Advances in Electronics and Electron Physics, Volume 28A: Photo-Electronic Image Devices, Proceedings of the Fourth Symposium

ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS VOLUME 28A

Advances in

Electronics and Electron Physics EDITED BY L. MARTON National Bureau of Standards, Washington, D.C.

Assistant Editor CLAIREMARTON

EDITORIAL BOARD T. E. Allibone H. B. G. Casimir W. G. Dow A. 0. C. Nier E. R. Piore

M. Ponte A. Rose L. P. Smith I?. K. Willenbrock

VOLUME 28A

1969

ACADEMIC PRESS

New York and London

Photo-Electronic Image Devices PROCEEDING8 O F THE FOURTH SYMPOSIUM HELD AT IMPERIAL COLLEGE, LONDON, SEPTEMBER 16-20, 1968

EDITED BY J. D. McGEE, O.B.E., Sc.D., F.R.S. D. McMULLAN, M.A., Ph.D. E. KAHAN, B.Sc., Ph.D. AND

B. L. MORGAN, B.Sc., Ph.D. Department of Physics, Imperial College, University of London

1969

ACADEMIC PRESS

London and New York

COPYRIGHT

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LIST OF CONTRIBUTORS H. D. ABLES, Flagstaff Station, U.S. Naval Observatory, Flagstaff, Arizona, U.S.A. (p. 1) R. W. AIREY, Applied Physics Department, Imperial College, University of London, London, England (p. 89) N. ARMAD,Department of Pure and Applied Physics, The Queen’s University of Belfast, Belfast, Northern Ireland (p. 999) H. ANDERTON,Westinghouse Electric Corporation, Electronic Tube Divkion, Elmira, New York, U.S.A. (p. 229) M. ASANO, Department of Electronic Engineering, University of ElectroCommunications, Chofu City, Tokyo, Japan (pp. 309, 381) H. BACIK, Applied Physics Department, Imperial College, University of London, London, England (p. 61) G. S. BAKKEN, Physic8 Department, Rice UniverBity, Houston, Texaa, U.S.A. (P. 907) M. E. BARNETT,Applied Physics Department, Imperial College, University of London, London, England (p. 545) J . R. BASKETT, A.C. Electronics, Defense Research Laboratories, General Motors Corporation, Santa Barbara, California, U.S.A. (p. 1021) c . W. BATES,JR., varian Associates, Palo Alto, California, U.S.A. (pp. 451, 545) W. A. BAUM,Lowell Observatory, Flagstaff, Arizona, U.S.A. (p. 753) W. BAUMOARTNER, Institut f u r Technische Physik, E T H , Zurich, Switzerland (P. 151) J . E. BECKMAN, Physics Department, Queen Mary College, University of London, London, England (p. 801). It, L. BEURLE,Department of Electrical and Electronic Engineering, Nottingham University, Nottingham, England (pp. 635, 1043) R. R. BEYER, Westinghouae Electric Corporation, Electronic Tube Division, Elmira, New York, U.S.A. (pp. 105, 229) 1’. BIED-CHARRETON, Observatoire de Paris, Paria, Prance (p. 27) A. BIJAOUI, Observatoire de Paris, Paris, France (p. 27) M. BLAMOUTIER, Compagnie Franpise Thomaon-Houston, Paris, France (p. 273) A. H. BOERIO, Westinghouse Electric Corporation, Electronic Tube Division, Elmira, New York, U.S.A. (p. 159) A. BOKSENBERG, Mullard Space Science Laboratory, Physics Department, U n i versity College, London, England (p. 297) I. S. BOWEN, M t . Wilson and Palomar Observatories, Carnegie Institution of Washington, California Institute of Technology, Paaadena, Calqornia, U.S.A. (P. 767) P. W. J . L. BRAND,Department of Astronomy, University of Edinburgh, Royal observatory, Edinburgh, Scotland (pp. 737, 783) F. LE CARVENNEC, Compagnie Gknkrals de Tklkgraphie Sans Fil, Paria, France (P. 265) W. N. CHARMAN, Atomic Energy Reaearch Establishment, Harwell, Berkshire, England (p. 705) P. A. CRATTERTON,Department of Electrical Engineering, University of Liverpool, Lancaahire, England (p. 1041)

vi

LIST OF CONTRIBUTORS

M. COHEN, Applied Physics Department, Imperial College, University of London, London, England ( p . 125) P. R. COLLINQS, Westinghouse Electric Corporation, Electronic Tube Division, Elmira, New York, U.S.A. ( p . 105) M . COMBES,Observatoire de Meudon, Meudon, France (p. 39) A. C. CONRAD,JR.,Physic8 Department, Rice University, Howton, Texas, U.S.A. (P. 907) J. M . LE CONTEL,Obaervahre de Paris, Paris, France ( p . 27) R. J. CORPS,Royal Aeronautical Establishment, Farnborough, Hampshire, England ( p . 827) G. I(. L. CRANSTOUN, Inorganic Chemistry Laboratory, University of Oxford, Oxford, England (p. 875) G. W . A. CZEEALOWSKI, Department of Medical Physics, University of Leeds, England ( p . 653) M . V . DANIELS, Department of Electrical Engineering, University of Nottingham, Nottingham, England ( p . 635) R. W . DECKER, Westinghouse Aerospace Division, Baltimore, Maryland, U.S.A. (pp. 19, 357) J. H . M . DELTRAP, Aerojet Delft Corporation, Melville, New York, U.S.A. ( p . 443) E. W. DENNISON, Mt. Wilson and Palomar Obseruatoriea, Paaadena, California, U.S.A. ( p . 767) K . DEUTSCHER, Ernst Leitz G.m.b.H. Optical Works, Wetzlar, West Germany (P. 419) P . DOLIZY, Laboratoires d’$lectronique et de Physique Appliqude, Limeil-Brdvannes, France ( p . 367) B. DRIARD, Compagnie Franpaise Thomson-Houston,Division Tubes .@ectroniques, Paris, Prance ( p . 931) M . DUCHESNE, Observatoire de Paris, Paris, France ( p . 27) M. DvoBAK, Tesla-VUVET, Prague, Czechoslovakia ( p . 347) D. W. EQAN,Jet Propulsion Laboratory, Caliifornia Institute of Technology, California, U.S.A. (p. 801) C. T. ELLIOTT, Royal Radar Establishment, Malvern, Worcestershire, England ( p . 1041) D. L. EMBERSON, Mullard Ltd., Mitcham, Surrey, England ( p . 119) L. ENGLAND, Applied Physics Department, Imperial College, University of London, London, England ( p . 546) G. ESCHARD, Laboratoires d’glectronique et de Physique Appliquke, LimeilBrdvannes, Prance ( p p . 499, 989) J. M. FAWCETT, Westinghouse Defense and Space Celzter, Baltimore, Maryland, U.S.A. ( p . 289) P. FELENBOK, Observatoire de Meudon, Meudon, France ( p . 39) J. R. FOLKES, English Electric Valve Co. Ltd., Chelmsford, England ( p . 375) D. P. FOOTE,Electro-Optical Systems, Inc., Paaadena, Calqornia, U.S.A. (p. 1059) K . G. FREEMAN, Mullard Research Laboratories, Redhill, Surrey, England ( p . 837) B. C. GALE, Department of Pure and Applied Physics, The Queen’s University of Belfast, Belfmt, Northern Ireland ( p . 999) B. R. C . GARFIELD,English Electric Valve Co., Chelmsford, England ( p . 375) R. K. H. GEBEL, Aerospace Research Laboratories, Wright-Patterson A i r Force Bme, Ohio, U.S.A. ( p . 685)

LIST OF CONTRIBUTORS

vii

A. GEURTS,N . V . Philips’ Gloeilumpenfabrieken, Eindhoven, The Netherlands (P. 616) R. GIESE,Phyaikaliaches Institut der Univeraitat Bonn, West Germany (p. 919) 0. GILDEMEISTER,Physikalischea Inatitut der Universitat Bonn, Weat Germany (P. 919) G. W . GOETZE, Weatinghouae Electric Corporation, Electronic Tube Division, Elmira, New York, U.S.A. (pp. 106, 169) A. W . GORDON,20th Century Electronics Ltd., Croydon, England (p. 433) J. GRAF, Laboratoirea d’dlectronique et de Physique AppliquBe, Limed - Brdvannes, France (p. 499) M . GREEN, Weatinghouae Electric Corporation, Electronic Tube DivCion, Elmira, New York, U.S.A. (p. 807) G. A. GROSCH, AEG-Telefunken, SoJinger Str. 100, 79 U l m (Donau), Weat Germany (p. 603) P. R. GROVES,Marconi Inatrumenta Ltd., Longacres, St. Albans, Hertfordshire, England (p. 827) J. GUERIN,Observatoire de Meudon, Meudon, France (p. 39) A. GUEST,Mullard Research Laboratories, Redhill, Surrey, England (p. 47 1) A. H. HANNA,Aerojet Delft Corporation, Melville, New York, U.S.A. (p. 443) J. R. HANSEN,Westinghouae Research Laboratories, Pittaburgh, Pennsylvania, U.S.A. (p. 807) W. HARTH, Inatitut f u r Technbche Electronik der Technischen Hochachule Munchen, Weet Germany (p. 636) P. HARTMANN, Laboratoirea de PhotoWectricitB dea Facultha dea Sciencea de Dijon et de Besanpon, France ( p . 409) 8. HASEOAWA, Department of Electronic Engineering, University of ElectroCommunications, Chofu City, Tokyo, Japan (p. 553) G. A. HAY, Department of Medical Phyaica, University of Leeda, England

(P. 663)

W . HEIMANN,Forachungslaboratorium, Wieabaden-Dotzheim, Weat Germany (P. 677) M . HERRMANN, Forachungslaboratorium, Wiesbaden-Dotzheim, West Germany (P. 955) W . HERSTEL,The Radiological Department, Univeraity Hoapital, Leiden, The Netherlands (p. 647) A. V . HEWITT,Flagstaff Station, U.S. Naval Observatory, Flagstaff, Arizona, U.S.A. (p. 1) R. L. HILLS,Department of Electrical and Electronic Engineering, University of Nottingham, Nottingham, England (p. 636) G. W . HINDER,Atomic Energy Research Eatablishment, Harwell, Berkahire, England (p. 966) M . HIRASHIMA, Department of Electronic Engineering, University of ElectroCommunicationa, Chofu City, Tokyo, Japan (pp. 309, 381) T. HIRAYAMA, Electron Tube Division, Nippon Electric Company, Tamagawa Plant, Kawasaki, Japan (p. 189) K . HIRSCHBERU, Ernst Leitz G.m.b.H. Optical Works, Wetzlar, West Germany (P. 419) E . L. HOENE, Forschungalaboratorium, Wieabaden-Dotzheim, Weat Germany (p. 677).

...

Vlll

LIST OF CONTRIBUTORS

R. T. HOLMSHAW, Mullard Research Laboratories, Redhill, Surrey, England (P. 471) H . HORI,Toshiba Reaearch and Development Centre, Tokyo Shibaura Electric Co. Ltd., KomuLai, Kawasaki, Japan (p. 253) P. IREDALE, Atomic Energy Research Establishment, Harwell, Berkshire, England ( p p . 689, 965) F . W . JACKSON, Research Laboratories, Electric and Musical Industriea Ltd., Hayes, Middleaex, England (p. 247) V . JAR&, Vacuum Electronics Research Institute, Prague, Czechoslovakia (p. 523) M. JEDLI~KA, Vacuum Electronics Research Institute, Prague, Czechoslovakia (P. 323) G. W. JENKINSON, Department ($ Electrical and Electronic Engineering, Nottingham University, Nottingham, England (p. 1043) A. S. JENSEN,Westinghouse Defense and Space Center, Baltimore, Maryland, U.S.A. (p. 289) J . M . JOHNSON, Research and Development Laboratories, Corning Glass Works, Corning, New York, U.S.A. (pp. 487, 507) J . A. JORDAN, JR., Department of Physics, Rice University, Houston, Texas, U.S.A. (p. 907) E . KAHAN, Applied Physics Department, Imperial College, University of London, London, England ( p . 725) Y. KAJIYAMA, Electron Tube Division, Nippon Electric Company, Tamagawa Plant, Kawasaki, Japan (p. 189) J. S. KALAFUT,Westinghouse Electric Corporation, Electronic Tube Division, Elmira, New York, U.S.A. (p. 105) T . KAWAHARA, Electron Tube Division, Nippon Electric Company, Tamagawa Plant, Kawasaki, Japan ( p . 189) H . KAWAKAMI, Matsushita Research Institute, Tokyo, Kawasaki, Japan (p. 81 ) B. KAZAN, I B M Watson Research Center, Yorktown Heights, New York, U.S.A. ( p . 1069) M . H. KEY, Department of Pure and Applied Physics, The Queen’s Iiniversity of Belfast, Belfast, Northern Ireland (p. 999) M . J . KIDGER,Applied Optics Section, Physics Department, Imperial College, University of London, London, England ( p . 759) Y . KIUCRI, Toshiba Research and Development Centre, Tokyo Shibaura Electric Co. Ltd., Komukai, Kawasaki, Japan (p. 253) T . KOHASHI, Matsuahita Research Institute Tokyo, Inc., Ikuta, Kawasaki, Japan (p. 1073) D. KOSSEL, Erndt Leitz G.m.b.H. Optical works, Wetzlar, West Germany (p. 419) J. K. KRIESER,A E G Telefunken, Soflinger Str. 100, 79 Ulm (Donau) West Germany (p. 603) G. E . KRON, Flagstaff Station, U S . Naval Observatory, Flagstaff, Arizona, U.S.A. (p. 1 ) W. KUHL, Philips Reaearch Laboratories, A’. V . Philips’ Gloeilampenfabrieken, Eindhoven, The Netherlands (p. 616) C. KUNZE,Porachungalaboratorium., Wiesbaden-Dotzheim, West Germany ( p . 955) W. KUNZE,AEG-Telefunken, 2 Hamburg 11, Steinhoft 9, West Germany (p. 629) A. LABEYRIE, Observatoire de Paris, 92-Meudon, France (p. 899) D. L. LAMPORT, Mullard Research Laboratoriea, Redhill, Surrey, England (p. 567) R. LEGOUX,Laboratoires d’&lectronique et de Physique Appliqude, LimeilBrdvannee, France (p. 367)

LIST OF CONTRIBUTORS

ix

B. T. LIUDY,Department of Pure and Applied Physics, The Queen's University of Belfaet, Belfaat, Northern Ireland (p. 375) I. D. LIU, AC Electronic.?, Defense Research Laboratories General Motors Corporation, Santa Barbara, California, U.S.A. (p. 1021) B. E. LONG,Mullard Ltd., Mitcham, Surrey, England (p. 119) J. L. LOWRANCE, Princeton University Observatory, Princeton, New Jersey, U.S.A. (P. 861) R. LYNDS,Kilt Peak National Observatory, Tucaon, Arizona, U . S . A. (p. 745) J. D. MCGEE,Applied Physics Department, Imperial College, University of London, London, England (pp. 61, 89) D. MCMULLAN, Royal Greenwich Observatory, Herstmonceux, Suasex, England (PP. 61, 173) H. MAEDA, Matsuahita Reaearch Institute, Tokyo, Kawaaaki, Japan (p. 81) B. W. MANLEY,Mullard Research, Laboratories, Redhill, Surrey, England (p. 471) R. MARTIN,Atomic Energy Research Establishment, Harwell, Berkshire, England (P. 981) H. MESTWERDT,United States A i r Force, Wright-Patterson A i r Force Baae, Ohio, U.S.A. (p. 19) K. MEYERHOFF,A E G Telefunken, 2 Wedel, Holstein, West Germany (p. 629) D. E. MILLER, Physics Department, University College of North Walea, Bangor, Walea (p. 513) K. MIYAJI, Matsuahita Electric Industrial Co., New York, U.S.A. (p. 1073) S . MIYASHIRO,Toshiba Research and Development Centre, Tokyo Shibaura Electric Go. Ltd., Kawaaaki, Japan (p. 191) E. MIYAZAKI,Matsuahita Reaearch Institute Tokyo, Inc., Ikuta, Kawaaaki, Japan (P. 81) B. L. MORGAN,Applied Physics Department, Imperial College, University of London, London, England (p. 1051) S . NAKAMURA, Matsuahita Research Institute Tokyo, Inc., Ikuta, Kawasaki, Japan (p. 1073) T. NAKAMURA, Matsushita Reaearch Inatitute Tokyo, Inc., Ikuta, Kawaaaki, Japan (p. 1073) M. J. NEEDHAM, Department of Physics, Queen Mary College, London, England (P. 129) P. D. NELSON,English Electric Valve Go. Ltd., Chelmsford, Essex, England (P. 209) A. C. NEWTON, Mullard Space Science Laboratory, Department of Physics, University College, London, England (p. 297) T. NINOMIYA, N H K Technical Research Laboratories, Setagaya, Tokyo, Japan (P. 337)

G. NIQUET,' Laboratoires de Photodlectricitd des Facult& dea Sciences de Dijon et de Beaanpon, France (p. 409) M. NOVICE, Westinghouse Electric Corporation, Electronic Tube Diviaion, Elmira, New York, U.S.A. (p. 1087) B. NOVOTN~T, Vacuum Electronics Reaearch Inatitute, Prague, Czechoslovakia (P. 523) Y . NOZAWA, Smithsonian fnatitution, Astrophyeical Observatory, Cambridge, Massachusetts, U.S.A. (p. 891) S. NUDELMAN, University of Rhode Island, Electrical Engineering Department, Kingston, Rhode Island, U.S.A. (p. 677)

X

LIST OF CONTRIBUTORS

T . W. O’KEEFFE, Weatinghome Research Laboratoriea, Pittaburgh, Pennaylvania, U.S.A. (p. 47) M . OLIVER,Applied Physics Department, Imperial College, Univeraity of London, London, England ( p . 61) J . v. OVERHAGEN,N . V . Philipa’ Qloeilampenfabrieken, Eindhoven, The Netherlands (p. 615) C. H.PETLEY, Mullard Research Laboratoriw, Redhill, Surrey, England (p. 837) J . P. PICAT,Obaervatoire de Meudon, Meudon, France (p. 39) R. POLAERT, Laboratoirea d’glectronique et de Phyaique Appliqude, LimeilBrBvannes, France ( p . 989) L. J . VAN DER POLDER, N . V . Philipa’ Qloeilampenfabrieken, Eindhoven, The Netherlands (p. 237) J . R. POWELL, Kitt Peak Natiortal Obeervatory, Tucaon, Arizona, U.S.A. (p. 745) D. L. PULPREY, Department of Electrical Engineering, Univeraity of Mancheater, England (p. 1041) W . P. RAFFAN, 20th Century Electronica Ltd., Croydon, Surrey, England ( p . 433) R. P. RANDALL, E.M.I. Electronica Ltd., Valve Division, Ruklip, Middlesex, England (p. 713) G. RETZLAFF, AEQ- Telejunken, Hamburg, West Germany (p. 629) G. T. REYNOLDS, Palmer Physical Laboratory, Princeton Univeraity, Princeton, New Jersey, U.S.A. (p. 939) E . A. RICHARDS, Signala Research and Development Eatabliahment, Chriatchurch, Hampshire, England (p. 661) E . W . T . RICHARDS, Atomic Energy Reaearch Eatabliahment, Harwell, Berkahire, England (p. 981) J . H . T . VAN ROOSMALEN, N . V . Philipa’ Gloeilampenfabrieken, Eindhoven, The Netherlanda (p. 281) D. J. RYDEN,Atomic Energy Research Eatabliahment, Harwell, Berkahire, England (p. 589) W .M . SACKINGER, Research and Development Laboratoriea, Corning Glaea Worka, Corning, New York, U.S.A. (pp. 487, 507) F . SCHAFF, C E R N , Geneva, Switzerland (p. 535) P. SCHAOEN, Mullard Reaearch Laboratoriea, Redhill, Surrey, England (p. 393) G. SOHUSTER, Phy8ikaliachea Inatitut der Univeraitiit Bonn, Weat B e m n y ( p . 919) S . SHIROUZU, Toahiba Reaearch and Development Centre, Tokyo Shibaura Electric Co., Ltd., Kawaeaki, Japan (p. 191) M . SCHMIDT, Mt. Wilaon and Palomar Obaervatoriea, Carnegie Inatitution of Waehington, California Inatitute of Technology, Pmadena, California, U.S.A. (P. 767) R. W . SMITH,Applied Optica Section, Physics Department, Imperial College, Univeraity of London, London, England (pp. 1011, 1051) W . A. SMITH,The Rutherford Laboratory, Chilton, Dideot, Berkahire, England (p. 1041) D. W . S . SMOUT,Atomic Energy Reaearch Ealablhhment, Harwell, Berkahire, England (p. 966) M . J . SMYTH,Univeraity of Edinburgh, Department of Astronomy, Royal Obaervatory, Edinburgh, Scotland ( p . 737) A. M . STARK, Mullard Reaearch Laboratoriea, Redhill, Surrey, England (p. 567) C. H. A. SYMS,Services Electronios Reaearch Laboratory, Baldock, Hertfordahire, England (p. 399)

LIST OF CONTRIBUTORS

xi

Z. SZEPESI,Westinghouse Electric Corporation, Electronic Tube Division, Elmira, New York, U.S.A. (p. 1087) H . TACHIYA,N H K Technical Research Laboratories, Setagaya, Tokyo, Japan (P. 337) K . TAKETOYHI, N H K Technical Research Laboratories, Setagaya, Tokyo, Japan (P. 337) D. G. TAYLOR, Mullard Research Laboratories, Redhill, Surrey, England (p. 837) M . TEPINIER,Laboratoires de Photodlectricitd des Facultb des Sciences de Dijon et de Besanpon, France (p. 409) R. F . THUMWOOD, Department of Physics, Queen Mary College, London, England (P. 129) G. 0. TOWER, Applied Physsios Department, Imperial College, University of London, London, England ( p . 173) S . TSUJI,Toahiba Research and Development Centre, Tokyo Shibaura Electric Go. Ltd., Komukai, Kawaaaki, Japan (p. 253) A. A. TURNBULL, Mullard Research Laboratories, Redhill, Surrey, England (p. 393) Y . UNO,Matsushita Research Institute Tokyo Inc., Ikuta, Kawaaaki, Japan (p. 81) B. P. VARMA, Applied Physics Department, Imperial College, University of London, London, England (p. 89) J . VINE, Westinghouse Research Laboratories, Pittsburgh, Pennsylvania, U.S.A. (PP. 47, 537) P. VERNIER,Laboratoires de Photodlectricitd des Facultds des Sciences de Dijon et de Besanpon, France (p. 409) S. VERON,Compagnie Gdndrale de Tdldgraphie s a w Pil, Orsay, France (p. 461) K . H . WAGNER,Department of Electrical Engineering, University of Salford, Salford, Lancaahire, England (p. 1033) M . F . WALKER,Lick Observatory, University of California,Santa Cruz, California, U.S.A. (p. 773) J . WARDLEY, Research Laboratories, Electric and Musical Industries Ltd., Hayes, Middlesex, England ( p . 247) G. WENDT,Compagnie Gdndrale de Tgldgraphie Sans Fil, Orsay, France (p. 137) W. L. WILCOCK, Physics Department, University College of North Wales, Bangor, Wales (p. 513) G. A. WILSON,Applied Physics Department, Imperial College, University of London, London, England (p. 1051) H . S . WISE, Atomic Energy Research Establishment, Harwell, Berkahire, England (P. 981) G . WL~RICK, Observatoire de Paris, Meudon, France (p. 787) R. D. WOLSTENCROFT, Royal Observatory, Edinburgh, Scotland (p. 783) A, W. WOODHEAD, Mullard Research Laboratories, Redhill, Surrey, England (P. 667) C . G. WYNNE, Applied Optics Section, Physics Department, Imperial College, University of London, London, England ( p . 759) P. M . ZUCCHINO,Princeton University Observatory, Princeton, New Jersey, U.S.A. (P. 851)

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FOREWORD Three years have elapsed since we presented in our Volume 22 Parts A & B the proceedings of the Third Symposium on Photo-Electronic Image Devices. It is a great pleasure to publish this record of the fourth one. As Professor McGee points out in his Preface, the important aspect is that this subject is still a growing field. With the development of extra-terrestrial astronomy (orbiting space telescopes, etc.) one would have expected a lessening of interest in aids to terrestrial devices. The contrary happened and, as usual, new developments stimulate new interest in older ones. Professor McGee and his associates succeeded again in organizing an outstanding Symposium and in collecting its material herein. I am certain to express the thanks of all my colleagues, who are users of these volumes, in emphasizing the devotion and amount of work required for this task. It is to be hoped that he and his collaborators will consider to renew their effort in the not too distant future. They have already earned the esteem of the scientific community; their continued efforts make these Symposia into an established feature of our scientific life, which we have come to appreciate at regular intervals. I should like t o use this opportunity to list some of our present expectations of new authors and subjects in our forthcoming volumes.

H. M. ROSENSTOCK

Study of Ionization Phenomena by Mass Spectroscopy J. P. BLEWETT Recent Advances in Circular Accelerators S. AMELINCKX Image Formation at Defects in Transmission Electron Microscopy Quadrupoles as Electron Lenses P. W. HAWKES J. ROWE Nonlinear Electromagnetic Waves in Plasmas G. BENEand E. HENEITX Magnetic Coherence Resonances and Transitions at Zero Frequency N. R. WHETTEN and D. H. DAWSON Mass Spectroscopy Using Radio Frequency Quadruple Fields Ion Beam Bombardment and DopD. B. MEDVED ing of Semiconductors xlii

xiv

FOREWORD

E. R. ANDREWand S. CLOUGH J. A. MERCEREAUand D. N. LANGENBERU S. DATZ

Nuclear and Electronic Spin Resonance Josephson Effect and Devices

Reactive Scattering in Molecular Beams Luminescence of Compound SemiF. E. WILLIAMS conductors Energy Beams as Tools K. H. STEIQERWALI) et al. Electron Precursors RICHARDG. FOWLER The Physics of Long Distance H. A. WHALE Radio Propagation Macroscopic Approach to FerroJ. FOUSEK and V. JANOVIC electricity Sputtering M. W. THOMPSON Plasma Instabilities and TurbuC. KEITHMCLANE lence Electron Polarization STEPHENJ. SMITH F. J. KERRand WM.C. ERICKSON Galactic and Extragalactic Radio Astronomy Light Interaction with Plasma HEINZRAETHER Superconducting Magnets P. F. SMITH Recent Advances in Field Emission L. SWANSON and F. CHARBONNIER Microfabrication Using Electron A. N. BROERS Beams The Measurements of Lifetimes of A. CORNEY Free Atoms, Molecules and Ions Energy Distribution in ThermioniB. W, ZIMMERMANN cally Emitted Electron Beams Information Storage in Microspace S. NEWBERRY Recent Progress on Fluidics H. BURKEHORTON Theory Network L. WEINBERG The Formation of Cluster Ions in W. ROTHand R. NARCISSI Gaseous Discharges and in the Ionosphere

L. MARTON Washington, D .C. June 1969

PREFACE We have pleasure in presenting in these two books, Volume 28, Parts A & B of Advances in Electronics and Electron Physics, the papers read and discussed at the Fourth Symposium on Photo-Electronic Image Devices held at Imperial College from 16th to 20th September 1968. The number of papers presented, and the attendance, at this Symposium are convincing evidence of the interest that is maintained in this field. The four Symposia in the ten years since the first was held have recorded the development of this field of electron physics from a rather limited one to one of world wide interest, in which many of the great laboratories are actively working. Inevitably some of the projects discussed in the earlier Symposia have dropped into oblivion, but many have prospered and these volumes record their progress and, in many cases, ultimate success. We like to think that these meetings, and the lively and objective discussion that they encourage, have done much to advance this subject. It is a considerable advantage that it has been possible to include the proceedings of all four Symposia inJthiswell known series Advances in Electronics and Electron Physics published by the Academic Press. By maintaining a certain uniformity and continuity, the subject matter in this field has been made more readily available to those who are interested. We thank the Editor-in-Chief, Dr. L. Marton, and Academic Press for their co-operation which has made this possible. We have endeavoured to maintain a reasonable uniformity of presentation throughout the volume while retaining the scientific sense as intended by the authors. While we have made every effort to correct accidental errors, the author has the final word as regards subject matter. The Editors wish to thank their colleagues of the Applied Physics Department of Imperial College for their assistance in running the Symposium and Miss Margaret Jones, secretary, for her help and meticulous care in dealing with the papers. We should also like t o put on record our great appreciation of the continuing interest and support from Professor Lord Blackett, O.M., C.H., President of the Royal Society, who opened the Symposium. We also thank all those who participated and contributed papers for the excellent spirit in which the meeting was conducted, making it both very informative and very enjoyable. J . D. MCGEE

D. MCMULLAN E. KAHAN B. L. MORGAN

London June 1969 xv

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CONTENTS LIST OF CONTRIBUTORS

.

V

FOREWORD.

xiii

PREFACE.

xv

CONTENTSOF VOLUME B

.

xxi

Electronography A Technical Description of the Construction, Function, and Application of the U.S. Navy Electronic Camera. By G. E. KRON,H. D. ABLESAND A. V. HEWITT

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Large-image Electronographic Camera. MESTWERDT .

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By R. W. DECKERAND H. 19

Sur Quelques Progrbs RBcents Apportee B la Camera l%xtronique B Focalisation $lectrostatique et sur son Application en Physique et en Astronomie. By P. BIED-CHARRETON, A. BIJAOUI,M. DUCHESNE 27 AND J. M. LE CONTEL

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Electronic Cameras for Space Research. By M. COMBES, P. FELENBOK, J. GUERINAND J. P. PICAT. 39 A High-resolution Image Tube for Integrated Circuit Fabrication. T. W. O’KEEFFEAND J. VINE

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By 47

Further Developments of the Spectracon. By J. D. MCGEE,D. MCMULLAN, H. BACIKAND M. OLIVER 61

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Cathode-ray Tube with Thin Electron-permoable Window. By Y. UNO, H. KAWAKAMI, H. MAEDAAND E. MIYAZAKI.

81

Image Tubes Cascade Image Intensifier Developments. By J. D. MCGEE, R. W. AIREY AND B. P. VARMA .

89

A Family of Multi-stage Direct-view Image Intensifiers with Fiber-optic Coupling. By P. R. COLLINGS, R. R. BEYER,J. S. KALAFUT AND G. W. GOETZE .

105

Some Aspects of the Design and Manufacture of a Fibre-optic Coupled Cascade Image Intensifier. By D. L. EMBERsoN AND B. E. LONG

119

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A Proximity-focused Image Tube. By M. J. NEEDHAM AND R. F. THUMWOOD

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129

INTIC, an Image INTensifying, Integrating and Contrast-enhancing Storage Tube. By C . WENDT

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A Light Amplifier with High Light Output. By W. BAUMGARTNER xvii

137

161

CONTEXTS

xviii

Signal Generating Tubes SEC Camera-tube Performance Characteristics and Applications. By . G. W. GOETZE AND A. H. BOERIO Some Properties of SEC Targets. By D. MCMULLANAND G. 0. TOWLER. Newly Developed Image Orthicon Tube with a MgO Target. By Y. KAJIYAMA, T. KAWAHARA AND T. HIRAYAMA . Electrostatically Scanned Image Orthicon. By S. MIYASHIRO AND S. SHIROUZO .

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173 189 191

The Development of Image Isocons for Low-light Applications. By 209 P. D. NELSON , Dynamic Imaging with Television Cameras. By H. ANDERTON AND 229 R.R.BEYER . Beam-discharge Lag in a Television Pick-up Tube. By L. J. v. D. POLDER. 237 A 13-mmAll-Electrostatic Vidicon. By J. WARDLEY AND F. W. JACKSON. 247 An Infra-red Sensitive Vidicon With a New Type of Target. By H. HORI, S. TSUJI AND Y. KIUCHI . 253 Recherche d’un Dispositif Nouveau do TBlBvision Thermique. By F. LE CARVENNEC . 265 Un Tube de Prise de Vues Sensible aux Rayons X. By M. BLAMOUTIER. 273 Adjustable Saturation in a Pick-up Tube with Linear Light Transfer Characteristic. By J. H. T. VAN ROOSMALEN . 281 Measurement of TV Camera Noise. By A. S. JENSEN AND J. M. FAWCETT.289 An Electromechanical Picture Signal Generating Device. By A. BOKSENBERU AND A. C. NEWTON . 297 Effects of Caesium Vapour upon the Target Glass of Image Orthicons. By M. HIRASHIMA AND M. ASANO .

Photocathodes and Phosphors

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Research on Photocathodes in Czechoslovakia. By M. JEDLI~EA Crystal Structure of Multialkali Photocathodes. By T. NINOMIYA, AND H. TACHIYA. K. TAKETOSHI Some Properties of the Trialkali Sb-K-Rb-Cs Photocathode. By M. DVO~AK. Decay of S.20 Photocathode Sensitivity Due to Ambient Gases. By R. W. DECKER A New Technology for Transferring Photocathodes. By P. DOLIZYAND R. LEUOUX Improvements to Photocathodes for Pulse Operation. By B. R. C. GARFIELD, J. R. FOLKESAND B. T. LIDDY Some Getter Materials for Caesiwn Vapour. By M. HIRASHIMA AND M. ASANO New Approaches to Photoemission at Long Wavelengths. By P. SCHAGEN AND A. A. TURNBULL . . , Gallium Arsenide Thin-film Photocathodes. By C. H, A. QYMS

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323 337 347 357 367 375

381 393 399

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CONTENTS

fitude de l’fimission Photoblectrique des Structures MBtal-IsolantMBtal. By P. VERNIER,P. HARTMANN, G. NIQUETAND M. TEPINIER. 409 Interference Photocathodes. By D. KOSSEL, K. DEUTSCHER AND K.HIRSCH419

BERG

The Development and Application of Interference Photocathodes for Image Tubes. By W. P. RAFFANAND A. W. GORDON . Image Intensifier System Using Reflective Photocathode. By J. H. M. DELTRAP AND A. H. HANNA . Scintillation Processes in Thin Films of CsI(Na) and CsI(T1) due to Low . Energy X-Rays, Electrons and Protons. By C. W. BATES,JR. Quelques Aspects des Essais de DBp6t de Photocathodes 5.20 et d’ficrans Fluorescents sur Fibres Optiques. By S. VERON .

433 443 451 461

Channel Multipliers and Secondary Emissions Channel Multiplier Plates for Imaging Applications. By B. W. MANLEY, A. GUESTAND R. T. HOLMSHAW . An Analysis of the Low-level Performance of Channel Multiplier Arrays. By W. M. SACKINGER AND J. M. JOHNSON . Quelques Problhmes Concernant les Multiplicateurs Canalis& pour Intensificateur d’Image. By G. ESCHARD AND J. GRAF Effects of Vacuum Space Charge in Channel Multipliers. By W. M. SACKINGER AND J. M. JOHNSON . Statistics of Transmitted Secondary Electron Emission. By W. L. WILCOCK AND D. E. MILLER

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471 487 499

507 513

Electron Optics Two Methods for the Determination of the Imaging Properties of Electronoptical Systems with a Photocathode. By V. JARE& AND B. N O V O T.N ~ 523 Computation of Imaging Properties of Image Tubes from an Analytic Potential Representation. By F. SCHAFF AND W. HARTH . 535 The Design of Electrostatic Zoom Image Intensifiers. By J. VINE . 537 Electron Optics of a Photoconductive Image Converter. By M. E. BARNETT, C. W. BATES,JR., AND L. ENGLAND 545

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CONTENTS OF VOLUME B Image Tube Assessment Resolving Power of Image Tubes. By S. Hasegawa. Calculation of the Modulation Transfer Function of an Image Tube. By A. M. Stark, D. L. Lamport and A. W. Woodhead. Intensifiers: Detective Quantum Efficiency,Efficiency Contrast Transfer Function and the Signal-to-noise Ratio. By S. Nudelman. On the Quality of Photographic Images Recorded with the Use of Image Intensifiers. By P. Iredale and D. J. Ryden. Leistungsgrenze eines Sichtsystems mit Bildverstiirker. By G. A. Grosch and J. K. Krieser. Information Transfer with High-gain Image Intensifiers. By W. Kiihl, A. Geurts and J. v. Overhagen. The Useful Luminance Gain of Image Intensifier Systems with Respect to Noise Limitations. By W. Kunze, K. Meyerhoff and G. Retzlaff. Image Intensifier Design and Visual Performance at Low Light Levels. By R. L. Beurle, M. V. Daniels and B. L. Hills. The Observation of Moving Structures with X-Ray Image Intensifiers. By W. Herstel. A Quadrature Spatial-frequency Fourier Analyser. By G. W. A. Czekalowski and G. A. Hay. Contrast-enhancement in Imaging Devices by Selection of Input Photosurface Spectral Response. By E. A. Richards. Improvement of Signal-to-noise Ratio of Image Converters with S.1 Photocathodes. By W. Heimann and E. L. Hoene. The Fundamental Infra-red Threshold in Thermal Image Detection a8 Affected by Detector Cooling and Related Problems. By R. K. H. Gebel. Cosmic Rays and Image Intensifier Dark Current. By W. N. Charman. Dark Current Scintillations of Cascade Image Intensifiers. By R. P. Randall. Comparison of the Efficiency of Image Recording with a Spectracon and with Kodak I r a - 0 Emulsion. By E. Kahan and M. Cohen. Linearity of Electronographic Emulsions. By M. J. Smyth and P. W. J. L. Brand. Methods of Increasing the Storage Capacity of High-gain Image Intensifier Systems. By J. R. Powell and R. Lynds.

