyionizing particles detection. The important radiation hardness tests are planned for the next future. Nevertheless, considering the extreme physical properties of diamond and the experimental data obtained from polycrystalline CVD diamond films [12], a sufficient high radiation dose tolerance is expected. Acknowledgments This work is supported by the EC Integrated Infrastructure Initiative Physics Project under contract number: RII 3-CT-2004-506078.The express their gratitude to Element Six, Ascot, UK for the SC diamond provision and to the Target Laboratory crew of GSI for the metallization.

Hadron authors samples samples

References 1. E. Berdermann, et al, ICATPP,2001, p246-251. 2. M. Marinelli.et al, Diamond Related Mater. 7 (1998) 519. 3. J. Isberg et al., Science 297 (2002). 4. Element Six, Ascot, UK 5. S. Ramo, Proc. IRE 27, 584 (1939). 6. M. Pomorski, E. Berdermann phys. stat. sol. (a) 202, No. 11, 2199-2205 (2005) 7. P. Moritz, E. Berdermann, K. Blasche, H. Stelzer, and B. Voss, Diam. Relat. Mater. 10, 1765 (2001).

8. H. Pernegger, et al, Journal of Applied Physics 97, 073704 (2005) 9. C. Jacoboni, C. Canali, G. Ottaviani, and A. Alberigi Quaranta, Solid-State Electron. 20, 77 (1977). 10. T. Lari, A. Oh, N. Wermes, H. Kagan, M. Keil, and W. Trischuk, Nucl. Instrum. Methods A 537, 581 (2005). 11. E. Berdermann, K. Blasche, P. Moritz, H. Stelzerl, and B. Voss, Diam. Relat. Mater. 10,1770(2001). 12. W. Adam et al. MM A: , Vol. 476, Issue 3, 11 January 2002, Pages 686-693

I M P R O V E M E N T OF PARTICLE IDENTIFICATION B Y E N E R G Y LOSS IN A STACK OF SILICON D E T E C T O R S

M. R E G L E R , R. F R U H W I R T H A N D M. F R I E D L Institute

of High Energy

Physics of the Austrian Academy Nikolsdorfer Gasse 18, 1050 Wien, Austria E-mail: [email protected]

of

Sciences,

We have studied the optimal use of information from silicon track detectors for particle identification, using a recent measurement with 4 GeV/c pions with low electronic noise. The raw data were modeled by the convolution of a density obtained by detailed simulation with a Gaussian that accounts for the measurement noise only. We observe excellent agreement with the data. Then a stack of fifteen detectors has been simulated from the experimental distribution. Using these data, the performance and robustness of some location estimators has been compared, including traditional methods such as truncated means and the Maximum Likelihood estimator, as well as a novel method, the optimal L-estimator.

1. Introduction The study of energy loss in silicon has a long history. Due to the complex structure of the cross section (thresholds and multiple modes in the low energy domain, and the possibility of very large energy transfer in a single collision leading to a large skewness), the number of collisions is too small for the Central Limit Theorem. Many attempts for practicable approximations have been made with some well-known milestones like the work of Landau 1 , Vavilov2, and Shulek3, along with the earlier work of Blunck and Leisegang4 who used a Gaussian sum approximation. The model used in this contribution is a probability density from a more refined simulation (without experimental noise), followed up by a convolution procedure, performed for us by Hans Bichsel 5,6 .

98

99

2. The experimental data 2.1. Detector

and experimental

set-up

A prototype silicon detector module with double sided readout was built for an upgrade of the BELLE experiment 7 at KEK (Tsukuba, Japan) and studied in a beam test with 4 GeV/c pions in April 2005. The sensor is 300 /xm thick and has AC-coupled strips with a readout pitch of 51 fxm on either side. The strip signals are amplified by the low-noise APV25 front-end chip 8 , which was originally developed for the CMS experiment at CERN (Geneva, Switzerland). 2.2. Raw data

analysis

Out of the various measurements performed in that beam test, we will only show data of the p-side measured at "standard" conditions like perpendicular incidence and moderate over-depletion. The raw strip data was processed with the usual pedestal subtraction and common mode correction algorithms, followed by a cluster finding procedure with two different thresholds: Once a seed strip with a signal above 5 times the RMS of the noise is found, neighbouring strips are added to the cluster as long as their signals exceed 3 times the RMS of the noise. Statistical tests were carried out to ensure that the noise of individual strips is largely uncorrelated as expected. Consequently, the noise of a cluster signal depends on the number c of strips involved. Assuming identical RMS noise ns on each strip (which is a good approximation of the real system), we obtain the RMS cluster noise nc by nc = y/cns. In the data presented here, we found an average RMS strip noise of 522 e-h pairs and a mean cluster size of 2.49. Hence, the cluster noise is expected to be 824 e-h pairs in average. By modifying the cluster finding procedure such that the same number of strips are summed up but displaced from the actual cluster strips above threshold, we found an RMS cluster noise of 739 e-h pairs. Note that the data has been taken in a clean test-beam environment. 3. The electron-hole pair distribution 3.1. The Bichsel

model of the energy

loss

We thank H. Bichsel for providing us with a (pointwise) probability density function of the energy loss corresponding to our experimental setup (4 GeV/c pions, detector thickness of 300 //m). The mode (most probable value) of the Bichsel density ist at Emode = 82.64 keV, corresponding to

100 22637 e-h pairs. We have "standardized" the Bichsel PDF by an afnne transformation such that its location (mode) b is at zero and its scale (one quarter of the full width at half maximum) a is equal to 1. The standardized Bichsel density can be approximated quite well by the convolution of a Landau density (with mode b = -0.26 and scale a = 0.791) with a Gaussian density with \i = 0 and a — §.711\.. This explains the apparent success of the Landau-Gaussian-convolution in modeling energy loss data. 3.2. Modeling model

the data by the Bichsel

and by the

Landau

The observed e-h pair counts are corrupted by the electronic noise. We have modeled the noise by Gaussian fluctuations of the counts with zero mean and a width to be determined from the data. The convolution of the Bichsel density with a Gaussian gives an excellent fit (Figure 1). The Bichsel density has mode b = 19785 and scale a = 2058. The standard deviation of the Gaussian is equal to
xio

x 10

Figure 1. The freqency distribution of the experimental e-h pair counts, normalized to 1, and the density of the fitted Bichsel-Gaussian convolution.

101 model quires mated makes

describes the data much better than the Landau model which rethe convolution of a Gaussian whose width is far beyond the estinoise width in order to account for the "Shulek braodening". This the estimation of low noise difficult.

4. Comparison of analysis methods In this section we investigate some methods of estimating the location parameter of the parent distribution from a small sample. As we have at our disposal only observations by a single detector, we have artificially generated 50000 random samples by drawing from the observed distribution of e-h pair counts. We use a sample size of fifteen, which would be a typical sample size from a silicon tracker. Following the work of Talman 9 , we concentrate on L-estimators, i.e. linear functions of the ordered sample values. Truncated means are special cases of the general L-estimator. 4.1. Equivariant

L-estimators

of

location

Let x = (xi < ... < xm) denote an ordered random sample of size m. An L-estimator of location C{x) = wTxis equivariant a if and only if J2wi = 1The mean of any subset of the ordered sample x and every truncated mean is equivariant. It should be noted that truncation is not tantamount to simply discarding part of the information — the order of the observations depends on all values in the sample. The mean of observations Xi to Xj is denoted by C(itj) • In the class of equivariant L-estimators of location with given expectation there is one, denoted by £0pt> that has the smallest variance. Under the constraints E(Copt) = t and ^Wi = 1 the optimal weights are given by wopt = A - 1 B(te — ft), where jl = E(x), B = G(/2e T - ep,T)G, G = [cov(x)] -1 , A = /uT-Be*and e is a vector of ones. The optimal weights can be estimated from the data. 4.2. Results from experimental

data

The separation of different parent distributions (particle types) is optimal if the ratio p = mean/std of the location estimator is maximal. We have investigated the robustness of various truncated mean estimators and of the optimal L-estimator by contaminating the samples with various amounts a

A location estimator l(x) is equivariant iff l(ax + b) — a l(x) + 6.

102 of noise, uniformly distributed between 0 and 100,000. The ratio p drops by only about 5 percent with a 2%-contamination of noise. Figure 2 shows the results for 2% contamination. The separation is optimal when the expectation of the L-estimator is equal to about 19200, somewhat below the mode of the parent distribution. The optimal Lestimator performs only slightly better than the best truncated mean estimators (£(2,5) and £(2,7))- Besides being not equivariant, the maximum likelihood estimator is slightly worse than the best L-estimators and much slower to compute.

<-< B—B

0-0 o-e

v-v A-A •

—

® 1.6

1.8 2 Expectation of estimator

•

Mean of subset (size=2) Mean of subset (size=4) Mean of subset (size=6) Mean of subset (size=7) Mean of subset (size=8) Mean of subset (size=10) Optimal L-estimator Maximum likelihood Bichsel mode

2.2 ,„4

Figure 2. The ratio mean/std of truncated mean estimators,the optimal L-estimator and the maximum likelihood estimator. The abscissa is the expectation of the estimators. The truncated mean estimators of size m are computed by taking the average of observations k to k + m — 1, starting at k = 1. The dotted vertical line is at the mode of the Bichsel PDF.

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

L. Landau, J. Phys. (USSR) VIII , 201 (1944). P.V. Vavilov, Sov. Phys.-JETP 5, 749 (1957). P. Shoulek et al., J. Nucl. Phys. (USSR) IV, 564 (1966). O. Blunck und S. Leisegang, Z. Phys. 128, 500 (1950). H. Bichsel, Rev. Mod. Phys. 60, 663 (1988). H. Bichsel, A method to improve particle identification in TPCs and silicon detectors (in preparation). 7. A.Abashian et al., Nucl. Instr. Meth. A 479, 117 (2002). 8. L.L.Jones et al., The APV25 Deep Submicron Chip for CMS Detectors, CERN-99-09, p.162-166, CERN, Geneva (1999) 9. R. Talman, Nucl. Instr. Meth. 159, 189 (1979).

STUDIES FOR A FAST RICH

P. ABBON*, M. ALEKSEEV*, H. ANGERER*, M. APOLLONIO" 1 , R. BIRSA m , P. BORDALO 3 , F. BRADAMANTE" 1 , A. BRESSAN" 1 , L. BUSSO', M. CHIOSSO*, P. CILIBERTI™, M. L. COLANTONI a , S. COSTA', S. DALLA TORRE" 1 , H. DESCHAMPS*, V.DIAZ" 1 , N. DIBIASE', V. DUIC m , W. EYRICH d , D. FASO', A. FERRERO*, M. FINGERS, M. FINGER JR^, H. FISCHER e , S. GERASSIMOV*, M. GIORGI" 1 , B. GOBBO" 1 , D. VON HARRACH' 1 , F.H.HEINSIUS 6 , S. HORIKAWA c , R. JOOSTEN 6 , B.KETZER*, K. KOENIGSMANN 6 , V. N. K O L O S O V ? I . KONOROV*, D.KRAMER^, F. KUNNE*, A. LEHMAN d , A.MAGGIORA*, A. MAGNON fc , A. MARTIN" 1 , G.MENON" 1 , A. MUTTER' 1 , O. NAEHLE 6 , D. NEYRET fc , F. NERLING e , P. PAGANO" 1 , S. PANEBIANCO fc , D. PANZIERI 0 , S. PAUL', J. POLAK/, P. REBOURGEARD*, P. SCHIAVON™, C. SCHILL 6 , W. SCHROEDER d , L. SILVA9, M. SLUNECKA^', F. SOZZI m t L.STEIGER 7 ', M. SULC^, F. TESSAROTTO" 1 , A. TEUFEL d a

INFN, Sezione di Torino and University of East Piemonte, Alessandria, Italy Universitat Bonn, Helmholtz-Institut fur Strahlen- und Kernphysik, 53115 Bonn, Germany C CERN, European Organization for Nuclear Research, Geneva, Switzerland Universitat Erlangen-Nurnberg,Physikalisches Institut,91054 Erlangen, Germany e Universitat Freiburg, Physikalisches Institut, 79104 Freiburg, Germany ' Technical University of Liberec, Liberec, Czech Republic 3 LIP, Lisbon, Portugal Universitat Mainz, Institut fur Kernphysik, 55099 Mainz, Germany % Technische Universitat Munchen,Physik Department, 85748 Garching, Germany 3 Charles University, Prague, Czech Republic and JINR, Dubna, Russia k CEA Saclay, DSM/DAPNIA, 91191 Gif-sur-Yvette, France INFN, Sezione di Torino and University of Torino, Torino, Italy m INFN, Sezione di Trieste and University of Trieste, Trieste, Italy

"on leave from IHEP Protvino, Russia, te-mail: [email protected]

103

104 The present and next generation of H E P experiments requires RICH detectors able to stand high beam intensities and trigger rates and to handle complex, crowded events: fast RICHes are needed. In the context of the future requirements of the enlarged luminosity of the COMPASS experiment, we are building a fast RICH detection system, with photon detection based on multianode photomultipliers and a system of lenses to collect the light from a much larger surface and to guide it to the P M T photocathode, preserving the position information. Albeit this approach is not new, we exploit a surface ratio of the order of 7, thus keeping the costs of the photon detection system reasonable, and we emphasize the fast response of the detectors and the associated electronics. We report about test beam results and laboratory studies of the multianode P M T performances.