Applications in Astronomy A Critical Comparison of Image Intensifiers for Astronomy. By W. A. Baum. The Design of Optical Systems for Use with Image Tubes. By C. G. Wynne and M. J. Kidger xxl

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CONTENTS OF VOLUME B

An Image-tube Spect,rographfor the Hale 200-in. Telescope. By E. W. Dennison,

M. Schmidt and I. S. Boweri. Performance of the Spectracon in Astronomical Spectroscopy. By M. F. Walker. Recent Astronomical Applications of a Spectracon. By P. W. J. L. Brand and R. D. Wolstencroft. Gtudes d’Astres Waibles en Lumiere Totdc avec la
Low Light-level Television Applications of the Image Isocon Tube. By P. R. Groves and R. J. Corps. Television at Low Light Levels by Coupling an Image Intensifier to a Plumbicon. By D. G. Taylor, C. H. Petley and K. G. Freeman. Integrating Television Sensors for Space Astronomy. By J. L. Lowrance and P. M. Zucchino. The Application of High-gain Image Intensification and Closed-circuit Television to Field-ion Microscopy. By G. K. L. Cranstoun.

Miscellaneous Applications Characteristics of a Television Photometer. By Y. Nozawa. An Image-tube Fourier Spectrograph. By A. Labeyrie. A Cascade Image Intensifier Camera for Beam-foil Spectroscopy. By J. A. Jordan, Jr., G. S. Bakken and A. C. Conrad, Jr. Test of a High Resolution brenkov Chamber with a Four-stage Image Intensifier. By R. Giese, 0. Gildemeister and G. Schuster. ContrGle des Monocristeaux par Tube Intensificateur de Luminance. By B. Driard. Photon Interference Experiments Utilizing Photoelectronic Devices. By G. T. Reynolds.

Multichannel Photon Counters A New Multiplier System with Forty Separate Channels. By M. Herrmann and C. Kunze. Position-sensitive Photon Counters. By P. Iredale, G. W. Hinder and D. W. S. Smout. Digital Read-out of an Image Intensifier Using a Vidicon or a Scanning Spiral Slit plus a Digital Memory Oscilloscope. By H. S. Wise, E. W. T. Richards and R. Martin.

Applications in High Speed Photography Tubes Obturateurs pour Photographie Ultra-rapide au Temps de Pose d’une Nanosecondc. By G. Eschard arid 12. Polaert.

CONTENTS OF VOLUME B

xxiii

Time Resolution Limitations in Single-stage Image Converter Photography. By N. Ahmad, B. C. Gale and M. H. Key. The Application of the Electron Image Store and Analyser to High-speed Photography. By R. W. Smith. A High-gain Time-resolving Spect,rograph for Diagnostics of Laboratory Simulated Re-entry Objects. By I. D. Liu and J. R. Baskett. Application of Image Intensifiers and Shutter Tubes to the Study of Gas Discharges. By K. H. Wagner. A High Speed Photographic Study of the Electrical Breakdown of Small Gaps in Vacuum. By W. A. Smith, P. A. Chatterton, C. T. Elliott and D. L. Pulfrey.

Applications in Computing A Charge Image Storage Tube for Character Recognition. By R. L. Beurle, and G. W. Jenkinson. A Storage Image Tube for Optoelectronic Computing. By B. L. Morgan, R. W. Smith and G. A. Wilson.

Solid-state Devices Rccent Developments in Field-effect Image Storage Panels. By B. Kazan and D. P. Foote. Recent Developments in Solid-atate Infra-red Image Converters. By T. Kohashi, T. Nakamura, S. Nakamura and K. Miyaji. Solid-state Radiographic Amplifiers and Infra-red Convcrters. Z. Szepesi and M. Novice. AUTHOR INDEX SUBJECT INDEX

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A Technical Description of the Construction, Function, and Application of the US. Navy Electronic Camera G. E. KRON, H. D. ABLES and A. V. HEWITT Flagataff Station, U.S. Naval Observatory, Flagstaff, Arizona, U.S.A.

INTRODUCTION Development of the instrument now known as the U.S. Navy Electronic Camera began ten years ago at the Lick Observatory, was continued during 1961 at the Mt. Stromlo Observatory, and has been brought to completion since 1965 at the Flagstaff Station. Two cameras have been in use for more than a year at the two large telescopes of the Flagstaff Station. Hence, it now seems timely t o undertake a rather comprehensive description of the camera itself, its properties, and the results of some applications undertaken at the Flagstaff Station. THE CAMERA The camera is a pure electronographic, electrostatically focused image tube of the Lallentand type,l but with a valve that can be closed to protect the photocathode during the recovery of the exposed plate - ~ valve, which distinguishes and the insertion of a new ~ n e . ~This the Navy camera from other types, is a simple gate-valve made entirely of metal, and operated by gravity. The valve incorporates a double seal; an ion pump acts continuously between the two seals serving, both as a pump to remove the gas that leaks in during plate change, and as a gage to warn of an abnormality that might place the photocathode in jeopardy. Experience has shown that the valve, because of its simplicity, is reliable to a high degree, and has a long life in service. Because it can be baked for cleanliness prior t o photocathode formation, it need not act as a source of contamination, and hence photocathodes of quite normal sensitivities can be formed in the camera. These photocathodes show little deterioration during time spans as long as 14 months; the longest we have tried to preserve one. The construction of the camera can best be described by reference to Fig. 1, a diagrammatic cross-section, The principal materials are 1

2 0.E. KRON, H. D . ABLES AND A. V. HEWITT

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THE U.S. NAVY ELECTRONIC CAMERA

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304 stainless steel and Corning 7052 glass (sealed to Kovar alloy) used for electrical insulation. The entrance window, on the inside surface of which the photocathode is formed, is of optical-quality crystal sapphire, with its optical axis oriented at 90” to the transmission axis in order t o eliminate the effects of bi-refringence. The sapphire is brazed into a Kovar mounting frame which is, in turn, heliarc welded into the Kovar front-fitting of the tube. The tube part is demountable a t its center, as shown in Pig. 1, to permit cleaning and loading the focusing electrode. The seal here is made with annealed pure gold wire held with a ring of 48 closely spaced small stainless steel screws. The central electrode, used for electronic focusing, is made from low-carbon steel annealed, demagnetized, and cleaned by heating it to above the Curie point in a vacuum. The electrode contributes to the magnetic shielding of the tube. Additional shielding is provided by the external shield shown in the diagram. Valve action is achieved by clamping or unclamping a copper disk, the thin stainless steel diaphragm shown in Fig. 1 allowing one of the valve seats to make the small movement needed for the clamping action. The force for clamping the disk is obtained from four capstan screws. When the valve is open, a condition used only during cathode formation and actual picture taking, the disk is stored in a cavity to one side of the tube axis. The disk is rolled into one position or the other by tilting the tube with its axis horizontal. Thus, no valve operating mechanism is needed, and because the disk rolls on its edge, there is no galling or scuffing of the completely unlubricated metal. The valve assembly is demountable, and is also sealed with gold gaskets. Two small ion pumps form an appendage projecting at rightangles to the tube axis; one of these acts on the volume between the seals, as described above, whereas the other pumps from the volume associated with the plate. When the valve is open, both pumps act on the entire volume of the tube. The plate is carried in a removable cassette which is fastened to the tube with six brass nuts and sealed with a lead wire gasket. The plate is circular, 2.75 in. in diameter, with space for six full-sized exposures. The plate is retained in the cassette on a turn-table and is advanced from picture to picture by means of an external magnet which applies a torque to the turn-table through a simple 12 to 1 reduction gear; the magnet is turned by hand and removed when not in use. The plate is covered with a removable metal lid having a hole to frame the picture. A vane shutter made of thin brass bears on the edge of the plate-retaining frame, and is dragged by motion of the turn-table until it hits one or the other of an “open” or “close” position stop; hence, the shutter is opened or closed by the same motion as the

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0.E. D O N , H. D. ABLES AND A. V. HEWITT

advance of the plate. As the diagram shows, the plate cassette is constructed so that a Dewar flask can be attached to cool the plate by thermal conduction, a technique borrowed from Lallemand to prevent vapor escaping from the emulsion during exposure. At times other than actual exposure, the photocathode is, of course, protected by closing the valve, so the electronic camera can stand-by indefinitely without use of a refrigerant and without any other attention. The plate cassette is of simple design, but this property makes it reliable and easy to use. The problem of designing and building a better cassette which would permit taking more pictures on conveniently shaped plates is an important one, but also a most difficult one, as anyone who has tried it knows. It is worth adding that spectra can be arranged radially on the round plate, and we have found that as many as 80 spectra can be recorded per plate without mutual interference. Thus, the present cassette needs little improvement as far as spectroscopists arc concerned. The camera is stored during stand-by on a preparation table that is fitted with an adsorption pump, a relatively large ion pump with some capability for pumping helium, and power supplies for the ion pumps. The cassette can be changed only on this apparatus, not on the telescope, and therefore only one plate is available for use at a time. However, when the camera is used for direct recording of astronomical fields, we usually find that efficient application of the technique calls for long exposures, and we seldom wish for more than the six available exposures, which permit two cycles of three colors or three cycles of two colors to be recorded.

Field Correcting Lens Because this camera has electrostatic electron optics, the photocathode surface must be curved towards the plate (away from the piature source); in addition, there exists pincushion distortion at the electronic focus. A field lens has been designed by Dr. D. H. Schulte that curves a flat optical field t o the 3-in. radius of the photocathode, and that also introduces just enough optical barrel distortion to compensate for the electronic pincushion distortion, an amount equivalent to a change in magnification along a cathode radius of about 2.8%. The correcting lens (not shown in Fig. 1) consists of three components: the cathode support itself, of optical sapphire, and two others of ultra-violet-transparent glass. One of the ultra-violet glass components has a strongly aspheric surface; other surfaccs ar0 all spherical. The surfaces of the components are coated with low reflectance layers, except for the inner surface of the sapphire part. The importance of the coatings is, of course, more to reduce unwanted light

THE U.S. NAVY ELECTRONIC CAMERA

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on the cathode resulting from re-reflexions among the various surfaces, than to save light. By means of this correcting lens, the electronic camera presents an apparently flat optical image plane to the telescope, and it produces a picture almost entirely free from distortion; thus, two of the worst features of electrostatic focusing have h e n virtually eliminated.

Photocathode Preparation The photocathode must be formed in the tube. This requires flooding the entire volume of the tube with cesium vapor, a process that usually results in the production of a tube with a high background intensity. The background has been kept low, however, by employing favorable geometry to minimize the clectrostatic field strength in the electrode gaps, and by a method of cathode preparation that minimizes the deposit of cesium on surfaces where it is not wanted. A high polish on the electrode surfaces and careful removal from the tube of all solid particle contaminants with a vacuum nozzle also help to keep the background low. The photocathode materials are deposited on the cathode support by means of a probe which is mounted in a Pyrex-glass tubular housing temporarily attached to the back of the electronic camera in place of the plate holder (Fig. 2). The glass is sealed to a steel fitting with a Housekeeper seal, and the fitting is temporarily sealed to the camera valve body with a gold wire seal. The probe carries a shrouded antimony-charged filament at one end, and an r.f. pick-up coil and magnetic slug at the other. Using a magnet, the probe is moved into the camera through the valve aperture just far enough to place the antimony at the center of curvature of the cathode support. The thickness of the antimony film, which is evaporated by energizing the filament with an r.f. generator, is monitored as usual by measuring its optical transmission. Cesium is released in a side tube and torch-driven in doses to condense on the end of the probe when it is in the withdrawn position; the cesium can then be carried into the tube body by the probe, where it evaporates and reacts with the antimony. Prior to cathode formation, the tube is pumped down through a side valve not shown in Fig. 1, but visible in Fig. 2. Pumping is done with a Welch turbo-molecular pump, aided by a Varian 8-l/sec ion pump which is also used for leak detection by noting the reaction of the pump current to application of ethyl alcohol. If there are no leaks, the tube is baked at 350°C for a t least 24 h with constant pumping; the probe appendage is baked at only 275OC t o prevent premature sublimation of the antimony. A t the end of the bake the cesium is released from its pellet by r.f. heating, and the debris sealed off in the side tube

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G . E. KRON, 11. D. ABLES AND A. V. HEWITT

and removed, while the entire structure is still hot. The tube is now cooled to room temperature, the oven removed, and the Varian 1-l/sec appendage pumps are started. If these pumps indicate that the pressure is of the order of torr, the antimony is deposited until its light transmission is 70%. The valve and center electrode region of the tube are now wound with heating tape, and are heated to 200°C. The cathode structure, which is isolated from the hot parts by glass,

FIG.2. Camera mounted for cathode production, showing attachment with antimony evaporator probe.

reaches a temperature of about 35OC, and the cathode is formed a t this temperature as soon as doses of cesium are brought into the tube with the probe. The high temperature of the tube tends to minimize adsorption of cesium, whereas the low temperature of the cathode support tends to cause migration of cesium to this structure, where it is wanted. Formation of the cathode takes several hours, and is complete when the sensitivity, which is monitored continuously, falls drastically with further cesiation. All heat is now turned off and the cathode slowly recovers with constant pumping over a period of about 24 h. The valve is now closed for the first time, the cathode-forming

THE U.S. NAVY ELECTRONIC CAMERA

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equipment removed, and the first plate is mounted, followed by pumpdown of the chamber associated with the plate. The description of cathode formation has been brief, as instructions sufficiently complete to enable an inexperienced person to make a cathode would be quite long and well beyond the scope of this paper.

The High Voltage System The accelerating voltage is properly distributed between the photocathode and the focusing electrode by means of a voltage dividing network made of wire-wound resistors. Focusing is accomplished by means of coarse and fine decade controls, the fine control giving increments of about 5 V when the overall voltage is 30 kV. The tube will be appreciably defocused if the setting is out by two fine steps, but the setting, once found in the laboratory, appears to be stable and permanent. Because most commercially available high voltage power supplies show considerable ripple in the output, a filter section is provided in the voltage divider circuit. Peak-to-peak ripple amplitude must be kept below -5V if it is not to affect the definition of the camera. The wire-wound units of the voltage divider are strung on nylon rods and arranged so that points of high potential difference are kept well separated. The resistor stacks are shielded by a box of Perspex, and various circuit elements are so designed as t o eliminate completely any tendency towards corona, as the current drawn by corona will spoil the focus. Cabling from the voltage divider to the camera consists of two 25-ft lengths of RG-8U shielded polythene insulated cables. The camera cathode and focusing sections are insulated from ground by black, opaque silicone rubber not less than Q in. thick, cast in place inside the magnetic shield. Some experience with polystyrene insulation seemed to indicate that a transparent insulating material could illuminate the tube from light-generating events taking place either in the insulating material, or between it and the grounded surface, thus increasing background. The high voltage system will operate indefinitely, without the appearance of corona, at 40 kV at elevations less than 4000 f t and at 35 kV below 7500 ft.

OPERATIONOF

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CAMERA

Because this camera is an electronographic device like the Lallemand camera, its operating characteristics will be similar to those of the latter, details of which may be found in its extensive literature. The unique properties of the Navy camera require some comment, however, as do also certain features such as linearity (which is still controversial),

a. E. KRON,

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H. D. ABLES AND A. V. HEWITT

storage capacity, and information-gain over photography, of which we have made new measurements. Ordinary unoxidized Sb-Cs cathodes of 10% quantum efficiency at 4200 can be made readily; because the antimony is deposited from the center of curvature of the cathode, it is very highly uniform in thickness. Because, in addition, the antimony is overcesiated, the chemical reaction is carried to completion over the entire cathode, and hence the sensitivity over the surface is uniform within the accuracy of measurement (-2%). Experience has shown that the photocathodes will last for more than a year (Fig. 3) and, barring accidents, apparently do not deteriorate with use. However, if the long life is a consequence of the tube being kept clean by gettering from residual cesium, then the

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Fm. 3. Quantum efficiency of a Rb-Cs-Sb bialkali cathode as a function of time (measured at 4200 A).

photocathodes may have a life limited by the availability of cesium. In either case, the life is long enough to render the camera a practical piece of equipment.

Background The background when the camera is held in total darkness is extremely low; at room temperature, with 30 kV accelerating voltage, L4 emulsion increases in density at the rate of about 0.002 units per hour in a tube that reduces the scale by 1.6: 1. Our grain counts? (Fig. 4) indicate this density rate to be the equivalent of about 10,000 grains per mm2per hour, or about 4600 electrons per hour per mm2 of cathode area, assuming that there is 1 grain clump per electron. The equivalent cathode current is about A/mma, about typical for a thin, simple alloy photocathode. During an exposure, the background is larger than

t

These grain counts were made at the Lick Observatory by J. Breckinridge. G. E. Kron and I. Pepiashvili.

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THE U.S. NAVY ELECTRONIC CAMERA

this. Since the operating pressure is of the order of torr, the mean free path is several kilometers in length, and it seems unlikely that the increase in background could be pressure induced. Hence, we believe that the background increase results from the return of light to the inside surface of the photocathode after being reflected from the polished surfaces of the electrodes, and that it, can be decreased by blackening the inside of the tube.

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Ilford L4,30kV -

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2 Gains /mm2 ( ~ 1 0 ~ )

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FIO.4. Grain counts for photography and electronography.

Linearity of Emulsions According to theoretical treatments by Frieser and Klein5 and by Valentine,e the density of an emulsion exposed to electrons is an exponential function of the exposure. The initial part of the density growth curve is nearly linear depending upon what fraction of the saturation density is reached. From silver content and grain-size data supplied by its manufacturers, we estimate that the saturation density of Ilford L4 nuclear track emulsion is greater than 30, implying that, according to accepted t h e ~ r y ,linearity ~.~ should be reasonable if one does not expose to a density greater than 6. We have measured density spots formed on L4 emulsion with exposure controlled by timing and development for 5 min in full strength D-19 developer at 20°C. When the densities are measured on a Joyce Loebl microdensitometer we find a linear relationship between density and exposure within the accuracy of the densitometer (about 1%) up to a density of five, and up to a density of six with much reduced accuracy. Such data are shown plotted in Fig. 5 , and are typical of many similar plates we have P.E.1.D.-A.

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a. E. KRON,

H. D. ABLES AND A.

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taken on this and coarser grained emulsions, except that the linear range is limited to lower densities for the latter. An important feature of electronic camera recording is the high information storage capacity available in the very fine grained L4 emulsion as compared with photography. The grain counts shown in Fig. 4, obtained with a high-power microscope, show that, at a given density, there are about ten times more grains available per unit of area on L4 electronographs (exposed at 30 kV) than there are on baked 6

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FIo. 5. A plot of density against exposure for photography (103a-J) and eleotronography (L4, 30 kV). The resulta for L4 lie on & straight line.

I I a - 0 photographs. Figure 5 shows that the ratio of the linertr density ranges of these emulsions is three or four to one, so that the total storage capacity within linearity is some 30-40 times larger for L4 than for IIa-0 emulsions, an important advantage even after being degraded by a factor of four because of the 1 : 2 scale reduction of the electronic camera. Informntion-gain

We have undertaken a new measurement of the information-gain of the electronic camera over photography. A Joyce Loebl microdensitometer was used to measure the noise and signal levels of electronographic and photographic plates, the signal being taken as the area under the density curve produced by a scan across the diameter of a recorded image. Photography was represented by an Eastman IIa-0

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THE U.S. NAVY ELECTRONIC CAMERA

plate taken with the 61411. ctstrometric reflector, and also by an Eastman 103a-J taken with the ZOO-in. Hale telescope and kindly placed at our disposal by Dr. Horace Babcock. Electronography was carried out with our electronic camera used at 30 kV on the 61411. telescope, with Ilford K5 and L4 emulsions. Rate of information recording was assumed to be proportional to the area under the density curve, and inverselyproportional to the square of the arbitrary noise level set by the amplitude of the “grass” of the microdensitometer scans. The scanning aperture was deliberately set to a small size so as to enhance the noise level for ease of measurement, and it was properly proportioned to take care of the scale factor of the electronic camera. Exposure times and telescope apertures were, of course, taken into consideration. The

70t

I

I

1

X

r

5

2ol 10

01 0

I

I

*IO3a-O (200-in.), L4 x 1030-0 (200-in3, K5 O I l a - 0 (61-in.),L4 I

2

I

3

I

4

Maximum photographic density

FIQ.6. Measurements of information-gain in blue light of the electronic camera over the photographic camera.

results are shown in Fig. 6, where we see that the minimum advantage over photography (for a photocathode of 10% quantum efficiency) is nearly a factor of 30. The advantage becomes larger at higher densities probably because loss of efficiency from grain exhaustion increases at a higher rate for the low-storage photographic emulsion than for the very fine grain nuclear track emulsion. It is interesting t o note that the superiority of the electronic method is not apeotacular if the pictures are appraised by eye, because the gain is present mainly by virtue of a lower noise level, which the eye does not appreciate.

APPLICATION OF THE CAMERA Electronio picture recording has traditionally been used by astronomers for recording spectra. Many publications have resulted, the most significant being by Walker’ who has made several important

12

0.E. RRON, H. D. ABLES AND A. V. HEWITT

contributions to astronomy with the Lallemand electronic camera. I n contrast, we decided to experiment with the direct recording of astronomical images, employing the astronomical telescope as the optical input device. The properties of the electronic camera, as described by others and also in the previous section of this paper, indicate that it should be useful for direct photometry and colorimetry of surfaces, and of stars. Especially, it should be useful for the most difficult applications of this kind. Walker7 has emphasized that astronomical spectrographs must be redesigned for full realization of the advantages of the electronic camera in spectroscopy. Similarly, efficient direct photometry, with the precision inherent in the method, will not be accomplished until a precise, fast micro-isodensitometer is available. Until such an instrument is built, the Joyce Loebl isodensitometer modification as furnished by Technical Operations, Inc. (Boston, Mass.) is adequate provided it is further modified both t o improve the long-term stability of the light source and to increase greatly the precision of the specimen table cross-feed.

Photometry

A t the U.S. Naval Observatory we have investigated the application of the electronic camera to the wide-band surface photometry of Eaint, difficult nebulae8 and globular clusters,e and narrow-band photometry of planetary nebulae.1° Stellar photometry has been attempted by Lallemand, Canavaggia and Amiot,ll and by Walker and Kron.12 The stellar photometry to be described here differs from the previous measurements in that the image was appraised neither through a round diaphragm, nor by a simple scan, but by extracting all available data with the isodensitometer. The star field measured was in Selected Area 61, which had been measured by direct photoelectric photometry by Purgathofer13with the 40-in. reflector at the Lowell Observatory. Exposures were taken in two colors with the electronic camera on the Navy 40-in. reflector, but only the “visual” (V) exposures have been measured, as they yield information sufficient for the present purposes. The images of 26 stars measured by Purgathofer were scanned with the isodensitometer and the areas of the density contours on all images were measured with a planimeter. The planimeter data gave the radii of equivalent circular areas for each contour, which could then be assembled analytically into a solid having two linear dimensions and one density dimension. The volume of each of these solids was then determined by mechanical integration. On the sumption that the electronic camera response is linear, the volumes of these solids will be proportional to the brightnesses of the stars, even if the

THE U.S. NAVY ELEUTRONIU UAMERA

13

original images were distorted. The process is laborious, but the purpose here is to point to a valuable method of photometry, not necessarily an easy one. The importance of using all of the data can be seen in Fig. 7. In this figure are shown the relatively noisy profile obtained by a single microdensitometer scan of an image of a star of 16-5 magnitude, as compared with the smooth profile formed analytically from a density solid constructed as described above from a single isodensity recording,

/TLb

ikaphotes, Mean profile a11from data in image

J

-e

2’

t

e‘-

0 Scanning aperture 4

-I

I sec of arc

Profile from

F I ~7.. Profiles of a stellar image (selected area 61, Star 46), showing the improvement in noise level by using all of the data in the image (top) as compared with the data from a single, narrow scan (bottom). Microdensitometer magnification x 200.

Next, the volumes were converted to tt magnitude scale, and a zero point was transferred from the photoelectric data. The resulting electronic camera magnitudes are plotted against photoelectric magnitudes in Fig. 8. When the original numbers are compared, it is found that the average deviation between the two sets of data is only f0.035 mag. over a range of more then six magnitudes. This indicates that the random errors present in the two sets of data must be about the same, as we know from experience that the average deviations of the photoelectric observations under the prevailing conditions must be between f0.02 and f0.03 mag. The electronic camera required about 25 min of telescope time (14 min for the exposure, the rest for operational purposes); the photoelectric results required about three nights of telescope time. A small systematic scale difference exists between the

14

0.E. KRON, R. D. ABLES AND A. V. HEWITT

two sets of data; its origin cannot be identified without additional photoelectric observations. An interesting and possibly significant new application for the electronic camera lies in the removal from an image of some of the effects of the instrumental profile, thereby improving knowledge of the real properties of the image. The mathematical process for separation of the effects of the instrumental blurring function from the object ~ ~ electronic camera is well profile is often called d e c o n v ~ l u t i o n . The suited for recording the original data, as high precision is required, and this implies an application for the high storage capacity of the nuclear track emulsion.

17

0

i

4

Id

1'5

1'6

7 ;

; 8 '

Magnitude V (Furgathofer)

FIG. 8. Electronic camera stellar magnitudes (selected area 51) plotted against photoelectric stellar magnitudes measured by Purgathofer.13

We have undertaken an experimental deconvolution of the profile of a distant globular cluster, Baade 282, in the galaxy M31. The profile of this cluster differs appreciably from that of a star nearby in the field, whose measured profile was employed to derive the blurring function, or instrumental profile. The profiles were obtained by recording on L4 emulsion and measuring star and cluster at a magnification of 200 :1 with the isodensitometer, planimetering the isophotes, reducing them to equivalent circular areas, and then analytically deriving profiles that represent all of the data in each image. We then undertook the (incorrect, in the strictest sense)15 deconvolution of these profiles as a simplified substitute for the correct process of treating the three dimensional figures. Deconvolution was accomplished by the method of serial products,14 and by Kahn's method;la the results of the latter are shown along with the original profiles in Fig. 9.

THE U.S. NAVY ELECTRONIC CAMERA

15

The curves in Fig. 9 are smoothed reproductions of the original data which showed very small random errors, as expected. The oscillation at the tail of the deconvoluted profile is typical, and was worse when deconvolution was accomplished by serial products. A comparison with the shape of the profiles of nearby, galactic globular clustersg indicates that the deconvoluted curve is at least of the right shape

mm

FIa. 9. DeconvoIution of the image of a globular cluster (Baade 282) in the Andromeda Galaxy, M31: A, stellar profile (instrumental profile); B, smoothed globular cluster profile; and C, deconvoluted globular cluster profile. Isodensitometer magnification x 200.

from its center to the onset of the oscillations, and that a factor of two or three may have been gained in definition over the original. It seems probable to us that systematic errors may be limiting the effectiveness of the method, so that further progress may require a study aimed at reducing errors of this kind. The result of such a study may well indicate that further practical progress will depend upon an isodensitometer of higher precision than any now available.

CONCLUSIONS It is possible to build a practical electronic camera that has a permanent or nearly permanent photocathode, reasonably good

16

a. E. KRON, H.

D . ABLE8 AND A. V. HEWITT

sensitivity, and very low background. The electronic camera is believed, at least by our group, to have a linear response if certain conditions of emulsion type and development are observed. Because of linearity, high storage capacity, relatively high quantum e%ciency, and low background, the electronic camera method is much superior to the photographic method for all astronomical photometry and colorimetry. Under some conditions the electronic camera is even superior t o the photomultiplier for photoelectric photometry, because it allows the observer to use telescope time more efficiently. Improved methods and equipment are needed for appraising the data on the electronic camera plates.

Future Developments Astronomers need an electronic camera with a photocathode having a diameter of about 10 cm. Such a camera is within the state of tho art, and can be developed by expending the necessary effort and funds. ACPNOWLEDQMENTS The project was sustained by a series of annual grants from the Office of Naval Research until 1967, by one grant in 1962 from the National Science Foundation, and by aid from the University of California, the Mount Stromlo Observatory, and the U.S. Geological Survey. It is now sustained by the U.S. Naval Observatory. The deconvolution by Kahn’s method was done by Dr. C. Dahn during an extended visit to the Flagstaff Station from the U.S. Naval Observatory, Washington, D.C., and we are indebted to him for permitting us to use his results.

REFERENCES 1. Lallemand, A., C. R . Aoad. Sci. 203, 243 (1936). 2. Kron, G. E., Publ. Aetron. SOC.Pacif. 71, 386 (1959). 3. &on, G. E. and Papiashvili, I. I., Publ. Aetron. SOC.Pacif. 72, 353 (1960). 4. Kron, G. E., i n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, W. L. Wilcock and L. Mandel, Vol. 16, p. 25. Academic Press, New York (1962). 5. Frieser, H. and Klein, E., 2. Angew. Phys. 10, 337 (1958). 6. Valentine, R. C., in “Advances in Microscopy”, ed. by R. Barer and V. E. Cosslett, Vol. 1, p. 180. Academic Press, London (1966). 7. Walker, M. F., i n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22B, p. 768. Academic Press, London (1966). 8. Ables, H. D., Thesis, University of Texas (1968). 9. Kron, G. E., Publ. Astron. SOC.Pacif. 79, 9 (1967). 10. Walker, M. F. and &on, G. E., in “International Astronomical Union Symposium No. 34 on Planetary Nebulae”, ed. by D. E. Osterbrock and C. R. O’Dell with the editorial assistance of E. F. Swan, p. 282 (1968). 11. Lallemand, A., Canavaggia, R. and Amiot, F., C . R. Acad. Soi. 262,838 (1966). 12. Walker, M. F., andKron, G. E., Publ. Astron. Soc. Paoif. 70, 651 (1967).

13. Purgathofer, A., private communication.

THE U.S. NAVY ELECTRONIC CAMERA

17

14. Bracewell, R. N., “The Fourier Transform and its Applications,” p. 30. McGraw-Hill, New York (1965). 16. Burr, E.J., Austral. J . Phys. 8, 30 (1966). 16. Kahn, F. D., Proc. Cambridge Phil. SOC.51, 619 (1955).

DISCUSSION w.

Will you please explain the meaning of SIN ratio, which is plotted as a function of various parameters in your diagrams? a. E. KRON: Small round test areas were exposed in the laboratory under controlled conditions. These areas were scanned with the Joyce Loebl microdensitometer, set with an analysing aperture so small that appreciable “noise” appeared in the record during a scan. The SIN was then “density/noise”, where we used peak-to-peaknoise. Both density and noise are, of course, arbitrary, so control was exercised so that 8 valid evaluation was made for each parameter studied in the optimization program. P. FELENBOK: For what reason is your SIN ratio lower a t 40 kV than a t 30 kV? a. E. KRON: We do not know; this is simply an experimental result. One could speculate that grain growth becomes disproportionately more disorderly with higher and higher voltages, thus increasing the noise level more than the density (i.e. the signal). L. WILCOCK:

This Page Intentionally Left Blank

Large-image Electronographic Camera R. W. DECKER Westinghouae Aerospace Division, Baltimore, Maryland, U.S.A. and

H. MESTWERDT United States Air Force, Wright-Patterson Air Force Base, Ohio, U.S.A.

INTRODUCTION The electronic camera, first reported in 1936 by Professor Lallemand,l has been under development for use with the astronomical telescope. The image sizes obtainable in present cameras are very small, and the photosurfaces have been limited to the less efficient varieties. The electronographic camera design described here is the result of a study programt aimed at designing a camera that will take many large pictures (44in. square) and utilize the resolution and sensitivity gain offered by the electronographic technique. The overall response of the camera system would be further improved by the use of an 5.20 photocathode. During the past 2 years we have studied the basic problems involved in recording large images directly on film with electrons from a photosurface; our proposed solution is an electronographic camera that uses a magnetically focused image section and an 5.20 photocathode.

PRELIMINARY STUDIES Initially a review of the work of Lallemand2 and Kron3 provided a camera design on which to base the experimental study. A series of experiments were then conducted to determine the conditions of operation of the camera. The materials required in the design were analyzed for their out-gassing rates in an ultra-high vacuum system with a residual gas analyzer to determine their outgassing constituents

t Sponsored by United States Air Force, Wright-Patterson Air Force Base, Ohio (contract 33(616)6166). 19

20

R.

W. DECKER AND

H. MESTWERDT

and the resultant partial pressures of the individual gases that would be in the camera and would be a limiting factor in its operation. The outgassed contaminants were thus analyzed into their pure gas equivalents. A special vacuum system? was constructed in which S.20 photocathodes could be made and individual gases introduced at known partial pressures to determine the partial pressure a t which contamination takes place. The materials studied were Estar, Kodak SO-159 film, Viton-A, Teflon, 304 stainless steel, aluminum, Mylar, copper, molybdenum sulphide, tungsten selenide, 7052 glass, and soda-lime glass. I n addition the outgassing caused by flexing a stainless steel bellows was analyzed.

FIG.1. Degassing spectrum measured with residual gas analyzer for SO-169 film.

Those materials in the camera that could be heated were baked and their outgassing rates then measured. The rates of all materials were tabulated and these data gave ameasure of the initial pumping required by the camera to pre-process the film cartridge, and the pumping speed required to maintain the camera in operation. Water was the principal constituent outgassed from materials such as Teflon, Mylar, Estar, SO-159 film, and Viton-A, but with other materials hydrogen, methane, nitrogen, oxygen, carbon monoxide, carbon dioxide, and some fluorine were given off. The gas analyzer spectrum for the gases from SO-159, Fig. 1, shows that hydrogen and water are the main constituents. All gases except water vapor can be eliminated during the processing of the components of the camera. The water comes from the surfaces of all materials that are not baked and from the volume of materials

t See p. 367.

LBRGE-IMAGE ELECTRONOGRAPHIC CAMERA

21

such as Mylar, Estar, and 50-159. To provide a long-life S.20 photocathode, all materials in the assembly of the intensifier section of the camera are baked to a high temperature, and the materials in the film cartridge are baked a t their maximum permissible temperature (40°C for gelatin film and 15OOC for evaporated films) and pumped until the partial pressure of water is held below 5 x torr. To maintain this pressure, titanium sublimation pumps are built into the camera: in this pressure range a t room temperature, each square inch of fresh titanium will pump water at a rate of 20 I/sec. On the basis of the collected data a camera was designed to keep the contaminating gas a t such a low partial pressure that a reasonable operational time could be obtained.

ELECTRONOGRAPHIC CAMERA DESIGN Large images of high resolution require the electron optics section to be of the magnetically focused type, which does not have the off-axis loss of resolution found in electrostatically focused devices. Also, to obtain high resolution, the accelerating field must be high. These two requirements necessitate a short image section with parallel-plate electron optics. Practically this places the image plane (film) directly opposite the photocathode with no baffling structure in between. To separate the film and photocathode during processing a valve is required between them.

FIQ.2. Simplified scale drawing of electronographic camera.

22

R. W.

DECKER AND € MESTWERDT I.

A simplified scale drawing of the proposed camera is shown in Fig. 2. The external dimensions are approximately 24 in. high and 35 in. wide to the end of the valve actuator. The body of the camera is 11 in. in diameter, the photocathode is 7 in. in diameter, and there is a hydraulically operated gate valve between the photocathode and the film supply cartridge. The camera is separated into two components for processing and assembly. The intensifier section shown in Fig. 3, includes the photocathode, the isolation valve, the electron optical section, and the outer chamber of the cartridge. The cartridge section, Fig. 4, includes the film supply and take-up reels, the film drive and guidance spools, the electrostatic hold-down plate, a cooling chamber and all necessary

FIQ.3. Intensifier section sohemstic.

feed-throughs, mounted on a base plate. The focus coils are removable and can be fastened rigidly t o the chamber. In the design of the camera a computer program has predicted that with a uniform focusing field of 330 G, 20 kV accelerating potential, and parallel electrostatic field, the resolution will be over 500 lp/mm. In operation the film must be held flat to within 0.001 in. and this is done by use of the electrostatic attraction of two conductors. The flexible flm is coated with a conductor as shown in Fig. 5. Built into the cartridge is a sandwich of two soda-lime glass plates of 0.040 in. thick with a conductor evaporatedon t o the inside surface of one of the plates. The outside surface of the plate is ground to form a flat image plane, on to which the f l m is pulled. When the film is in position to take a picture, it is held tight to the surface by applying a d.c. potential of 500-1000 V between the two conductors, flattening the film against the glass plate. When the film is to be moved to the next image, the voltage is removed and the film is released. Standard films can be coated

LARGE-IMAQE ELECTRONOGRAPHIC CAMERA

23

Hormonic drive

FIa. 4. Film cartridge Schematic. Empision

Film

Conductor

&a. 6. Schematic of electrostatic hold-down.

chemically with a copper deposit without affecting the emulsion, and the coating is removed during film development.

ASSEMBLY OF ELECTRONOGRAPHIC CAMERA The data obtained in the study program showed that the photooathode must be protected from the initial outgassing of the film and therefore the film must be pre-evacuated. The life of the photocathode is influenced only by the time the valve is opened to the film and the

24

R. W. DECKER AND H. MESTWERDT

condition of the film. Therefore the camera must be assembled in a manner that permits the proper conditioning of the film and the processing of the S.20 photocathode. The photocathode must be inserted into the intensifier section by the use of a remote processing technique. A schematic of a remote processing system developed at Westinghouse for the assembly of magnetically focused image intensifiers is shown in Fig. 6: a system based on this principle will be required t o attach the 5-20 photocathode to the intensifier section of the electronagraphic camera. The photocathode

FIQ.6. Remote photocathode-processing system.

face-plate is inserted into the turn-table and the body of the intensifier section is mounted in the processing chamber. The photocathode is processed and evaluated in the processing chamber, and when an acceptable surface is obtained it is rotated into position over the intensifier section and is sealed by exerting an upward pressure on the cold-pressed indium seal. The intensifier section is then removed from the remote processing system and placed in a dry chamber. The cartridge section is loaded with film as a separate operation and attached to a simple vacuum system. It is processed by baking to an acceptable temperature and by evacuating to remove the water from the film and cartridge walls. The degassing measurements indicate that silver halide film can be baked at 40°C without fogging for the 50-100 h

LARGE-IMAGE ELECTRONOGRAPRIC CAMERA

26

that are necessary for the removal of water so that a pressure of 6 x torr can be maintained by a small holding pump. Other films such as evaporated silver bromide can be baked to 160°C and the partial pressure of water will be an order of magnitude lower. The assembly of the camera is completed by attaching the cartridge to the intensifier as shown in Fig. 7. The pre-processed cartridge and the intensifier section are bolted together in the dry chamber to reduce water absorption on the exposed walls of the two sections. The assembled camera is then evacuated until an acceptable pressure is obtained in the film cartridge section; since the film has been previously Dry nitrogen

Pre-processed

_--

---

FIa. 7. Assembly of camera in dry chember.

outgassed and the water content has been controlled during assembly, this evacuation does not take as long as it would if the film had not been pre-baked.