1. Introduction The COMPASS experiment 1 , at CERN SPS, is a high luminosity experiment dedicated to hadron physics, with a full research programme including studies about nucleon spin structure and charm spectroscopy. As for many modern HEP experiments, particle identification in COMPASS is a fundamental requirement; for this purpose, RICH-1 2 detector is operating in the experiment since 2001. The detection technique relies on MWPCs with Csl photocatodes, and the read-out system is based on the front end chip Gassiplex3. The long memory time of this detection system (~3//s) causes the presence of high pad occupancy due to background photons from uncorrected particles. Due to the muons of the beam halo, the background is very large in the detector central regions causing a sensitive reduction in the RICH performances for forward particle trajectories. For the second phase of COMPASS data taking, planned to start in 2006, high beam intensity (up to 10 8 /sec) and trigger rates (up to a maximum of 100 kHz) as well as complex events are foreseen: this calls for an upgrade of the RICH detector. The RICH-1 Upgrade consists of two complementary projects: in the external part of the detector, the present MWPCs will be read by a read-out electronics with APV-based architecture 4 , allowing to reduce the detector dead time; for the central part, a system based on Multi Anode Photo Multiplier Tubes (MAPMT) 56 , lenses and a reduced dead time readout system will result in an important increase in the number of detected photons and in a reduction of the background from uncorrelated events. In the following, we will describe test beam and laboratory studies related to the up-grade of the central region. 2. Description of the photodetection system The use of MAPMT matrices as photon detectors for RICH applications

105 has been already proposed (HeraB, studies for LHCb): respect to these approaches, we increase significantly the ratio between the surface of the entrance lens and the MAPMT photocathode surface. The MAPMT chosen is the R7600-03-M16 by Hamamatsu, characterized by a bialkali photocathode with 18 x 18 mm 2 active surface, 16 pixels and an UV extended glass entrance window to increase the range of the detectable Cherenkov light spectrum (200-600 nm). MAPMTs are coupled to a two lens system that allows to increase the geometrical acceptance and to minimize image distorsions; lenses are made of fused silica in order to match the wavelength range of the PMT. Home made resistive dividers are mounted on MAPMTs; the small front-end boards equipped with the MAD4 amplifier/discriminator chip 7 , which are used to amplify and digitize the MAPMT signals, are directly linked via a connector to the resistive divider boards. Power and threshold are distributed via a deck PCB. F l TDC chips8 arranged in digital boards, provide time resolution better than 130 ps. 3. Test b e a m results The principle of the photon detection system proposed for a fast RICH has been validated with two test beams at CERN SPS, T i l beam line, during 2003 and 2004. In the 2004 test, the Cherenkov photon source is a fused silica radiator placed on the axis of a UV paraboloidal mirror whose aim is modify the photon trajectories, so to decrease the opening angle. The amount of detectable photons is controlled with a cylindric aluminium screen. The detection system is formed by 8 MAPMTs, each coupled with a single thick fused silica lens. A typical threshold curve 56 exhibits a long plateau; the peak at low threshold values includes both electronics noise and signal generated by the cross-talk from adjacent MAPMT pixels. The plateau indicates a wide range for non critical threshold setting, allowing a rejection of cross-talk signal without affecting PMT efficiency. The long plateau has been observed also at the level of single MAPMT channel. 4. Laboratory studies on M A P M T performances RICH-1 detector is located in the residual field region of one of the COMPASS spectrometer magnets. The maximum value of the magnetic field B in the region where the MAPMTs will sit is ~200 Gauss. Figure 1 shows the efficiency of single photoelectron detection for different values of the longitudinal magnetic field B, normalized to the efficiency measured at B=0; the different sets of points correspond to a group of 4 external pixels

106

g

4 8 12 16

enc

| Photoelectron detection efficiency ya magnetic field B \I Photoelectron detection efficiency vs magnetic Held B |

3 7 11 15

y channels 1-4 9 channels 5-8 1 total channels

V channels 1-4 # channels 5-8 • total channels

2 6 10 U 1 5 9 13 0.8 0.6

" •'\^*'

0.4 0.2 -150

-100

-50

Figure 1. Efficiency of single photoelectron B, normalised to the efficiency measured at a group of 4 external pixels (1-4), 4 internal of a M A P M T . At right magnification of the

detection versus longitudinal magnetic field B = 0 ; different sets of points correspond to pixels (5-8) and the totality of the 16 pixels low B values region.

(1-4), 4 internal pixels (5-8) and the totality of the 16 pixels of a MAPMT. The plot at left shows a photoelectron efficiency reduction at high B values, while for values lower than 20 Gauss (plot at right), there is no efficiency reduction for the sets of 16 pixels and 4 internal pixels. For the external pixels, we can observe increased efficiency or reduced efficiency, according to the field sign; the behaviour is opposite for the corresponding group of 4 pixels at the opposite MAPMT edge. Globally, at low magnetic field, there is no efficiency reduction, while the photoelectron collection is partially moved from a pixel to the adjacent one. All the data points have been obtained producing threshold curves in the different field conditions. A soft iron shielding to house PMTs has been validated: inside the box, B value is below 10 Gauss even for an external field around 200 Gauss. The maximum photoelectron rate per MAPMT channel foreseen at COMPASS during the 2006 run is ~2 MHz. We have studied the MAPMT response at high rate to validate the design of the voltage divider. The effect of high photoelectron rates on gain has been studied monitoring the dependence of the anodic signal amplitude on the amount of photons per channel, operating the MAPMT at different HV values (Fig. 2). The high photoelectron rate is generated with a Halogen lamp and it is controlled applying calibrated neutral filters. No amplitude reduction is observed up to 950 V and photoelectron rates as large as at least 5 MHz per channel. For RICH application, the MAPMTs will be operated between 850 V and 900 V, HV values that guarantee maximum photoelectron collection efficiency.

107 I

Anodic signal amplitude vs photon rale 1

C

m

1

B s »^ * n 1 v _ _J&L 45 w 40

%

E

ill*

pk.AA 1 A 15 i 10

i

5|

0

..A.

I

* *

# " H V = 1000 V

A H V = W«V • jHV = 850V 2

8 10 photon ra1e(MH*/ch

Figure 2. Anodic signal amplitude vs photoelectron rate per channel, at different HV. 5. C o n c l u s i o n s Test beams and laboratory studies have confirmed the validity of a fast system for p h o t o n detection based on UV extended M A P M T , read by M A D 4 front end chip a n d coupled with q u a r t z lenses. Applications for large R I C H detectors are promising. Acknowledgments We acknowledge help from the C E R N / P H groups P H / T A 1 , TA2 and D T 2 and T S / S U a n d the support of B M B F (Germany) a n d of the E u r o p e a n Community-Research Infrastructure Activity under the F P 6 "Structuring t h e E u r o p e a n Research Area" programme (Hadron Physics, contract number RII3-CT-2004-506078). References 1. The COMPASS Collaboration, Proposal, CERN/SPSLC/ 96-14, SPSLC/P 297 March 1 , 1996; The COMPASS Collaboration, Addendum, CERN/SPSLC/ 96-30, SPSLC/P 297 May 20 , 1996; 2. E. Albrecht et ai, "Status and characterization of COMPASS RICH-1", Nucl. Instr. Meth. A 553 (2005) 215-219. 3. C. Santiard et al., "Gassiplex: a low noise analog signal processor for readout of gaseous detectors", 6th Pisa Meeting on Advanced Detectors, La Biodola, Isola d'Elba, Italy, May imi.Phys. Rev. C52, 2072 (1995). 4. P.Abbon et al, "Past read-out of the COMPASS RICH CsI-MWPC chambers using APV25 amplifier chips", proceedings of Beaune05, to be published in Nucl. Instr. Meth. A and reference therein. 5. M. Alekseev et al., "Studies for a fast RICH", Nucl. Instr. Meth. A 553(2005) 53-57. 6. P.Abbon et al., "Design and status of COMPASS FAST-RICH", proceedings of Beaune05, to be published in Nucl. Instr. Meth. A and reference therein. 7. F. Gonnella and M. Pegoraro, CERN-LHCC-2001-034, pp.204-8. 8. H.Fischer et al. IEEE Trans. Nucl. Sci. 49, (2002) 443. H.Fischer et al. Nucl. Instr. Meth. A 461, (2001) 567.