SUMMARY OF CAMERAOPERATION The foregoing description of a large-image electronographic camera has shown that the camera will operate if a required processing schedule of the components is adhered to. To assemble the camera, two processing vacuum stations are required in addition to the camera mechanism: first, a photocathode remote processing system to attach the photocathode to the intensifier section; secondly, a film processing system to degas the film and cartridge section and to evacuate the cartridge after assembly of the camera. The intensifier section is prepared by processing a photooathode and

R. W. DECKER AND H. MESTWERDT

26

attaching it to the intensifier section; this section can then be stored for later use. The cartridge is loaded with film and processed in vacuum until the film is degassed to an acceptable level. The camera is then assembled in a dry box and again evacuated by the vacuum pumps. The camera is kept ready to take pictures by small built-in titanium holding pumps. In operation the gate valve in the intensifier section is opened before exposure and kept closed when pictures are not being taken. This isolates the photocathode from the film cartridge which may be cooled to reduce the water vapor pressure still further, the life of the photocathode depending on the partial pressure of the water vapor and the length of time the cathode is exposed t o it.

REFERENCES 1. Lallemand, A,, 0.R. Acad. SOL 203, 243 (1036). 2. Lallemand, A., Duchesne, M., and WIBrick, G., Image Intensifier Symposium, Fort Belvoir, p. 111, Oct. (1958).

3. Kron, G. E., private communication.

DISCUSSION 0. W L ~ R I C K :I n

your project, what exposure time have you anticipated? R. w. DECKER: The exposure time is expected to be a small fraction of one second. The drive mechanism for advancing the fXm is designed to move the film from frame to frame in one second including exposure. J. A. COLLINSON: What resolution has been achieved in the image section? R. w. DECKER: Our tests are not oompleted but we have measured greater than 200 lp/mm; a t this point our instrumentation is limited. P. FELENBOK: What is your accelerating potential? R. w. DECKER: 20 kV with a magnetic field of 330 G and an electron path of 4.5 cm.

Sur Quelques ProgrCs RCcents Apportbs h la CamCra alectronique il Focalisation Electrostatique et sur son Application en Physique et en Astronomie P. BIED-CHARRETON,A. BIJAOUI, M. DUCHESNE et J. M. LE CONTEL Obaervatoire de Paria, Paris, France

MODIFICATIONSA LA C A M ~ R A ~LECTRONIQUE Au cours des dernihes annBes plusieurs modifications ont Bt6 apport6es a la cambra 61ectroniquel a focalisation Blectrostatique pour amBliorer sea performances et assurer une plus grande s6curit6 dans sa mise en muvre. 1. Le systbme focalisateur de la cam6ra Blectronique est un objectif immersion 6lectrostatique; avec ce type d’objectif le champ de l’instrument n’est pratiquement limit6 que par la courbure de champ. Nous avons Btudi6 systdmatiquement l’influence du profil et de la position relative des 6lectrodes sur la forme des surfaces tangentielle et sagittale. Les r6sultats obtenus ont permis d’augmenter le diambtre utile de la photocathode et de le porter de 18 ii 30mm (pouvoir r6solvant 30 pl/mm). En augmentant dans des proportions raisonnables lea dimensions de l’optique Blectronique nous esp6rons pouvoir utiliser dans un avenir proche des photocathodes de 40 mm et peut-&re de 50 mm. Pour des champs plus importants la focalisation Blectromagn6tique semble indispensable. 2. Le remplacement depuis plusieurs ann6es des getters au baryum par du chmbon active convenablement dispos6 et refroidi2 beaucoup simplifiBla prbparation de la camBm Blectronique et permet d’entretenir, conjointement avec la pompe ionique, un excellent vide pendant toute la dur6e d’une expbrience; des couches cbium-antimoine ont ainsi pu &re conserv6es pendant plusieurs semaines sans 6volution de leur sensibilit6. Le dispositif utilis6 pour la liberation de la photocathode a Bgalement BtB am6liorB; il comporte maintenant un double volet escamotable qui Bvite la projection d’6clats de verre dans lea lentilles 6lectroniques au moment de la casse de l’ampoule; ces morcemx de verre Btaient quelquefois 8, l’origine de dBcharges entre Blectrodes qui 27

28

P. BIED-OHARRETON, A. BIJAOUI, M. DUOHESNE ET J. M. LE OONTEL

produisaient une Bmission parasite prohibitive. La fiabilite du rkcepteur est maintenant telle qu’en utilisant alternativement deux cameras, il est possible d’observer chaque nuit dans les meilleures conditions. Au cours de nombreuses missions It l’observatoire de Haute Provence les seules limitations dans l’emploi de l’instrument ont 6t6 causees par les mauvaises conditions atmospheriques; en septembre 1967, par exemple, nous avons pu travailler 18 nuits consBcutives sans aucune difficult6 d’ordre technique. 3. La cam6ra Blectronique doit &re mise en position de travail, sous tensions, plusieurs heures avant les poses astronomiques. Cette pr& caution est indispensable, car le fait d’agiter la camem augmente 1’6missionparasite; en prenant cette pr6caution celle-ci reste negligeable et pour les poses les plus longues que nous avons effectu6es ( 2 It 3 h) la densit6 du voile parasite est g6neralement rest6e infdrieure It 0.02, pour des plaques Ilford G5, une tension acc616ratrice de 25 kV et des couches SbCs, dont la sensibilit6 Btait de l’ordre de 50 It 70 pA/lm. Pour les poses de longue duree, il eat n6cessaire de remplir d’azote liquide, plusieurs fois au cours d’une meme pose, les vases de Dewar qui assurent le refroidissement de la cathode et des plaques Blectronographiques. Pour cette op6ration nous avons realist5 un dispositif automatique qui permet ce remplissage sans produire de lumibre parasite et sans engendrer de vibrations m6caniques qui pourraient alt6rer la qualit6 des images. 4. Les propri6t6s remarquables de 1’6lectronographie (grande sensibilit6, absence de seuil, faible turbidit6, densit6 proportionnelle b la lumination (dans certaines limites), courbe CaractBristique ind6pendante de la longueur d’onde, etc.) font de la camera Blectronique un outil trbs bien adapt6 8, la photometric et nous nous sommes efforc6s de r6aliser les meilleures conditions pour son utilisation en photomBtrie de pr6cision. (a) Les ampoules renfermant les photocathodes ont Qt6modifi6es de manibre b pouvoir r6aliser des couches photodmissives de sensibilit6 plus homogbne et d6pourvues de structure. On peut, en principe, tenir oompte de ces dBfauts en utilisant un cliche de la photocathode obtenu en lumibre uniforme, m i s l’exploitation des clichBs astronomiques est rendue beaucoup plus laborieuse. (b) Pour diminuer la lumibre diffus6e 8, 1’intBrieur de la cam6ra, on applique un traitement anti-reflet approprid It la glace de fermeture ainsi qu’b la face d’entr6e du m6nisque sup lequel est d6poGe la couche photoBmi~sive.~ Lorsqu’on s’intdresse seulement i% la partie bleue du spectre, on peut remplacer les 6lBments des lentilles Blectroniques ghnhralement en acier inoxydable par des 616ments en cuivre, metal pour lequel le coefficient de rhflexion en lumibre bleue est plus faible.

LA CAM6RA 6LECTRONIQUE A FOCALISATION 6LECTROSTATIQUE

29

Actuellement, nous testons Bgalement differents traitements de surface des electrodes de maniere B diminuer les reflexions parasites dans une bande plus large. DES PLAQUES ~~LECTRONOQRAPHIQUES LINBARITE La linearit6 de la reponse de la camera Blectronique est liee it la linearit6 de la rBponse des plaques Blectronographiques. Plusieurs expkrimentateurs ont determine les courbes caracteristiques de divers types de plaques ionographiques exposees it, des electrons d’knergie moyenne (20 a 40 keV); les resultats sont malheureusement contradictoires: pour les uns4s5la plaque Ilford G5, par exemple, aurait en fonction de la lumination une reponse lineaire jusqu’A des densites de l’ordre de 4, alors que pour d’autressv7cette linearit6 serait limitee aux densites inferieures B 1. Nous avions nous-mbmes trouv@ pour le meme type de plaque une caracteristique lidaire, it quelques pourcents prks, jusqu’b une densite de l’ordre de 1.5 et qui presentait ensuite une courbure concave vers l’axe des abscisses. On peut penser que ces contradictions sont dues, soit it des variations de certains details de fabrication des emulsions qui modifieraient leurs propriBt6s d’un lot it l’autre, soit it la methode de mesure de la densit6 photographique, soit enfin a la technique du developpement. Nous avons donc, pour divers lots de plaques Ilford G5 et K5, que nous utilisons le plus gBneralement, repris ces mesures en amdliorant leur precision e t en determinant les densites photographiques avec deux microphotombtres de type different, un microphotom&tre de Moll et un densitometre de Joyce Loebl. Nous avons utilise une methode it flux constant; les plaques ont Btt5 exposBes dans la camera Blectronique it -140°C environ, tension d’accBl6ration 25 kV et developp6es 5 min B 18°Cdans le r6velateur Ilford ID-19. Nous avons pris plusieurs pr6cautions. (i) La methode de mesure permet, pour les differentes plaques, de determiner la densite dans une region des images Blectroniques qui correspond it la meme position sur la photocathode, de manitre P n’avoir pas A tenir compte d’une inhomog6n6itB Bventuelle de celle-ci; elle permet kgalement d’hliminer l’influence des petits “trous”, souvent trks nombreux pour ces emulsions riches en bromure, surtout pour les densites BlevBes. (ii) Nous avons pris bien soin d’eliminer la lumikre diffusee par effet Schwarzschild-Villinger et nous nous sommes assures que pour le microphotomktre de Moll la lumibre r6flbchie par les lentilles et leurs montures n’introduisait pas d’erreur, mbme pour des densites de l’ordre de 4. (iii) Les poses courtes et les poses longues ont BtB alternees de manikre it Bliminer une influence possible sur la forme de la caracteristique, soit d’une fatigue, soit d’une sensibilisation de la couche photoemissive par la l ~ m i e r e . ~ (iv) Pour le microphotomhtre

30

P. BIED-CHARRETON, A. BIJAOUI, M. DUCHESNE ET J. M. LE CONTEL

;

5

4-

3-

‘._ 2

-

Y)

D)

P

-

2-

I (+) Mars 1964

-

2(+) Juillet 1966 2(01 Mars 1966 l -

2(0) Moi 1962

-

3(+) Novembre 1966 3 ( x ) Septembre 1967

0

I 5

I 10

I 15

I 20

I 25

I 30

I

35

I 40

I 45

Temps (sec)

FIG.1 . Densit6 en fonction de la lumination pour different8 lots de plaques Ilford G5.

de Moll, dont le rdcepteur est un couple thermodlectrique, nous avons vdrifid la linearit6 de la rdponse. Les resultats obtenus sont rdsum6s dans les Figs. 1 et 2.

Plaques llford G5 Les resultats se rapportent iL six lots et les expositions ont BtB faites au cours d’une m8me experience de manibre iL avoir un flux dlectronique identique. On voit que la sensibilith varie d’un lot B l’autre: pour les lots extr6mes le rapport des sensibilitd est de 2.3. Lea courbes cmactdristiques, d’autre part, presentent une courbure, pratiquement identique pour tous les lots, qui est concave vers les abscisses; cette concavitd est faible: pour D = 1.5 l’dcart A lin6aritd n’est que de 2 B 3%, mais si l’on supposait une caracthristique lindaire jusqu’B D = 3 l’erreur sur la d6termination de 1’8nergie serait de 16 ZL 20%. Les

LA CAMERA ELECTRONIQUE

FOCALISATION fLECTROSTATIQUE

31

I , Mai 1961 2. Avril 1965

3.0

3, Aobt 1968 4, Ocfobre 1966 5, Avril 1967 6, Octobre 1967

2.5

2-0

1.5

-

0.5

-

0

20

40

60

a0

100

120

Temps ( s a c )

FIG.2. Densit6 en fonction de la luminstion pour diffbrents lots de plaques Ilford KS.

courbes en trait plein se rapportent aux mesures effectudes avec le microphotombtre de Moll et la courbe en trait pointill6 aux mesures faites pour 1’6mulsion de juillet 1966 avec le microdensitombtre de Joyce Loebl. On constate que les deux caracteristiques relatives it la m6me plaque sont W6rentes’ ce qui n’est pas surprenant, car 1’6mulsion photographique &ant un milieu diffusant la convergence des faiisceaux intervient dans la mesure de la densit6 optique. On a par exemple D, = D,q, oh q est le coefficient de Callier, D, et Dd sont respectivement lea densitds mesurees en lumibre parallhle et en IumiAre diffuse. Leur courbure, d’autre part, est identique (les caract6ristiques coincident si I’on change 1’6chelle de temps pour I’une des courbes), ce qui montre que le coefficient de Callier pour les densites consid6r6es et le mode de developpement utilis6 est pratiquement ind6pendant de la densite.

Plaques I lford K 5 Les rhultats se rapportent Bgalement it six lots et comme pour lea plaques G5 I’exposition aux Blectrons a Bt6 faite au cours d’une m6me expdrience. On voit que la sensibilit6 peut varier consid6rablement d’un lot B l’autre; pour les Bmulsions de mai 1961 et d ’ a d 1966 le

32

P. BIED-CHARRETON, A . BIJAOUI, M. DUCHESNE ET J . M. LE CONTEL

rapport des sensibilit6s pour D = 1.5 est de 21; pour un mBme lot de plaques la sensibilit6 peut Bgalement Btre assez diff6rente; pour l’kmulsion d’avril 1967 nous avons trouv6 pour deux Bchantillons tir6s de deux boites diff6rentes un rapport de sensibilitd de l’ordre de 2.3 pour D = 0.5 (Fig. 2, courbes 5(a) et 5(b)). Seule 1’6mulsion de mai 1961 a une caract6ristique linBaire au moins jusqu’b une densit6 de 3; pour tous les autres lots les caract6ristiques pr6sentent une courbure qui b l’inverse de celle des G5 est concave vers les ordonn6es; cette courbure est d’autant plus prononc6e que la sensibilit6 de 1’6mulsion est plus faible; la courbe 4(b) en trait pointill&,par exemple, reprdsente la caractdristique de 1’6mulsion d’octobre 1966 (courbe 4(a))t r a d e en comprimant 1’6chelle des abscisses de telle sorte qu’elle coupe pour D = 1.9 la caractdristique de 1’6mulsion plus sensible d’avriI 1965. Le sens de la courbure des caract6ristiques observe pour la plupart des lots de plaques K5 peut &re expliqu6 par la formation de sous-germes d’image latente pour les faibles luminations; la proportion de sousgermes par rapport aux germes est d’autant plus grande que 1’6mulsion est moins sensible, d’oh une courbure plus prononc6e de la caract6ristique. Pour les plaques G5 qui sont trhs sensibles et pour lesquelles la formation de sous-germes d’image latente doit &re faible nous avons trait6 le probleme thdoriquement en supposant que les grains d’argent, aprbs ddveloppement de 1’6mulsion,ne sont pas parfaitement opaques et qu’ils sont rbpartis, suivant une certaine distribution, dans une couche d’6paisseur non nulle. Ce calcul permet de rendre trhs bien compte de l’allure des caract6ristiques des plaques G5.

OBSERVATIONS ASTRONOMIQTJES L’installation b l’observatoire de Haute Provence d‘un spectrographe b tres grande rdsolutionlo a permis d’6tudier les possibilit6s qu’offrait l’dlectronographie en spectrophotom6trie, en particulier, pour le trace des profils de raies. La Fig. 3 represente la camera Blectronique en position de travail derriere ce spectrographe. Nous avons d6velopp6 deux programmes d’observation. Le premier programme est consacr6 b la recherche de composantes interstellaires des raies D du sodium dans le spectre de certaines 6toiles de types A et B; gr&ceB la grande rdsolution du spectrographe et b l’excellent rapport signal-bruit du r&cepteur, on a pu mettre en Bvidence, par exemple, dans le spectre de tc Cygni (AzP,m, = 1.3) cinq composantes nettement s6par6es; la dispersion Btait de 0.58 &mm sur la photocathode (grandissement Alectronique 0.7), plaque Word G5 et tenips de pose 1 h 30 min (transparence moyenne, turbulence atmosph6rique de plusieurs secondes). L’autre programme est consacre B l’6tude des variations

LA

CAMBRABLECTRONIQUE A

FOCALISATION

BLECTROSTATIQUE

33

pBriodiques du spectre des Btoiles du type /3 Canis Majoris. Nous avons plus particulierement BtudiB, jusqu’it prBsent, 1’Btoile y PBgase (B,,,, m, = 2.87) avec une dispersion de 2 Blmm; dans ces conditions le champ du spectrographe n’est que de 25A et nous nous sommes limit& ti quelques domaines spectraux pour lesquels nous espBrions obtenir le maximum d’informations. GrBce it la trAs faible Bmission parasite du rBcepteur nous avons pu enregistrer trois spectres de

FIQ. 3. Camera Bleotronique en position de travail derriere le spectrographe A tres grande rbsolution de I’Observatoire de Haute Provence.

1’6toile sur la meme plaque. La comparaison des spectres obtenus en photographie Blectronique et en photographie classique sur plaques IIa-0 chauffkes montrent de tres grandes diffkrences likes aux propriBtBs remarquables de l’dectronographie. y PBgase, en effet, est une Btoile variable dont la pBriode d’oscillation est de 3 h 38 min, l’amplitude de variation de la vitesse radiale est de 7 km/sec et celle de 1’Bclat de 0.015 magnitude; 1’6tude de cette Btoile nBcessite donc des spectres ii grande rBsolution obtenus avec des temps de pose relativement courts par rapport ii la pBriode. La camera Blectronique g r h e &

34

P. BIED-CHARRETON,A. BIJAOUI, M. DUCRESNE ET J. M. LE CONTEL

sa grande sensibilite a permis d’obtenir des cliches bien poses avec

des temps de pose inferieurs it 12 min; nous avons pu prendre plusieurs cliches au cours de la pdriode, de temps de pose identiques de maniere que le lissage dii au temps d’intkgration soit le m6me pour tous les spectres; en raison des conditions atmospheriques variables au cows de la p6riode leur densite n’etait pas identique, mais la linearit6 de la rdponse du recepteur nous a cependant permis de conserver une bonne precision dans la mesure des largeurs Bquivalentes. Ces cliches mettent en evidence des variations rapides des largeurs equivalentes W Aet de la forme des profi1s;ll pour Si I11 A4552 et Si I11 A4568 ( W Amoyennes voisines de 115 m.A et 100 d) ces variations sont de l’ordre de 10% et il semble qu’il y ait correlation entre cette variation et celle de la vitesse radiale. Enfin, il a dt6 possible d’observer des details tres fins dans le spectre: des composantes analogues it celles dBjit observees dans le spectre d’autres &toilesdu meme type apparaissent it certains moments de la periode dans les raies Si I11 A4552 et X4568, et Ies doublets A4481.13 et X4481-33 de Mg I1 et A4479.89 et X4479.97 de A1 I11 ont BtB s6parBs pour la premiere fois dans le spectre de cette

Nous avons Bgalement utilise la camera Blectronique en photographie stellaire directe. La determination des magnitudes des Btoiles est, en effet, un problbme delicat si l’on veut 6viter les erreurs syst6matiques. Dans le cas de l’utilisation classique de la plaque photographique il y a plusieurs sortes d’erreurs systematiques. (i) La reponse est non lindaire; on peut pour determiner la courbe caractkristique utiliser une sequence photoelectrique Btalonnde, mais dans le cas de telescopes paraboliques de grande ouverture l’effet de coma introduit des erreurs likes it la distance it l’axe; de plus, oette caracteristique dependant de la longueur d’onde il faudrait, en principe, une sequence photoelectrique pour chaque type spectral; enfin l’extrapolation pour les magnitudes &leveesest impossible. (ii) I1 est tres difficile, malgre l’utilisation des sequences Btalonnees de tenir compte de I’absorption atmosph6rique. (iii) La diffusion de la lumibre dans 1’6mulsion(turbidite) joue un grand r81e en photographie; les courbes caracteistiques determinees, d’une part, it partir de plages uniform6ment Bclairees et, d’autre part, it partir d’une sequence d’etoiles de magnitudes variables sont diffdrentes. GBnBralement, on utilise cet effet de diffusion pour determiner la magnitude (methode des diambtres); de nombreux facteurs interviennent dans l’exploitation de oette m6thode : turbulence atmospherique, defauts de mise au point, de guidage, coma, diffusion chromatique de l’atmosphbre, etc. Toutes ces erreurs systematiques font que la photographie ne permet pas d’obtenir une precision supkrieure iL environ 10% dans la mesure du flux des Btoilea.

LA C A M ~ R A~LECTRONIQUEA FOCALISATION &LECTROSTATIQUE

35

Pour pallier les nombreux d6fauts de la plaque photographique, on utilise le plus souvent une cellule photo6lectrique; dans ce cas le signal est proportionnel au flux et la r6ponse spectrale n’en depend pas. Malheureusement, on ne peut pas former d’images et il est impossible de mesurer la magnitude d’astres rapprochds (amas globulaires, par exemple). L’Blectronographie permet d’allier les avantages des deux m6thodes avec, toutefois, par rapport L la cellule photo6lectrique, quelques restrictions dues aux inhomog6n6itBs de sensibilit6 de la couche photosensible et de la plaque 6lectronographique; de plus, il a BtB vBrifi6 en utilisant une m6thode appropri6e13que pour les 6toiles la densit6 int6gr6e est proportionnelle it I’dclat dans un domaine de 5 magnitudes (it partir de la magnitude limite enregistrBe sur le clich6). Aprbs l’installation de la camBra Blectronique au foyer Newton du t6lescope de 193 cm de l’observatoire de Haute Provence par G. Wl6rick et ses collaborateurs, nous avons entrepris un programme sur les a m s globulaires; les premiers r6sultats obtenus sur M13 et M15 montrent tout l’intBr€!t de la photographie Blectronique pour 1’6tudedu centre de ces objets; en particulier la faible turbidit6 observBe dans l’enregistrement des Blectrons sur plaques ionographiques a permis de mieux rBsoudre ces objets en Btoiles. On pourrait encore amhliorer la r6solution en effectuant une d6convolution que permet la lin6arit6 de la rBponse; l’emploi de cette m6thode pose toutefois un problbme d6licat de digitalisation et de traitement de l’information en raison de la nBcessit6 d’une Btude it 2 dimensions.

APPLICATION EN SPECTROSCOPIE Le c h a p d’application de la cam6ra Blectronique a Bgalement 6t6 Btendu L la spectroscopie atomique de laboratoire. GBnBralement, lee spectres atomiques B trbs haute r6solution sont enregistres par des spectrombtres 6quip6s de grands rBseaux ou utilisant des techniques Fabry- P&ot

Filtre multicanal Miroir parabolique

eseau par transmission Camera

Dirperreur croir6

Fro. 4. Scheme de principe du spectromhtre Fabry-PCot S.I.M.A.C.

36

P. BIED-UHARRETON, A. BIJAOUI, M. DUCHESNE ET J. M. LE CONTEL

interfBromBtriques et le recepteur est constitue par un ou plusieurs photomultiplicateurs. I1 est, cependant, difficile de multiplier le nombre des photomultiplicateurs pour augmenter beaucoup le nombre d’informations enregistrees simultan6ment par plusieurs canaux. La camera Blectronique pouvant 6tre consideree comme un rkcepteur kquivalent ii la juxtaposition d’un tres grand nombre de recepteurs photoBlectriques BMmentaires, nous l’avons adaptde B un spectrometre Fabry-PBrot rBalis6 sous le nom de S.I.M.A.C.14-16 Le spectre de

FIQ.6. Vile g6n6rale du spectrom&re Fabry-P&ot S.I.M.A.C. et de son r6cepteur. A, Interf6romBtre de Fabry-PQrot. B, Disperseur principal. C, Disperseur crois6. D, Diasporambtre assurant le d6placement de l’image. E, Objectif. F, Couche photoBlectrique. G , Objectif B immersion Qlectrostatique. H, Magasin de plaques itlectronographiques.

la source iL Btudier est BchantillonnB par 1’interfBromhtreet les BlBments spectraux transmis par les diffkrents pies de la fonction d’appareil sont disperds sur la photocathode par un disperseur b deux rBseaux croisBs; l’exploration des intervalles entre les ordres du Fabry-PBrot se fait par deplacement simultank de la fonction d’appareil (en changeant la pression du gaz ii I’intBrieur de 1’interfBrombtre)et de l’image projetde. La Fig. 4 represente un schema de principe de l’appareil et la Fig. 6 une vue generale du spectrometre et de son rkcepteur. Les rBsultats

LA CAMI~RA~LECTRONIQUEA FOCALISATION ~LECTROSTATIQUE

37

obtenus dans le cas du terbium confirment que le spectromhtre Bquip6 de la cam6ra Blectronique est trhs bien adapt6 8, l’enregistrement rapide des structures hyperfines; il permettra d’atteindre des corps qui sont inobservables 8, la r6solution demandbe par la spectrosaopie atomique, isotopes rares ou isotopes instables par exemple. RI~FERENCES 1. Lallemand A., et Duchesne, M., C.R. Acad. Sci. 233, 306 (1951). 2. Duchesne, M., Dane “Advances in Electronics and Electron Physics”, ed. par J. D. McGee, W. L. Wilcock et L. Mandel, Vol. 16, p. 19, Academic Press, New York (1962). 3. WlBrick, G. et Grosse, A., Dane “Advances in Electronics and Electron Physics”, ed. par J. D. McGee, D. McMullan et E. Kahan, Vol. 22A, p. 466. Academic Press, London (1966). 4. Vernier, P., ThBse, Paris (1968). 6. Ables, H. D. et Kron G. E., Abstracts of Papers presented a t The Pasaden8 Meeting, p. 423, June, 1967. 6. Jeffers, S. et McGee, J. D., Dane “Advances in Electronics and Electron Physics”, ed. par J. D. McGee, D. McMullan et E. Kahan, Vol. 22A, p. 41. Academic Press, London (1966). 7. Wilcock, W. L., Communication personnelle. 8. Duchesne, M., J . Obeervateurs 50, 123 (1967). 9. Duchesne, M. J . Phye. 26, 117 (1966). 10. Baranne, A., Bastid, J., Bijaoui, A., Duchesne, M. et Le Contel, J. M., J . Obeervateurs 60, 289 (1967). 11. Duchesne, M., Herman, R. et Le Contel, J. M., C . R . Acad. Sci. 265, 1213 (1967). 12. Le Contel, J. M., Duns “Non-Periodic Phenomena in Variable Stars.” Bd. par L. Detre, Academia, Budapest (1968). 13. Lallemand, A., Canavaggia, R. et Amiot, F., C.R. Acad. Sci. 262, 838 (1966). 14. Chabbal, R. et Pelletier, R . , Japan. J . Appl. Phya. 4, Supplement 1 (1965). 16. Chabbal, R., Bied-Charreton, P., et Pelletier, R., J . Phye. 28, C2-209 (1967). 16. Bied-Charreton, P., These de Docteur-IngBnieur, Orsay (1967).

This Page Intentionally Left Blank

Electronic Cameras for Space Research M. COMBES, P. FELENBOK, J. GUERIN and J. P. PICAT Observatoire de Meudon, Meudon, France

INTRODUCTION Two Lallemand cameras have been built for balloon-borne experiments, one using electrostatic and the other electromagnetic focusing. I n this paper only the latter is described in detail. As with previous models of the Lallemand camera a new photocathode is needed after each pump-down. The nuclear plates are inside the vacuum chamber which has t o be brought back to atmospheric pressure after each experiment in order t o allow the recovery of the plates. The whole instrument is immersed in liquid helium during the experiment; the photocathode and the plates are thus cooled to a very low temperature, and there is a powerful cryo-pumping action inside the camera. A special technology has been used employing only stainless steel and alumina, instead of the traditional glass envelope which would not be suitable for use in a balloon.

USE OF LIQUIDHELIUM COOLING Cryopumping An ultra-high vacuum of 10-lo torr has been achieved in the alumina-metal camera without prior bake-out. This allows a drastic reduction in preparation time and the vacuum is maintained for long periods of time by the sole effect of cryo-pumping. Residual oxygen is completely suppressed by the cryogenic pumping. We expect that the exoelectrons which are thought t o be produced in the oxygen-caesium reaction1 will therefore be eliminated. This should reduce the background and increase the lifetime of infra-red-sensitive photocathodes.

Superconducting Materials These can be used in two ways. (i) They can constitute a very efficient screen against external magnetic fields, which, as is well so

40

M. COMBES, P. FELENBOK, J. QUERIN AND J. P. PICAT

known, can greatly impair the image quality. Accordingly the camera optics are enclosed in a thin sheet of superconducting material such as lead. (ii) The magnetic field necessary for the electromagnetic camera can be obtained through the use of a superconducting solenoid. I n this manner it is no longer necessary to carry a power supply aboard, since the current can be trapped in the solenoid before the flight. Furthermore, trapped currents of this type provide a very stable field and dissipate negligible power. Finally, a much more intense magnetic field can be obtained with much less weight than when a conventional solenoid is used. It will be shown that increasing the magnetic field

:19-

c

I*OOZ

-

1.001

-

d (em1

FIG. 1. Inhomogeneity H ( d ) / H ( 3 . 6 )of the magnetic field on the axis of the superconducting solenoid.

helps to improve the image quality. It is expected that the use of superconducting screening will also protect external apparatus near the camera. A high field coil, with a high degree of homogeneity, has been built for the magnetically focused tube. The homogeneity has been improved by adding compensating windings a t the ends of the solenoid. The characteristics (length, diameter, number of turns) were computed on an IBM 7040 computer. The superconducting Nb-Zr wire, 0.01 in. in diameter ( I max = 50 A), is wound on a stainless-steel core in 7 layers of 625 turns each. The compensating windings consist of 14 additional layers of 39 turns each. The internal radius of the solenoid is 5.65 cm and the external radii are

41

ELECTRONIC CAMERAS FOR SPACE RESEARCH

5.95 cm in the central part and 6.56 cm a t the ends. The length of each compensating winding is 1.91 cm, the total coil length being 24-16 cm. As can be seen in Fig. 1 a magnetic field of 7 kG has been achieved with a homogeneity of better than 0.2% over a length d = 10 cm on the axis. Theory shows that this degree of homogeneity is attained over the whole volume that is used.

DESCRIPTION OF THE TUBES Both cameras use the same basic technology as far as the body, the system for introducing the photocathode, and the plate holder are concerned.

Technology The body of the tube is formed from alumina elements hermetically sealed together so as to remain leak-free when immersed in liquid helium. Stainless steel flanges are sealed a t both ends of the alumina body. These flanges support the other parts of the camera (entrance window, pumping assembly, and plate holder) which are sealed by metal gaskets. Plate Holder Plate holder motion is obtained by means of a mechanism actuated externally by an electric motor. Motion through the vacuum-tight envelope is achieved by linear or rotative feed-throughs. This system is much lighter than the magnetically actuated one previously used since the power required is very much reduced. Furthermore it can be remotely controlled with complete reliability. Photocathode Introduction System The photocathodes are prepared outside the camera and stored in evacuated glass ampoules (Fig. 2) where they are held in place by a groove in the metal disc (1). Grid (3) permits the photocathode sensitivity to be measured. The ampoule is placed in the introduction system (Fig. 3) which is then evacuated. The glass envelope (2) (Fig. 2) is placed in a fixed cylinder (5) (Fig. 3) t o prevent it from tipping. The metal disk is held in a support (6) to prevent sideward movements. A twisting motion on the disc then breaks the glass-metal seal (4)(Fig. 2) without breaking the glass envelope which falls into a basket ( 7 ) . This method thus eliminates tho risk of introducing glass powder into the electron optics. The freed photocathode is then taken by remotely controlled pincers (8) and placed in the cathode holder. P.E.1.D.-A.

3

42

M. COMBES, P. FELENBOK, J. QVERIN AND J. P. PICAT

FIa. 2. Photocathode ampoule.

The tube is pumped through the photocathode introduction tube (9) which is made of copper. A pinch-off seal can be made a t this point and when this is done the pumping system and the photocathode introduction mechanism are mechanically separated from the body of the camera. This reduces the weight and overall dimensions by eliminating the two appendices characteristic of the previous version of the Lallemand camera.

Fro. 3. Photocathode introduction system attached t o the electrostatic tube.

ELECTRONIC2 CAMERAS FOR SPACE RESEARCH

43

Electromagnetically Focused Tube The camera shown in Fig. 4 has 20 electrodes 1 mm thick (internal diameter 5 cm) separated by 4-5 mm. The potential difference between two adjacent electrodes is set by an adjustable regulated supply. There are therefore 19 similar supplies in series giving a total voltage of 19 kV. The photocathode and the plates are rectangular (1.5 x 4.5 cm). The photocathode introduction tube is angled at 60" to the tube axis in order to clear the top of the solenoid which has a length of 26 cm, the useful part being 10 cm long. The field strength is

FIG.4. The electromagnetic tube with its plate holder. The potential of the 20 electrodes are set by 19 separately adjustable regulated supplies arranged in series.

7 kG. No experimental results have yet been obtained with this set-up but our calculations indicate that the use of large magnetic fields should improve the image quality.

Electrostatically Focused Tube A Lallemand-Duchesne optical system has been used with a 20-mmdiameter photocathode. The plate-holder can contain twelve 1 x 1 in2 nuclear plates. Preliminary measurements have demonstrated a, resolution of 30 lp/mm over the whole field. The magnification is about 0.7.

TREORETIUAL STUDYOF HIQH-FIELD MAGNETICALLY FOCUSED TUBE Calculations have been made in order to determine the chromatic aberration component of the electron density distribution in the

44

M. COMBES, P. FELENBOK, J. QUERIN AND J. P. PICAT

focused spot. The distance d between the photocathode and the target, the wavelength A of the incident light and the intensities of the magnetic and electric fields B and E were taken as parameters. The two fields were considered perfectly homogeneous. The electron velocity distribution has been taken into account. The angular distribution is assumed to be Lambertian. For the energy distribution we have used the experimental results of Apker, Taft and Dickey2 for the SbCs, photocathode. If a is the angle between the initial velocity vector and the axis, and v is the electron speed, the number JV of electrons per unit area a t a distance r from the centre of the spot is given by:

11

r + Ar

N ( d , r, E , B, A) =

2nr A r

~

n/2

n[A,v(a, d, r, E ,

B)]sina cosa da dr,

0

r-Ar

(1)

where n[A, v, (a,d, r , E , B ) ] is the probability of an electron having a speed v, and N is the total number of electrons emitted from the object point on the photocathode. The quantities v, r, d, a, E and B must satisfy the relation r=--

2 v sin a m

B

e

sin

[g (v

cos a

-)I,

2dE e+ ,/cosz a + a V m

(2)

where e is the electronic charge and m the electronic mass. The electrons at a distance r from the spot centre, for a given value of d , E , and B, are those for which the initial velocity vector (v, a) satisfy Eq. ( 2 ) . Knowing the probability distribution n (A, v ) of the emission process, Eq. ( 1 ) gives the required intensity distribution. In order to calculate JV ( d , r , E , B , A) the probability function n[A, v (a, d , r , E , B ) ]is made equal to zero beyond the values of the solution to Eq. ( 2 ) . The values of JV have been numerically calculated on an IBM 7040 computer. The results of these calculations give the theoretical resolution as a function of the distance d , and the depth of focus. Calculations with B = 300 G and E = l o 5 V/m show that it is possible to obtain a very high theoretical resolution (about 1000 lp/mm) but only in an exactly defined position of the image plane (d = 0.1 1248 m for 3 focusing loops). To achieve high resolution, the magnetic and electric fields must be very stable because the optimum value of d varies very rapidly with B and E. The relationship between Ad, and A B , and between AdE and A E are given by Ad, AB -d= 2 B '

ELECTRONIC CAMERAS FOR SPACE RESEARCH

45

and

With the above fields, a resolution of 250 lp/mm implies a stability (AB/B)<0.005 for the magnetic field and a stability (AE/E)
FUTURE DEVELOPMENTS We are presently studying the background intensity and the quality of the images in both types of tube. I n the near future we expect to have an operational model of each type. The first tests conducted with the electromagnetic tube together with the results obtained in the study of the superconducting solenoid and the calculation of electron trajectories have encouraged us to start the design of an electromagnetically focused camera with a field 10 cm in diameter. The electrode configuration (shape and number) and the length of the tube will be optimized by numerical computations.

REFERENCES 1. Duchesne, M., Notes et informations (Publication de I'Observatoire de Paris) 17, divers No. 2 (1964). 2. Apker, L., Taft, E., and Dickey, J., J . Opt. SOC.Amer. 43, 78 (1953).

DISCUSSION How many loops of focusing are employed in the electromagnetically focused tube? J. P. PICAT: In the electromagnetically focused tube 47 loops of focusing are employed (with a magnetic field of 7 kG and an electric field of 2 x lo5 V/m). J. VINE:

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A High-resolution Image Tube for Integrated Circuit Fabrication T.W. O’KEEFFE and J. VINE Westinghouse Research Laboratories, Pittsburgh, Pennsylvania, U.S.A.