PROPERTIES OF SCINTILLATOR STRIP WITH WAVELENGTH SHIFTING FIBER AND SILICON PHOTOMULTIPLIER E.TARKOVSKY Institute for Theoretical and Experimental Physics B.Cheremushkinskaya ui, 25 Moscow, 117218, Russia The detector consisting of the 200x2.5xlcm3 plastic scintillator strip, the wavelength shifting (WLS) fiber and two novel photodetectors called Silicon Photomultipliers (SiPMs) has been constructed and tested. Light yield and light attenuation measurements are presented. More than 13 photons are detected by two SiPMs placed at the fiber ends for cosmic ray particles at normal incidence. This provides >99% efficiency with a small noise rate. SiPM has several advantages over photomultipliers. It is not sensitive to magnetic field and hence light transportation is not required. Its gain is determined easily by observing peaks corresponding to different number of fired SiPM pixels. It is cheaper than one channel in multichannel photomultiplier. A possibility to use such a new technique in the muon systems or calorimeters with tracking information is demonstrated.

1. Introduction Use of plastic scintillator with WLS fiber and multichannel photomultiplier for detection of charged particles is a well established and reliable technique (see, e.g. [1]). However this method has some disadvantages: • photomultiplier can not operate in magnetic field. So one has to use clear fibers to guide light to photodetectors placed outside magnetic field and light loss in long fibers is inevitable. • assembly of hundreds or thousands fibers in bundles to be attached to PMTs is not easy and creates problems when exchange or disassembly of fibers is needed • conventional PMTs require HV in 1000 V range (cables, safety ...etc) In this work we used a novel solid state photodetector Silicon photomultiplier (SiPM) [2] which is free from these drawbacks. Due its small size a SiPM may be embedded into scintillator without noticeable loss of efficiency. There is no need to use clear fiber for light transportation. It is insensitive to magnetic field (has been tested up to 4T field ). Operational voltage of a SiPM does not exceed several tens volts.

108

109

2. Properties of the Silicon Photomultiplier The SiPM is the matrix of 1024=32x32 independent silicon photodiodes (pixel) operating in Geiger mode. The total area covered by diodes is 1 mm2. Each diode is connected to a common bus via a quenching resistor of the order of few hundred k£2. A reverse bias voltage of 40-60 V is applied to pixels. If bias voltage exceeds the breakdown voltage then the Geiger discharge starts when a free charge carrier appears in the p - n junction depletion region. The resulting output signal is Qpixei = Cpixei x (Vbias - Vbreakdown). Typically Cpixei ~ 50 fF and AV=Vbias - Vbreakdown ~ 3 V yielding Qpixei ~ 10 electrons. When several pixels are fired then the sum signal is proportional to the number of fired pixels because characteristics of all diodes are very similar. This is illustrated in Fig. lb where SiPM response spectrum is shown. SiPM is illuminated by weak flashes of a light emitting diode (LED). The spectrum has peak structure. Each peak corresponds to the number of fired pixels, so there is an excellent possibility to find the SiPM gain from LED spectrum. Due to finite number of pixels saturation effects should be taken into a when number of detected photons is comparable with number of pixels. The SiPM photodetection efficiency depends on the value of overvoltage AV and light wave length. Typical value of efficiency is 10-15% for the green light. It is product of silicon photoefficiency (~80%), efficiency of Geiger avalanche development (-60%) and 300 ADC counts geometry efficiency (-35%). There is an interpixel cross talk which may reach 30% this limits possible gain. SiPM 300 has a significant noise of about ADC counts 2 MHz at -0.1 pixel level. However the noise rate drops fast with increasing threshold. The drop rate is determined 600 800 ADC counts mainly by interpixel cross talk. The SiPM is a very fast device Figure 1. Response of SiPM to various trigger - the discharge development time is less than 1 nsec.

110 3. Test of scintillator strip 3.1. Experimental set-up Figure 2 shows schematically the set-up. The detector was a 200 x 2.5 x 1 cm strip with 2.5 mm deep groove in the middle of the strip. The scintillator was produced by "Uniplast" company in Vladimir, Russia by extrusion technique from the granulated polystyrene with two dopants (0.01% of POPOP 3

Cosmic particle

2 trigger counters 25*23x15 mm3 Fiber

SiPM

Figure 2. Schematic view of the experimental set-up.

VLS fiber of 1 mm diameter placed in the groove. Two SiPMs were attached to the fiber at both fiber ends. The gap between sensitive SiPM surface and fiber was 200 u. The scintillator was wrapped in 3M Superradiant VN2000 foil. Signals from SiPM were integrated by LeCroy 2249A ADC with the gate of 120 ns . There were two trigger counters of 23 x 25 mm above and below the tested strip. Moving them along the strip one might test strip response at various distances from strip end. Due to angular spread the mean value of cosmic particles path length in the strip is 10% larger than strip thickness. This was taken into account during data analysis. 3.2. Data analysis During a run we collected SiPM response distributions for different triggers - one for trigger from cosmic particles, another - for LED light and the third one for random trigger. This is illustrated at Fig. 1. We determined the SiPM gain from the LED spectrum. It was equal to the distance between adjacent peaks. After that we expressed the MIP response in pixels. To estimate the light yield of the detector for the minimum ionizing particles we used the maximum of Landau distribution. The average value is approximately 1.1 times larger than

111 position of the maximum due to Landau tail. We did not correct result for this value in order to be conservative in our estimates of detector performance. In order to convert the number of fired pixels into number of detected photons the • • response of each SiPM was divided by • • • : • factor taking into account the inter pixel • • cross talk. This factor was equal to 1.43 '» and 1.28 for the left and right SiPMs, " " correspondingly. The obtained values 6 ' ' " were divided by 1.1 because of inclined t tracks selected by trigger counters. The number of detected photons at normal incidence is plotted at Fig. 3, where the sum signal is shown as well. The minimal Figure 3. Average number of value of sum signal is equal to 13.7 for the detected photons for normally position of trigger counters in the strip incident cosmic particles vs center. One may see that light position of trigger counters. transportation from the far end attenuates the light by about factor of 2. The sum signal is uniform in the 13% limit. In case of Poisson distribution with mean value of 13.6 detected photons the 98% efficiency is reached when the threshold is set to 7 photons. In order to determine the threshold in terms of fired pixels the simple modeling was done. We simulated response for each SiPM equal to 6.8 detected photons distributed according to Poisson law. Then number of fired pixels was calculated by Left SIPM

Right SIPM D

D

A

D

°

Strip canter

Position ol trigger count«ra, cm

Table 1. Data of strip response at various positions of trigger counters. x-coordinate, cm

5

Number of cosmic trigger

270

Number of cosmic events A„a < 9

0

Non-efficiency, %

0. 3

60

100

140

170

607

199

645

199

517

142

4

3

11

1

5

0

0.7

1.5

1.7

0.5

1.0

0.