INTRODUCTION One of the most precise techniques used in the fabrication of integrated circuits is the so-called photoresist technique. It is used t o manufacture a required arrangement of devices on the surface of a silicon slice by transferring patterns from a photographic mask t o an oxide layer grown on the silicon. The organic photoresist material is usually sensitive to ultra-violet light in such a way that when spread as a thin layer over an oxidecoated silicon slice and exposed to an ultra-violet image of the desired pattern, the exposed areas can be selectively removed. The remaining resist then acts as protection for the underlying oxide allowing the oxide from the exposed areas t o be removed, thus forming the required pattern. The oxide then forms a mask against the diffusion of the dopants necessary to produce the emitter, base, collector, and other regions of the integrated circuit. A complete integrated circuit may require as many as 7 or 8 different patterns to be copied either in an oxide or metal layer a t the various process steps on each silicon slice. The usual procedure is to place the photographic pattern in direct contact with the photoresist and then flood the whole with ultra-violet light, thus forming a contact print. The width of the finest line that 3 to can reliably be produced in the photoresist by this method is 5 pm. The limit is set mainly by the thickness of the emulsion, which is usually 4 t o 6 pm, and the inherent diffraction limit a t these dimensions determined by the wavelength of the light. Another major disadvantage of this method is the physical contact involved which causes damage t o both the mask and the resist, and leads t o short mask life and low yield of perfect circuits. High resolution is important for several reasons. As devices became smaller they became faster, leading to higher data rates and lower

-

47

T. w. O’KEEFFE AND J. VINE

48

costs. Smaller devices also enable more complex circuits to be fabricated in the same area. The yield is also improved since there is less likelihood of including a foreign material or process defect in the area occupied by the device.

Light Projection The problems associated with contact printing can be avoided by light-optical systems using image projection. However, a system which produces 2 t o 3 pm resolution over a typical wafer of dimensions of 1 t o 2 in. is not easy t o obtain, especially in the ultra-violet region, although such systems are being developed. They will nevertheless still be inherently limited by diffraction a t these wavelengths and will require optically flat silicon slices for exact focus over the whole of the area since the depth of focus is of the order of the resolution, i.e. 2 to 3 pm.

Electrons Diffraction effects of electrons of even only a few keV energy are negligible on the scale of 1 pm. Electron beams of very small diameter, for example 0.1 pm diameter, are available in instruments such as the scanning electron microscope. Such instruments have high resolution and great depth of focus, as evidenced by the beautifully detailed micrographs of three-dimensional objects that they produce. The use of such equipment to produce patterns in a photoresist type material is the basis of current research in our laboratories and elsewhere. However, although large current densities are available in the beam, 1 A/cm2 being typical and higher values feasible, the technique requires that a large area, i.e. the silicon slice, be scanned sequentially and this takes a considerable time, even a t such high current densities, due to the small area exposed a t any instant. Further, the field of such microscopes is limited, and the area that can be scanned is much less than the silicon slice size, so that a step by step procedure must be used. However, very fine lines of 1-pm width and less can be produced in a resist in this way, the limit not being so much the diameter of the primary beam, but the effects of back scattered electrons from the substrate which also cause resist exposure.

-

Large Area Image The use of image tube techniques appears to solve the problems mentioned. The basic idea is t o form a source of electrons in the desired pattern a t a photocathode, accelerate them towards the resistcoated silicon slice and bring them to a focus using the required electromagnetic fields.

IMAQE TUBE FOR INTEGRATED CIRCUIT FABRICATION

49

Analysis of a simple tube structure indicated that large areas could be achieved, resolution could be made better than 1 pm, the depth of focus could be large enough to allow conventionally polished silicon slices t o be used and the electromagnetic fields required could be produced easily by conventional equipment. The basic tube structure could be made very simple and hence inexpensive. As far as exposure time is concerned, the resists available would require charges of about C/cm2 using typical accelerating potentials, implying an exposure time of 10 to 100 sec at a very modest photocurrent density of 1 pA/cm2. DESIGNSPECIFICATION The design specification of the tube for integrated circuit definition and especially for large scale integration in which upwards of 100 devices are fabricated on a 0.04 x 0.04 in.2 chip of silicon are typically: (a) ability to define lines 1 pm or less in width over areas of diameter as large as 2 to 3 in.; (b) a depth of focus of 25 pm, i.e. the typical height variation over the slice surface; (c) the image should be distortionfree over the whole field to tolerances of the order of 0.2 pm so as to allow accurate registration of the image with a pre-existing pattern on the silicon slice from a previous fabrication step.

Unusual Features The tube is quite different from the conventional image tube. Some of the unusual features are as follows. (a) The sensitivity or gain of the tube is not important per se, since within reason, the level of illumination is arbitrary. (b) The “output” is a very high resolution resist layer having high contrast and effectively a high gamma in photographic terms. (c) Since silicon slices must be introduced into and withdrawn from the vacuum, the photocathode must be resistant to contaminants and stable in air. (d) The pattern to be imaged must be placed either in or on the photocathode/window assembly thus avoiding the problem of imaging the pattern optically on to the photocathode. (e) Rather trivially, the tube is not required to be portable so that large solenoids can be used to produce the focusing magnetic field. TUBE THEIMAGE A very simple tube design, of which Fig. 1 is a schematic diagram, proved to be adequate. It is based on a 1 : 1 imaging system with plane parallel electrodes 2.5 in. in diameter and placed 1 cm apart, providing a very uniform axial electric field. The axial magnetic field is provided by a coil from a 4-in. electromagnet which provides the 1059 G, single-loop focus-field when the electric field is 10 kV/cm. The anode

50

T.

w.

O’KEEFFE AND J . VINE

is normally at ground potential, the cathode being held negative a t 10 kV. The solenoid is 4-5 in. inside diameter, 13 in. outside diameter, and 4.5 in. long. The experimental tube is of simple re-entrant design, the cathodewindow, of quartz 2.5 in. in diameter and Q in. thick, is demountable and vacuum sealed with an O-ring. The anode has provision for mounting various targets including, of course, silicon slices. The glass tube wall is sealed to the ends using black vacuum wax. The re-entrant -IOkV

Ground

,Torget

Cathode, plate u

.

v

.

plate

-To

0

pump

\Gloss tube (meta1,Iized1

FIQ.1. Schematic diagram of image tube.

wall a t the anode end contains ports for evacuation of the interelectrode space. A photograph of the tube with the anode removed is shown in Fig. 2. Computer Analysis

The theoretical performance of the tube was determined in part, by electron path computation using the Burroughs 5500 computer a t the Westinghouse Research Laboratories and also by analysis where this proved feasible. The computer program consisted of two basic parts. Initially, accurate field plots were calculated for both the electric field based on electrode shapes and for the magnetic field based on the known current distribution in the solenoid. The electric field departs from perfect uniformity due to the edge effects of the finite diameter electrodes. The solenoid has a typical magnetic field distribution, that on axis being strongest at the midpoint. The effects of the non-uniformities were found to produce the usual distortions and aberrations. The electric and magnetic distortions

IMAUE TUBE FOR INTEURATED CIRCUIT FABRICATION

51

FIG.2. Photograph of image tube with anode removed.

were, however, found to be independent a t the level of accuracy required, namely 0.1 pm. I n general, the magnetic field non-uniformities lead t o linear effects such as rotation and magnification. The electric field non-uniformities give rise t o non-linear effects such as pincushion or barrel distortion and S-type distortion.

RESULTS Resolution and Depth of Focus The basic formulae involved in such a tube are very simple. The single-loop focus condition is given by d = (10.59/B)V112,where d

52

T.

w.

O’KEEFFE AND J. VINE

is the cathode-anode separation (cm), V is the potential (V) and B is the magnetic field (G). This result is obtained by equating the electron transit time t o that required for electrons t o complete a loop in the magnetic field B, and since the loop time is independent of the transverse velocity, the focal plane is the same for all electrons leaving the photocathode. However, because of the distribution of axial velocities, up to some maximum corresponding t o a voltage V,, the transit time for all electrons is not the same and this produces a minimum spot size of the image of a point source. The diameter of this disc of confusion is given by

FIa. 3. Comparison of a line on the photocathode (left) with its reversed image in the resist layer (right). Line-width, approximately 2 pm. E = 2d( V,/V ) . Taking Vi = 0.1V leads t o E = 0.2 pm for the system described. The quality of the image resolution that can be obtained is illustrated in Fig. 3 which compares a pattern placed a t the photocathode with its image as reproduced in the resist material. The lines are -2 pm wide, and were produced by scribing a fine line on an opaque layer on the photocathode. At present it is difficult to make patterned cathodes with details fine enough to test the resolution fully; images of fine lines of width approximately 0.5 p m have been observed but are difficult to photograph. The depth of focus f is given by the expression

IMAGE TUBE FOR INTEGRATED CIRCUIT FABRICATION

53

The stability required of the high voltage and solenoid supplies can be derived from considerations of the depth of focus. Figure 4 shows five exposures at different magnetic field settings. The center line is recorded at the calculated focus field and the others are at fields differing from this by & 1 yo and f 3%. As a percentage of the cathode-anode separation d , the depth of focus f is given by 100-f = 200

d

J"- - 0.6%

in this case, and from the focusing condition given previously a change in the magnetic field of 0.6% produces a change in focal length of this magnitude. It can be seen that the sharpness of the f 1% lines is

FIG.4. Line exposures at different magnetic field settings. Center line at calculated focus field.

roughly consistent with the 0.2-pm resolution predicted and thus with the assumption of 0.1 eV effective maximum emission energy. The focal length is only dependent on the electric field to the one-half power and the allowable variation in it is thus 1.2%. Such levels of stability in the magnetic and electric fields are easily attainable. We have observed that focus is good over areas as large as 1.5 in. in diameter or more; again this is consistent with the region of the magnetic field that is uniform to 1 % as shown by both direct field plots and by prediction from the computer analysis.

Rotation and Magnijcation As mentioned previously, the non-uniformity of the magnetic field leads to rotation and magnification of the image. Rotation was measured by making two exposures with the magnetic field in opposite

54

T. w. O’KEEFFE AND J. VINE

directions. Since the sense of rotation reverses on field reversal, the separation of the two images can be measured as a function of radius and the amount of rotation determined. S-type distortion due to electric field non-uniformity can also be determined in this way. The rotation and S-type distortion measured for two different electrode configurations are shown in Fig. 6 . IT-1 refers to a design in which the O-ring sealing the cathode was mounted on the inside face of the window and necessitated a large lip to accommodate it. IT-2 had the O-ring sealing the outside face and only a small lip was necessary to support

Radial distance from axiskm)

FIQ.6. Comparison of 8-type distortion in image tubes IT-1and IT-2.

the force of the atmosphere on the window. The difference is significant and in IT-2 the distortion was less than 1 pm out to a radius of 1 cm. Further increases in the distortion-free diameter have since been accomplished using the same basic 2.Bin.-diameter electrode. The computer predictions are given in Fig. 6. Barrel distortion and magnification can be determined by direct comparison of the photocathode and resist image placed in contact and viewed with a microscope. Both were found to follow closely the theoretical predictions.

Simple Analysis The simple solenoid used for the focusing field was of rectangular cross section. Such a- coil does not produce a uniform magnetic field.

15-

I

(b) 10

-

5 -

+4

0

0-5

Radius (crn)

0

O

1-0

0.5

m

Radius (cm)

2'5

Radius ( c r n )

FIQ.6. Image megnificetion and rotation, (a,b) for tube (IT-I)and (c,d) for tube (IT-2). A, Electric and magnetic fields non-uniform. B, Magnetic field non-uniform. C, Electric field non-uniform.

66

T. w. O’KEEFFE

AND J. VINE

There is, however, a plane perpendicular to the coil axis and at the mid-plane of the coil in which the fist derivative of the z(axia1) fieldcomponent is zero. Such a plane is a plane of symmetry for an ideal coil and contains the point on the axis of maximum field strength. For some distance on either side of this plane the axial field nonuniformity may be described by the second derivative of the axial component. Higher derivatives may be neglected. With this model of the magnetic field, and assuming uniform electric

(Unity magnificotion

gnification variation from unity

(M-I) x104 ( p m / c m )

FIG.7. Rotation as a function of magnification, showing dependence on field-uniformity and position.

field, the behavior of the electrons can be analyzed. The magnification and rotation are thus found to depend on two parameters: 1, the value of the second derivative of the axial magnetic field, d2B,/dz2, and 2, the position on the z-axis of the symmetry-plane with respect to the electrodes. Figure 7 shows how rotation and magnification depend on these parameters. The parallel lines are lines of constant second derivative given by R 1 d2 1 d2B - (M-1)--.--. r 7 r 47r B dza

IMAGE TUBE FOR INTEURATED CIRCUIT FABRICATION

57

The radial lines are lines of constant symmetry-plane position and are given by -

R _ -r

-848 - 5 T(-348 - 5)

x ( M - 1).

The parameter 5 is the distance of the symmetry-plane from the photocathode as a fraction of the inter-electrode spacing d, M is the magnification and R is the shear at a distance r from the axis. There are two interesting positions for the symmetry-plane: 1, at 5 = 0.348 giving unit magnification for any value of d2B,/dz2, and 2, at 5 = 0.848 giving zero rotation for any value of d2B,/dz2. Three experimental points are plotted for three different positions of the symmetry-plane along the tube axis. The points lie well on a line corresponding to B" = -16 G/cm2, the value predicted from the computer estimation of the field distribution. As far as practical applications are concerned, it is possible to so position even a single coil, i.e. at 5 = 0.348, and produce exact replication of the image with respect to its size. This is much more important than rotation as it can be compensated by correct orientation of the target. Hence, absolutely uniform magnetic fields are not necessary.

PHOTOCATHODES The air-stable photosensitive materials used to date have been either palladium or gold, the former appearing to be more useful. These materials have work functions close to 5 eV and hence are nominally sensitive only below 2500 to 2600 d. Conventionally, lowpower, low-pressure mercury lamps are used as the ultra-violet lightsource. The sensitivity of a 40-d thick palladium layer on 4in. thick quartz is approximately 200 pA/W at the 2537 d mercury resonance line. The sensitivity of the resist is such that a charge in the region of 50 to 100 pC/cm2 is necessary for full exposure. Such doses have been provided in times of the order of one minute using several low pressure lamps as the source. I n order to shorten the exposure time to the order of seconds, highpower, high-pressure mercury lamps, closely coupled optically to the photocathode have produced photocurrents as high as 350 pAlcm2. The heat output from such relatively inefficient ultra-violet lamps is high, however, and cathodes quickly deteriorate in sensitivity. By cooling the outer surface of the window with water, however, current levels of over 100pA/cm2 can be maintained for tens of hours and probably indefinitely.

58

T.

w. O’KEEFFE

AND J. VINE

CONCLUSIONS I n practice the required pattern can be defined in an evaporated metal layer deposited on the quartz window by resist techniques. Conventional photoresist techniques can be used for the 3 to 5 p m resolution patterns. For finer patterns the scanning electron microscope can be used to produce the pattern in the resist. Techniques for accomplishing the latter under computer control are being developed in our laboratories. As an example of this technique Fig. 8 shows a micrograph of a pattern etched in a silicon dioxide layer produced by exposure in the image tube. It is a portion of an integrated circuit of typical size, the smallest dimension being of the order of 6 pm.

FIG.8. Integrated circuit pettern etched in silicon dioxide.

To summarize, we have shown for the purposes of integrated circuit fabrication the following advantages of the image tube approach: 1, a simple tube can be used; 2, 1-pm resolution is practical; 3, unit magnification can be obtained from a simple solenoid; 4, distortion can be essentially eliminated; 5 , area is limited only by magnet size; 6, magnetic and electric focus field stability is easily attainable; 7, a considerable depth of focus is available; 8, this technique provides a non-contact projection system; 9, cathode sensitivities are sufficient to give reasonable resist exposure times. N

ACENQWIJCDGDNT This work was supported in part by the Air Force Avionics Laboratory, Air Force Systems Command, USAF.

IMAGE TUBE FOR INTEGRATED CIRCUIT FABRICATION

59

DISCUSSION J. A. LODGE: Do any problems exist in the fogging of the resist by the ultraviolet radiation and has the voltage of 10 kV bean chosen to minimize scattering of electrons in the resist? T. w. O’KEEFFE: The resist is a specially developed “electro-resist” and is insensitive to ultra-violet radiation. The voltage of 10 kV was not chosen by scattering considerations. However, work a t 10 kV in the scanning electron microscope had shown no particular problem associated with this energy range. P. FELENBOK: What is your magnetic field homogeneity? What is the quality of parallelism between magnetic and electric fields? T. w. O’EEEFFE: As shown in Fig. 7 the uniformity of the field could be described by d2B,/dz2 = - 16 G/cm2 or roughly 1% over the “active” volume of the tube. Theoretically a misalignment of the axes of the fields of 10% is allowed a t the 0.2 pm resolution level. H. o. LUBSZYNSKI: How do you produce the master pattern in the first place: by stop and repeat methods? Have you actually produced patterns with resolution better than 1 x 1 pm2? T. w. O’KEEFFE: The images of integrated circuit patterns shown were projected from sources produced by conventional photoresist techniques, which do involve stop and repeat methods for the initial production of the photo mask, and was then contact printed on the cathode. Regular patterns of, for example, micronwide lines have not as yet been produced although micron resolution has been demonstrated in the image. J. HARRICK: Which photocathode was employed? How was the mask pattern applied on to the photocathode? Can one achieve the “mask aligning” with this tube required for multiple photoresist processes? T. w. O’KEEFFE: The photoemissive layer consisted of a palladium metal layer 40 A thick deposited by routine evaporation over times of the order of 1 to 2 min from a tungsten wire. The pattern was defined by photoresist technique in an ultra-violet opaque layer deposited on the quartz of the cathode window. Image position can be adjusted by deflexion coils over a range sufficient to allow registration to a pre-existing pattern a t the target. Rotation of the image is also feasible. Detection of the image position has also been demonstrated. E. F. LABUDA: How uniform is the photoresist exposure? T. w. O’KEEFFE: Resist exposure is as uniform as the ultra-violet illuminating the cathode. No significant changes in image quality as recorded in the resist is detected for a 60% “over exposure” a t least.

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Further Developments of the Spectracon J. D. McGEE, D. McMULLAN,? H. BACIK and M. OLIVER Applied Phyaica Department, Imperial College, Univeraity of London, England

INTRODUCTION Since the last report1 on the Lenard window image tube, now known as the Spectracon, no major change has been made t o the normal tube but many refinements in internal design, processing, encapsulation and operation have led to considerable improvements in all aspects of its performance. These will be briefly outlined in this paper. One major development is an attempt t o make a tube with a much larger effective area, with a mica window, say, 1-in. diameter and -4-p" thick and t o use it in the same manner for contact electronography. Preliminary results of these experiments are reported.

PRESENT TUBEAND SOLENOID DESIGN This is shown diagrammatically in Fig. 1. The dimensions of the Spectracon tube, 28-cm long by 5-cm diameter, have not been changed. The tube 1 is shown in Fig. 1 mounted in a co-axial Perspex cylinder 2, which is closed a t the front, or photocathode, end with a high-quality fused silica plate 3 and at the back or mica-window end with a Perspex annulus, to which the applicator case can be attached. The potentialdivider resistor chain 4 is attached t o conducting bands on the external surface of the tube opposite the internal conducting metal bands t o which the annuli 5 make electrical contact. The potentials of the annuli are maintained by the electrical leakage through the soda-lime glass walls of the tube, except for the first annulus and the photocathode, for which platinum seals are used in order t o give good conductivity for photocathode sensitivity measurements. The resistors of the potential divider are kept within limits of -&- 2% since greater variation than this can adversely affect the uniformity of the accelerating field and hence the electron-image geometry. The high-voltage lead 6 is taken from the photocathode contact,

t

Now at the Royal Greenwich Observatory, Herstmonceux, Sussex, England. 61

FIG.1. A Spectrecon in screened focusing solenoid, Mk III(b).

FURTHER DEVELOPMENTS O F THE SPECTRACON

63

between the tube and the wall of the Perspex cylinder and through the Perspex disc at the back end of the encapsulation. From where it emerges it is screened with an earthed copper braid 7 for safety in case of a fault developing in the insulation. The space 8 between the tube and the Perspex cylinder is filled with a cold-curing silicone rubber, “Silastomer”. The silica end-window 3 of the encapsulating cylinder is coated internally and externally with very light, slightly conducting tin-oxide (Nesa) coatings. Together these absorb <5% of transmitted light. The inner surface coating is connected to the tube photocathode and maintained at its working potential of -40 kV while the mica-window end of the tube is operated at earth potential. The outer layer is connected electrically to a conducting coating of Aquadag applied to the whole outer surface of the Perspex cylinder, which in turn is in contact with the inner brass cylinder 10 of the solenoid mounting, which is earthed. Thus there is a potential difference of 40 kV across the insulating/encapsulating cylinder. This is safely sustained by the encapsulation provided there are no flaws in the Perspex and the joint of the Perspex cylinder to the silica window is made carefully. As the background of the tubes was reduced by methods previously described,l it became clear that the main residual source was the corona discharge into the air from the silica plate in front of the photocathode. Even with the very high resistance of the silica plate, -lo1* !2cm2, there is sufficient electrical leakage through it with 40 kV across it to give an appreciable discharge into the surrounding air a t a position from which some of the photons so excited can easily reach the photocathode, liberate photoelectrons and so produce background. Now, with the external surface earthed, this leakage current is dispersed and no discharge can take place. Another major advantage of this technique is that other electrical or optical objects can be brought as close as desired to the outer surface of the encapsulation without trouble due to corona discharge.

Solenoid The focusing solenoid has been designed t o give a magnetic field of -160 G, uniform to within f 1%, between the photocathode 9 and mica window 18 with effective cooling of the photocathode and adequate screening of the tube from external magnetic fields. The solenoid resistance is -1 5Q and requires a current of -2 A for a field of 160 G, when the heat dissipation is -60 W. The solenoid windings are mounted on a former constructed from two concentric, precision-bore brass tubes, 10 and 11 at the front end and a solid copper cylinder 12 at the back end. The inner brass tube

64

J. D. MCQEE, D. MCMULLAN, H. BAUIK A N D M. OLIVER

has an internal diameter such that the encapsulated tube is a snug, sliding fit in it. Between the inner and outer brass tubes is a space through which coolant can be circulated, being piped in through the tube 13 and out through the tube 14. Thus the coolant circulates only through the front 60% of the coil-former and hence cools only the front part of the tube, and especially the photocathode. This is the only part of the tube that it is really necessary to cool t o reduce thermal electron emission. The rear 40% of the coil-former is solid copper so that the heat generated by the coils is conducted t o the tube and warms the mica-window end of the tube. Thus the mica window can be kept 10 to 15°C warmer than the photocathode so that, for moderate cooling of the latter, it can be kept above the dew point and hence a film of condensed water will not form on the mica. This is very important in practice because if a strip of emulsion is pressed against a wet window it may stick t o the mica and when removed can cause it to rupture. The focus coil was first wound in sections, as shown, so that the number of windings per unit length could be easily adjusted. Thus, by trial and error, the uniformity of the magnetic field could be adjusted by varying the number of turns on particular coils. However, it was soon found that the mu-metal screening cylinders 15 and 16 (which have their front ends partially closed by apertured discs 17) that are used t o screen off external magnetic fields, drastically altered the field distribution at the photocathode end of the coil and helped greatly in maintaining the field uniform t o much nearer that end of the coil. This effect is due to the very high permeability of the mu-metal which induces the magnetic lines of force t o spread more uniformly over a cross-sectional area up t o a distance from the aperture equal t o -60% of its smallest dimension. If this end of the screening cylinders is closed completely, the magnetic field remains very uniform right up to the surface of the mu-metal. Of course, there must be an aperture to allow the light image t o reach the photocathode but if, as illustrated in Fig. 6 of a companion paper,t this is a slot 30 x 7 mma the field can be kept adequately uniform up to -7 mm from the inner surface of the mu-metal end-plate, and so the tube can be mounted in a position where the optical image can reach the photocathode. Again where an optical system, such as that as shown in Fig. 7 of the same paper,? has a protruding cylindrical back element and forms the optical image 6 t o 8 m m outside its rear surface, the magnetic field can be kept adequately uniform t o this image-plane by keeping the diameter of the aperture in the mu-metal screen as small as possible. Finally, the design shown above in Fig. 1 is such that the field-flattening optical

t See Paper No. 76.

65

FURTHER DEVELOPMENTS OF THE SPECTRACON

elements required by a Bowen-type coudi? spectrograph, can be mounted without difficulty in front of the tube window. The dimensions of the solenoid are: length 38 cm, external diameter 10.8 cm, internal diameter 7 cm. I t s weight is -10 kg and hence it is not prohibitively heavy for mounting on a telescope or other apparatus. The measured axial magnetic field strength is shown plotted in Fig. 2, the full line giving the field with the mu-metal screens which in this case have circular apertures 4 cm in diameter opposite the photocathode, while the dotted line gives the axial field without the screens.

1-

With mu- metal screens

I

\\

I

- 1 0

1

I I

I l l

4

Photocathode plane

\ I

I 8

I

I

12

I

I

16

,

I

20

I

I 24

I

I

28

Distance from mu-metal disc (cm)

I

I

I

32

I

I \

36

k 1 window P ie

FIQ. 2. Axial field of solenoid with and without mu-metal screening cylinders. The same coil current was used throughout.

The approximate positions of the photocathode and mica-window on this graph are shown by arrows and it can be seen that in the space between these the axial field is constant t o within & 1%. The off-axis longitudinal field is also constant t o within & 1% over the plane of the photocathode and over its full area. It rapidly becomes much more uniform over transverse planes towards the centre of the solenoid. It can be seen from the two curves of Fig. 2 how effective the mu-metal screens are for maintaining the uniformity of the magnetic field to very near the end of the coil.

66

J. D. MCOEE, D. MCMULLAN, € BACIK I. AND M. OLIVER

Magnetic Screening It is essential for the use of the Spectracon on a telescope moving in the Earth’s magnetic field or in the presence of other variable magnetic fields that the tube should be effectively screened magnetically. For spectroscopic applications the most objectionable fields are t’hose transverse to the tube axis in the direction of dispersion of the spectrum since the electron image is deflected approximately in that direction by an amount 6 = B,/B, x L, where B, is the transverse field, B, is the axial field and L is the distance from cathode to anode. For an image resolution of 50 lp/mm a deflexion of 6 m 5 pm would be significant. Hence, a varying transverse field of f 2-5 mG would be undesirable. As the Earth’s field may vary by f 0.5 G it is clear that a screening factor of -500 is desirable.

FIQ. 3. Superimposed exposures of resolution chart with two different transverse magnetic fields.

The double layer screening by cylinders 15 and 16 shown in Fig. 1 was chosen because two such shells separated by a small air-space are more effective than a single shell of twice the thickness. It has been asserted that such mu-metal screens will be saturated by the return flux of the solenoid and hence will cease to be effective screens. In fact for the given conditions the mu-metal is very far from saturation and its screening efficiency is not impaired. The efficiency of the screening is best measured by observing the shift of an image in a tube in operation. One method of doing this is the following. A tube is set up with a Baum test pattern focused on the photocathode and the electron image on the mica-window so as to give good resolution (>80 lplmm). A pair of Helmholtz coils are mounted so as to produce a fairly uniform transverse magnetic field over the whole volume of the solenoid and in a direction perpendicular to the lines of the resolution test pattern. The field transverse to the axis of the solenoid, in its absence, is measured for a given current in the Helmholtz coils. A short exposure of the test pattern is recorded with a given deflecting current and a superimposed exposure of equal density

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67

is immediately made with the field reversed. The result is that, as shown in Fig. 3, at some point one electron image is moved by just one line-width relative to the other, and a t this point the modulation approaches zero. I n the example shown in Fig. 3 the modulation is near zero at -40Ip/mm. Thus for the double deflexion 26 due to f B, a relative image shift of 1/80 mm has resulted. From such observations it was established that the screening factor of this design of coil was 500. Thus the field penetration from the Earth’s fieId, is not likely to be > 1 mG, which, allowing for complete reversal, and in its worst possible direction, will produce a negligible reduction in spectrum resolution.

-

Spectrum Scanning

It is frequently desirable to broaden a star spectrum in a direction perpendicular to the direction of dispersion sometimes simply to enable more information to be recorded but also t o enable variations in the light distribution with time to be displayed.2 This may be done by “trailing” the star image along the spectrograph slit, by moving the plate perpendicular to the dispersion or other methods. However, the Spectracon (and other image tubes) allows this to be done by electronic deflexion which has some considerable advantages. A simple flat rectangular coil, of -20 turns of 30 s.w.g. copper wire, is wound outside the focusing solenoid but inside the mu-metal screens, with its plane passing through the axis of the solenoid and extending along its whole length. A current passing through this coil produces a magnetic field perpendicular t o its plane and to the direction of flight of the image electrons. This field deflects the electron image in a direction at about f.9” to the normal to the scanning-coil plane, depending on the polarity of the axial field. Thus by suitably adjusting the plane of this coil at f 9” relative to the direction of dispersion and passing a current of saw-tooth waveform through the coil the electron image of the spectrum can be scanned perpendicular to the direction of dispersion. Figure 4 shows seven images of the fine resolution end of the Baum test pattern extending from 30 to 90 Ip/mm recorded as it was deflected in steps of -0.3mm along the mica window of a Spectracon. The resolution, which is > 80 lp/mm, is not visibly affected by the deflexion, and a similar result is obtained if the image is deflected across the small dimension of the window. Figure 5 shows a simulated spectrum varying from 8lp/mm to 90lp/mm in its recorded dimensions. I n (a) it is recorded statically and in (b) scanned to -20 times its original width by a saw-tooth wave-form scanning field over an amplitude of -0.5 mm. The exposure required for (b) is -20 times that for (a) and the amount of information

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J. D. MCQEE, D. MCMULLAN, H . BACIK AND M. OLIVER

recorded is greater by about the same factor. The image resolution is well maintained in the scanned image and as far as can be measured at present there is no bending of the lines which would complicate microphotometer measurements. Since the scanning is electronic its frequency can be varied from very low (-1 scan/h) to quite high (-lo2 scans/sec) with ease and precision.

FIG.4. Section of Baum resolution test chart deflected to seven positions in steps of 0.3 mm along the mica window.

FIG.6. (a)Static resolution dot pattern from 8 to 90 lp/mm. (b) Same pattern scanned perpendicular to its length by transverse saw-tooth magnetic field.

For example it could be done quite slowly to show the variation in spectral light distribution known to occur with certain variable stars2 which vary in times of the order of an hour, or quite fast for objects varying rapidly in brightness. For example if an image, or a spectrum, of a Pulsar were scanned a t its known frequency of pulsation of radio emission, the spread-out image or spectrum would show if there is any variation a t the same frequency in the optical intensity or colour.

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Since in using this method the star image would not move on the spectrograph slit, it would therefore be practicable t o close the slit except over that small length on which the star image falls and so exclude the sky-glow light which a t present contributes an unwanted background. THE FILMAPPLICATOR Because of the danger of breaking the mica window of the tube by the recording operation, the problem of pressing an emulsion-covered Melinex film against it has been the subject of continuous investigation. Any design of applicator must aim a t the use of the minimum possible pressure on the mica window; the pressure must be very uniform but sufficient t o give intimate enough contact between the emulsion and the mica surface t o ensure good image resolution. It must be easily loaded in a semi-dark room and reliably manipulated a t the tube which may, for example, be attached t o a telescope. This latter problem is helped by the fact that L4 and G5 emulsions can be handled in relatively bright amber illumination while XM can be exposed to fairly bright green light. The first applicator used consisted of a foam-rubber covered cylinder of slightly smaller radius of curvature than the mica window which, over most of its length, is deflected by the atmospheric pressure into a cylindrical form. A strip of film is wrapped around this roller and on being pressed with light spring-loading against the mica window, it slightly deforms t o assume the shape of the window. This device gave satisfactory and reliable operation when stripping emulsion was being used and very few mica windows were broken. However, in order t o achieve greater stability of image geometry the emulsion is now coated on 50-pm thick Melinex film; this has proved t o be much harsher and is more liable to cause damage when pressed against the 4-pm mica window. I n an attempt t o design an applicator with a more uniform and lighter pressure an extended series of experiments was done using pneumatic pressure to expand a rubber diaphragm which pushed the film against the window. While this method was made t o work quite well it seemed t o be, if anything, rather less safe than the original method and considerably more inconvenient t o use. Consequently, the spring-loaded rubber-covered roller was reinstated in a modified design to fit the new, small solenoid. The whole device is made of plastic and rubber and a simple mechanism is provided t o allow the film t o be advanced by the required amount between each of about eight exposures. This device is fairly satisfactory; the total force on the mica is only

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J . D. MCQEE, D. MOMULLAN, H. BACIK AND M. OLIVER

-200 g and quite uniform. Good image definition is regularly obtained but there are occasional breakages of mica windows. Consequently other, better methods are being sought.

Film-loop Applicator This is the most promising new method tried so far. It is illustrated in Fig. 6. A strip of emulsion-coated Melinex film is cut t o the exact dimensions required, found by trial and error t o be, for example, 25 x 36mma. This film 1 is formed into a loop with the emulsion outermost, its ends accurately lined up and inserted between the jaws 2 and 3 of the holder 4. Its ends are pushed down between the jaws of the

FIG.6. Diagram of film-loop applicator.

holder until they rest firmly against the shoulder 5 . The jaws are then tightened with the thumbscrew 6 to grip the film firmly. If it has been cut accurately and inserted properly the film will now take up a fairly accurately repeatable shape as illustrated. The film holding device 4 is carried on a base 7 which fits accurately into the normal applicator chamber attached to the tube and its height can be adjusted by another screw (not shown). On insertion into the applicator chamber the film loop is offered up to the window centrally and with their long axes parallel. The film loop is initially adjusted so that its minimum radius of curvature is less than that of the window. As it comes into contact with the window it easily deforms to a greater radius of curvature and its final position is adjusted until it is accurately the same as that of the window over most of its area.

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With proper adjustment of the loop satisfactory contact of the emulsion with the window over its whole cylindrical area can be achieved with a total force of -50 g, that is, about one-quarter of the pressure required for the rubber-covered roller. This is believed t o represent a very large reduction of the hazard to the mica window and experience has confirmed this expectation. The device has not yet been designed to permit the film to be advanced between exposures; only one exposure can be made on each loop. A design to provide this facility is being developed. However, at present the main requirement for quick successive exposures is for the preliminary “focusing run” when a series of exposures of a resolution test pattern are made with small adjustments of the optical or electrical focus. It is found that this procedure can be carried out equally effectively by using a small test pattern and deflecting it to different parts of the window as the progressive focus adjustments are made, as was done for the images in Fig. 4.

COOLINGOF THE PHOTOCATHODE As will be described below, moderate cooling of the photocathode has a marked effect in reducing the thermal current from the photocathode and now that sources of spurious background have been greatly reduced, this becomes important. Moderate cooling can be obtained by passing refrigerated liquid through the cooling jacket of the solenoid and in this way a photocathode temperature of -0°C can be reached. Since it is necessary to cool only the photocathode itself to reduce thermal emission, with the new form of tube encapsulation it became possible to mount Peltier devices in the focus coil with their cold surfaces in contact with the tube end-window and their hot surfaces in ~ a suitable pair of contact with the water jacket of the ~ o l e n o i d . For such devices a current of 2 A gave a temperature difference of 25 to 30°C. So, if tap water at 15°C is passed through the solenoid the cold plate of the device will reach about -13”C, and if ice-cold water is - 25°C. Howpassed through the solenoid the cold plate will reach ever, the photocathode surface is thermally isolated from the endwindow and heat transfer to and from the cathode is solely by radiation. The equilibrium temperature of the cathode was found to be a few degrees below the mean of the temperatures of the cooled end-window and -15°C and the body of the tube. Thus it was at --1*5OC respectively under the two conditions specified above. This is adequate for most photocathodes except possibly the S.1. Since the solenoid described above operates a t current of 2 A and this is the operating current of the Peltier device, it is convenient t o operate these in series from the same power supply.

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J . D. MCQEE,

D. MCMULLAN,

H. BACIK AND M. OLIVER

It was thought possible that the magnetic field associated with this current in the near neighbourhood of the photocathode might produce enough stray magnetic field to disturb the focusing conditions. However, measurements show that this field is -0.15 G and no distortion of the electron image has been detected. The only slight disadvantage in the case of this device is that it competes with the optical system for space immediately in front of the tube. However, it could probably be used fairly effectively with most of the optical systems envisaged. OF PHOTOCATHODES TRANSFER The transfer procedure already describedl has continued to be satisfactory for S.9, S-11 and S-20 photocathodes but the S.1 photocathode generally loses an appreciable proportion (about one third) of its efficiency especially in the infra-red region, during the process. There is often an appreciable recovery of its sensitivity after it is mounted in its operating position. The reason for this and the exact point in the operation at which the loss occurs is not yet known. Experiments on this problem continue. The other photocathodes mentioned retain their sensitivities with very little change until the tube is put into operation. Then if an excessive photocurrent is taken, especially with a high applied voltage, a rapid drop in the photocathode efficiency may occur. This is relatively slight in the cases of S.9 and S-11 photocathodes but can be very large for the 5.20. Even currents of a few microamps with the collector at a few hundred volts, as used in routine photocathode measurements, are sufficient to produce a detectable effect. However, with the tube at full operating voltage, even a much smaller current ( ~ 1 0 - ~ A can) cause a large drop in photocathode efficiency. Such a drop could be the result of some external or internal electrical breakdown causing sparks or corona which produce excessive spurious current in the tube. This was the main cause of failure of early S.20 cathodes in Spectracons. Since the adoption of the much better encapsulation and more reliable electrical contacts to the tube electrodes this trouble has been greatly reduced and tubes with S.20 cathodes can now be made and used with good photocathode sensitivity. Care must be taken to avoid the passage of excess photocurrent during operation and even during measurement of the photocathode. To facilitate this a special device was designed and made using an interrupted light beam, an amplifier and phasesensitive detector. This enables the photocathode sensitivity to be measured while drawing a photocurrent of only -lWQA and with an electric field of -80 V/cm in front of the photocathode. Also, with this device the photocathode sensitivity of an encapsulated tube can be

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73

measured although there is no direct external contact t o the f i s t annulus. The exact cause of the loss of photocathode sensitivity following the passage of large currents is still uncertain but it is thought to be due mainly to the release of adsorbed gas from tube electrodes when bombarded by electrons. The gas may leave the surface partly as ions and partly as neutral gas atoms. The former will be accelerated back to the neighbourhood of the photocathode from which they appear to release bunches of -10 electrons which are then recorded as an intense “ion spot” if the tube is in electron-focus, This effect is frequently observed when undue background is generated in a tube with an S.20 photocathode. It is very much less in evidence in tubes with S.9 or Sell photocathodes. Again the reason is unknown but it may be speculated that this is the result of the very low electron affinity of the 5.20 surface. Moreover, the bombardment of the 5.20 surface by ions is known from gas-filled photocell experience to be very destructive t o its sensitivity. Also any neutral gas atoms may reach the photocathode, react with the surface caesium, and so impair its efficiency. These effects are observable in tubes with S.9 or S.11 photocathodes, but are of much less importance, probably because these surfaces are inherently more stable.

TUBEPERFORMANCE CHARACTERISTICS Resulting from the modifications described above there have been improvements in all the main characteristics of the tube compared to those reported a t the last Symposium.l These are summarized briefly below. Photocathode Xeneitivity This is now 50 to 80 pA/lm for S.11 photocathodes, 100 to 150 pAI1m for S.20 but only 10 to 20 pA/lm for S.1 photocathodes. Thus there has been an increase in quantum efficiency of -50% in the blue region and an extension of sensitivity into the infra-red t o a wavelength of -1.3 pm. Speed-gain The mica window thickness is now 3.5 to 4 p m compared with -4.5pm which results in a substantial increase in the number and energy of the photoelectrons that penetrate it. H.ence this gives a useful increase in “speed” of recording of the tube. The speed-gain as compared with baked I I a - 0 emulsion is now 10 to 20 for G5 and 50 t o 100 for XM emu1sion.t These figures depend on the exact cathode sensitivity and mica thickness in any particular case and do not include

t See p. 725. P.E.1.U.-A.

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J. D. MCGEE, D. MCMULLAN, € BACIK I. AND M. OLIVER

additional relative gain due to reciprocity failure of the photographic emulsions.

Resolution This is now fairly consistently: -60 lp/mm for XM, -90 lp/mm for G5 and 120 lp/mm for L4. These values are for limiting resolution and this is fairly constant over the whole window area. Background Provided the electrical connexions and the encapsulation remain intact the background is now independent of the tube’s surroundings. It depends only on the temperature of the photocathode and for 8.9, S.11 and 5-20 cathodes the rate of blackening of an emulsion is reduced by a factor of > 10 on cooling from 15 to 0°C. The background a t 0°C may reach a density of -0.1 in a 10-h exposure on XM, the fastest emulsion. This corresponds t o a photocathode thermal current density of -50 electrons cm-2sec-1. The equivalent exposures on G5 and L4 are 50 h and 250 h respectively. The background for an S - 1 cathode tube is several orders greater but it improves by a larger factor, (-30 times) on cooling from 15 t o 0°C. At 0°C reasonably long exposures can be made with acceptable background. Image Geometrg Improvements in the uniformity of both the magnetic and the electrical fields, especially in the near neighbourhood of the photocathode, has given marked improvement in image geometry. As noted in another papert the image geometry can now be sufficiently accurate to enable astronomical measurements requiring great precision t o be performed satisfactorily. Uniformity of Photocathodes Though little attention has been given t o this problem uniformity of 5% is often obtained over the working area of the photocathode. With more care being taken and with some seleotion a uniformity of f 1% might be achieved. Image Stability There is a small amount of drift in image position during the first 30 min after switching on the tube and operating circuits but thereafter the electron image is quite stable over long exposures provided that the focus current, the applied voltage and the temperature remain constant. f See p. 773.

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LAROE-AREA MICA WINDOWS As its name implies, the Spectracon was originally developed for the recording of spectra. While the rectangular shape of the window aperture is ideal for that purpose, the fact that the mica forms a cylindrical surface (except at the two ends where it is curved spherically) eases the problem of bringing the electron-sensitive emulsion into intimate contact with the window. As described above the electronsensitive emulsion on a Melinex base (50-pm thick) can be pressed against the mica by a soft rubber-covered roller having the same radius of curvature as the window, or simply by forming the film into an unsupported loop, again of approximately the same radius of curvature. As has been shown by Kront and other^,^.^ electronography may be applied with considerable advantage to the direct recording of astronomical images and to stellar photometry, A Spectracon designed for these purposes would ideally have a circular window; however it is also practicable to use one with a rectangular window of greater width than is normal for spectrographic recording. Whether the window is circular or rectangular, the aim niust be to achieve the maximum useful area consistent with adequate strength. By useful area is meant the area over which the emulsion can be brought into intimate contact with the mica. The relative advantages of rectangular and circular windows will now be discussed.

Rectangular Window It can be showne that under a differential fluid pressure a thin rectangular membrane will take up a cylindrical shape except a t the edge of the membrane. The curvature at the edge depends on the way the membrane is clamped; it is our practice to seal the membrane of mica to a cylindrical surface milled in the chrome-iron end-plate (see 18 in Fig. 1) and to some extent this reduces the bending of the mica near the edge. Sealing to a cylindrical surface also gives some initial curvature to the mica and this, as is shown later, reduces the working stress. The stress in the mica over the central cylindrical portion of the window where the curvature is constant can be shown' to be p = -

PR t '

where P is the pressure differential, t is the thickness of the mica, and R is the radius of curvature. The deviations due to bending a t the edge do not affect the final

t See p. 1.

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J. D. MCGEE, D. MCMULLAN, H . BACIK A N D M. OLIVER

shape of the window significantly since the width of the region where bending occurs is a small fraction of a millimetre. The stress in this region is probably somewhat higher than in the remainder of the window, but since it is doubtful whether it could be accurately calculated and since in practice failure of a window does not necessarily occur at the edge, we feel justified in taking the stress a t the centre of the window as the significant parameter. The radius of curvature of typical windows 0.5 cm wide and 4-pm thick ranges from 1-1cm t o 1.6 cm when the pressure differential is one atmosphere. Substituting these figures in Eq. (1) shows that the stress ranges from 2.5 x lo3 kg/cm2 to 4 x lo3 kg/cm2. The variation in radius of curvature with different samples is a consequence of the difficulty in controlling the amount of initial curvature during sealing. Calculations show7 that if the mica was completely flat when sealed t o the chrome-iron end-plate the radius of curvature of a window of this size under one atmosphere pressure differential would be 2 cm ( p = 5 x lo3 kg/cm2). The figure for the elastic limit of mica quoted in the literature* is 3.5 -3.9 x lo3 kg/cm2; although the working stress in the windows mentioned above in most cases exceeds this figure, windows of these dimensions have proved to be reasonably robust and generally will withstand a pressure differential of several atmospheres. It can be shown7 that the stress is proportional t o P2I3 so that the ultimate tensile stress of a good specimen of mica is therefore a t least 1.5 x lo4 kg/cm2. That some yielding takes place at a lower stress ( - 7 x lo3 kg/cm2) is shown by the appearance of strain lines, in the longitudinal direction, which remain after the pressure differential has been reduced t o zero. The stress in a rectangular window can also be shown to be proportional to the two-thirds power of the width. Thus the stress in a 2-cm wide window will be about 2.5 times that in the 0.5 cm wide window, i.e. -8 x lo3 kg/cm2 if the mica is curved while sealing. This is well below the ultimate tensile stress and windows of this size should have adequate strength. A window of this width will offer a square field 2 x 2 om2 if the length of the window is sufficient. Only the central section can be used because the ends of the window take up a spherical shape; the Melinex can be curved only in one dimension by the roller of the applicator and it cannot be made to conform to the spherical surfaces a t the ends (except in the special case of a circular window, see below). The total length of window that is unusable through this cause is approximately equal to the width, so that the 2 cm wide window would need to be 4-cm long. A window of this size cannot be accommodated in the

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present design of Spectracon; 3.5 x 1.5 om2 would appear to be the maximum possible, giving a useful area of 3 cm2.

CIRCULARWINDOW A circular membrane under uniform pressure takes up a spherical shape.6 The stress in the membrane can be shown' t o be: p = -P R 2t ' where the symbols are the same as for Eq. (1). Unlike the rectangular window it is not possible to increase the initial curvature by bending the mica during the sealing process. The mica can be sealed only t o a flat surface. For a window 2 cm in diameter and 4-pm thick, the radius of curvature with 1 atmosphere differential is 7.5 cm so that p = 9.4 kg/cm2, a value that is well below the ultimate tensile stress. Windows 3 cm in diameter have been made to withstand atmospheric pressure, but success with these has been variable. Such a window 3.6-pm thick had a radius of curvature of 8-9 cm so that p = 1.24 x lo4 kg/cma, which is rather close to the ultimate tensile stress of the mica; 3 cm is probably the absolute maximum diameter for windows of this thickness. However even a 2.5-cm diameter window affords a useful increase in available area over that of a 3.5 x 1.5-cm2 window (4.9 cm2 as compared with 3 cm2), as long as the film can be brought into contact with the whole area of the window. A method of achieving this is described in the next section. Application of Filvn With a circular window the film must present a spherical surface t o the window. One way in which this can be done is to clamp the film a t its edge and deform it by a uniform pressure (conveniently atmospheric pressure). The film which is generally used has a Melinex base 50-pm thick. It can be shown4 that the elastic constants of Melinex and mica are so proportioned that for the same differential pressure the radius of curvature of a Melinex membrane 50-pm thick will be of the same order as that of a mica window 4-pm thick. By reducing the pressure in the iriterspace between the film and the mica window, the film takes up a spherical shape; with a suitable choice of dimensions the film and the mico will come into contact when their radii of curvature are equal. Figure 7 shows the cross-section of an applicator for a 2-cm diameter window. The body of the applicator is mounted on the chrome-iron

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J. D . MCOEE, D . MCMULLAN, H. BACIK A N D M. OLIVER

end-plate of the Spectracon with a Perspex ring, and a vacuum-tight seal is made with a flat rubber ring. A disc of film 4.2-cm diameter is held t o the perforated face of the applicator plate by atmospheric pressure (tube A t o vacuum pump). The applicator plate is now offered up to the body and the tube B connected to the vacuum pump. When the pressure within the applicator has fallen by about 0.5 kg/cm2 the film will be tightly clamped around its edge between the body and the plate, and tube A can be opened to atmosphere thus deflecting the film towards the mica. A further reduction in pressure brings the film

FIG.7. Cross-section of film applicator for use with Spectraconhaving a 2-om-diameter mica window. The internal electrode structure of the tube is not ahown.

into contact with the window. The opening of A t o the atmosphere is done automatically by a solenoid valve controlled by a pressure-switch connected t o B. To remove the film the cycle of operations is reversed.

Practical Tests A number of windows 2 cm and 3 cm in diameter have been made and tested. The 2-cm windows present little difficulty and specimens have undergone several thousand cycles of film application without breaking. Windows 3 cm in diameter have not proved to be so successful although one or two specimens have withstood a few hundred cycles. The viability of windows of this size is critically dependent on the quality of the mica; with careful selection it may be possible to make satisfactory windows of this size and thickness.

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ACKNOWLEDGMENTS We wish to express our thanks to many colleagues for their assistance with this work, including Messrs. R. Barr, J. Beisiegel, I. Coole, G. Matthews, J. Osborne, W. Simpson, and especially Mr. N. Curtis who has been responsible for the making and processing of the tubes and Mr. B . Weekly who has processed many of the photocathodes used and given much assistance in other problems. Two of us, H.B. and M.O., are indebted to the Carnegie Institution of Washington for a maintenance grant during our work on this project.

REFERENCES 1. McGee, J. D., Khogali, A., Ganson, A. and Baum, W. A., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 11. Academic Press, London (1966). 2. Walker, M., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22B, p. 768. Academic Press, London (1966). 3. McMullan, D. and Oliver, M., J . Sci. Inatrum. 1, 1255, (1968). 4. Lallemand, A., Canavaggia, R. and Amiot, F., C.R. Acad. Sci. 262, 838 (1966). 5. Walker, M. F. and Kron, G. E., Publ. Aetron. SOC.Paoij. 79, 551 (1967). 6. Prescott, J., Phil. Mag. 43, 97 (1922). 7. McGee, J. D. and McMullan, D., J . Sci. Instrum. 2, 36 (1969). 8. Knoll, M., “Materials and Processes of Electron Devices” p. 239. SpringerVerlag, Berlin (1959).

DISCUSSION M. F . WALKER: The background observed with the Spectracon I have tested at Mt. Hamilton has always been greater than you report in this paper. This could be due to the fact that to avoid condensation on the Lenard window, the temperature of the photocathode has never been lower than about 40°F during the observations. What is the rate of change of background with temperature in this temperature range? At what temperature were the measurements made of background reported in your paper? How was the temperature of the photocathode determined? Were actual thermocouple measurements made? In addition to the uniform background, the Spectracons tested a t Mt. Hamilton have tended, after some weeks or months of use, to develop an irregular background, with one or more regions of the film exposed at the Lenard window showing a blackening in a matter of seconds or minutes. It is this type of defect which usually terminates the useful life of a particular Spectracon. Would you comment on the cause of this effect? Would you give more details concerning your tests of the magnetic shield of the Spectracon? Is it certain that the tests you have made adequately reproduce the conditions under which the Spectracon would be used on a moving telescope, i.e., do they show the effect of those flux lines which enter through the open ends of the shield as the aspect of the tube in the Earth’s magnetic field changes? J. D . MCGEE: The degree of background that you report for two of the tubes sent to you is far worse than that recorded in tests before they were dispatched to you. We know from subsequent examination that this was due to a breakdown in the enoapsulation in two of these tubes and only subsequently have we devised a form of encapsulation that is really reliable. Also, if a resistor in the potential

+

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J. D. MCaEE, D. MCMULLAN, H. BACIK AND M. OLIVER

divider, which is embedded in the encapsulation, changes value seriously, or contacts become intermittent, spurious background can appear. The rate of change of background with temperature has not been plotted. The tests are usually done at about tap water temperat,ure, --15"C, and 0°C. The cooling jacket of the coil surrounds the tube over an axial distance of 25 cm and after the cooling system has been running for 1 h it is found that the tube background reaches a fairly steady value which is assumed to be that of the surrounding cylinder and is checked by a thermometer placed near the tube window. The temperature of the actual photocathode inside the tube, of course, cannot be measured. I n the cooling system using the Peltier device the situation is different since the cold face of the device is much the coldest object in the neighhourhood of the photocathode which must take up a temperature intermediate between this arid the surrounding bodies. I n this case the actual temperature of the photocathode surface was measured by a thermocouple in an experimental tube. The tests of the magnetic screening are more fully described in the toxt and we can see no reason for doubting that they reproduce adequately the conditions of a Spectracon on a telescope moving in the Earth's field. The residual image deflexion due to a magnetic field is probably almost all due to the flux lines that penetrate into the neighbourhood of the photocathode through the aperture in the photocathode end of the screens, which of course cannot be avoided. The effectiveness of the screening, when the direction of the external field is changed relative to the coil axis, has been tested carefully and no increase in transverse deflexion can be detected. As is to be expected, a field applied at a n angle other than 90" to the axis has the combined effects of its resolved components perpendicular and parallel to the axis which produce image shift, and change in the image focus, respectively. Clearly the image shift must be less than when the same field is applied perpendicular to the axis and the change in axial field < 0.1 G is too small to produce a detectable change in image resolution.

-

-

Cathode-ray Tube with Thin Electron-permeable Window Y. UNO, H. KAWAKAMI, H. MAEDA and E. MIYAZAKI Matsushita Research Institute Tokyo, Kawaaaki, Japan

INTRODUCTION The penetration of electrons and other charged particles through matter has been the subject of theoretical study and experiment for many years. Many attempts have been made to use this phenomenon in electron-beam devices. In 1936, Knoll1 described a cathode-ray tube with an electron-permeable window which would withstand a pressure differential of several atmospheres. Since then a number of electron-beam devices with thin windows have been described, in particular tubes with mica window^.^-^ In the field of electron-image recording, several methods other than the conventional cathode-ray tube with electron-sensitive plates in vacuo, or with a phosphor screen and photographic camera, have been described. They include cathode-ray tubes with fibre-optic face-plates, those with face-plates having conducting pins for electrostatic recording, as well as those with electron-permeable windows for direct recording on electron-sensitive emulsions in air. The latter have the advantage of the high sensitivity of cathode-ray tubes with electron-sensitive emulsions in vacuo without the disadvantage of air locks and pumping equipment. The electron-sensitive emulsion is in air, and the cathoderay tube can be sealed-off. The tube to be described is of this type.

PENETRATION OF THINFILMS BY ELECTRONS As mentioned above, electron penetration of matter has been studied by many workers. A good summary of these researches has been given by Sch~macher.~ When high energy electrons are injected into matter they gradually lose energy and are scattered. The practical range of an electron in matter is defined as the depth at which the electron just stops. It depends only on the density of the material, and on the initial electron energy; for a given electron initial energy it may be stated in units of 81

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Y. UNO, € KAWAKAMI, I. H. MAEDA AND

E. MIYAZAKI

the mass per unit area, An experimental value of the range RE of an electron having energy E eV is given by the expression5 RE = 4.57 x mg/cm2. If the density of the film is d , g/cm3 the penetration depth is L, = 4.57 x d;l pm. Kanter6 has shown that the transmission properties of a material in the form of a foil can be summarized in a single universal transmission curve which holds over a wide range of film thicknesses. We have measured the electron transmission of nickel foils of various

FIQ. 1. Calculated and measured values of the fractional transmission as a function of electron energy for nickel foils of various thicknesses t .

thicknesses. The results of aome of these measurements are plotted in Fig. 1 together with curves derived from Kanter’s universal curve for nickel.6 Electron scattering will occur not only in the foil itself but also in the air-gap between the foil and the recording emulsion. The effect of the air-gap is reduced in our tube by the design of the window, as is described later.

THERECORDING HEAD The recording head replaces the face-plate of an otherwise conventional cathode-ray tube having a well-focused beam and working with a total accelerating potential of 20 to 30 kV. The head has a thin

CATHODE-RAY TUBE WITH ELECTRON-PERMEABLE WINDOW

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electron-permeablewindow which must of course withstand atmospheric pressure without leakage. The choice of window material is most important. Knoll1 suggested several materials including metals and plastics, while Seehof' proposed bentonite clay supported by a nickel mesh. McGee et aLa and Wheeler and Emberson3used mica 3 to 4 pm thick. Thin foils of all these materials are very fragile and they must be supported by either a plate having a narrow slot or a number of small holes, or by a mesh. A measure of the strength of a thin foil covering a circular aperture is

?-JsaCT3 -

-3

E P2 where a is the radius of the hole in cm, t is the film thickness in cm, C is a constant (0.25-0.34), T is the yield stress in dyne/cm2, E is Young's modulus in dyne/cm2, and P is the pressure differential in dyne/cm2. t

FIG.2. Recording head of cathode-ray tube.

Experiment showed that aluminium foil 2500 A thick on an aperture 1OOpm in diameter would just withstand one atmosphere. Safety would seem to require a thickness of at least 5000 d. Nickel was chosen 88 the window material for our tube because the ratio altis about 20 times greater than that for aluminium and it is more easily handled. The thickness of the nickel foil is 1 pm and it is supported by a plate 30 pm thick having an array of holes 100 pm in diameter pitched at 200 pm. This thickness of nickel absorbs 20 to 30% of incident 30 keV electrons; there is no danger of burning or melting the window unless there is a failure of the scan. The effect on resolution of electron-scattering in the nickel foil

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Y. UNO,

H. KAWAKAMI, H. MAEDA AND

E. MIYAZAKI

could be serious because of the gap between the window and the recording paper. This air gap is essential to prevent the thin nickel from being damaged by the moving recording paper. However, the array of holes in the window support plate confine the scattered electrons and the resolution is determined by the pitch of the holes. The support plate with the nickel foil is sealed with an epoxy adhesive to a nickel-iron block having a slit 150 pm wide and 8 cm long, This block is sealed with frit to the soft-glass envelope of the tube (see Fig. 2). The fabrication of the window and support plate is carried out using a micro-photoresist process followed by electroplating. It is essential that there should be no pin-holes or clogged holes. A micrograph of a section of the window is shown in Fig. 3.

FIQ.3. Micrograph of section of recording head.

This array of holes was chosen for its mechanical strength and because its resolution was adequate. However, an array of holes in parallel lines would facilitate electron beam alignment, but would of course be less robust. TUBECONSTRUCTION

A photograph of the tube is shown in Fig. 4. At present the tubes can be baked only a t a low temperature and because of this a large gettering area or continuous pumping with an ion pump is necessary to protect the cathode. Other sealing methods which would permit a high-temperature bake are under investigation. The electron gun which operates with a second anode potential of 20 kV gives a spot diameter of 100 pm a t several microamps and -200 pm at 100 p A . The second anode and the recording head are normally grounded.

CATHODE-RAY TUBE WITH ELECTRON-PERMEABLE WINDOW

85

FIG.4. Cathode-ray tube with thin-window recording head.

EXPERIMENTAL RECORDINGS

A block diagram of the experimental recording equipmeat is shown in Fig. 5. The beam can be modulated either by applying the video signal to the gun cathode (or grid) in the conventional way, or by deflecting the beam away from the line of holes in the window support plate. The beam is deflected horizontally along the line of the holes Recording paper

Recording Paper drive

c

, Dynamic coil current

current

coi I current

current

1 Synchronizing pulse generator

Electron gun ower SUPPh

-

Isolated video amplifier

-

1

Video signal modulator

-

Video signal generator

J

Fm. 5. Block diagram of recording equipment. 1 , Modulating deflexion coil; 2, horizontal tleflexion coil; 3, static focusing coil; 4, dynamic focusing coil.

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Y . UNO, H. KAWAKAMI, H. MAEDA AND E. MIYAZAKI

FIG.6. Recording of test chert.

by a conventional electromagnetic deflexion system. If the scan fails the beam is automatically turned off so as to avoid burning the window. Alignment of the beam with the window is facilitated by pressing a phosphor screen against the recording head. Experimental recordings were made on silver-halide recording paper; the video signal was derived from a facsimile apparatus. The scan-rate

FIG.7. Magnified are& of a recording.

OATHODE-RAY TUBE WITH ELECTRON-PERMEABLE WINDOW

87

was 5 lines/sec and there was a total of 650 lines. Figure 6 shows a test pattern recorded with the tube. It should be noted that the scan lines run vertically, the line length being 75 mm. The recording paper was driven a t about 1’25 seclmm and the recording time was 130 sec. The accelerating potential was 30 kV and the beam current a few microamps. As can be seen in Fig. 6 more than 450 lines are clearly resolved in the direction perpendicular to the sweep, but only 300 lines along the sweep. This loss of resolution appears to be due to the periodic spacing of the holes. The moire pattern effect appears to be due to fluctuations in the driving speed of the recording paper. A magnified section of a recorded pattern is shown in Fig. 7 and it can be seen that, as would be expected, the recording is made up of dots.

CONCLUSION The tube which has been described is still in the experimental stage but it has demonstrated its potential for high-quality recording of electron images directly on to electron-sensitive emulsion in air.

REFERENCES 1. Knoll, M., U.S. Patent, No. 2,036,532 (1936). 2. McGee, J. D., Khogali, A., Ganson, A. and Baum, W. A., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 41. Academic Press, London (1966). 3. Jeffers, S. and McGee, J. D., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 41. Academic Press, London (1966). 4. Wheeler, B. E. and Emberson, C. J., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 51, Academic Press, London (1966). 5. Schumacher, B. W., I n “Electron and Ion Beam Science and Technology”, ed. by R. Bakish, p. 5. Wiley, New York (1964). 6. Kanter, H., P h p . Rev. 121, 461 (1961). 7. Seehof, J., Smithberg, S. and Armstrong, M., Rev. Sci. Inatrum. 29, 776 (1958).

DISCUSSION J. D. MCUEE: Do you have difficulty due to fine holes in the nickel films causing air leaks and hence vacuum trouble? H. MAEDA: In the past we suffered from air leakage due to pin-holes appearing in the thin nickel film while processing, but a t present carefully developed filmforming techniques almost eliminate pin-hole troubles. About 0.5 pm thick epoxy resin or photoresist thin film with a 500 A thick conductive aluminium layer coated on the inner surface of tho nickel film stops leakage through pinholes and reinforces the fragile nickel window.

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Cascade Image Intensifier Developments J. D. McGEE, R. W. AIREY and B. P. VARMA Applied Physics Department, Imperial College, University of London, England

INTRODUCTION This paper deals first with the improvements that have been made since the previous report, in 1965,l in the design and performance of the three-stage cascade image intensifier developed a t Imperial College, and secondly with the development and preliminary testing of a considerably modified design of tube especially suitable for spectroscopy.

TUBE THE STANDARD Resolution and Gain The phosphor screens used in the cascade intensifier are made by electrophoretic deposition,2using a P.11phosphor, E.M.I. type MA 214. Screens of high energy-conversion efficiency were made, for the tubes described previously, by coating the phosphor layer with a highly reflecting evaporated aluminium film, floated off a water soluble substrate. This backing technique was found to be unsuitable for very high resolution screens because the floated aluminium film does not come into intimate contact with the phosphor particles, and hence considerable lateral spread of reflected light is possible, resulting in reduced resolution. Attempts were made to increase the resolution of these screens by making the phosphor layer thinner. Some improvement in resolution can be obtained in this way, and intensifier tubes with a limiting resolution of -40 lp/mm were made with phosphor screens of 0.5 to 0.7 mg/cm2. Electrons begin to penetrate right through the phosphor screens in this thickness range, and hence efficiency is lost, when the tube is operated in the region of the maximum working voltage. The overall gain of the tube tends to saturate and does not continue to rise as the operating potential is increased (Fig. 1). ao

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J. D. MCOEE, R. W. AIREY AND B. P. VARMA

A considerable improvement in image resolution has now been obtained, a t the cost of a slight loss in screen efficiency, by using an adaptation of the well-known organic film processt to apply the aluminium backing. This improved resolution is obtained because a nitrocellulose film containing a plasticizer moulds itself t o the granular structure of the phosphor layer and when this is removed the remaining aluminium adheres closely to the phosphor grains, whereas the floatedon aluminium film touches only the tops of the outer phosphor particles. I 06

I

I

I

lo5

/

k

\ Tube with thick

Io4

.-C m U

& ._

lo3

3 0)

m

102

10

'0

I II 10

I 20

I

I

I

30

40

50

Phosphor screens were made in the thickness range 0.5 to 1.5 mg/cm2 using aluminium backings formed on nitrocellulose films. Tests on these screens showed that the optimum thickness for the best compromise between resolution and energy conversion efficiency a t a working voltage in the region of 12 to 15 kV per stage, or an overall tube

t I n this technique the phosphor s ~ r e e nis irnniersecl in water, and a very thin film of nitrocellulose formed on the water surface by droppirig on to it tl specially prepared solution, which spreads uniformly. When the film has partially dried the water is removed, allowing the nitrocellulose to coat the surface of the phosphor screen. The filmed screen is allowed to dry and the nitrocellulose layer is used as a support for a continuous aluminium backing, produced by vacuum evaporation in a demountable system. Before use in a device the nitrocellulose layer is decomposed by baking in air.

91

CASCADE IMAGE INTENSIFIER DEVELOPMENTS

0

I

I I

I 2

I

3

1

Phosphor thickness ( mg/cmz)

FIQ.2. Screen efficiency as a function of tliickiiess at four operating voltages for electrophoretically deposited P.11 phosphor type MA214, aluminized by the organic film process.

potential of 36 to 45 kV, is 0.9 to 1.0 mg/cm2 (Fig. 2). A limiting resolution in excess of 50 lp/mm is typical of current tubes.

Photocathod es Efforts have been made to increase the gain of the multiplying screens by using a photocathode with a higher quantum efficiency in the emission band of the Pall phosphor, than the simple S-9 (Sb-Cs) photocathode used initially. The S.20 photocathode obviously recommends itself but is difficult to make. Attention was therefore turned to an antimony-caesium photocathode formed on an oxidized manganese substrate layer, which has been described r e ~ e n t l y . ~This .~ cathode, sometimes known as a “Super S - l l ” , has a photoresponse which differs from that of the normal S-9 in that it has a much higher quantum efficiency in the blue region of the spectrum. It was found that these cathodes can be made with a quantum efficiency of -15% a t the Pall response peak. The Sb-Cs[MnO] cathode was found to be a better match to Pall phosphor than all but the best 5-20 photocathodes (Fig. 3). Three-stage tubes with a blue light gain in the region of lo5 to lo6 at 40 to 46 kV overall potential are currently being made. The electron gain of each cascade is 50 to 100 a t an operating potential of -13 kV per stage.

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J. D. MCQEE, R. W. ATREY AND B. P. VARMA I

I

Wavelength

I

(8)

FIG. 3. Coinparison of the spectral responses of P.11 phosphor wibh S.9, S.20 arid Super S.11 photocathodes.

Background The so-called “bright” scintillations which are often found a t the output of cascaded image intensifiers can be shown to consist of a bunch of 2 to 20 electrons emitted a t the same time from the same point on the surface of the phot0cathode.l These events are probably produced by positive ion bombardment of the photocathode, or by interaction with soft x-ray^.^ I n a device under high vacuum, ion desorption and soft X-ray production depend upon bombardment of structures in the tube by spurious electrons. These can be generated by field emission. It follows that a reduction in the number and the energy of stray electrons passing between electrodes will minimize the secondary effects and reduce the number of “bright” scintillations. I n the earlier design of the cascade tube each annulus was mounted with a small gap between its edge and the glass wall. Spurious electrons travelling near the walls could pass through these gaps arid reach

CASCADE IMAGE INTENSIFIER DEVELOPMENTS

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energies of several thousand electron volts before striking one of the internal electrodes. This led to a relatively high “bright” scintillation level. A dramatic decrease in background noise has been obtained by a method of construction shown in Fig. 4 in which the metal annuli, which serve primarily to define the equipotential planes in the accelerating electric field but also to screen electrically the working region of the tube, are mounted by their being sprung into slots cut Platinum tapes

Multiplying screens

Slots in glass wall

Fro. 4. Diagram of a standard cascade tube showing modified oonstruotion of the annuli.

-0.15 mm into the glass walls. This procedure prevents unwanted electron migration in the region of the tube walls. The accelerating annuli, multiplying screen holders and the outer surface of the aluminium backings on the phosphor screens are now coated with a thin layer of evaporated carbon to suppress optical reflexions, raise the work function of surfaces, thus reducing field emission, and also to bind down traces of caesium by chemical combination. Tubes have been made with varying numbers of annuli (up to 12) in the first stage, each annulus being sprung into a slot in the glass wall. A typical background level for the latest “quiet” tubes is -1 bright

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J. D . MCQEE, R . W. AIREY A N D B. P. VARMA

scintillation cm-2 see-l, and the best result obtained is two orders of magnitude less than this. The rate of single electron background scintillations seems to be of the order of the thermal noise in the primary photocathode, and can be reduced by cooling the tube. A figure of 75 t o 80 electrons cm-2sec-1 from the primary photocathode is usual a t room temperature.

Focusing Solenoid A small light-weight solenoid (Fig. 5 ) has been designed to focus the cascade intensifier, using low resistance windings, with cooling water circulating between the coils and the tube. A double-skin mu-metal shield reduces the penetration of external magnetic fields by a factor of -500 permitting the tube to be moved in the earth’s magnetic field Focus coils (graded windings)

rout

Encapsulated cascade image intensifier

r in

Brass cooling jacket

FICA5. Diagram of the new light-weight solenoid.

during exposures. By adjusting the distribution of turns along the axis and using the effect of the mu-metal screens on the fields a t the from the first photoends of the coil, a field uniformity of <&I% cathode to the output screen can be achieved. The use of this solenoid with tubes having improved electrode structures has led to a marked improvement in image geometry. Camera

A plate camera is a t present under development for image intensifier photography using the lens designed by Dr. C. G. Wynne of the Applied Optics section of the Physics Department, Imperial College, and manufactured by Messrs. Wray Optical Company. This lens which is described in detail elsewhere,t is designed to give optimum m.t.f. a t

t See p. 705.

CASCADE IMAGE INTENSIFIER DEVELOPMENTS

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unity magnification using light with the spectral distribution of a Pa11 phosphor. It covers a field of 35 mm diameter with an aperture , the photons emitted from a scintillation will be of f l 2 . 8 . Thus O . S ~ of collected by the lens. Hence if the electron scintillations a t the output of the cascade intensifier produce 2.5 x lo6 photons, the lens will focus about lo4 of these on to the recording emulsion; enough to make about 10 grains developable. The complete tube inside the solenoid and the camera is shown in Fig. 6. The camera clamps on to the end of the focusing solenoid (Fig. 6). A photographic plate is introduced in a light-tight plateholder, and when the dark slide is removed the plate is pressed, by means of springs, into contact with a flat reference plane in which the image is located. After exposure, the plate is removed in its holder for subsequent development.

FIG.0. Plate camera attached to the light-weight solenoid.

When such a high definition optical system is used for image recording it becomes necessary to consider the optical quality of the input and output windows of the tube envelope. Steps are being taken t o replace the present Pyrex glass face-plates with window materials which are optically more homogeneous.

A SPECIALCASCADEIMAGE INTENSIFIER FOR ASTRONOMICAL SPECTROSCOPY A cascade image intensifier ha5 been developed with dimensions similar to the Spectracon,t for the special purpose of recording astronomical spectra. I n such a tube it is only necessary to produce a primary photocathode and electron multiplying screens in the shape of a small rectangular strip (for example 20 x 5 mm2). This opens up the possibility of processing the photocathodes in a separate photocell and subsequently transferring them into the tube prior to seal-off from the Pump. Many advantages arise from the use of this procedure. (1) The processing compartment, necessary in the larger Imperial College tubel

t See p. 81.

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J. D. MCQEE, R. W. AIREY AND B. P. VARMA

with 40-mm circular cathodes, is now eliminated, making all stages of the tube equal, and disposing of the field-free region which causes some geometrical distortion of the image. (2) No alkali vapours are generated within the tube; processing of the photocathodes takes place elsewhere and thus a possible source of background is eliminated. (3) Perhaps the most important advantage of the “in-vacuum’’ assembly procedure is that it permits the introduction of a primary photocathode of the S.1, S-20, or some other type, for use in a particular region of the spectrum while retaining the Super S-11 cathodes for the two multiplying screens. I n this case the primary photocathode is processed in a cell separate from that containing the multiplying screens and all the cathodes are transferred into the tube a t the same time. (4) Photocathodes can be tested before transfer and their spectral response determined. Poor photocathodes are rejected a t an early stage leading to a considerable saving in effort and better final results.

The Tube Design The tube envelope is 27.5 cm long with a diameter of 48 mm and a wall thickness of about 1 mm (Fig. 7 ) . These dimensions were chosen to approximate to those of the Xpectracon. Both tubes can therefore be used with the same focusing solenoid and power supplies. The electron image focus is obtained a t a field 140 G for equal stage lengths of -9 cm, using single-loop focusing. Electrical connexions in the standard cascade intensifier are established by means of thin platinum tapes sealed through the Pyrex glass walls. This is a mismatched seal and must be made with great care to prevent leaks. I n order to reduce the number of metal connexions which must be sealed through the glass walls it was decided to adopt a technique of tube construction already developed in this department. A lime-soda glass envelope is used in which the internal accelerating electrodes are maintained a t their working potential by conduction through the glass, using concentric bands of metallizing material (bright platinum paint) 2 fired on to the outer and inner walls. Those electrical contacts which are required to carry relatively large currents are provided by platinum tapes 1 sealed through the glass. These seals are very reliable since platinum and lime-soda glass have almost the same expansion coefficient. Only five platinum tapes are used, corresponding to the positions of the primary photocathode 7 , the multiplying screens 8,9, the output phosphor 6 and a test point for photocathode measurement. Flat lime-soda glass discs, 2.5 mm thick, are sealed a t each end of the tube with a low melting point solder glass, It is an advantage of the N

CASCADE IMAGE INTENSIFIER DEVELOPMENTS

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end of tube

FIQ.7. Diagram of the small cascade tube.

use of this material that lime-soda glass face-plates of high optical quality are readily available. The electric field gradients in the tube are maintained by a series of thin stainless steel annuli 3, 1 cm apart, which are held in position by Pyrex glass spacers 4. These spacers are made to fit closely within the internal diameter of the tube. Each annulus carries a thin flexible spring contact which presses against one of the internal bands of platinum paint.

The Photocathode Transfer Operation A f h r assembly the tube is sealed on to the pump together with the cell or cells containing the photocathode 9 and the two cascade screens 8 which have been prepared previously (Fig. 8). The cell is closed off from the rest of the system by a glass break-bulb 2, to which is attached

J. D. MCGEE, R. W. AIREY AND B. P. VARMA

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a long side arm 1 leading to the tube. Only that portion to the right of XX in Fig. 8 is subjected to the bake-out procedure, while the photocathodes are maintained a t room temperature. After baking for 40 h the tube is allowed to cool and a movable evaporator 3, operated by an external radio-frequency induction heater is used to deposit barium getter on to a flat surface a t the back of the output-phosphor holder 6. The break-bulb on the cell is then fractured with a magnetically operated hammer 7 so that the photocell is brought into direct communication with the tube. One of the

I

FIO.8 . The arrangement for photocathode transfer.

photocathodes is then transferred in its magnetic holder along the side arm and released a t a point a few inches from the tube envelope. A small magnetic slug is used t o manoeuvre the cathode through the flattened part of the side arm 5 and into the tube until it lies face down on the glass plate coated with barium getter, where it is held in position by means of clips. This procedure is repeated with each photocathode in turn until the primary photocathode and both cascade screens are all lying side by side on the barium getter plate. The tube is then sealed off from the pump, and after a period has elapsed to allow the getter to take up traces of seal-off gas, the cathodes are released in turn, manoeuvred along the tube and locked in their appropriate positions.