30

195

6.4

13.3

3.9

12

4.4

12

3.2

Number of random events Asura > 8

3

9

7

6

1

11

4

Noise probability, xlO"4

5

7

18

5

2

9

12

Number of random trigger, xlO

multiplying of number of detected photons after simulation of cross talk value. Then the width of single photoelectron peak from LED spectrum was .added to have the final SiPM response. It turned out that one might have more than 98%

112 efficiency if sum response was larger than 8 pixels. This was checked with data. As the data were taken with inclined tracks we had to increase the threshold by factor 1.1. Numbers of cosmic triggers and number of events with Asum < 9 are given in the Table 1. One may see that for all data the detector non-efficiency is less than 1%. In order to estimate the noise rate at the threshold > 8 pixels we used the spectra obtained with random trigger. Number of random trigger and number of events with amplitude higher than 8 pixels are also given in the Table 1. -4 All runs data gave 1.7*10 noise probability with 120 ns gate width. The noise rate may be reduced if one uses for SiPM pulses coincidence faster electronics. 4. Conclusions The detector consisting of 200 x 2.5 x 1 cm3 plastic scintillator, the WLS fiber and two novel photodetectors (SiPM) has been tested. Such kind of detectors may be used in muon systems. It has high efficiency and low noise rate as compare with resistive plate chambers often proposed as muon detector. The SiPM has similar gain and efficiency as the traditional photomultiplier. Despite Contrary to vacuum photomultipliers the SiPM can operate in magnetic field, its small size allows incorporate it into scintillator body, SiPM operational voltage does not exceed several tens volt. The light collection efficiency may be increased by gluing of a fiber into the groove [3], References 1. 2.

3.

P Adamson et al., The MINOS scintillator calorimeter system, IEEE Trans. Nucl. Sci. 49, 861-863 (2002). G.Bondarenko et al., Limited Geiger-mode silicon photomultiplier with very high gain, Nucl. Phys. Proc. Suppl. 61B, 347-352 (1998) G.Bondarenko et al., Limited Geiger-mode microcell silicon photodiode: new results, Nucl. Instr. Meth. A442, 187-192 (2000) P.Buzhan et al., An advanced study of silicon photomultiplier, ICFA Instr. Bull. 23, 28-41 (2001) P.Buzhan et al., Silicon photomultiplier and its possible applications, Nucl. Instr. Meth. A504, 48-52 (2003) V.Andreev et al. A high-granularity scintillator calorimeter readout with silicon photomultipliers, Nucl. Instr. Meth. A540 368-380 (2005) R.Wojcik Embedded Waveshifting Fiber Readout of Long Scintillators, Contribution to the III International Conference in High Energy Physics, Sep.29-Oct. 2, 1992, Corpus Christi, Texas

Astroparticle Experiments Organizers: D. Saltzberg and A. Capone

E. Andreotti

S.N. Ahmed

J.K. Becker M. Brigida S. Capelli S. Cecchini R. Cerulli J.V. Dawson A. De Sio O. Dragoun S. Dusini L.S. Esposito E. Ferrer Ribas L. Gambicorti D. Grandi

M. Hareyama S. Hasan

The Underground Baradello Laboratory: Characterization of the Site and Early Results on Gamma-Ray and Neutron Spectrometry Measurements The Art of Detecting Neutrinos, 2 km Underground The Three Phases of the Sudbury Neutrino Observatory Neutrino Physics at the South Pole - Recent results from the AMANDA Experiment First Results from GLAST-LAT Integrated Towers Cosmic Ray Data Taking and Montecarlo Comparison Background Analysis of Cuoricino in View of the Future Experiment CUORE Application of Nuclear Track Detectors in Astroparticle and Nuclear Physics Recent DAMA Results Double Beta Decay Measurement with COBRA Ultraviolet Diamond Photodetector Status of the KATRIN Experiment The RPC System for the OPERA Spectrometers Electromagnetic Shower Reconstruction in Emulsion Cloud Chamber A Low Background Micromegas Detector for the CAST Experiment Imaging Optical Adapters for Multi-Anode Photo Multipliers Detectors A 2D Stochatsic Montecarlo for the Solar Modulation of GCR: A Procedure to Fit Interplanetary Parameters Comparing to the Experimental Data Spatial Distribution of Energetic Heavy Ions near the Earth A Photon Tag Calibration Beam for the AGILE Satellite 113

114 N. Hasebe A. Haungs

M.H. Israel Yu.G. Kudenko C. Leroy J. Lizarazo F. Lucarelli M. Messina

V. Mikhailik

M. Pedretti M. Ruppi O. Tarasova

Observation Program of Isotope Composition in Ultra Heavy Cosmic Rays High-Energy Cosmic Rays Investigated by Air-shower Measurements with KASCADE, KASCADE-Grande, and LOPES Scintillating Optical Fibers for Astroparticle Physics The T2K Experiment: Status and Instrumentation of the 280m Near Detector Results and Status of the PICASSO Experiment Data Acquisition System and Trigger Electronics for CACTUS The Central Pixel of the MAGIC Telescope for Optical Observations Status report of ARDM Project: a New Direct Detection Experiment, Based on Liquid Argon, for the Search of Dark Matter Development of Multiple Photon Counting Coincidence (MPCC) Technique for Characterisation of Scintillators for Cryogenic Applications Results from Cuoricino Experiment and prospects for CUORE Timing Calibration of the NEMO Optical Sensors Design, Production, and First Results from the IceCube Digital Optical Module

T H E U N D E R G R O U N D BARADELLO LABORATORY: CHARACTERIZATION OF T H E SITE A N D EARLY RESULTS O N G A M M A - R A Y A N D N E U T R O N SPECTROMETRY MEASUREMENTS

E. A N D R E O T T I Universita

dell'Insubria, Dipartimento di Fisica e Maternatica, INFN- Laboratori Nazionali del Gran Sasso

Como

A. G I U L I A N I , M. P E L L I C C I A R I , C. R U S C O N I Universita

dell'Insubria,

Dipartimento

di Fisica e Maternatica,

Como

A. C E S A N A , L. G A R L A T I , M. T E R R A N I Politecnico

di Milano,

Dipartimento

di Ingegneria

Nucleare,

Milano

G.L. R A S E L L I INFN-

Sezione

di Pavia,

Pavia

An underground station for the measurement of low level radioactivity is in operation in the town of Como under the Baradello hill. The laboratory is covered by 100 m of rock (~ 300 m w.e.). The main activities carried out at the laboratory concern the measurement of the radioactive content in different kind of matrices by means of an HPGe detector shielded with radio-pure copper. Another research subject of the laboratory is the analysis of fast neutron flux produced by natural radioactivity of the rocks using liquid scintillator cells. This paper reports about the preliminary measurements carried out to characterize the site. Moreover gamma and neutron spectrometry set-up are described and some measurement results performed on low level radioactivity samples and on fast neutron flux are also reported.