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Encapsulation The tube is encapsulated in a manner very similar to the Spectrac0n.t The tube, with a potential divider attached is enclosed in a, cylindrical Perspex case with a silica window closing the photocathode end and the space between the tube and the case filled with Silastomer. Separate leads are brought out at the back (output) end of the tube from the primary photocathode and the two cascade screens. Thus the optimum focusing conditions can be achieved by adjusting the potentials of these screens by means of an external potentiometer. Performance The tube is usually operated a t an overall potential of 40 t o 45 kV in an axial magnetic field of about 140G, using single-loop focusing in each stage. This tube operates satisfactorily in the solenoid used for Spectracons and with the same optical systems. The main difference in use being that, instead of the film applicator used with the Spectracon, the screen is photographed by a conventional camera. Gain The blue-light gain of current tubes lies in the range of lo5 to log a t 40 to 45 kV operating potential, the spread in performance depending mainly on photocathode sensitivities. Resolution Two recent tubes of the design described were able to resolve - 5 5 lplrnm. The resolution falls to about 40 lp/mm near the ends of

Distance from centre (mm)

Overall potential (kV)

FIG.9.(a) Resolution as a function of distance along the slot. (b) Resolution m . a function of overall operating voltage.

rectangular working area (Fig. 9(a)). When the overall operating potential of the tube is reduced from 40 kV to 20 kV the change in the limiting resolution is scarcely detectable (Fig. 9(b)). This shows that

t See p. 61.

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J. D. MCOEE, R. W . AIREY AND B. P. VARMA

the contribution to the disc of confusion due t o aberrations in the electron image is very small compared with the effects due t o the thickness of the phosphor screen and the mica sheet.

Image Geometry Because the three sections of the tube are almost identical dimensionally and electrically and because care has been taken to avoid structural details that would affect the uniformity of the electric fields, the geometrical distortion introduced into the image is quite small, especially when operated in the solenoids with very uniform magnetic

Fra. 10. Photograph taken with small cascade tube showing image geometry.

fields. Some S-distortion is detectable but has been kept within reasonable limits as can be seen from Pig. 10.

Background In a recent tube about one bright scintillation per minute is observed over the whole field. Some signal-induced bright scintillations have been seen. These appear to increase in number when signal electrons st'rike the metal frame of the multiplying screen. The reason for this is not clear, but it may be that ions are released more readily from the surface of the metal frame than from the carbon-coated aluminium backing. Electron-multiplication Statistics A measurement was made of the distribution in intensities of single primary electron pulses observed a t the output. The photocathode was illuminated with a faint source of light and the numbers of output scintillations occurring in a given time were recorded on a photographic emulsion a t different apertures of the recording lens and then counted. Figure 11 shows the number of scintillations as a function of Iff2 where f is the relative aperture of the lens. The curve remains constant over a considerable range of values off, then falls sharply over a small

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CASCADE IMAGE INTENSIFIER DEVELOPMENTS I

I

I

I

1

I

f/l.

005

0.1

0.15

0.2

1

0.25

O.!

l/f2

FIG.11. Number of recorded scintillations plotted against l/f2.

range below which nearly all scintillations become invisible, indicating that they are of fairly uniform intensity. Hence the multiplication factor is fairly constant.

Recording Eficiency The electron recording efficiency of the tube was determined in the following way. The primary photocathode was illuminated so that the measured photocurrent leaving it was of the order of 10-llA. This current was reduced by a large, accurately determined factor by attenuating the light, passing it through a small aperture. The output scintillations were recorded on a photographic emulsion with the lens fully open and the number of those which were recorded so clearly that they could be counted reliably was determined as a fraction of the known number of electrons that left the photocathode. A value of 68 5% was obtained for the tube a t 40 kV which agrees well with other estimates. Long Exposures and Photographic Gain Several exposures of 1 h at very low light-levels have been made with the tube running at 40 kV and the recording lens set a t fl5.6 and f/8. The image is very satisfactory and background is undetectable. A recent tube was compared with Tri-X emulsion. All the photographs were taken a t 10-sec exposure time to avoid reciprocity failure in the emulsion. Speed gain factors over direct photography were 5400 at f l l . 2 and 676 a t fl5.6.

102

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J. D. MCUEE, R . W. AIREY AND B. P. VARMA

12. The effect of overall system-gain upon image quality. (a) Direct photograph

of the test pattern. The remaining photographs are taken with decreasing recording lens aperture: (b)fll.2; (c)f/2; (d)f/2.8;( e )f/S; (f) f/lf3.

A series of photographs of the Baum' test pattern, taken from the output screen of a tube as the copying lens was stopped down from fll.2 to f/lS, is shown in Fig. 12 and can be compared with a direct photograph of the same pattern in Fig. l2(a). The maximum density in each picture was adjusted to be approximately the same. These pictures show the progressive change from the first, (b), a t fll.2, the

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quality of which is limited by photon shot-noise, to the last, (f), a t f/l6, in which the shot-noise is negligible and the image quality comparable with that of the direct photograph (a) but with a speed gain of -66. This does not take into account the additional gain factor that would usually accrue due to reciprocity failure when using direct photography for really long exposures.

Astronomical Tests Tubes of this type have been subjected t o preliminary tests at the coudk spectrograph of the 304x1. telescope a t the Royal Greenwich Observatory, Herstmonceux using the experimental system installed for tests of the Spectracon. The results have been very satisfactory.

Fro. 13. Photograph of the spectrum of star (HD 26162, type K1, M,, = 7.2) using the small cascade tube in the wavelength region 4340 A at 3 A/mm dispersion. This spectrum was obtainedwitha 30-in. telescope at the Royal Greenwich Observatory, Herstmonceux, exposure time I0 min.

The tube is stable under moderately warm and humid conditions of operation. Though cooling to about 0°C gives a great improvement in background, exposures of -1 h are practicable a t room temperature. I n Fig. 13 is reproduced the spectrum of a star (HD26162 type K1 of M,, = 7.2) in the wavelength region of 4340A and a t a dispersion of 3A/mm. The exposure time was 10 min and aperture f/5.6. The estimated exposure time required for a similar exposure direct on baked I I a - 0 emulsion was > 100 h. This represents a speed gain of about 600. I n practice this factor would usually be smaller because the effective aperture of the copying lens is usually reduced to improve the quality, especially the granularity, of the recorded image.

104

J. D. MCQEE, R. W. AIREY AND B . P. VARMA

ACKNOWLEDGMENTS We should like to express our thanks to many colleagues in this department who have assisted in this work. I n particular we would like to thank Mr. F. C. Delori, Mr. B. Weekly, Mr. J. Beisiegel, Mr. G. E. Busby, Mr. N. Curtis and Mr. G. Matthews. One of us, B.P.V., would also like to acknowledge receipt of a Research Fellowship from Bihar State University Commission, Patna (India)and from the Applied Physics Department, Imperial College, London, for a period of two years.

REFERENCES 1. McGee, J. D., Airey, R. W., Aslam, M., Powell, J. R. and Catchpole, C. E.,

2. 3. 4.

5. 6.

I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 113. Academic Press, London (1966). McGee, J. D., Airey, R. W. and Aslam, M., In “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 571. Academic Press, London (1966). Stanley, V. A., I.E.E.E. Trana. ATucEearSci.NS13, No. 3,63 (June 1966). Wooten, F., J. AppZ. Phya. 37, 2965 (1966). JedliEka, M., Czech. J. Phya. B16, No. 2, 132 (1966). Baum, W. A., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, W. L. Wilcock and L. Mandel, Vol. 16, p. 391. Academic Press, New York (1962).

DISCUSSION M. COMBES: What is the thickness of the metal R. w. AIREY: These components are made from

annuli in your tube? 0.008-in. molybdenum sheet.

A Family of Multi-stage Direct-view Image Intensifiers with Fiber-optic Coupling P. R.COLLINGS, R. R. BEYER, J. S. KALAF’UT and G. W. GOETZE Westinghouse Electric Corporation, Electronic Tube Division, Ehkira, New York, U.S.A.

INTRODUCTION It has long been recognized that, in order to obtain maximum useful amplification from a direct-view image intensifier, a gain of several tens of thousands has to be achieved. I n order to obtain such a high amplification two distinctly different approaches have been pursued in the past. The first approach is amplification by cascading several single-stage image intensifiers, in each of which the light gain is achieved by accelerating the photoelectrons to impinge with high energy on the phosphor screen. The second is electron multiplication by transmission secondary electron emission from thin foils positioned between the photocathode and phosphor of a single image intensifier. The more promising approach seems to have been the cascaded type of multi-stage image intensifier. The various possible ways of constructing a cascade image intensifier are classified in Fig. 1 according to the main categories of design concept. For a more detailed discussion the reader is referred to a series of excellent review articles on this ~ u b j e c t . l - ~ It can be concluded that certain of these possible approaches are inherently less attractive. For example, it seems apparent that the high power consumption and weight associated with magnetically focused intensifiers generally more than offsets their superior performance. Electrostatically focused, multi-stage image intensifiers using thin films for phosphor-photocathode coupling in a single vacuum envelope, suffered initially from the conflicting requirements for oppositely curved cathode and anode surfaces. This predicament was overcome by the advent of fiber-optic plates which could easily accommodate oppositely curved surfaces. Fiber-optics were used at first solely as phosphor-photocathode coupling elements in single envelope devices. However, an inherent disadvantage of the single P.E.1.D.-A.

105

6

106

P. R . COLLINQS, R . R . BEYER, J.

s. KALAFUT

AND Q.

w. QOETZE

envelope multi-stage image intensifier is the low production yield in making several sensitive photocathodes in the same vacuum envelope where the failure of one photocathode results in the failure of the whole tube. The use of vacuum-tight fiber-optic plates (now generally available) as input and output windows for individual stages, was a logical development leading to the modular type of cascaded image intensifier. The concept of using electrostatic demagnification in the individual stages of fiber-optically coupled cascaded image intensifiers seemed particularly attractive to us. This approach offers design freedom that did not exist before. It now becomes possible to make an unrestricted Fiber optically r c o u p l e d devicesMogneticolly focusgd m ! t a t i c a l l Y Single envelope devices

--f

Cascade photocathode-phosphor image intensifiers Fiber optically coupled multienvelope or modular devices

LThin film coupled devicesFiber optically coupled devicesThin film coupled devices-

7-

Magnellcally focused

Electrosibtically focused 1 -[.Fractional

rUnity magnification

-

magnification-

Fra. 1. Classification of cascade type image intensifiers according to design approach.

choice of the input image size required for the application and maintain standardized output image sizes which generally do not exceed 40 mm in diameter. A further advantage of this approach is that the brightness gain obtained by demagnification can be substituted for the acceleration gain of an additional stage, thus minimizing the number of subcomponents which adversely affect the modulation transfer function. A variety of electrostatically focused, demagnifying image intensifiers suitable for fiber-optic coupling to form multi-stage units has therefore been developed, some of which have been equipped with the additional features of zoom and gating. As an example of the general performance characteristics of this family of intensifiers, a two-stage demagnifying device is described and one of its possible applications as a “starlight scope” is illustrated.

IMAGE INTENSIFIERS WITH FIBER-OPTIC COUPLING

107

DESCRIPTION OF THE IMAGE INTENSIFIER AND ITS LABORATORY EVALUATION The intensifier is shown schematically in Fig. 2. Figure 3 shows a, photograph of the coupled units which include the spring loaded ring assembly used for holding the two mated fiber-optic plates together. The unit consists of only two stages of intensification each of which is a simple electrostatically focused diode. The first stage demagnifies the electron image from 40 mm to 25 mm in diameter while the second

FIG.2. Schematic diagram of a two-stage demagnifying image intensifier.

FIG.3. Photograph of a two-stage demagnifying image intensifier.

stage demagnifies the electron image from 26 mm t o 16 mm in diameter producing an overall magnification of 0-4. As will be apparent from Figs. 2 and 3, the second stage intensifier is merely a scaled down version of the first. The only difference in construction is in the output window of the second stage intensifier where the fiber-optic plate has been replaced by a clear lime-soda glass window. Both phosphor screens are of the P.20 type; the first photocathode is of the S.25, and the second of the 5-20 type. The ultimate performance of any type of intensifier is critically dependent upon the first photocathode. The device described here uses

108

P. R .

coLLmas, R.

R . BEYER, J.

s. KALAFUT

AND G .

w.

GOETZE

the 5-25 type of photocathode4 because of its superior performance in the near infra-red region of the spectrum. The 5.25 photocathode is a derivative of the 5-20 type using the same basic chemical constituents; it differs only in the processing technique. Figure 4 shows absolute spectral response curves for S.25 and S-20 photocathodes processed in the envelopes shown in Fig. 2. The white-light response for the 5.25 photocathode is typically of the order of 200-250 pAIlm but sensitivities as high as 300 p A / h can be obtained. The responses to the blue and green regions of the spectrum are generally slightly lower than the corresponding values for the S.20 photocathode, but at wavelengths of SOOOA and 8500A the responses are considerably higher and are typically 15 and 7 mA/W respectively. We have found also that the

Wavelength ( A )

FIG.4. Absolute spectral response curves for typical 5.26 and 5.20 photocathodes.

thermionic emission from the 5.25 photocathode is comparable with that from the 5.20 type, that is about 6 x A/cm2 at room temperature. It is also interesting to compare the response of the 5.25 photocathode with that of the S.1 type. At a wavelength of 9000 A, for example, the response of the 5.25 type is about 1.5 mA/W while that of a standard S.1 photocathode is 2 mA/W.5 Having obtained the primary photoelectrons by the use of a high quantum efficiency photocathode having the desired spectral range, it is then necessary to introduce sufficient gain into the device to present the information to the human eye with optimum efficiency. It has been establishedempirically that a brightness gain of about 3 x lo4 is sufficient for a human observer to operate effeotively under starlight illumination. Figure 5 shows the brightness gain of the image intensifier, plotted against the operating voltage. This gain is defined as the output

IMAGE INTENSIFIERS WITH FIBER-OPTIC COUPLING

109

phosphor screen brightness (measured in lm/ft2) divided by the input photocathode illumination measured in the same units. For these measurements, the source of input illumination was a calibrated tungsten lamp running a t 2870°K and the photometer used to measure the brightness of the output screen had approximately the same spectral response as that of the human aye. The brightness gain is independent of the input illumination for values below 5 x lm/ft2; for higher illumination levels the intensifier shows signs of saturation. At an operating voltage of 30 kV the brightness gain for this device is 3

x

104.

The equivalent background illumination referred to the first photo-

Operating potential ( k V )

FIG.5. Brightness gaiii as a furlottion of operating potential for the two-stage image intensifier.

cathode is normally less than 2 x lm/cm2 a t 30 kV operating voltage and this is equivalent to a photocurrent of about 4 x A/cm2 from this photocathode. Field emission and stray light contribute t o this value in addition to thermionic emission, but there are very few ion scintillations, generally less than 15 sec-l cm-2. I n addition to quantum efficiency and brightness gain there is a third important parameter, the resolving power of the intensifier. A typical square wave modulation transfer function is shown in Fig. 6. At high light levels, and for a 100% contrast test-pattern, the limiting resolution (defined as the 3% response point) referred to the first photocathode is typically 20 Ip/mm at the center, falling slightly to about 18 lp/mm at the edge of the picture. This corresponds to an output resolution of 50 lp/mm and 45 lp/mm at the center and edge respectively. There

110

P . R. UOLLINGS, R . R. BEYER, J. S. KALAFUT AND G . W . GOETZE

Resolution ( t p / r n r n l

FIG.6. Square wave tnorlulation transfer function of the two-stage image intensifier (resolution is referred to the first photocathode).

is no observable astigmatism. It should be noted that the results shown in Fig. 6 were obtained with a device using three fiber-optic plates each having a 16-pm fiber pitch. If fiber-optic plates were used with smaller diameter fibers, then the modulation transfer function would improve. 30 20

-E € E

I

I

I

100%Contrast

101 0

1

\

[I I 2

.6 +

I

-

-

-

5 5-

3 -

w

-

n"

2-

I 10-6

IO-~

Faceplate illurnination (Irn/f t2)

FIG.7. Itesolution referred to the first photocathode as a function of face-plate illumination for different test pattern contrasts.

While photocathode quantum efficiency, brightness gain, and modulation transfer function provide important information for the purpose of tube design evaluation, most users are principally concerned with the performance of the device described in more practical terms. An accepted measure of this is a plot of discernible resolution of the intensifier as a function of face-plate illumination. With such data, it is

IMAGE INTENSIFIERS WITH FIBER-OPTIC COUPLING

111

simple to determine the angular resolving power of a device for any given illumination and an optical system of known parameters. Unfortunately, this type of information, if provided a t all, is frequently limited to lOOyo contrast while in practice scene contrasts are usually much less than 100 yo. Figure 7, therefore, shows discernible resolution in lp/mm referred to the first photocathode plotted against photocathode illumination for test pattern contrasts of 100yo,600/, and 30% respectively.

FIG.8. Photograph of a mesh pattern reproduced by the image intensifier illustrating the very low geometrical distortion.

Figure 8 shows the good geometrical fidelity of the two-stage device.

It is a photograph of a mesh as reproduced by the image intensifier. At the very edges of the picture the distortion D is less than 2%. Distortion is defined as D = [ ( M I- M c ) / N c r ] x looyo,where M , is the paraxial magnification and M r is the averaged magnification measured over the total useful output diameter. The brightness uniformity of the output phosphor screen is also

112

P. R. UOLLINQS, R. R. BEYER, J. 9. KALAFUT AND 0.W. QOETZE

very satisfactory. For 70% of the total screen area the brightness lies within 10% of the maximum.

FIELD EVALUATION OF A STARLIGHT SCOPE EMPLOYINGITHE TWO-STAGE IMAGE INTENSIFIER In order to field test the intensifier, the “starlight scope” shown in Fig. 9 was constructed. This unit is self-contained, incorporating a battery operated power supply. It consists of an fl0.87 objective lens with a field of view of 30°, the two-stage image intensifier, a high voltage power supply and an erecting eyepiece system. The overall angular magnification is 1.5. The angular acuity of the device was measured as a function of the illumination level and of scene contrast.

FIQ.9. Photograph of a “starlight scope” conatructed to field test the image intensifier.

The results are shown in Fig. 10, which also includes similar measurements made using an objective lens of f/O-87, and 15’ field of view, In order to convey a practical idea of what these curves represent, photographs were taken of the output of the intensifier. A t the time that these photographs were taken, there was no moon and about 20% cloud cover so that the major source of illumination was true starlight. In addition, there were traces of ground mist. The test range terrain was typical of up-state New York, that is, pastoral and remote from big city lights and sky scatter. A camera with a 50-mm focal length, fl2.8 lens was used. To enable the scene to be reproduced as nearly as possible as it appeared through the erecting eyepiece to the observer, exposure times were 1 to 3 sec. For Fig. 11, a 30’ field of view, fl0.87 objective lens was used. The black and white barred targets and neighboring detail seen at the center

Scene brightness (Lm/ft2)

FIQ.10. Angular acuity as a function of scene brightness and contrast for two different objective lenses.

FIG.11. Starlight scene. Illumincteion conditions: starlight, 20% cloud cover, slight ground mist; range: 100 m; field of view: 30" (70-mm,f/0-87 objective lens).

FIG.12. Starlight scene. Illumination conditions: starlight, 20% cloud cover, slight ground mist; range: 200 m; field of view: 30" (76-1nm,flO.87 objective lens).

FIQ. 13. Starlight scene. Illumination conditions: starlight, 20% cloud cover, slight ground mist; range: ?QO m; field of view: 4.5" (500 mm,f/4 objective lens).

IMAGE INTENSIFIERS WITH FIBER-OPTIC C O U P I J N G

115

of the photograph were a t a distance of 100 m from the starlight scope. Figure 12 shows the same scene but a t a range of 200 m. With this 30" field of view objective lens and the 100yocontrast targets used, an angular acuity of 2 minutes of arc was obtained, which is in fair agreement with the laboratory measurements. It is worth noting that on the night that the photographs were taken, little more than the skyline could be distinguished by the unaided eye. Results obtained with a 50-om focal length, f/4 reflective lens, which

FIG.14. Starlight scene. Illumination conditions: starlight, 20% cloud cover, slight ground mist; range: 1000 m; field of view: 4.5" (500-mm,f/4 objective lens).

provided a 4.5" field of view, are shown in Fig. 13. The distance of the targets from the starlight scope in this case was 200m. Figure 14 shows the same scene but taken a t a range of 1000 m with the same objective lens. The angular acuity obtained with this objective lens was 0.3 minute of arc. Immediately to the right of the targets, the figure of a man can just be distinguished.

FAMILY OF IMAGE INTENSIFIERS Table I shows the principal features of the complete family of image intenaifiers developed. As can be seen the WX-30677 and the

116

P. R . c o L L m a s , R. R . BEYER, J.

s. KALAFUT AND a. w.

OOETZE

TABLEI Principal design features of a family of image intensifiers

Tube silhouette

Tube type

Photocathode diam. (mm)

Phos- Par. phor axial Length Diam. screen image Gating (in.) (in.) diam. magni-

WX-30946

170

25

0.15

WX-30958

80

40

WX-30957

80

40

0.5

WX-30956

80

25

0.31-1.0 Variable

WX-30836

80

25

WX-31443

40

25

WX-30677

40

25

WX-30920

26

16

yes

0.5-1.0 yes Variable

12.4

8.75

8.125

5.75

no

6.4

5.375

yes

7.125

5.75

0.31

no

5.5

5.375

0-63-1-3 Variable

yea

6-0

4.0

0.64

no

5.4

4.0

0.63

no

3.5

2.75

IMAGE INTENSIFIERS WITH FIBER-OPTIC COUPLING

117

WX-30920 are the first and second stages respectively of the device described previously. Input formats and output formats vary from 170 mm to 25 mm and from 40 mm to 16 mm, respectively. Several intensifiers have the additional features of zoom and gating. Thus, a considerable variety of multi-stage devices can be assembled depending on the particular requirements of a specific application.

CONCLUSIONS The construction and practical performance of a two-stage cascade type image intensifier has been described illustrating the advantages of modular construction using electron-optical demagnification. The brightness gain introduced by electron-optical demagnification is sufficient to replace the electron gain of an additional stage of intensification as shown by the ease of observation of single photoelectron events at the output of the device. The very good resolution and contrast performance can be attributed t o the use of a minimum number of sub-components which adversely affect the modulation transfer function. The 40116 mm two-stage intensifier described in detail was but one example of a family of demagnifying image intensifiers which offers to the system designer the flexibility of a “building block” approach in the selection of the most suitable combination of intensifiers for his particular application. ACKNOWLEDQMENTS The authors gratefully acknowledge the help and encouragement rendered to this project by their colleagues at the Westinghouse Electronic Tube Division, Elmira, New York.

REFERENCES 1. McGee, J . D., Rep. Prog. Phye. 24, 167 (1901). 2. Morton, G. A., Appl. Optics 3, 651 (1964). 3. Soule, H. V., “Electro-Optical Photography at Low Illumination Levels”. Wiley, New York (1968). 4. Joint Electron Device Engineering Councils (JEDEC), Announcement of Electron Device Type Registration, Release No. 5663, January 15, 1968. 6. Engstrom, R. W., RCA Rev. 21, 184 (1900).

DISCUSSION Your tubes do not have a grid for gating. Is it possible tfo supply high voltage pulses without inducing additional dark current? a. w. QOETZE: Yes. w. HEIMANN: What are the typical values for the thermionic emission of the 5.25 cathode? It should be higher than that of the S.20 cathode. Are the values around 10-16A/cm2? Q. w. GOETZE: Typical values for the thermionic emission of the 5.25 cathodes have been measured to be about 6 x 10-1BA/cm2, R. QIESE:

118

P. R . COLLINOS, R . R. BEYER, J. S. KALAFUT A N D 0 . W. OOETZE

w.

Have you used the intensifier with a signal generating tube? Wc have not used the 40/16mm two-stage image intensifier with a signal generating tube. However, extensive work has been done using single-stage image intensifiers, and, in particular, using the WX-30677, 40/25 mm image intensifier which is the first stage of the image intensifier described in the paper. This has been fiber-optically coupled to SEC camera tubes. Results have been given by G. W. Goetze and A. H. Boerio*, H. Anderton and R. R. Beyert, and M. Green and R. Hansent. J. H. M. DELTRAP: What is an S.25 photocathode? Does it have the same composition as the 5.209 a. w. a o E T z E : The 5.25 photocathode does not have the same composition as the 5-20 photocathode although it uses the same basic chemical ingredients; namely, antimony, potassium, sodium, and cesium. We do not know the precise composition of the 5.25 photocathode but believe that it contains a higher proportion of sodium and antimony than does the S.20. J. D. SCHUMANN: What optical system did you use to image the output phosphor picture on to the film? a. w. QOETZE: We used a 50-mm focal length, f / 2 4 lens. Exposure times were of the order of 1 to 3 secs and the photographic film used was Kodak Plus X. This was an attempt to reproduce as nearly as possible the appearance of the output as viewed directly by the eye. J. F. LINDER: Would the image, directly observed with the eye, instead of photographed, be as bright? G . w. COETZE: Assuming the output of the image intensifier when viewed by the eye to be a t a distance of l o i n . and the pupil of the eye to be 4 m m in diameter, then the illumination of the image incident upon the eye’s retina would be approximately 5 times less than the illumination of the image incident upon the film of the photographic system described in the answer to the previous question. E.

a . w.

DENNIBON:

aoETZE:

* See p.

159.

t

See p. 229.

t

Bee p. 773.

Some Aspects of the Design and Manufacture of a Fibre-optic Coupled Cascade Image Intensifier D. L. EMBERSON and B. E. LONG Mullard Ltd., Mitcharn, Surrey, England

INTRODUCTION The principle of using fibre-optic windows on image converter tubes to enable several to be cascaded t o produce an image intensifier is well known. The major advantage accruing from the use of fibre-optic windows is that they allow the tubes t o be coupled with much better optical efficiency than would be obtained by the use of large aperture lenses and with a considerable saving in bulk and weight. It also allows the critical first stage (input) tube to be selected for the optimum sensitivity t o the input light and low background equivalent-input illumination. The authors have been primarily concerned with a particular application (the use of image intensifiers as an aid t o vision when the object brightness is extremely low), which required an image intensifier tube to meet certain specific requirements. The most important of these were as follows. 1. The complete assembly should be rugged, comparatively small, portable and suitable for use in a direct viewing system. 2. The useful cathode area should be approximately 25 mm in diameter. 3. The assembly should have sufficient gain for the eye to make use of all the available information in the signal from the first photocathode, 4. The assembly should operate from a transistorized oscillator and voltage-multiplier unit with minimum current drain on the batteries. These requirements necessitated an electrostatic, selffocusing diode as the basic module. Two basically similar designs have been investigated, the first possessing low distortion properties, and the second a redesigned form to give improved luminance uniformity. Three such modules are coupled by fibre-optics to produce the image intensifier with the required luminous flux gain. This combination, because of the image inversion in each individual tube, enables an upright image to be obtained with simple input optics and viewing eyepiece, a5 illustrated 119

120

D. L. EMBERSON AND B. E. LONQ

in Fig. 1, without recourse to erecting prisms or inverting fibre optics and the consequent increase in weight, length and complexity. IMAQE

TUBEDESIQN

All electrostatic, self-focusing diodes suffer from a degree of imagesurface curvature, and so consideration is immediately given to using the properties of fibre-optic windows to maintain the edge resolution of the tube by matching the fibre-optic curvature t o that of the electron image surface. Further, by suitable choice of the photocathode curvature (an increase of which tends to flatten the image surface) and screen curvature, the overall distortion produced by the tube may be minimized. An electron-optical design that fulfils the requirements indicated above, within the allowed limit of overall length, is as shown in each of the three coupled tubes (Fig. 1). The main features of this design f-=:

Fra. 1. Diagram of direct-view image intensifier system.

relevant to the present discussion are that the photocathode and phosphor screen have identical radii of curvature and the distortion, defined as distortion (%) = (magnification a t 10-mm radius) -(magnification a t 1-mm radius) x 100, (magnification at 1-mm radius) is about 2%. If, however, a cascaded assembly of three tubes of this type is made, it will be found that there is a considerable non-uniformity in the luminous-flux gain, which varies generally by a factor of about ten from the centre of the field to the edge. This non-uniformity arises, not from any fault in the photocathode or screen processing, but from a systematic radial variation in the transmission properties of the fibre-optic plates for Lambertian radiation. This is brought about by the fact that as the distance of a fibre from the centre of the curved surface of the plate increases, the angle between the fibre axis and the normal to its end-surface increases and this reduces the eeciency with which the light emitted from the phosphor is trammitted through the fibre.

A FIBRE-OPTIC COUPLED CASCADE IMAUE INTENSIFIER

121

These transmission properties arise, qualitatively, in the following manner; firstly, the fluorescent phosphor screen in each tube can be regarded as a light source which is a close approximation t o a Lambertian emitter, with the majority of the phosphor grains not in optical contact with the substrate. It can easily be shown that with such a source all the light that is refracted into the glass substrate will travel in directions within a cone of semi-angle 0, about the normal t o the interface (Fig. 2(a)), where 0, is the critical angle for the vacuum t o core-glass interface. Secondly, for a fibre-optic plate with a meridional N.A. equal t o unity, the refractive indices of the core cladding glasses are such that only those rays which are travelling within a cone of semiangle 0, about the axis of the fibres will be totally internally reflected a t the core cladding-glass interface and thus transmitted. Cladding glass

/

(a1

(b)

FIG.2. Diagrammatic representation of meridional flux transmission of clad fibre; (a) interface perpendicular t o fibre axis, and (b) interface at angle to fibre axis.

Combining these two facts, it can be seen that the light from the phosphor, considered as a Lambertian emitter, is transmitted through a fibre most efficiently when the fibre axis is normal to its end surface. When the fibre axis is not normal to the end surface (see Fig. 2(b)), as will be the case for the off-axis fibres of a plate with a curved input surface, the light flux refracted into the fibre is still distributed symmetrically about the normal to the interface but only that fraction of the light travelling within the cone of half angle, 0,, about the fibre axis will be usefully transmitted. Therefore, the Lambertian transmission of a plane-concave fibre-optic window is reduced as one moves away from the centre. The above qualitative description is approximate because it takes no account of skew rays in the fibres. The quantitative calculation of the relative transmission function cannot be arrived a t analytically and we are indebted to J. A. Clarke of Mullard Research Laboratories for a numerical integration of this problem using a computer.

122

D. L. EMBERSON AND B. E. LONG

The result of this computation for the case of a fibre-optic window with N.A. = 1 is shown in Fig. 3. This curve indicates the fractional amount of light emitted by a phosphor, which enters the fibre and is then totally internally reflected, plotted as a function of the angle 8 that the axis of the fibre makes with the normal to the face of the fibre. I n this application, we are not concerned with Fresnel reflexion losses a t the exit face, which would further modify the distribution, because in practice the intermediate output and input fibre-optic plates are

0

10

I

I

20

30

I 40

I

50

I 60

70

Angle of incidence (degreeslof principal ray

FIo. 3. Fraction of light accepted by fibre (N.A. = 1) a8 a function of the angle of incidence of the principle ray on the fibre.

cemented together to ensure optical contact and a t the output fibreoptic plate the viewing system is of relatively low N.A. so that reflexion losses at the exit face are small. For purposes of tube design, it is more convenient to re-draw Fig. 3 in terms of the relative transmission of an input, with Lambertian distribution, as a function of the distance from the centre expressed as a fraction of the radius of curvature of a fibre-optic plate with unit radius of curvature on the input surface. Then with the assumption that all other factors such as photocathode sensitivity, and screen efficiency are independent of radius, the cumulative effect of cascading three such fibre-optic tubes will be as shown in Fig. 4. For the ratio of field diameter to curvature of the low distortion type of tube, the luminous flux gain falls by a factor of 13 between centre and edge.

A FIBRE-OPTIC COUPLED CASCADE IMAGE INTENSIFIER

123

To reduce the non-uniformity of the flux gain i t is necessary to choose a smaller ratio of field diameter to radius of curvature of the screen fibre-optic plate. For example, if the radius of curvature of the screen is doubled, the fall in flux gain from centre to edge is now only a factor of 2, but, from a recomputation of the electron optics, this is obtained at the expense of firstly, some loss of edge resolution because the electron-image surface is not now a close match to the screen curvature and secondly, some increase in distortion.

I

0

0.1

0.2

0.3

O

0.4

I

0.5

0.6

0.7

Distance from centre in terms of R

FIG.4. “Relative transmission” as a function of distance from centre of three cascaded Lambertian-emitter/fibre-opticplate combinations with unit radius of curvature input surfaoe.

So far only the performance of the assembly in terms of the uniformity of the luminous flux gain in so far as it is a function of the transmission through different areas of the fibre-optic plates has been considered, but the uniformity of the output screen luminance will also depend on the distortion characteristics of the tube. For the two designs discussed, the relative distortion characteristics for both the radial and tangential directions are reproduced in Fig. 5 . From these curves, the relative output luminance variation of the two designs can be computed and in Fig. 6 this is shown compared with experimental values measured on actual tubes. It can be seen that there is reasonable agreement between the theoretical and practical performance when it is remembered that the

124

D. L. EMBERSON AND B . E. LONG

latter also includes radial variations of the photocathode sensitivity. It was found, for example, that the photosensitivity to collimated light of an 5-20 photocathode on a plano concave fibre-optic plate increases towards the edge by an amount dependent on the thickness of the photocathode layer, whereas the sensitivity to a source with a Lambertian distribution tends to decrease towards the edge. These two effects tend to balance each other out in a three-stage assembly. It should be noted that an analogous effect to that observed a t the

0

I

I

1

2

4

I 6

1 8

I I 1 0 1 2

Distance from centre (mrn)

FIG. 6. Incremental magnification ( R = radial direction, T = tangential direction) as function of radius for three cascaded tubes: A, low distortion design; B, redesigned tube.

phosphor screen does not occur a t the photocathode, because the latter is in optical contact with the substrate and its effective refractive index is greater than that of the core g1ass.l I n conclusion it can be said that in the low distortion design the output luminance varies by a factor of about 10 : 1 between the centre and a 10-mm radius. The major contribution to this non-uniformity arises from the non-uniformity in luminous flux gain, whereas in the second design, the luminance uniformity is improved to a variation of about 5 : 1, the result of approximately equal contributions from

A FIBRE-OPTIC COUPLED CASCADE IMAGE INTENSIFIER

125

the luminous flux gain variation and magnification changes. This means that an object moving across the field of view of the assembly in the first design remains approximately the same size but varies in luminance by about 10 : 1, whereas for the second design its luminance changes by 5 : l from centre to edge but its area increases by about 2.5: 1, so that the latter assembly should give better off-axis target detection.

0

L

I 2

I 4

I

I

6

8

I I 1 0 1 2

I

Distance from centre (rnm)

PIG.6. Relative output luminance as function of radius at the photocathode. Curve A, low distortion design (calculated); + , experimental values. Curve B, redesigned tube (calculated); 0, experimental values.

GENERAL TUBEPERFORMANCE Tube-assemblies have been made to both the above designs but efforts have been concentrated on the second. Illustrated in Fig. 7 is the final form of an assembly which consists of the three tubes encapsulated in silicone rubber together with a 45-kV output voltage-multiplier provided with taps at suitable points to allow series connexion of the tubes. This encapsulated unit is then contained within an outer Perspex sleeve provided with an insulating window at the phosphor end designed to withstand 45kV, ao that all of the viewing eyepiece can be at earth potential. The voltage required

126

D. L. EMBERSON AND B. E. LONG

by the multiplier is provided by a separate transistorized oscillator operating from a 6.75-V dry battery, and is applied between the pin contact visible on the side and the earthed photocathode flange. The total weight of the assembly excluding the oscillator and battery is approximately 0.8 kg. As stated, this method of construction allows the first stage tube t o be specially selected t o provide 5-20 photocathodes with typical sensitivities of 250 pA/lm to a tungsten Iamp operating a t 2854°K and radiant sensitivities of 20 mA/W a t 8000 A and 10 mA/W a t 8500 A. At room temperature and the nominal operating voltage, the overall background equivalent input illumination is better than 1.0 plux. Under these conditions, the minimum luminance gain is 36,000 measured

FIG.7. Final form of three-stage cascaded assembly, showing the fibre-optic input.

over a central diameter of 14 mm with a luminous flux gain uniformity between centre and edge better than 3 : 1. The output phosphor screen is a green fluorescent P.20 type. With present techniques of phosphor screen preparation and tube assembly, limiting visual resolutions (for a black and white bar pattern of unity mark-space ratio) of the complete assembly are typically 25 lp/mm on the axis falling to between 16 and 23 lp/mm a t 7-mm radius. CONCLUSIONS It has been shown that the use of a fibre-optic plate with a curved input surface gives rise t o appreciable radial non-uniformities in the transmitted flux from a Lambertian emitter such as a phosphor screen. Consequently, in the design of cascade intensifiers based on electrostatic self-focusing diodes, a compromise has to be made between the

A FIBRE-OPTIC COUPLED CASCADE IMAGE INTENSIFIER

127

luminous flux gain uniformity and the distortion for a given field of view and overall length. A considerable improvement in the uniformity of luminous flux gain can be achieved a t the expense of increasing distortion, but the corresponding improvement in output Iuminance uniformity is not so great because of the increasing contribution due t o the magnification changes. However, it is believed that the increasing magnification towards the edge of the field is useful as it assists in target detection and the latter compromise is therefore to be preferred. ACKNOWLEDGMENTS The authors wish to acknowledge the assistance given by their colleagues in both E.O.D.L. Mitcham and Vacuum Physics Division, Mullard Research Laboratories, in particular, Messrs. J. A. Clarke, A. Stark and Miss D. Lamport for various computer calculations; also the Directors of Mullard Ltd., for permission to publish this paper. This work was performed under a C.V.D. development contract.

REFERENCE 1 . Kondrashov, V. E. and Shefov, A. S., Bull. Acad. Sci. U.S.S.R., Phye. Ser. 28, 1349 (1964).

DISCUSSION What is the spectral light transmission of the fibre-optic plates? D . L. EMBERSON: The collimated spectral light transmission of the fibre-optic plates is within 6% of their maximum transmission over the region 4600 A to R. CIESE:

6500 A. N. s. KAPANY: You said thet you ignored the influence of Fresnel reflexions in

calculating the light transmission efficiency of fibres with curved ends. In your computations did you take into account two other important parameters; namely the skew ray propagation and the degree of optical contact between the phosphor and the fibre end surface? The latter parameter modifies the angular distribution of light to make i t non-Lambertian. D. L. EMBERSON: Although for simplicity in the qualitative description of the effect of curving the fibre ends, I only considered the case of meridional rays, the computation performed by Mr. J. A. Clarke took account of the effects of skew ray propagation. For these calculations it was assumed that the phosphor was not in optical contact with the substrate. We have measured the percentage of the phosphor in optical contact with the substrate for phosphors prepared by our standard process and find this to be of the order of 20%. This degree of optical contact will not make a significant difference to the results quoted. o. w. GOETZE: Please quote figures for the distortion for both tube designs. How do you define distortion? D . L. EMBERSON: The distortion is defined as: distortion (per cent) = (Mlo M , ) / M , x 100, where MI, = magnification at 10-mm radius and M I = magnification at 1-mm radius on photocathode. For individual tubes, the distortion is approximately 2% for the low distortion design and 6% for the redesigned tube. This results in a total distortion for three cascaded tubes of about 6 % for the first design and 21y4 for the second design,

-

128 P. R. COLLINS:

D. L. EMBERSON AND B. E. LONG

What was the diameter of the individual fibres of your fibre-optic

plates? D. L. EMBERSON: We have used several types of fibre-optic plates whose individual fibre diameters range from 6 to 10 pm. J . VINE: Do you consider that the non-uniformity of the gain might be due to non-uniformity of the phosphor produced by the curvature of the surface on which it is deposited? D. L. EMBERSON: A small contribution to the non-uniformity of luminance gain with radius may be due to non-uniformity of the phosphor deposition produced as a result of the curvature of the surface. However, measurements on tubes with phosphors prepared by our standard process on curved plain glass windows indicate that this is not more than a few per cent and leave no doubt that the major cause is the vignetting in the fibre optic, which I have described.

A Proximity-focused Image Tube M. J. NEEDHAM and R. F. THUMWOOD Queen Mary College, Univereity of London, England

INTRODUCTION The simplest possible image converter tube consists of a photocathode and phosphor screen separated by a gap of a few millimeters. Such a tube was described by Holstl in 1934 and was made in large numbers during the course of the Second World War,2 when it was used as the principal component of an infra-red wavelength converter system. Its limited resolution restricted the possible applications for other than infra-red use and little development took place for some years. It now seems that more modern techniques of tube construction have produced a new interest in proximity focusing.

THEORETICAL ANALYSIS The resolution of a proximity-focused image tube depends on the distribution both of the energies and directions of emergence of the photoelectrons from the photocathode surface, and on the field in the gap between the cathode and screen. Equations for the modulation transfer function assuming arbitrary energy and velocity distributions are developed in the Appendix and these equations have been solved by computer for some particular distributions. I n the cases of Maxwellian velocity and Lambertian directional distributions it is possible to obtain solutions in terms of tabulated function^.^ Calculations have been made for a Gaussian distribution of energies and a Lambertian distribution of directions. The results of these calculations are shown in Figs. 1 and 2. It may be that the limits of performance are set by the allowable electric field in the gap between photocathode and screen, in which case the resolution at constant contrast and constant field varies inversely as the square root of the applied voltage. The gain of the system is also a function of the applied voltage, and Fig. 3 illustrates the basis of a compromise between gain and resolution for some 129

130

M. J. NEEDHAM AND R. F. THUMWOOD

Normalized units

fd fi

FIQ. 1. The modulation transfer function for a Lambortian distribution of directions of emission and Gaussian distribution of photoelectron energies with standard deviations u = 0.2 eV and 0.04 eV, and mean energy 0.4 el'.

0

e

e

0

0.4

-

0.2

-

0

0.2

0.3

Normalized units

0.4

0.5

Id fi

FIQ.2. The modulation transfer function for various angular distributions of photoelectrons and a Maxwellian distribution of energies.

assumed conditions. I n practice it is not possible t o increase the resolution indefinitely a t the expense of gain because of the necessity for an aluminium backing to the phosphor screen in order t o avoid optical feedback, unless the photocathode is blind to the phosphor emission. Thus a certain minimum potential is required to ensure that the electrons penetrate the aluminium film.

131

A PROXIMITY-FOCUSED IMAGE TUBE

Photocathode-phosphor screen spacing (mm)

FIo. 3. Variation in gain and resolution versus the photocathode to phosphor screen spming for a constant electric field. The shaded portion is unusable aa in this region the electrons cannot penetrate the aluminium backing on the phosphor.

CONSTRUCTION As shown in Fig. 4, the tube is fabricated in two halves, which are finally joined by argon arc welding immediately prior t o the caesiation of the photocathode. Antimony is deposited on the photocathode surface before the final assembly and the SbCs3 photocathode is formed 1 torr is used to in the usual way except that argon at a pressure of N

FIG.4. Components and completed image tube.

132

M. J. NEEDHAM A N D R. F. THUMWOOD

reduce the mean free path of the caesium molecules to a value less than the tube gap width, Without the addition of argon the process of photocathode formation takes many hours and usually results in uneven sensitivity. The argon is, of course, pumped away at the conclusion of processing. Early tubes employed conventional powdered phosphor screens deposited by electrophoretic means, and these screens limited the possible applied field to 4.5 kV/mm. Various attempts to improve bhe bonding of the glass-phosphor-aluminium sandwich were unsuccessful and most tubes were destroyed by portions of aluminium foil becoming detached and bridging the gap. More recently screens have been prepared by vacuum evaporation of calcium tungstate.* Phosphors prepared in this way are sufficiently smooth to permit the direct evaporation of aluminium with very much improved adhesion. Break8.5 kV/nim down of tubes employing these screens occurs a t fields of and even when breakdown does occur the result is not complete destruction of the tube. Microscopic examination revealed that damage is limited to small areas where the aluminium has melted and fused into a bead on the surface of the phosphor. The exact cause of this breakdown has not been determined. N

MEASUREMENTS AND RESULTS The resolution of the tube was measured by imaging a slit of light on to the photocathode and measuring its spread function a t the phosphor screen. The modulation transfer function was then found by calculating the Fourier transform of the spread function with the aid of a digital computer. The results shown in Fig. 5 are for two typical tubes; tube A with a conventional powdered phosphor, and tube B with an evaporated calcium tungstate phosphor.

CONCLUSIONS From the graphs shown in Fig. 2 it can be seen that the resolution does not depend very strongly on the angular distribution of photoelectrons for the distributions tried so that it is possible that departure from a Lambertian distribution5 might not be easily detected. The theoretical curves obtained have shown, that for the distributions studied, the resolution varies inversely as the square root of the most probabIe energy. The published work on measured energy distributions shows that they are fairly well fitted by a normal (Gaussian) distribution for photocathodes subject to visible radiation ((2.4 eV). However, with ultra-violet radiation the electrons appear to be subject to internal collisionss and the majority are concentrated in the region

133

A PROXIMITY-FOCUSED IMAGE TUBE

of lower energies ((1 eV) so that a Maxwellian distribution may be more realistic. The variation in resolution with the standard deviation of the normal distribution is shown in Fig. 1 and it may be seen that the only effect is a slight change in contrast ratio at the higher spatial frequencies. This indicates that differences between the assumed normal and actual distributions would produce only small variations in the modulation transfer function. From the equations developed in the Appendix it can be seen that the resolution varies directly as the square root of the applied voltage V and inversely as the spacing d between the photocathode and the phosphor screen and Figs. 1 and 2 have been plotted to show this.

Tube A 0 2.4kV/rnm v 3.lkV/mrn Tube B 6*OkV/mrn o 7*3kV/mm

I

0

5

10

15

20

25

I 30

35

Cycles/rnm

FIG. 5. Comparison of the measured modulation transfer function for two typical tubes. Tube A with a powdered screen and 2.9-mm. spacing. Tube B with an evaporated screen and 14-mm spacing.

For d.c. operation, the spacing has been found to be limited by the mechanical strength of the aluminium-to-phosphor bond, and a compromise between resolution and gain has had t o be made. With the tubes that have been constructed a resolution of 20Ip/mm at 60% contrast ratio has been attained. Unoxidized antimony-caesium photocathodes (S.9) have been formed in these tubes by the methods already described with sensitivities of up to 25pA/lm, and the efficiencies of the evaporated screens have been about a third of the powder screens of the same phosphor. Although no measurements of the gain of these tubes have been made it has been calculated that a gain of three a t 10 kV should

134

M. J . NEEDHAM AND R . F. THUMWOOD

be achieved (4400 d radiation). However, with better phosphors this could be considerably improved. These tubes, which are quite robust and should be cheap to produce, may be adapted for high speed photography although no tubes have yet been operated under pulse conditions. ACKNOWLEDUMENTS

The authors wish to express their appreciation to the English Electric Valve Co. Ltd., for much practical help and advice and to Mr. J. Honour for his technical help. Professor M. W. Humphrey Davies provided us with the facilities and one of us (M.J.N.) was supported by an S.R.C. studentship.

REFERENCES 1. Holst, G., DeBoer, J. H., Teves, M. C. and Veenemans, C . F., Physica, 1, 297 (1934). 2. Pratt, T. H., J . Sci. Inatrum. 24, 312 (1947). 3. Grant, J., Proc. In&. Elect. Electronic Engrs, 54, 801 (1966). 4. Feldman, C. and O'Hara, M. J . , Opt. Soc. Amer. 47, 300 (1957). 6. Petzel, B., Phya. Status Solidi, 12, 103 (1965). 6. Apker, L., Taft, E. and Dickey, J., J . Opt. SOC.Amer. 43, 78 (1953).

APPENDIX The motion of the electrons may be determined with the aid of

I"

Photocathode plane

Phosphor screen plane

FIG. 6. Coordinate system of the motion of the photoelectrons between the photo-

cathode and phosphor screen.

Fig. 6. Let the probability of emission of an electron with energy w, at the angles 6' and 4, be P(6'5 4, w)d6' d4 dw

= p(6')p(4)p(w)d6'd4dw,

(1)

135

A PROXIMITY-FOCUSED IMAGE TUBE

if 8,

and w are independent,6 and n/2

R/2

s P(4dO = s P(+)d+= 0

0

m

SP(W)dW = 0

(2)

1.

Now.

e(z)

W

112

2d sin

if w

< eV,,

where d is the distance between the photocathode and phosphor screen, e is the electronic charge and V , the applied voltage.

For any given initial energy w the maximum possible value of R is given by Eq. 3 with 8 =77/2: 1iz

Rmaz

=

2 d ( ~ )

(4)

5

therefore,

The total probability density for an electron landing a t radius R, weighted according to the energy of emission is m

P‘ (R) = SP (R)P(W)dW.

(7)

0

Due to the form of the expression for p (R) this becomes

WO

Since the area of the annulus a t radius R is 2rRdR, the electron density a t R is p’(R)/BnR. This quantity is known as the point spread function. The electron density for a line of point sources may be found by changing the system into rectangular coordinates (R2 = x2 y2), assuming a line of sources a t x’ = 0, and the total density found by the summation from all points to a line y = 0; thence

+

+m

-m

m

wo

136

M. J. NEEDHAM AND R . F. THUMWOOD

The modulation transfer function is the Fourier transform of the line distribution, therefore, m

0

I m

--m

m

wo

DISCUSSION c. A.

What type of screen was evaporated and what was its efficiency? M. J. NEEDHAM: A calcium tungstate (P.5) phosphor was evaporated with an efficiency of approximately one third of the original powdered phosphor. No absolute measurements were made. P. FELENBOK: What impurities were present in the argon gas used? M. J. NEEDHAM: High purity (commercial) argon (99495yb pure) was used in these experiments. I t was found that the caesium gettered the residual reactive gases. u. W L ~ R R I C KWhat : kind of application were you anticipating when developing the tube? M. J. NEEDHAM: The image tube was developed for use in the field of high speed photography although it has not yet been used under pulsed conditions. J. D. MWEE: What is the effect of exposure to air of your Sb layer on its activation, and what precautions do you take to reduce this? M. J . NEEDHAM: No effects of the exposure of the antimony to the air were noticed as the photocathodes were not intensively studied. However, exposure to the air was kept to a minimum. Also attempts were made to protect the antimony during the argon-arc welding process, with dry argon. M. F. WALKER: What is the uniformity, in per cent, of the sensitivity over the surface of photocathodes made by the process you have described? From what distance were the Sb and Cs layers deposited? Did I understand correctly that the use of the I-rnm pressure of argon decreases the time of caesiation of the cathode? M. J. NEEDHAM: No accurate measurements of the sensitivity were made, but by illuminating the whole of the photocathode evenly, images with a uniform brightness were observed a t the phosphor screen. The antimony layers were always deposited from a single source a t a distance greater than twice the diameter of the photocathode, that is from about 12 cm. It was found that the argon reduced the caesiation time considerably for this type of image tube. W. E. TURK: Did I understand you to say that 40 pA/lm was the maximum sensitivity obtained? What is a typical value? M. J . NEEDHAM: The maximum sensitivities obtained were 40 pA/lm for test cells and 25 pA/lm for tubes. A typical value in a test cell for an unoxidized antimony-caesium photocathode was 30 pA/lm but no typical values can be quoted for an image tube as only a few tubes were constructed. T. NINOMIYA: What was the wavelength of the light used to assess this tube? What is the threshold wavelength of the S-9 photocathode? M. J. NEEDHAM: White light was used to assess the performance of the image tube from a standard lamp with a tungsten filament at 2870°C. No threshold measurements of these photocathodes were made. OROSCH:

INTIC, an Image INTensifying, Integrating and Contrast-enhancing Storage Tube G. WENDT

Cornpapie QBnSrale de Tdligraphie Sans Fil, Orsay, Frame

INTRODUCTION The contrast of the image observed on the last screen of a multistage intensifier is often extremely low. This is often due to the poor contrast in the scene itself as well as the deterioration inherent in the image transfer by the intensifier and it is therefore highly desirable to enhance this contrast. A further disturbing effect may be the presence of glare due to strong light sources in the scene and it is obviously advisable to limit this. The characteristics of a system permitting such an improvement are shown schematically in Fig. 1. At top left, the luminance of the intensifier output screen along a line crossing the image (or the corresponding potential pattern if the intensifier output is a target) is represented. The transfer characteristic of the system should permit clipping of both the unwanted image background and the signal levels which are too high. The cut-off voltage of this characteristic should be variable. The lower, right diagram shows the luminance variation appearing on the system output screen. Such image conversion can be achieved with a television pick-up tube, the signal peak and base clipping being effected in an amplifier circuit. Although systems of this kind have already been described by GebeP2 some years ago, it is useful to achieve a similar effect with a tube which requires no scanning or video-frequency circuits and uses only direct current supply units. One such system can be obtained by replacing the writing gun of a well-known storage tube3 by a photocathode (Fig. 2), on which the image t o be transmitted is projected either directly or through a fibre-optic coupling to a conventional image intensifier. The working cycle of such a tube, which can be divided into three steps, is summarized below. During the exposure, or writing time, the electrons emitted by the P.1.E.D.-A

137

6

a. WENDT

138

photocathode are focused on to a metal grid covered with an insulator, the storage grid, where, by secondary emission, they produce a positive charge pattern, and hence a potential pattern geomet'rically similar t o the image on the photocathode. During the reading time, a uniform electron flux (yielded by the same photocathode or by flood guns) reaches the storage grid with a very low electron velocity so that, on passing through the grid, it is reflected by unwritten areas and modulated by the others according to the density of deposited charge.

L U

w #

:n PiE eQJ

5

1-

-I

2%

o n

I

---- ------

I

I

I I

1,

I

I I

System 3utput (phosphor screen)

FIG.1. Contrast enhancement by means of background and high-light clipping.

Each mesh of the grid then acts as a micro-lens which reflects or modulates the flux without allowing it t o disturb the charge on the insulating layer. After their passage through the grid, the electrons are strongly accelerated towards the tube screen where the stored image can be observed for a fairly long period of time. The desired level of the signal stored on the insulator may be rendered visible on the screen by varying the storage-grid bias voltage. To erase this image, the storage-grid potential is made sufficiently positive t o enable the reading electrons t o strike the insulating layer over the whole of its surface and so charge it uniformly t o cathode potential. The grid is then ready t o receive new information.

139

INTIC, INTENSIFIER STORAGE TUBE

Investigations were carried out on the three types of INTIC tube illustrated in Fig. 3; 1, type M using precise focusing by uniform electric and magnetic fields both between the photocathode and the grid, and the grid and the screen; 2, type E using precise electrostatic focusing between the photocathode and storage grid, and proximity focusing between the grid and the screen; 3, type P using a photoconductive layer deposited on the storage grid.4 I n the latter, the Direct viewing storage tube

Electrostatic

INTIC

i

I_

Writing gun

Deflexion coils

-

Photocathode

0“

4 ’ ’ LLOL I \

Uniformity grid Collector grid Insulator Storage grid

Phosphor screen

FIG.2. INTIC tube operating principle compared with that of the direct view storage tube.3

surface of the photoconductor is uniformly charged during the erasing phase and partly discharged during projection of the luminous image on this surface. The reading electrons are proximity focused between the grid and the screen. Several versions of each of these types were designed and developed but the most detailed work was carried out on the M-type tube because this was the most promising as regards resolution, contrast, sensitivity and duration of observation. Contrast enhancement is closely related to the shape of the characteristic curve of the reading current density passing through the

a. WENDT

140

storage grid as a function of the surface potential of the insulating layer. The greater the slope and the curvature of the toe of this characteristic, the greater is the contrast enhancement and ease of detection of very weakly contrasted detail in the image. A very important problem was therefore the theoretical and experimental determination of the relations between the shape of the characteristic curve and the dimensions of the grid, its potential, and the surrounding electric fields. Type M I Object

SCr.

FIG.3. The three types of INTIC tubes studied. Type Musing precise electron focusing by uniform electric and magnetic fields both between the photoelectrode and the storage grid, and between the latter and the viewing screen. Type E, using precise electrostatic focusing between the photocathode end the storage grid, and proximity focusing between the grid and the screen. Type P, using a photoconductive layer deposited on the storage grid, the reading electrons beirig proximity focused between the storage grid and the screen. L, Objective lens; PC, photocathode; CG, collector grid; SG, storage grid; UG, uniform-field grid; PCL, photoconductive layer; FG, flood gun; Scr., viewing screen.

I n the following, the first section deals with the methods and processes used to find these relations and summarizes the results obtained. Then follows the description of the tube (using uniform electric and magnetic fields) finally adopted and its performance. Lastly, possible improvements and applications are discussed.

DETERMINATION OF THE STORAGE GRID CHARACTERISTIC This investigation, which is t o be the subject of a detailed report, was carried out in several steps.

INTIC, INTENSIFIER STORAGE TUBE

141

Electrolytic tank measurements were made of the electric field configuration associated with each micro-lens formed by the meshes of the grid, while allowing for the influence of the network as a whole by appropriate positioning of the tank walls. Many measurements were made t o determine the dependence of the electrical characteristics on the grid geometry. From ( l ) ,the reading current cut-off voltage was determined. This storage grid cut-off voltage varies linearly with the potential INTIC tube EE2 US,,=4kV UcG=300V Theoretical curves

Experimental curve

Shifted theoreticbl

-2

-I

0

I

2

FIG.4. Typical calculated and measured read-out current versus storage grid voltage characteristics of an E type INTIC tube. The lower end of the measured curve cannot be approximated by an exponential function. ( t o is the most probable initial energy of the photoelectrons.)

difference between the insulating layer and the metal structure of the grid and with the root of the ratio El t o E,, these being the electrical fields in front of and behind the grid respectively. 3. Since the modulation of the reading current by deposited charges cannot be calculated from paraxial trajectories a method developed by Glaser and Schiske5 was used, which permits such a determination to be carried out without trajectory calculations. The distribution of the initial velocities of photoelectrons and eventual magnetic fields were taken into account. The effects of space-charge can be neglected. Assuming a Maxwellian distribution of the initial

142

G . WENDT

velocities, the toe of the characteristic can be expressed by an exponential function, the main section being a straight line. 4. A comparison of the measured characteristics with the results obtained by computation showed some differences in the behaviour of electrostatically and magnetically focused tubes. As regards the electrostatically focused tubes (Fig. 4), the curve slope is correctly reproduced but the curve itself is shifted horizonta'lly, which may be due partly to some contact potential and partly to the distribution of initial velocities. On the other hand, the toe of the measured characteristic cannot be fitted t o an exponential function. This is due to the asymmetrical INTIC tube MD 3 L&=IOkV Uc,=350V Theoretical curve

77-

curve

u, ( V ) FIQ.6. Typical calculated and measured read-out current versus storage grid voltage characteristics of an M type INTIC tube. The lower end of the measured curve can be approximated by an exponential function. ( co is the most probable initial energy of the photoelectrons.)

arrangement of the flood guns which results in the electrons being given transverse components of velocity on their entry into the storage-grid region. The same test, carried out with a storage tube having a flood gun located on the tube centre-line, produced a curve, the tail of which can be approximated by an exponential function with a most probable initial energy of 0.2 eV. I n the case of the magnetically focused tube (Fig. 5 ) the slope, which is now much steeper, is also correctly reproduced. The horizontal shift of the curve is less, probably because the results of measurements in the electrolytic tank had to be extrapolated in this case. The toe can be fitted to an exponential function with a most probable initial energy of 0.15 eV. The charac-

INTIC, INTENSIFIER STORAGE TUBE

143

teristic of this tube is therefore much more suitable for yielding a large contrast enhancement. Summarizing these results, the contrast increases inversely with the width of each mesh and the grid input field, and also increases with the reduction in the spread in electron energies. Other more complicated mechanisms also affect the contrast but cannot be dealt with here.

ELECTRIC AND MAGNETICFIELDS INTlC TUBEUSINGUNIFORM The tube using uniform fields has been adopted as the preferred design owing to its better performance.

r

Object

Flood light

Objective lens

\

/

Focusing coils

FIG.6. Schematic design of an M type INTIC tube.

The tube (Fig. 6) is operated in an axial magnetic field of about 160 G produced by stacked focusing coils and the uniform axial electric field is produced by disc-shaped electrodes fed from a potential divider. The image is projected on to the type S.20 photocathode with a useful surface 80 mm in diameter, deposited on a transparent, conductive, chromium underlayer. The storage grid on which electrons are focused during writing is an electroformed nickel grid with 400 meshes/mm2 covered by a double insulating layer, the outer layer being made of MgF, in order to achieve maximum secondary emission. The primary

144

a. WENDT

electron energy is of the order of 400 eV, the secondary electrons are collected by the collector grid, also made of nickel, with 400 meshes/ mm2. All the electrodes, the collector grid, and the aluminium coating of the screen are blackened in order t o prevent any light transmitted through the semi-transparent photocathode from being reflected back on to it.

FIG. 7. Variation of contrast of the stored image during reading by variation of the storage grid bias from 4.9 V in (a) to 5.9 V in (f).

During the reading phase, the photocathode is uniformly illuminated and brought to a potential such that the photoelectrons are reflected a t the unwritten regions of the storage grid and pass through the other regions in numbers varying with the charges previously deposited, without actually reaching and altering them; this number of electrons can be controlled by varying the grid bias (a few volts). The electrons are then accelerated by a voltage of 10 kV towards the type Pa20 screen

INTIC, INTENSIFIER STORAUE TUBE

145

where there appears an image which is the replica of the charge on the grid meshes and therefore a positive image corresponding t o that projected on to the photocathode. The magnification is unity because the fields are plane parallel. The geometrical distortion is extremely low. The definition is limited by the structure of the storage grid and is better than 10 lp/mm; that is 1600 TV lines per diameter. The duration of observation of the stored image without substantial contrast loss is about 15 min; the pattern written on the insulating layer may be stored for as much as several days providing all the voltages are switched off.

FIQ. 8. Contrast improvement using the INTIC tube. (a) Contrast of the image as projected on the photocathode of the INTIC tube. (b)Different parts of the same scene, the grid bias for each being selected to obtain the maximum information.

146

Q. WENDT

Figure 7 illustrates the variation of contrast during reading when varying the grid bias. The voltage difference between any two successive images is 0.2 V. At the lowest bias only the strongest signals are visible; at the highest bias these signals are clipped and the weakest signals appear. A definite contrast enhancement can be seen in Fig. 8. The upper image (a) shows the contrast in the image as projected on the photocathode; the lower image (b) taken from the tube screen, consists of different parts, each corresponding to the grid bias selected to obtain the best information in that part of the scene.6 It is particularly interesting to compare the details within the oval track pattern on the left in both photographs.

1 0

I

I

I

5

10

I5

I

20

Input modulation m, (70)

FIG.9. Contrast enhancement M versus input modulation m, m,, output modulation; ,,E

mean exposure; L,, mean screen luminance.

The contrast enhancement may be expressed quantitatively by means of the ratio of the modulation in the output image to that in the input image. For low input image modulations, ratios of about 20 to 1 and 30 to 1, depending upon the input image luminance, were achieved (Fig. 9). As in the case of photographic emuIsions, the sensitivity is expressed in lux sec. The sensitivity threshold is of the order of 10 mlux sec, and an acceptable image requires 50 to 100 mlux sec. The sensitivity is limited by the inhomogeneities in the active layers in the tube, i.e. the photocathode, screen and insulating layer on the grid, but mostly in the latter since inhomogeneities in the two former layers seldom exceed 10%. Indeed, such inhomogeneities become much more visible when the contrast is enhanced and greatIy exceed the noise inherent in the photoelectron fluctuation. I n order to approach the limits

INTIO, INTENSIFIER STORAGE TUBE

147

given by these photoelectron fluctuations, two intensifier stages should be coupled to the tube. A device using a single intensifier stage with fibre-optic coupling is shown in Fig. 10. The fibre diameter is 15 pm, and the intensifier has a resolving power of 28 lp/mm and a gain of 25 (flux ratio). With fixed scenes, however, the possibility of increasing the exposure time enables very faint images to be recorded by the tube alone, without an intensifier. This time is only partly limited by the photocathode Object

Fro. 10. Schematic design of an INTIC tube coupled by fibre-opticsto an image intensifier.

dark-current, which corresponds to a uniform cathode illumination of about lO-*lux, as the effect of the uniform charge produced by this current on the storage grid may be eliminated in the recorded image. Exposure times of the order of one hour have been achieved without difficulty.

APPLICATIONS AND POSSIBLE IMPROVEMENTS The tube may replace applications: 1, for instant of the image background; in a continuous way after

a photographic camera in the following reproduction of the image; 2, suppression 3, variation of the contrast in the image the completion of exposure; 4, operation

148

0.WENDT

with an extremely short exposure time by using an electron shutter (t <0.1 psec);? 5 , operation with a very long exposure time t o minimize photoelectron noise; 6, recording of images in a spectral region (infra-red, for instance) in which the photographic plate is not very sensitive. Compared with closed-circuit television, the suppression of background and contrast enhancement and control is common to both, so that the advantages afforded by the INTIC tube are: 1, highly simplified circuits and the ability to dispense with a pick-up tube; and 2, practically noise-free reading current. The disadvantages still exhibited are (a) residual inhomogeneities in the active layers, in particular the insulating layer on the storage grid; (b) relatively low sensitivity; and (c) difficult erasure of the image. The methods envisaged t o overcome these are as follows. Sensitivity is closely related to inhomogeneities so that if they could be eIiminated the sensitivity could be greatly improved by merely increasing the flood illumination. Further, changing from a 20 mesh/mm grid t o one of 40 mesh/mm doubles not only the definition from 1600 to 3200 TV lines per diameter but aIso the slope of the characteristic (see above), that is the “amplification”. The next problem is therefore t o improve the technology which, so far, has remained a t the laboratory stage. Moving images may be observed by causing the writing, reading and erasing phases to follow in a television sequence. This experiment has not yet been undertaken, as technological improvements of the tube are still required. ACKNOWLEDUMENTS This work was jointly supported by the U.S. Avionics Laboratory, Wright Patterson Air Force Base, Ohio, U.S.A. and the Direction des Recherche8 et Moyens d’Essais du Ministere des Armees Francaise, Paris.

REFERENCES 1. Gebel, R. K. H. and Devol, L., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee and W. L. Wilcock, Vol. 12,p. 195. Academic Press, New York (1960). 2. Gebel, R.K . H., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, W. L. Wilcock and L. Mandel, Vol. 16, p. 451. Academic Press, New York (1962). 3. Knoll, M. and Kazan, B., “Storage Tubes and Their Basic Principles”. W h y , New York (1962). 4. Gebel, R. K.H., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 209. Academic Press, London (1966).

t The possibility of synchronizinga very short exposure time with a flash illuminating the scene, also reduces the effect of a diffusing foreground such as haze, fog, etc. (ranging).

INTIC, INTENSIFIER STORAGE TUBE

149

5. Glaser, W., I n “Handbuch der Physik”, ed. by S. Flugge, Vol. 33, p. 14G.

Springer, Berlin (1966). 6. Gebel, R. K. H., Mestwerdt, H., Charles, D. and Weridt, G., Bull. Amer. Phys. SOC.(11) 12, 243 (1967).

DISCUSSION a. w. QOETZE: How many distinguishable shades of grey does your storage tube display a t its output? a. WENDT: There are about five distinguishable shades of grey in the part of the stored signal made visible on the screen of the storage tube. The stored signal itself (a sort of latent image) may include many more steps in electric potential, their number depending on the exposure. The number of these steps reproduced in the visible image is limited by the still existing inhomogeneities of the storage layer and may be increased by further research.

This Page Intentionally Left Blank

A Light Amplifier with High Light Output W. BAUMGARTNER Institut fiir l'echnische Physib, ETH,Zurich, Switzerland

INTRODUCTION Any large-scale projection of a picture signal requires the modulation of an intense light flux. Several methods involve using a high power standard light source which is modulated either by absorption, as in the Sciatronl or by diffraction, as in the Eidophor.2 I n the latter case modulation of the light flux is by means of Schlieren optics, this being essentially based on a diffraction effect due to distortions produced by the signal beam in a thin oil film. The principles of operation of the Eidophor are essentially the same as in the system where the oil layer is replaced by a gel and this is described in the next section. ARRANGEMENT USINGA PRISM There are various methods of producing the signal-induced deformations in the gel and one example, the Elmicon3 (ELectro-Mechanical Image CONtrol) will now be described. The gel, about 150 pm thick, is deposited on a transparent electrode (Fig. 1) and separated from the photoconductor, which is also mounted on a transparent electrode, by a space of about 20 to 50 pm. This space allows distortion of the gel t o take place. The purpose of the grating is made clear later. An a.c. potential is applied between the electrodes. Variations in intensity of the signal beam striking the photoconductor produce local variations in the electric field, these producing corresponding distortions in the gel. To convert the mechanical distortions into light modulation the following optical arrangement is used (Fig. 2). The high intensity light source 1 is focused by a lens 2 on to the diaphragm 3, containing an aperture /3. A first collimator lens 4, a 90" prism 5 , a second collimator lens 4' and a stop 6 follow. The single hole /3 and the single stop 6 are used for ease of explanation but these can, in practice, be replaced by a system of parallel slits. The base of the prism carries the gel 8, the working surface (which is shown already distorted) being ABCD. The 161

152

W. BAUMGARTNER lnterspace (dry air) Photoconductor Diffracted light

Gratinq

;

Projection 'light

: . :

Picture sig

Defdrmotions

FIQ.1. Diagram of Elmicon (not to scale).

signal beam 0, which falls on the photoconductor 9, produces the distortions. In the undistorted state, total internal reflexion occurs a t ABCD and light from aperture /3 is focused on to the stop 6 a t /3' so that no D

FIG.2. Optical arrangement: 1, projection light source; 2, 4, 4', 7, lenses; 5 , 90" prism; 6,stop; 8, gel layer; 9, photoconducting sheet; 10, glass plate; 11, transparent electrode; 12, interspace.

A LIGHT AMPLIFIER WITH HIGH LIGHT OUTPUT

153

light is collected by lens 7. If the surface ABCD is now disturbed, secondary images will be diffracted to /3" so that lens 7 will now collect light, and since the screen and surface ABCD are conjugate pIanes of lens 7 , local distortions will appear as bright spots on the screen. However, a uniformly bright signal will produce a uniform displacement of ABCD and thus produce no output signal. To overcome this the grating shown in Fig. 1 is introduced. This usually has a bar-to-space ratio of unity. The pitch D of the grating obviously limits the resolution of the device and is an important parameter. Another important parameter is the space bet'ween the gel and the photoconductor which should not be more than D / 6 . On the other hand the thickness of the photoconductor

FIG.3. Picture reproduction by the Elmicon using a prism.

should be a t least 0 1 6 . This requirement results in an excessively thick photoconductive layer but methods of reducing it have been d e ~ c r i b e d . ~ Figure 3 demonstrates the picture quality obtained with the Elmicon. This arrangement suffers from two defects, however. First, because the gel layer lies obliquely to the optical axis, blurring and distortion occur, thus necessitating additional optical elements t o compensate these defects. Secondly, some of the scattered light strikes the gel a t a low angle of incidence and is therefore not internally reflected. This reaches the photoconductor and causes further deterioration of the image. The latter defect can be remedied by using an opaque layer with high electrical resistivity deposited on the photoconductor. Both defects can be circumvented by depositing on the gel a metal layer with the following properties: high reflectivity, low transparency, sufficient deformability and adhesion to the gel. A solution t o the problem has

154

W. BAUMGARTNER

been found, using a layer of mercury between two gel layers. This will now be described.

ARRANGEMENT USING A MERCURYLAYER Figure 4 shows a new arrangement (b) which incorporates a reflector in the form of a layer of mercury5 between two layers of gel, gel I and gel 11,and, for comparison, the old arrangement (a) is shown alongside the new.

Gel

V-

Interspace Photo-conductor

--- __-__ _ _------

Photo-conductor

The a.c. potential is now applied between the mercury layer and the photoconductor. Deformations produced in gel I by the signal are transmitted to gel I1 via the mercury and thus modulate the light beam from the high-intensity source. The signal distortions are illustrated in Fig. 5 : (a) without signal and, (b) with signal. De Hailers has shown that Gel E

Hg-layer

-

Gel I

I H v

Photoconductory \ T r o n s p o r e n t

(a)

(b)

FIG.5. Deformtttion of the Hg layer. (a)Without, and (b) with incident signal. (Not t o scale.)

there is a close correspondence between the distortions of gels I and 11. The relationship between the two distortions is given in the Appendix and from it the following conclusions may be drawn. (i) The amplitude depends on the frequency of the a.c. field. There are two frequencies a t which large deformations occur (see Fig. 6); the separation of these resonant frequencies increases from zero to infinity for decreasing thickness of the Hg layer. The respective modes of oscillation are illustrated in Fig. 8. (ii) A local deformation of gel I produces an analogous one

155

A LIGHT AMPLIFIER WITH HIGH LIGHT OUTPUT

in gel 11,if the viscosity of the gels is high enough and if the thickness of the Hg layer is not too large. I

I

I

1

I

IOOpn

I 1.5

I

2.0

I 2.5

I

I

3.0

3.5

Frequency (kHz1

FIQ.6. Dependence of the deformation amplitude 20" (in arbitrary units) on the frequency of the applied voltage for different thicknesses of the Hg layer (experimental).

A device has been constructed based on these considerations, the details of which are as follows: Photoconductor Bi,S,, 0.3-pm thick Gel I Silicone rubber, shear modulus 2 x lo4 dyne/cm2 90pm 150 pm Gel I1 30 to 70 pm Hg layer 2 to 3.5 kHz according to the thickness of Ax. frequency the Hg layer lo4 V/cm Field strength 10 x 10 cm2, with 500 grating bars. Size of the image <0.1 sec, determined by the time lag of Response time the photoconductor. It was possible to reproduce a moving film without visible blurring due to time-lag. The first results are illustrated by the photographs in Fig. 7.

I

CONCLUSIONS This report describes a light amplifier suitable primarily for the high power amplification of medium power light signals. It is believed that

156

W. BAUMQARTNER

FIG.7. Picture reproduction by the Elmicon incorporating an Hg layer. The coarse line structure in the centre is caused by an accidental reflexion in the Schlieren optics.

A LIGHT AMPLIFIER WITH HIGH LIQHT OUTPUT

157

Gel 11

Gel

I

Lower mode

Gel

I

Higher mode

FIG.8. Oscillation modes of the Hg layer.

further applications of the Elmicon will be found if its design is adjusted according t o the purpose in mind and if an adequate photoconductor is available. Present limitations are determined to a large extent by technological limitations, but these may be reduced by further development. AOPNOWZEDOMENTB The author is grateful to Professor E. Baumann, head of tho institute, for his interest, and to the members of the staff for their careful co-operation. The work reported was sponsored by the Gesellschaft zur Foerderung der Forschung an der ETH, Zurich.

REFERENCES Rosenthal, A. H., J . SOC.Motion Picture Tekvia. Engra 45, 218 (1945). Baumann, E., J . Brit. Inst. Radio Engre 12, 69 (1952). Baumgartner, W., 2. Angew. Math. Phye. 18, 31 (1967). U.S.Patent 3,137,762 (June, 1964). Swiss Patent 424,009 (November, 1966). de Haller, A., “Nouvel amplificateur de brillance avec Eidophor ti miroir liquide”. Thesis, EPF, Zurich (1965). 7. Alfrey, T., “Mechanical Behaviour of High Polymers”, p. 538. Interscience, New York (1948).

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

APPENDIX If the normal stress u at the surface of the gel I is given by u (x, t ) = uo sinrcx exp (jut), and the deformation W a t the surface of the gel I1 by W(x, t ) = W osin K X exp ( j u t ) ,

158

W. BAUMCIARTNER

the following relationship between the amplitudes uo and Wo holds:

r

where 3 = q / K , 1;2 = p W 2 / 2 p K 2 , = tch, and K is the wave number, is the angular frequency, p is the density and h is the thickness of the Hg layer, x is the coordinate in the plane of the gel, t is the time variable, the shear modulus and the viscosity are p and -q respectively. It has been shown7 that the elastic properties of the gel are sufficiently described by the shear modulus and the viscosity. The formula is derived making the following assumptions: (i) incompressibility of the gels and the mercury, (ii) infinite thickness and negligible inertial mass of the gel layers, and (iii) the Hg layer has negligible viscosity and adheres to the gels. The low and the high resonant frequencies are given approximately bY w

flow =f m

where

Jtanh

r

zi

fhigh = f m

JCOth

r

59

.-

With K = ( 2 n / D ) w 3 x lo2 cm-l, p = 2 x 104 dyne/cm2 (static value), p = 13.5 g/cm3, we find f m w 2.6 kHz. The constants p and depend on w in an unknown manner. A complete derivation including the mercury viscosity will be given elsewhere.