1. Introduction In the frame of an interdisciplinary project funded by the University of Insubria, a station for the measurement of low level radioactivity was developed starting from the year 2002. The aim of the project was the characterization of the environment surrounding the town of Como, with particular attention for the distribution of natural radioactive nuclides. The 115

116 results obtained from preliminary measurements allowed to perform some other activities, in connection with fundamental research experiments. The chosen site for the installation of this station is an artificial cave under the Baradello hill housing the water purification plant of Como. A small area of about 10 m 2 in the deepest point of the main underground hall is dedicated to the low background laboratory. In that point the rock cover reaches about 100 m, corresponding to about 300 m water equivalent. The air conditioning system operates continuously and in standard condition the temperature is 13 °C and the humidity level about 80%.

2. C h a r a c t e r i z a t i o n of t h e site Some preliminary measurements were carried out in order to evaluate the potentiality of the site as described in Ref. 1. The measurements concerned the following items: radon ( 222 Rn) concentration in air, gamma ray background, muon background, neutron background, natural radioactivity of soil, rocks and concrete. Radon concentration was measured inside the underground hall at several different points, using detectors based on different operations principles. The Radon concentration resulted to be about 34 Bq/m 3 , i.e. a rather low value in agreement with the high forced ventilation rate (31.000 m 3 /h). Gamma-ray background was measured approximately at the centre of the area housing the Laboratory with an unshielded HPGe detector. A simple model able to account for the contributions from rock and atmosphere to the main gamma peaks enabled to extract useful informations on the activities of the rocks forming the cave walls. Activity values resulted to be about 20-30 Bq/Kg both for 238 U and 2 3 2 Th, in agreement with concentrations found in some rock samples by means of high resolution gamma spectroscopy. Measurements performed with Nal detector showed that the muon flux is reduced by about two orders of magnitude with respect to the outside sea level value (result consistent with the 300 m w.e. rock cover). Thermal neutron flux was measured at the centre of the Laboratory by a BF3 proportional counter showing a value reduced by a factor of ~ 103 with respect to sea-level external laboratories. Fast neutron flux, measured using the set-up described further on in this article, resulted to be a factor ~ 102 suppressed with respect to sea level values. These results confirmed the suitability of the underground site located under the Baradello hill for scientific activities for which a low level radioactivity is required. In fact, in spite of an observed higher radioactivity content in the constituent rocks with respect to other underground sites (that can anyway

117

be made harmless by a proper shielding), the 300 m water equivalent cover ensures a sizeable reduction of various components of the cosmic radiation. 3. Gamma spectrometry measurements 3.1. The HPGe

detector

A high purity Ge detector (sensitive volume ~ 148 cm 3 and 30% relative efficiency at 1.33 MeV) was then installed at the Baradello Laboratory under a in shield of ultra-pure OFHC copper with a minimum thickness of 20 cm: thanks to this configuration the background was reduced by about four orders of magnitude with respect to that obtained without shield. A metallic box flushed continuously with nitrogen surrounds the detector, in order to keep it in a Radon free atmosphere. 3.2. Comparison

with other

sites

Comparisons between the Baradello background spectrum and those of some similar above ground and underground installations were performed. In Table 1 we report the count rate (Counts/d/Kg) obtained with p-type HPGe detectors in the characteristic energy range of gamma spectrometry, (between 40 and 2700 KeV) for some Low Background European Laboratories. 2 We point out the lower background counting rate in the case of Baradello Laboratory, obtained thanks to the ~ 300 m w.e., with respect to the above ground ACR Austrian Laboratory: in that case the developement of a high sophisticated passive and active shielding was needed in order to reduce the HPGe background. On the contrary Baradello background is worse with respect to those of the other underground laboratories, mainly due to the much more thicker rock cover and to the more absorbing shielding used, in most cases provided by a 15 cm thickness external lead shield plus an electrolitic copper core. Table 1. Count rate (Counts/d/Kg) in the energy range 40-2700 KeV for some Low Background European Laboratories. ARC

Baradello

IRMM

PTB

LNGS

LSCE

Seiberdorf

Laboratory

HADES

UDO

(Italy)

Modane

(Belgium)

(Germany)

(Austria)

(France)

Counts/d/Kg

8200±200

7000±17

260±4

277±4

87±1

186±2

Depth m w.e.

ca. 1

300

500

2100

3800

4800

118

3.3. MDA calculations and upper bounds on content for CUORE

radioactive

Finally, to give a quantitative estimation of the potentiality of the Baradello site, Minimum Detectable Amount (MDA) were calculated for different typical environmental matrices and different isotopes, using the procedure given in Ref. 3. As an example we report MDA calculations in the case of soil, obtained assuming SHO2, p= 2.5 g/cm 3 , a cylindrical geometry and considering a hypothetical measurement time of ~ 1 day. Sensibility resulted to be about 1 mBq/g for 226 Ra, 0.7 mBq/g for 40 K and 0.1 mBq/g for other radionuclides ( 228 Ac, 137 Cs, 60 Co, 26A1). MDA calculations are also useful in order to derive upper bounds on radioactive content in the case of very high purity materials, allowing to select those suitable for fundamental research experiments. The CUORE experiment aims at the study of neutrinoless Double Beta Decay of 130 Te using TeC-2 crystals as bolometric detectors: the reduction of background induced by bulk radioactivity in the detectors themselves and in the other components which constitute the set-up is of paramount importance for the experimental results. So at the Baradello Laboratory we performed some measurements on a Te02 crystal (mass ~ 200 g), costantan wires (mass ~ 7 g) and connectors (total mass ~ 60 g). Results for TeC-2 crystal are given in Table 2 together with reference activities, calculated by means of Monte Carlo code in order to produce a background counting rate not exceding the one expected for the fulfilment of CUORE radiopurity goal.4 In this case the upper limits attainable at the Baradello Laboratory are greater than the required values (and the same happens in the case of costantan wires), so a further analysis is needed, for example by means of measurements performed at LNGS; however a first selection can be provided, as it is evident from the results on connectors, also given in Table 2: in that case a large amount of radioactive contaminants was found, thus precluding their usage in the experimental set-up. 4. Fast neutron flux measurements 4.1. Set-up

description

The tecnique used for the estimation of fast neutron flux inside the laboratory is based on proton recoil in an organic liquid scintillator. The equipment consists of three commercial cells, each one viewed by two photomultipliers, filled with liquid scintillator (Bicron B501A) manifactured by Bicron with total sensitive volume of about 1 liter. This type of scin-

119 Table 2. Radioactive upper bounds in g/g for the TeC>2 crystal (measured upper bounds are derived from MDA calculated considering a measurement time of 5 days) and radionuclides concentrations found in connectors (Bq/Kg). Nuclide

23S

Measured upper bound

Required upper bound

Activity in connectors

for T e 0 2 g/g

for T e 0 2 g/g

Bq/Kg

s

13

U

232 T h 60

Co

40

K

< 1.2 x 1 0 " <

1 5

x

10 -s

< 1 x lCT <

5

x

1 0 -13

2.4±0.2 1.2±0.15

< 1.2 x lCT 1 8

< 4.8 x 1 0 - 2 4

< 0.06

< 2.7 x 1 0 " 9

~ 10-8

0.09±0.009

tillator has been chosen for its enhanced emission of delayed light which allows optimal pulse shape discrimination suitable for neutron spectrometry in intense gamma radiation background. The spectrometer operates in the neutron energy range from 0.5 MeV up to 20 MeV and over.5 The two PMT pulses of each cell are summed up by means of a unit which also provides the neutron identification logic signal. Moreover for each cell the summed PMT signal is splitted in two branches, thus allowing the simultaneous integration of the whole PMT pulse shape (Total Charge) and of the delayed pulse tail (Delayed Charge) by means of two time gates, separated by a fixed delay. Data analysis is then based on neutron selection criteria according to pulse shape discrimination graphical cuts, by means of a comparison between Total and Delayed Charge. All data are grouped in 12 energy bins 1 MeV wide. The final neutron spectrum is then obtained by running a deconvolution iterative program starting from a zero order spectrum, determined via Monte Carlo simulation and based on U and Th concentration in the rock. In fact at the considered depth (~ 300 m w.e.) it is realistic to consider neutron as produced for the most part by fission and (a,n) reactions in the rock.

4.2. Flux

variability

Neutron flux measurements, in the energy range from 2 to 18 MeV, were performed during two periods: from May till September 2002 and from April till October 2003. In the first case the integral flux resulted to be (0.94±0.33) x 10~ 5 c m - 2 s - 1 while in the second case the result was (1.68±0.44) x 1 0 - 5 c m - 2 s _ 1 . Due to the fact that the two considered periods were characterized by very different weather conditions (Summer 2002 very rainy, Summer 2003 very dry) the hypothesis of flux variation induced by differences in rock humidity gained credit. 6 So it was decided to

120

study anticorrelation between experimental data, using proton recoil data in order to be sure that the fluctuations were not caused by deconvolution process, and rainfall data. Since a small amount of a emitters (about 1 0 - 1 1 atoms/scintillator atom) was found in the liquid scintillator, thus inducing an intrinsic background up to about 3 MeV proton equivalent, only the proton recoil spectrum above 3.5 MeV was considered. This analysis was performed using data, grouped into 9 days long runs, from April 2003 till January 2005. A maximum anticorrelation coefficient was found when recoiling proton data were shifted backward by about 20 days, while no correlation was found when no shift was applied. Figure 1 shows these results.

0.2

,

A a

-o.i

W! fir

\\

20 days 20

<50

Shift (Days)

Figure 1. Anticorrelation coefficient between recoiling proton and rainfall data versus shift applied (days).

References 1. C. Brofferio et al., J. Environ. Radioactivity 71, 159-173 (2004). 2. M. Laubenstein et al., Appl. Radiat. Isotopes 61, 167-172 (2004). 3. D. Strom, Continuing education lecture, Health Physics Society, MinneapolisMinnesota, (15 July 1998).

121 4. CUORE Collaboration, CUORE: A proposal submitted to the INFN, the us doe and the us nsf by the international CUORE Collaboration, http://crio.mib.infn.it/wig and hep-ex/0501010 (2005). 5. O. Barnaba et al., 2002 Symposium on Radiation Measurements and Applications, Ann Arbor, Michigan USA, (21-23 May, 2002). 6. A. Borio di Tigliole et al, Europhys. Lett. 67, 1045-1049 (2004).

T H E ART OF D E T E C T I N G N E U T R I N O S , 2 KM U N D E R G R O U N D - T H E T H R E E P H A S E S OF T H E S U D B U R Y N E U T R I N O OBSERVATORY

S. N. A H M E D * Department

of Physics, Queen's University 99 University Avenue, Kingston, ON K7L 3N6, Canada E-mail: naeem@surf. sno. laurentian. ca

The Sudbury Neutrino Observatory is a lOOOt heavy water neutrino detector built in a 2 km deep underground nickel mine in the Canadian province of Ontario. With its about 8500 operational photomultiplier tubes and 40 long helium filled proportional counters, it is capable of differentiating the interactions of electrontype neutrinos from the total interactions of all types of neutrinos, thus giving insight into the phenomenon of neutrino flavor transformations. Two phases of the experiment, namely the D 2 0 Phase and the Salt Phase, have been successfully completed giving precise measurements of the parameters associated with neutrino oscillations. With the deployment of the 40 neutral current detectors, the Observatory is now taking data in its final phase and is expected to make more precise measurements of the neutral current interactions of neutrinos in the pure heavy water. In this talk an overview of the SNO detector, operated with the newly installed neutron detectors, is presented and the physics results obtained so far are discussed.

1. Introduction The Sudbury Neutrino Observatory (SNO) is a heavy water Cherenkov detector built 2 km underground in a nickel mine in the Canadian province of Ontario *. This paper describes the basic building blocks of the detector including the neutral current detectors installed in the final phase of the experiment. The physics results obtained in the first two phases of the experiment are also discussed. The physics motivation of the SNO experiment include neutrino oscillation, solar neutrino flux measurement, day-night asymmetry in neutrino flux, measurements of atmospheric neutrinos and muons, and detection of *On behalf of the SNO collaboration.

122

123 antineutrinos. Of course this list is not complete and the full SNO data analysis program contains a number of other interesting topics as well. The active detection medium of the detector is 1000 tonnes of highly pure heavy water kept in a large acrylic vessel. The vessel is surrounded by about 9600 highly sensitive photomultiplier tubes. The heavy water is continuously sampled, monitored, and purified to keep the background radioactivity and other contaminations at extremely low levels. The SNO experiment was designed to be carried out in three distinct phases corresponding to three ways of detecting neutrons. The neutrons are generated in heavy water as a result of neutral current interaction of neutrinos with deuterium. In the first phase (D2O phase) the detector was filled with pure heavy water. To increase the neutron detection efficiency, 2 tonnes of purified salt was added in the second phase (salt phase) of the experiment. These two phases have been completed and their results have been published. The third phase is characterized by installation of 40 long helium filled neutral current detectors inside the acrylic vessel. Since these detectors can directly detect neutral current neutrons therefore this phase is distinct from the other two phases where statistical separation was needed to identify neutral current signals.

2. D e t e c t o r Overview The SNO detector is located underground in a nickel mine in the Canadian province of Ontario. The center of the detector is at a depth of 2092m from surface. The main detection medium is 99.92% isotopically pure heavy water, which is contained in a 12 m diameter acrylic vessel. The walls of the vessel having a thickness of 5.5cm are transparent to light. The neutrino interactions in D2O result in generation of Cherenkov light. The photons thus produced are detected by an array of 9456 Hamamatsu R1408 photomultiplier tubes installed outside the acrylic vessel. A stainless steel geodesic structure having a diameter of 17.8m supports these tubes. Each PMT is equipped with a non-imaging light concentrator to increase the effective light collection area. With these light concentrators the effective photocathode coverage of the complete PMT array is about 54% of Ait. Both the acrylic vessel and the PMT support structures are immersed in ultra pure light water. The light water shields the detector against radioactive background from the surrounding rocks and other materials in the cavity. A few additional PMTs have also been mounted on the geodesic sphere facing outward to act as cosmic veto counters.

124 During the first phase of the experiment the detector was operated with pure heavy water. In the second phase, nearly 2 tonnes of NaCl was added to the heavy water to increase the neutral current detection efficiency. These two phases have been completed and their results have been published. The third phase of the experiment is characterized by installation of 40 helium filled long proportional counters in the D%0. 36 of these counters are filled with 3He while the remaining four are filled with 4He. The former are used to directly count neutrons produced during the neutral current interaction of neutrinos in heavy water. The helium-4 counters are used to assess the background. The three flavors of neutrinos interact with heavy water through the following reactions. vx + e

> vx + e

Elastic Scattering (ES)

ve + d

• p +p + e

Charged Current Interaction (CC)

vx + d

• vx + p + n

Neutral Current Interaction (NC)

Here the subscript x stands for any of the three neutrino flavors: electron (e), muon (/i), or tau (r). The elastic scattering reaction, though written with a subscript x, is primarily sensitive to electron neutrinos with very weak sensitivities to the other two flavors. Since the neutral current reaction is sensitive to all types of neutrinos therefore this channel can be exploited to measure the total flux due to all neutrino flavors. Furthermore, comparison of this flux with the flux measured through the charged current reaction can give insight into neutrino flavor transformations. 2.1. Impurities

and Background

Suppression

The extremely low neutrino interaction probability in heavy water warrants the suppression of background to very low levels. These backgrounds are mainly due to decay of uranium and thorium chains in heavy as well as light water. Apart from these radioactive impurities, other non-radioactive impurities could severely affect the optics of the water. The heavy and light water are therefore continuously monitored and purified to assure that the impurity levels remain below the limits set by event detection. The two isotopes in the thorium-232 and uranium-238 chains that can produce 7-rays having energy greater than the photodisintegration threshold of deuterium are 2 0 8 T/ and 2UBi. To keep their concentrations at very low levels, water is regularly sampled, assayed, and purified. Fig.l shows the historical levels of 238U and 232Th in heavy water as obtained by

125

in-situ and ex-situ techniques. In the in-situ technique data are analyzed for background identification and separation while the ex-situ technique is characterized by sampling the water and analyzing its contents through different processes. The high concentration of radon near day 580 is due to the radon added for calibration purposes. It is apparent that the average concentrations of these isotopes have remained well below the limits set by event separation criteria.

a) 232 Th Q t>0

• In-situ * MnOx

• HTiO n

Goal:<3.7xlO' gTh/gD ? 0

Wl I" u b) 238U

•?„ 10

In-situ Radon

Goal:<4.5x10' 4 gU/gD : ,O

tL

a

* Wf \oo

i^i

MJ^

200 300 400 500 Days since 26 July, 2001

600

700

Weighted average

Figure 1. Backgound levels of uranium and thorium in heavy water as a function of time as deduced by in-situ and ex-situ techniques. The high radon concentration near day 580 is due to radon added to heavy water for calibration purposes.

126 2.2.

SNO

Electronics

SNO PMT electronics chain is shown in Fig.2. The analog pulse from each PMT is transported to the front end through 32m long 750 coaxial cable. The same cable carries the high voltage to the PMT as well. The signal cable goes into a photomultiplier interface card that can handle up to 8 channels. This card is connected via backplane to a daughter board that has signal processing components for all eight channels. Four such daughter boards are installed on a motherboard that occupies one of 16 slots in a VME-like SNO-specific crate. The whole setup contains 19 such crates arranged in 11 racks. The global trigger and timing are handled by several units arranged in a separate rack.

SNO_R»*JJOT|CW*>_

Figure 2.

_

r

Timing Rack

Sketch of SNO electronics setup.

The signal processing components of the front end card include on-board discriminators, integrators, timing units, analog memory, and logic units. Each crate contains its own trigger module. The discriminator thresholds are usually set by special electronics calibrations but can also be tweaked

127 manually. Whenever a PMT pulse having an amplitude higher than the set threshold arrives, the corresponding discriminator fires and initiates the integration. The integration continues until either a global trigger arrives or 400T?5 have passed. In case of no global trigger, the channel gets reset automatically. Integration leads to four types of integrated pulses, which differ in gains and integration times. The relative time of the firing of the channel is determined by charging of a capacitor relative to the global trigger. The event timing is determined by the time of issuance of global trigger with respect to the 50Mfi system clock. Global time synchronization is done through a dedicated GPS unit. Details of the electronics and data acquisition can be found elsewhere 1. 3. D20

Phase D a t a Analysis and Results

In this pure heavy water phase of the experiment, the NC neutrons were detected through their capture on deuterium, which resulted in the following reactions. n + d —> t* t* —y t + 7

(6.25MeV)

Here d and t stand for deuterium and tritium respectively. The Q.25MeV photon thus produced had the capability to Compton scatter an electron. The scattered electron, having high enough energy, produced Cherenkov light, which was detected by the PMT array. The phase 1 of the experiment corresponded to a total livetime of 306.4 days of neutrino interactions in pure heavy water. The main target of this phase was to test the null hypothesis of no neutrino flavor transformation. To retrieve the ES, CC, and the NC+background events from the dataset, extended maximum likelihood method was used. The probability density functions were generated from Monte Carlo simulations. The distributions in Teff, the volume weighted radial variable p = (R/RAV)3, and cos# 0 were then simultaneously fitted to these PDFs. Here RAV is the radius of the acrylic vessel and 6Q is the angle between the reconstructed direction of the event and the instantaneous direction from the Sun to the Earth. Under the assumption of undistorted 8B energy spectrum 5 , the electron neutrino flux and the total flux of muon and tau neutrinos in units of 106 cm~2s^1 were found to be 4>e = 1.76+°;°5(Stat.)l°;0°3(SySt.)
128 Since 0^ T is 5.3(7 above zero, it provides a strong evidence for rejection of our null hypothesis of no flavor transformation (see, for example, 2 ' 3 ) . The total flux measured in this phase confirmed the solar model predictions. 4. Salt Phase Data Analysis and Results In the second phase of the experiment, about 2 tonnes of highly purified NaCl was added to the heavy water. The idea was to enhance the neutron detection efficiency through the capture of neutrons on 35 C7. Such a capture results in the emission of a number of 7-rays with a total energy of 8.6MeV according to the following reactions. n + 35Cl —• 36

36

C7*

36

Cl* —+ Cl + 2.5 7

(8.6MeV)

There are two main advantages of neutron capture on chlorine as compared to deuterium. Firstly the NC neutron capture cross section for Chlorine is about 446, which is much higher than the 0.5mb for deuterium. The higher capture cross section increased the capture efficiency in the second phase to about 40.7% as compared to 14.4% in phase 1. The other main advantage of using NaCl can be understood by noting that here on the average 2.5 photons are produced as compared to single photon during neutron capture on deuterium. More photons means scattering of more electrons and consequently production of more Cherenkov cones. Hence the total Cherenkov light produced in salty water is more isotropic than in pure heavy water. This isotropy was used to statistically separate the NC signals from the CC signals, which removed the need for energy constrained analysis that was performed in the first phase of the experiment. Specifically, the variable used to determine the isotropy was 0u = (3\ +4/34, where N~\

N

Here N is the total number of triggered PMTs in the event, 8ij is the angle between triggered PMTs i and j relative to the reconstructed event vertex, and Pi is the Legendre polynomial of order I. The result of this analysis is shown in Fig.3. The isotropy