DISCUSSION w. EEIMANN: Did you try other semiconductors of the selenide type which should be faster than the sulphides? W. BAUMOARTNER: Not yet, but the use of photoconductors of the selenide type should not give rise to serious problems. J. D. MWEE: Do you consider the problem of background faults to be serious? w. BAUMGARTNER: The specks visible on the slides are due to very faint gas bubbles in the gel and are of an accidental nature. Q. w. GOETZE: What is the temporal response (time constant) of these light amplifiers? w. BAUMGARTNER: Generally it is nearly equal to that of the photoconductor, the latter being the slowest component. II. G. LUBSZYNSKI: On your last slide I saw a number of vertical black and white lines. Were they due to the grating? w. BAUMGARTNER: No, it is just a parasitic reflexion of the slit system in the Schlieren optics.

SEC Camera-tube Performance Characteristics and Applications G. W. GOETZE and A. H. BOER10 Westinghouse Electric Corporation, Electronic Tube Division, Elmira, New York, U.S.A.

INTRODUCTION At the “Second Symposium on Photoelectronic Image Devices” we first reported on a new type of thin film target consisting of low-density deposits of insu1ators.l At that time the basic physical and electrical properties of such layers were described. The more interesting characteristics of these layers were the very good storage properties and the high gains achieved by free secondary electrons. Their transmission secondary emission characteristics were described in more detail in 1964.2 At the last Symposium in 1965 we reported laboratory results obtained with developmental camera tubes using these targets in a reverse-bias mode, introducing the term ‘‘Secondary Electron Conduction”, or SEC.3-5 I n the past three years, work has continued on the advanced development and also in establishing production capabilities of SEC camera-tubes. This paper presents the results of our efforts to produce several types of SEC tube in a factory. Although it completes, in a sense, a six-year effort on this project, further detailed improvements will undoubtedly be forthcoming, especially in making modifications required for specific applications.

PERFORMANCE CHARACTERISTICS Four types of SEC camera-tubes, shown in Fig. 1, are currently in production at Westinghouse and are available “off-the-shelf ”. All of these tubes are designed for extreme low light-level television and do not necessarily represent the optimum configuration for other applications. Nonetheless, as will be illustrated in the second portion of this paper, present SEC tubes are being used successfully in many applications. 159

160

G. W. GOETZE AND A. H. BOER10

The tubes shown in Fig. 1 all use magnetic focusing and deflexion in the scanning section. The tube a t the extreme right has an electrostatic image section with an image zoom capability of 2 : 1. The second tube uses a magnetic focusing image section and is characterized by superior performance in resolution and geometrical fidelity. The two tubes t o the left both use diode-type electrostatic image sections and differ only in the size of their photocathode diameters, 25 mm and 40mm respectively. I n order t o illustrate the capabilities of SEC tubes, the performance parameters for the smallest, the type WL-30691 camera tube, will be discussed in some detail. The WL-30691 uses a 1-in.

FIG.1. Standard production-type SEC camera-tubes.

vidicon type gun and a 16-mm diameter SEC target. The higher performance of the larger tubes is available from the commercial data sheets but can also be estimated by simple scaling. It should be emphasized that all values reported here are typical values and do not represent measurements on selected samples. They are in fact taken directly from commercial data sheets. Probably the most important characteristic of a camera tube is its transfer curve which, for this tube, is shown in Fig. 2. The tube is intended for operation with photocathode illumination from approximately to lm/ft2. The linear portion of the transfer curve indicates a gamma of close t o unity. It can also be seen that saturation takes place around lm/ft2a t the “knee” in the characteristic which is similar to that of the image orthicon. A signal current of 150 nA is delivered a t a photocathode illumination of about 5 x Im/ft2. Since SEC tubes were until recently almost exclusively used for

SEC CAMERA-TUBE PERFORMANCE A N D APPLICATIONS

161

Face-plate i Ilumination (lrn/ft2)

FIG.2. Light transfer characteristic of the SEC tube type WL-30891.

extreme low light-level television, particular care was taken to reduce lag as much as possible by proper adjustment of target parameters. Figure 3 shows the temporal response as a function of signal current. The residual signal was measured in the third field, corresponding t o 50 msec after the illumination was removed. Under normal operating 20 oQ1

I

1

I

I

I

I

I

- 3 In-

EP

Photocathode voltage 7-8kV 1/30sec frame rate

- .._= w L .9 c

Q S 10 m--

----

"p

br,

0

25

50

75

100

125

150

175

Signal current ( n A )

FIG,3. Lag characteristic for the SEC tube type WL-30891.

conditions the lag signal in the third field is only about 5% of the firstfield signal and is due only to discharge lag. There is no build-up lag. A further improvement could be obtained if necessary by sacrificing storage capacity. However, the performance as indicated in Fig. 3 makes it possible to use an SEC tube for such demanding applications as field-sequential color transmission a t a scan rate of 180 frameslsec. If one combines the information contained in the last two figures with a resolution measurement, one obtains an indication of the

a. W.

162

QOETZE AND A. H . BOER10

practical sensitivity as shown in Fig. 4. The discernible resolution is plotted for a black and white bar test-pattern as a function of photocathode illumination. The three curves are for a static picture and for the cases where an image point traverses the picture width in a period of 20 and 10 sec respectively. An increase in illumination of about 24 times and 34 times is needed for the 20-sec and 10-sec per picture-width movement, respectively. When the image moves much faster than 10 sec per picture-width, the results are dominated by signal mixing on the target due t o exposure smear and are therefore not very meaningful in evaluating the basic dynamic performance of a camera-tube.? The loss in sensitivity due to either camera or object movement is

Face-plate illumination (trn/ft21

Fro. 4. Static and dynamic characteristics for SEC tube type WL-30891.

important if low light-level operation is required. I n addition very few practical scenes contain objects of 100% contrast so that even greater sensitivity is required to view real scenes under conditions such as starlight. The SEC camera-tube alone is just not quite sensitive enough for televising night scenes. This problem can be overcome, however, by the addition of a one-stage image intensifier coupled by fiber optics to the camera-tube. Figure 5 shows such an assembly intended for night-time television. The image intensifier is of the 40125 mm variety with an 5-25 photocathode on fiber optics as described elsewhere in this volume.$ Figure 6 shows the performance of this combination in television lines per picture height plotted against face-plate illumination in lm/ft2 for

t See p. 229. 3 See p.

117.

163

SEC CAMERA-TUBE PERFORMANCE AND APPLICATIONS

100% scene contrast (solid lines) and 30% scene contrast (dotted lines). Both contrast measurements were made for static scene conditions (upper curves) as well as for a movement of 20-sec per picture-width (lower curves). I n most practical cases low contrast, dynamic conditions prevail, These curves indicate that the intensifier/SEC tube combination described here is a practical device for night-time viewing.

FIG.5. Image intensifier/SEC camera-tube combination (WL-32000).

I n order to further improve on this performance a combination of larger tubes could be used. Before discussing applications the general characteristics of production tubes will be summarized. The typical sensitivity is 15,000pA/lm, with a limiting resolution of 201p/mm, a distortion of typically 2% for the electrostatically focused tubes and a residual third-field signal 700-

2

OI ._

2

600 -

2 t

._ B Q 0 C ._ > l-

500 -

-

I

1 1 1 1 1 1 1 1

I

1 1 1 1 1 1 1 1

I

1 1 1 1 1 1 1 1

I

1

'

I

'

I ' l ~ t l L I

1

1

~

Operating voltage 2 2 - 2 3 kV Illumination 2870'K tungsten Equiv. noise current 6nA Bandwidth IOMHz 1/30sec frame rate Dynamic rate 20sec/picture width

300-

C ._

C

.o

-a t

200

-

3

g

IOO0

I

I

' 1 1 1 1 "

'

I 1 [ l l l l '

' ' t l l f i

164

0. W. 00ETZE AND A. H. BOER10

of 5%. The gamma is unity. There is no target leakage current and all dark current originates in the image section due t o light feedback, field emission and thermionic emission. It is therefore necessary t o distinguish between simultaneous operation with the electron gun “filament on”, and sequential operation for integration and read-out with the “filament off” during integration. The dark currents are measured in the target lead and are A/cm2 and A/cma of photocathode area with filament “on” and “off)’ respectively. These currents are very small compared with those of vidicon-type devices. The maximum integration times resulting from these two modes of operation are 2 min and 30 min respectively for tubes using S.20 type photocathodes. The warranty on tube life is 500 h with an expected life of several thousand hours. The thermal extremes which can be tolerated are -62°C and +85OC. The mechanical stresses the tubes can be exposed to under hard mounting are 15 g shock and 10 g sinusoidal vibration up t o 500 Hz.

APPLICATIONS A typical image intensifierfSEC camera for low light-level television is pictured in Fig. 7. This camera and similar versions are widely used

FIG.7. Camera head for image intensifier/camera tube combination.

in many forms of night time-observations and surveillance. Figure 8 shows four monitor pictures generated by such alow light-level television camera using electrostatic zoom of -3 : 1 in the image section of the SEC tube. It also gives an impression of the general picture quality.

SEC CAMERA-TUBE PERFORMANCE AND APPLICATIONS

165

An entirely different application is illustrated in Fig. 9 which shows an electron microscope coupled t o an image intensifier which in turn is coupled to an SEC camera, in each case through fiber-optic face-plates. This arrangement is very useful for working with extremely low current densities in the microscope. The fairly large input format and integration properties of the SEC camera-tube are very desirable features for this application. The integration capability as well as the sensitivity

FIG.8. Monitor pictures generated by an SEC tube with electronic zoom.

are illustrated by the various Fresnel diffraction patterns shown in Fig. 10. The nine patterns cover exposure times of 0.2, 10, and 20 sec a t microscope current densities of 4.0 x 5.75 x and 1.16 x 10-15A/cm2. An image intensifier/SEC camera-tube combination (WL-32000) was also used with the 36-in. McDonald telescope. A typical result is shown in Fig. 11 which is a comparison between a direct photograph taken on a IIa-D plate on the right-hand side and a picture from the television monitor using the WL-32000 system on the left-hand side.

FIG.9. Camera tubelelectron microscope arrangement (courtesy of Dr. K. Herrmann, Siemens).

Fra. 10. Fresnel diffraction patterns generated by the arrangement shown in Fig. 9. Scanning time, 1/30 sec. Current density (top t o bottom): 1.16 x A/ci.na, 5.76 x A/cm2, 4.0 x A/cma. Exposure time (left t o right): 0.2 sec, 10 sec, 20 sec. (Courtesy of Dr. K. Herrmann, Siemens.)

SEC CAMERA-TUBE PERFORMANCE AND APPLICATIONS

167

FIG.11. The M 51 galaxy as recorded on the McDonald telescope using an intensifier/ camera tube sensor (left) and a IIa-D photographic plate (right) with 7000 times longer integration. (Courtesy of M. Green and J. R. Hansen, Westinghouse.)

FIG. 12. Hand-held lunar SEC-camera (courtesy Westinghouse Aerospace Division).

168

0. W. GOETZE AND A. H. BOER10

The information on the left-hand side was recorded with an exposure time 1/7000th of that used for the photograph. A detailed description of the use of SEC camera-tubes in astronomical applications appears elsewhere in this vo1ume.t I n addition to ground-based astronomy, SEC tubes have been selected for five U.S. and one European space program. An example of this application is the hand-held “Apollo” camera, Fig. 12, t o be used for on-board operation and t o be carried on the lunar surface by a U.S. astronaut.

Fra. 13. Field-sequential SEC-camera (courtesy of Dr. R. H. McMann, CBS Laboratories).

The SEC tube has also made possible a very impressive development by the CBS Laboratories, shown in Fig. 13. This is a field-sequential camera for closed-circuit color television transmission. This camera uses a single SEC camera-tube and operates a t 180 frames/sec. The results of a preliminary experiment are shown in Fig. 14. This is a picture generated with a triple-SEC tube, NTSC color camera. The scene illumination was 7 lm/ft2 at an iris setting of f/4. The superior performance as regards lag of SEC tubes is illustrated in the photographs of Fig. 15. The candle a t the left is a monitor presentation generated by See p. 807.

FIG.14. Monitor picture from an experimental three-tube color camera. The scene illumination was 7 Im/fta.

FIG. 15. Monitor pictures from a three-tube SEC color camera (left) and Plumbicon color camera (right). P.E.1.D.-A

E

three-tube 7

170

G . W. OOETZE AND A. H. BOER10

the same SEC color camera, while that at the right was generated with a Plumbicon color camera which, because of lag, presents several images of the flame. The applications summarized above are representative of ways in which SEC tubes are now being used. Because of the particular combination of performance characteristics which SEC tubes possess, there are many more emerging. ACKNOWLEDGMENTS The results preaented here were made possible through the combined efforts of a devoted group of colleagues. Their numerous contributions are hereby gratefully acknowledged.

REFERENCES 1. Goetze, G. W., In “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, W. L. Wilcock and L. Mandel, Vol. 16, p. 145. Academic Press, New York (1962). 2. Goetze, G. W., Boerio, A. H. and Green, M., J . Appl. Phys. 35, 482 (1964). 3. Goetze, G. W., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 219. Academic Press, London (1966). 4. Boerio, A. H., Beyer, R. R. and Goetze, G. W., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 229. Academic Press, London (1966). 5. Beyer, R. R. and Goetze, G. W., ln “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 241. Academic Press, London (1966).

DISCUSSION How is the definition of the tube defined? G. w. GOETZE: The resolution versus light-level curves (Figs. 4 and 6) were determined by several observers viewing a monitor presentation of the Westinghouse Resolution Chart No. EL 1338. This chart consists of square-wave bar petterns with each pattern containing groups of four black and three white bars of various line numbers. These curves are affected by several system parameters, like preamplifier noise-current and bandwidth which must also be specified. The inherent resolving power of SEC camera-tubes is defined by the squarewave amplitude-response curves provided in commercial data sheets. For example, the response measured at 400 TV lines per picture height is typically 20 and 40% for the WL-30691 and the WX-30654 respectively. J. A. LODGE: How far is the resolution of the SEC tube limited by the target, and how far by the electron optics? G. w. UOETZE: The intrinsic resolution of the SEC target was measured with an optically scanned device described at the previous conference. More recent measurements made with this device show that the resolution capability of the SEC target is greater than 60 lp/mm. The resolution of SEC camera-tubes is limited by the suppressor mesh spaced close to the target surface and operated a t a low voltage in order to limit the target surface potential. We are currently conducting experiments designed to J. D . MCUEE:

SEC CAMERA-TUBE PERFORMANCE A N D APPLICATIONS

171

increase the first cross-over potential of the target or to make it “self-stabilizing” without compromising other performance parameters. If these experiments are successful it will be possible to either increase the suppressor mesh voltage or eliminate the mesh entirely and thereby achieve a considerable improvement in resolution. G . 0.TOWLER: Could you tell me the value of the signal-plate potential, for the data given on lag, and what is a typical working value for the signal-plate potential? 0. w. GOETZE: All performance parameters reported here or in commercial data sheets pertain to the same fixed operating voltages. The operating potential for the target, signal-plate for a specific tube can vary within the range of 10 to 15 V.

This Page Intentionally Left Blank

Some Properties of SEC Targets D. MoMULLANt and G. 0. TOWLER Applied Physics Department, Imperial College, London, England

INTRODUCTION It is now several years since low-density insulating layers were first used as targets in television camera tubes.lS2 Since then camera tubes ~ comwith SEC targets have become commercially a ~ a i l a b l e ,but paratively little has been published on the mode of operation of the target.4*5Neither have there been any reports of targets having better characteristics than those of the original potassium chloride target. I n this paper we discuss the question of target stability, and a target having a much improved performance in this respect is described. Some experiments are reported on charge integration with singleframe read-out. These help in understanding the mode of operation of the target and in particular the internal distribution of charge. I n the course of these experiments a method was found for producing negative images; it is thought that this may have an application in the correlation of optical images. Some measurements on target lag are reported and finally target resolution is discussed and attention drawn to a theoretical limitation.

TARGET STABILITY All SEC camera-tubes which have so far been reported in the literature have used a low-density layer of potassium chloride as the target material. Such targets have a high gain (up to several hundred) but suffer from the disadvantage that, if the intensity of the imaging beam is too high, the potential of the target surface can rise above the first secondary-emission cross-over potential. The reading beam will then charge the target to anode potential instead of discharging it, and the resulting high voltage across the layer will destroy it.5 This effect can be prevented by mounting a stabilizing mesh, at a potential less than the first cross-over potential, in front of the target. Because the crossover potential of potassium chloride is only 1 5 V , the mesh must be 7 Now at the Royal Greenwich Observatory, Herstmonceux, Sussex, England. 173

174

D . MCMULLAN AND G . 0. TOWLER

mounted very close to the target surface if the focus of the reading beam is not t o be impaired by the lowering of the electric field a t the target surface. On the other hand, if the spacing is small, the layer may be damaged by touching the mesh, the mesh may be in focus, and the increased capacitance can give rise to microphony and shunting of the signal current. I n practice, a spacing of about 0.3 mm is a not entirely satisfactory compromise. The use of materials with a higher first cross-over potential than that of KC1 would permit a higher mesh potential, and hence a larger spacing. Unfortunately, the secondary-emission coefficients of such materials are inevitably lower than those of KC1, and the target gains are much less. For example, a low-density layer (20 pm thick) of ZnS

-\ A-

'<

- j

F

-

Imaging beam (7keV)

-Scanning

Al,O, substrate 5 0 0 8 thick

beam

L o w density ZnS 0 . 3 - 2 p m thick

I

Aluminium signal plate 5008 thick

Low\ density K C I 7-15pm thick

FIG.1. Cross-section of

a two-layer target.

has a cross-over potential of 100 V, but a maximum gain of only 10, which is rather low; it is therefore not a very attractive target material. We have found that it is possible to prepare a high-gain target, which also has a high cross-over potential, by covering a low-density layer of KCl with a very much thinner layer of low-density ZnS (see Fig. l).s Both the layers are deposited by the well known method of evaporation in an inert gas a t a pressure of a few t ~ r r . ~ Depending .~ on the thickness of the ZnS, the gain of the target is reduced, but the cross-over potential is raised; the reduction in overall gain is quite small for a thickness of ZnS giving a cross-over potential of 50 V (mean thickness of the order of few tenths of a micron). This increase in cross-over potential permits the suppressor mesh to be a t a higher positive potential, and hence to be mounted farther away from the target.

SOME PROPERTIES O F SEC TARGETS

175

With a slightly greater thickness of ZnS, a target can be made that is completely stable, i.e. the surface potential cannot be raised by the imaging electrons above the first cross-over potential, and a suppressor mesh is therefore no longer required.

Inherently Stable Target The condition for a target to be inherently stable will now be discussed. The gain G of the target, i.e. the ratio of the charge in the target to total charge in the imaging beam during the period between reading scans, has two components: the SEC gain GSEC due to the collection of secondary electrons by the signal-plate, and the transmitted secondary emission or TSE gain GTSE, due to those secondary electrons which leave the surface of the target on the reading-beam side and are collected by the nearest positive electrode. Hence : GTSE. (1) When the signal-plate is at a positive potential Vsp with respect to the reading gun cathode, G T s E is negligible compared with GsEc because the comparatively high electric field within the layer causes all the secondaries, except those produced very close to the surface, t o flow to the signal-plate. Under overload conditions, that is if the imaging beam is very intense, the surface potential will rise to a high positive value, and the internal field will diminish and then reverse. This is assuming that the potential rise is not limited by a low-potential stabilizing mesh close to the target surface. As the internal field falls, GsEc will fall and will also reverse, whereas GTs, increases because more of the secondary electrons can leave the surface. When the potential across the layer reaches the equilibrium potential YE, GSE, is equal and opposite to GTSE, and G is zer0.l The maximum value of the potential excursion A V of the surface from its stabilized value (gun-cathode potential) is = GSEC

+

d v m a x = V, vsp* (2) If dVma,> V,, the first cross-over potential, the reading beam will charge the surface even more positively, unless there is a stabilizing mesh at a potential less than Vl. However, if

the reading beam will always be able to discharge the target following an overload. For a 7.5-pm KCI target, V, N 15 V and V, _N 80 V with normal mesh spacings and potentials, SO that relation (3) cannot be satisfied with any positive signal-plate potential (which is necessary for normal

176

D. McMULLAN AND C . 0. 'FOWLER

high-gain operation). With the KCl covered by a 1.5-pm layer of ZnS, not only is V , increased to 9OV, but V , is only 25V because CTSE, which depends on the secondary electrons produced near the surface, is also reduced. C,,, is also less with this thickness of ZnS, but V,, can be increased to offset this. With the above values of Vl and V,,

FIQ.2. Static gains of KCl-ZnS target as a function of voltage excursion on target surface; signal-plate potential V s , = + 80 V. A, SEC gain; B, TSE gain; C, total gain.

for the target to be atable V,, must be less than 65 V; in practice, with the above layer thickness, 50V has been found to be the maximum acceptable value, since a t 55 V the reading beam starts to penetrate right through the layer t o the signal-plate, giving rise to spurious white signals. At V,, = 50 V there is a substantial amount of lag (see later in this paper); for negligible lag Vsp must not exceed 30 V. The gain of a typical composite target with Vsp = 50V is about 100, and a t

SOME PROPERTIES O F SEC TARGETS

177

30 V it is 25. Figure 2 shows GSEC,GTSEand G plotted as functions of A V for Vsp = 5 0 V , with AV,,, marked at the potential where ,G,--, = CTSE. The stability margin 1.7, is 15 V, which is ample to

take account of the transient condition when the tube is switched on, and potentials may rise above the normal values because of interelectrode charging currents. It is also sufficient to ensure good acceptance of the reading beam, and hence a rapid discharge of the target, if AV,,, is reached.

CHARGEINTEGRATION The very high resistivity of SEC targets makes them particularly suitable for television cameras which are to be used for integrating low light-level images. The integration time is not limited by the SEC target since it can hold charge patterns for several days with no loss in information content. Rather, it is the dark current in the image section that is the limiting factor. The usual mode of operation is to integrate the image while the scanning beam section is turned off and then t o read out the charge pattern as a single frame. Before integration is commenced the surface of the target will have been brought to gun-cathode potential by the scanning beam. This normally will mean that all the charges deposited during the preceding integration will have been erased. However, we have found that under certain circumstances a residual charge is left which can increase the gain of the target. How this can occur is best described by considering the case in which the previous integration is of a uniform flood beam. The target is flooded with a uniform beam of electrons from the photocathode while the reading beam is switched off, and the potential of the target surface is allowed to rise so that it is more positive than the signal-plate, i.e. the electric field in the target is reversed. The surface potential is limited either by the potential of the nearest mesh (which is set more positive than the signal-plate) or, in the case of a two-layer target, by its equilibrium potential. When the surface has reached its limiting positive potential the flood beam is switched off and the surface is reduced to gun-cathode potential by turning on the reading beam. With a conventional KC1 target the signal-plate potential may also have t o be varied during this stabilization in order t o prevent the reading beam striking the target surface while it is above the first crossover potential. However, with a two-layer target the cross-over potential is so high that this precaution is unnecessary. If the target is now used in the normal way to integrate a charge image, the target gain will be found to have increased by an amount which depends on the signal-plate potential Vsp. With the two-layer target described in

178

D. MCMULLAN AND G . 0. TOWLER

+

the previous section, at V,, = 4 V the increase in gain is about four times, at VSp = 10 V it is doubled, but at V,, = 30 V there is no enhancement. The gain of this target a t the above signal-plate potentials is low (4 to 25) but the effect has also been demonstrated with KCl targets with considerably higher gains a t low signal-plate potentials. Unfortunately the effect does not appear to give a gain higher than can be realized merely by increasing the signal-plate potential. It does however help in the understanding of the multiplication process and it may also have a practical application since, as is described later, negative images can be formed. The reason for the gain enhancement after the preparatory procedure is believed to be due to positive charges stored within the body of the target. Figure 3 shows the potential distribution V ( z )which is believed t o exist through the target for two different signal levels, zt being the target thickness (signal-plate to target surface). I n each case the signalplate potential is $20 V. I n Fig. 3(a) the imaging beam is producing a voltage excursion A V = 5 V on the target surface. (This is considered a strong signal and is near the maximum that can be used before beam-bending effects become troublesome.) Most of the positive charge is being stored near the target surface for two reasons. First, a major proportion of the secondary electrons are produced in this region by imaging electrons which have lost most of their initial energy. Some of these secondaries leave the surface, but the majority are collected by the signal-plate after travelling through the target (SEC electrons). Both components leave a positive charge near the surface. Second, any positive charges produced inside the target will tend to be neutralized by SEC electrons which originated nearer the target surface and were travelling towards the signal-plate under the influence of the internal field. I n Fig. 3(a), A-B-C is the potential distribution after integrating the signal, and A-D-E is that after reading. Figure 3(b) shows the potential plot for what would normally be considered an overload condition, but which also occurs during the enhancement procedure. The surface potential is higher than that of the signal-plate, and the SEC electrons travel towards the surface since the electric field is reversed. Positive charge is stored on the surface, but there will also be positive charges within the target. Since the rate of production of secondaries increases as the imaging electrons pass through the target (in consequence of their energy loss) the number of positive charges produced in a layer 6z will increase with z. However, the number of SEC electrons entering the layer will be less than the rate at which positive charges are produced and hence a net positive charge will build up. (When the surface is more negative than the signal-plate the SEC electrons travel in the opposite direction and

+

+

179

SOME PROPERTIES O F SEC TARGETS

exceed in number the positive charges produced in the layer 6z, and hence no internal charge results.) This will be especially the case near the signal-plate where the rate of production of secondaries will be very low. Equilibrium will be established when the field due to these charges causes electrons t o leave the signal-plate. The potential distribution is shown as the curve A-B-C, and after cathode potential stabilization by A-D-E.

v,,

+

A

=20v I\

+I

+ + AV>2OV

(a)

(b)

-

FIG.3. Potential distribution through the target for target surface potentials: (a)5 V, and (b) 40 V before and after cathode potential stabilization.

It can be seen that after cathode-potential stabilization there is a difference in the internal field gradient for the two cases shown in Fig. 3(a) and (b), that for the latter being the greater (except near the signal-plate). Since the gain of the target is dependent on the electric field in the region where the secondaries are being produced (recombination is less likely to occur when the field is high) the gain of the target in the condition shown in Fig. 3(b)will be the higher initially. However, during the integration of an image, the SEC electrons

180

D. MCMULLAN AND

a. 0.TOWLER

travelling towards the signal-plate will combine with and neutralize the internal positive charges; thus the internal field, and hence the gain, will fall. Eventually, after many charging and read-out cycles, most of the internal positive charges will be neutralized and the potential distribution will be shown in Fig. 3(a). This can be demonstrated by internally charging the target and then observing the amplitude of the video waveform when a constant optical image is focused on the photocathode and the target is scanned a t 25 frameslsec.

0

1

I 2.0

I

I

5

10

I 20

I

50

I 100

I

200

Time (secl

FIG.4. Decay of target gain with time after internal charging. Scan rate 25 frames per sec.

Figure 4 shows that the gain decays to one-half its initial value with a time constant of about 5 sec (125 frames). I n normal use with repetitive scanning, little internal charge builds up unless there is a gross light-input overload (e.g. by a flash bulb in the field of view) and unless the overload is of long duration, the build-up of internal charge will be small and no difficulty with persistent images (positive or negative) is experienced. As mentioned earlier, the degree of enhancement is dependent on the signal-plate potential. This is thought t o be a consequence of the neutralization of positive charges within the layer by the scanning

181

SOME PROPERTIES O F SEC TARQETS

beam which can penetrate right through the layer to the signal-plate when the signal-plate potential is high. Under this condition the internal positive charges produced during the flooding process will be neutralized when the scanning beam is used to return the surface to cathode potential and there will be thus no gain enhancement.

Negative Images If a target is internally charged with a uniform beam, as described above, and the internal charge is then partially neutralized by an electron image (with the scanning beam on) a negative image can be produced during subsequent read-out after the target has again been flooded uniformly. This will now be described in detail.

%P=

+@; Target surface Signal-plate

/

FIG. 5. Potential distribution diagrams illustrating formation of negative images: (a) target prepared before writing image, (b) writing and reading white vertical bar image, ( c ) cathode-potential stabilization after writing, and (d) after uniform flooding.

Figure 5(a)shows the potential along one line of a target with internal charge and with the surface cathode-potential stabilized. This corresponds with A-D-E of Fig. 3(b) but with the line direction x on the target surface included. If a vertical white bar image is now scanned for a few seconds a t 25 frameslsec, the internal charge in the illuminated area will be neutralized to some extent. Figure 5(b) shows the potential distribution after several scans, with the light image on (just before the line is scanned), and Fig. 5(c) that after the light is switched off so that the whole surface is a t cathode potential. The internd charge in the illuminated area has to some extent been neutralized and the gain in

182

D. MCMULLAN AND G . 0. TOWLER

this area will therefore be less; an example of this decrease in gain during continuous scanning is shown in Fig. 4. If the target is now flooded with a uniform beam of electrons, the potential of the previously unilluminated areas will rise higher than those that were illuminated, as shown in Fig. 5(d),i.e. a negative image will be formed. I n Fig. 5(d) the potential of the surface has been allowed to rise 10-15 V in order t o give a 5-V signal amplitude. To read out the charge pattern the surface potential must first be reduced to the range 0-5 V by lowering the signal-plate potential by 10 V. Otherwise severe beam-bending would occur and the contrast would be low. Two optical images could be correlated by using one of them t o discharge selectively an internally charged target, and the other in place of the uniform flood-beam in the final operation described above. The final read-out will then give a video signal which depends on the difference in luminance between corresponding points in the two images.

TARGET LAG When a television camera is used for dynamic imaging it is desirable that the reading beam should completely discharge each picture point, and that there should be no charge regeneration after read-out. Incomplete discharge (or “discharge lag”) occurs when the current in the reading beam is insufficient; normally it is only significant with highcapacity targets. SEC targets of the usual thickness (10-20 pm) have a comparatively low capacitance and discharge lag does not occur at normal scanning speeds. Charge regeneration (or “target lag”) is a major problem in camera tubes with electron bombardment induced conductivity (EBIC) targets because of the comparatively long decay time of the EBIC effect. Again, SEC targets do not exhibit charge regeneration when operated at the signal-plate potentials (<-30 V) normally used. However, if an attempt is made to increase the gain by raising the signal-plate potential, lag effects become evident. It has been suggested* that both the high gain and the lag are produced by EBIC processes in the target. Since another effect, beam electron conduction (BEC), has been proposed7 as the reason for the high gains measured under these conditions, we decided to investigate whether there was any evidence of EBIC effects. The method employed was t o measure whether the charge regeneration was time-dependent. As noted above, the decay of EBIC is by no means instantaneous after the bombarding electron beam is turned off, but persists for a comparatively long time (in the range from several milliseconds to many seconds). A block diagram of the experimental arrangement is shown in Fig. 6. A staircase-waveform generator is used as a frequency divider to pulse on, for 20psec every tenth

183

SOME PROPERTIES O F SEC TARGETS

frame, a cathode-ray tube having a short-persistence P.16 phosphor. The resulting pulsed light spot, which is stationary and also defocused, is imaged by a transfer lens on to the photocathode of an SEC camera tube having a two-layer target of the type described earlier in this paper. This is scanned at 200 lines, 50 frames/sec, and in odd frames one line is selected and displayed on an oscilloscope. By means of the delayed pulse generator the time between the light pulse and the first scan of the selected line can be varied in the range 0 to 20 msec.

Camera tube

[

Q

,

-

'# Lull

Monitor

w Adding amplifier

Delayed pulse Light _generator trig-

tt

I

Z00msec

/Ln T=IOOpSeC

Staircase generator

-IT+

Pulse inverter and amplifier step control

40msec

& Line selector C.R.T. cathode

100,usec

t

Line trig

t

Frame trig

FIG.6. Block diagram of lag-measuring equipment.

Figure 7 shows the oscilloscope waveforms for four different signalplate potentials and with the light pulse timed to occur 25 psec before the first read-out. When the signal-plate potential is 5 V or 10 V a negative pre-pulse can be seen; this is caused by the charging current due to transmitted secondary emission (TSE) electrons. This effect has been described by Filby et aL7 At the higher signal-plate potentials the TSE gain is lower and the pre-pulse is very much smaller. As can be seen in Fig. 7, at the three lower signal-plate potentials there is no evidence of charge regeneration. However, at 40V (and 30V) there are residual charges which are read out on the second (not displayed), third

184

D. MCMULLAN AND G . 0. TOWLER

FIG.7 . Oscilloscope waveforms showing target lag; scans superimposed. Signal-plate potential: (a) 5 V, (b) 10 V, (c) 20 V, and (d) 40 V. A negative charging pulse can be seen in (a)and (b).

and subsequent scans. The profile of the lag can be displayed more clearly if the succeeding scans are displaced by a signa,l from the staircase generator to the X-deflexion amplifier of the oscilloscope (see Fig. 8). When the time delay between the light pulse and the first scan is increased, the amplitude of the subsequent scans relative to the first could be expected to decrease if there were EBIC effects. I n fact, no

FIG.8. Oscilloscope waveforms showing target lag; scans displaced to show lag profile. Signal-plate potential: (a) 10 V, (b) 20 V, (c) 30 V and (d) 40 V.

185

SOME PROPERTIES O F SEC TARGETS

such time-dependence has been found when the delay is increased to 20 msec; in a separate test the delay was increased to 10 sec and even then there was only a very slight change. It therefore seems likely that EBIC occurs to only a very small extent in the SEC target. The signal-plate potential at which lag becomes evident is close to that at which the gain curve starts to rise steeply (see Fig. 2). As was mentioned above it has been suggested that the cause of the latter is beam electron conduction (BEC). This is the penetration of the scanning beam right through the target to the signal-plate when the electric field within the target is high. The charge pattern on the surface of the target modulates the BEC electrons. At still higher signal-plate potentials there is penetration even in the absence of a surface charge pattern. As a qualitative explanation of the large increase in the slope of the gain curve, BEC would seem to be plausible. On the other hand it is not so obvious how it can be evoked to account for the increase in lag. Even if part of the scanning beam penetrates to the signal-plate there would appear to be no reason why it should be any less effective in neutralizing the surface charges. A possible explanation is that the penetrating electrons produce positive charges within the target by secondary emission. The neutralization of such charges would be a slow process because the penetrating electrons would have energies in the region of the first secondary emission cross-over energy of the KCI (- 15 eV). With rather higher signal-plate potentials it might be impossible to neutralize the charges, and this could be occurring when there is permanent penetration by the scanning beam.

RESOLUTION The resolution of a camera tube is often limited by the read-out section rather than by the target. It is interesting to consider how far the resolution could be increased if the read-out section were improved, or in other words, what is the resolution limit of the target. At first sight the resolution of an SEC target would appear to be limited only by the scattering of the imaging electrons in penetrating the target. However, this scattering has been shown to be more directive than a Lambertian distribution,8 and it is of minor importance. Also, because of the very high resistivity of the target there can be no loss of resolution through redistribution of charge by conduction. The actual limitation, which is a fundamental one that is common to all electrostatic devices, arises because the read-out electron beam senses the potential distribution rather than the charge distribution on the target surface. Krittman9 has shown that the transformation of a charge pattern into a potential pattern can in itself be considered as an P.E.1.D.-A

8

186

D. MGMULLAN AND 0.0. TOWLER

imaging process, and the theoretical aperture response derived. By the use of Fourier transforms he has shown that for a target of thickness q , on a conducting signal-plate, the sine-wave response with electron-beam read-out is given by 1 W

S

)

=

- exp

-

(- 4rfszt)

47?f8%

7

(4)

where fs is the number of cycles/unit length. The electron beam is assumed to have a very small cross-section and its aperture response is not taken into account.

Spatial fraquency(cycles/mm)

FIG.9. Sine-wave response calculated using Eq. (4) for target thickness zt

= 12 pm. Crosses indicate response reported by Beyer and Goetzelo for an optically scanned tube.

An optically scanned SEC camera tube designed for high resolution has been reported by Beyer and Goetze.lo The read-out beam was of very small diameter and beam-pulling effects were reduced by using a strong magnetic field for focusing. They estimated the sine-wave response of the target from the measured response of the complete system by making allowance for the responses of the optical lens, the imaging section, the read-out beam, and of the amplifier. The points plotted in Pig. 9 have been taken from their estimated response curve, The expression for R(f,), Eq. (4))is also plotted in Fig. 9 with zt = 12 pm, the value which gives the best fit to the experimental points at low frequencies where it can be expected to be most accurate.

SOME PROPERTIES OF SEC TARGETS

187

As can be seen, the fit is extremely good; unfortunately the thickness of the target is not stated, but 12 pm would seem to be a reasonable value. The fall-off at low spatial frequencies was remarked on by Beyer and Goetze who, however, did not advance an explanation. Krittman also reported measurements on image orthicons. He found that although there was good agreement with the theoretical curve for the first scan of a fixed pattern, the response was better for subsequent scans, particularly a t low spatial frequencies. He attributed this to charge redistribution on the surface of the target, the time constant of this equilibrium being determined by the electrical characteristics of the target. If this is the case then one would not expect to observe the improvement with SEC targets because of their high resistivity. I n Beyer and Goetze’s tube the target was completely discharged by a flood beam between successive scans, so that their measurement was always for the first scan. An alternative hypothesis to explain the improvement in repetitive scanning is that not all the electrons deposited by the first scan of the read-out beam combine with the positive charges of the charge pattern. These electrons could form areas of negative charge between the positive charges, and could so modify the potential field a t the target surface that on subsequent scans the read-out beam electrons land only on the positive charges of the charge pattern. If this were so, then the resolution of SEC targets would also improve after the first scan. ACKNOWLEDUMENTS We should like to express our gratitude for the interest and advice of Professor J. D. McGee in whose department the work was carried out. We also thank Mr. J. Westlake and Mr. W. Simpson of the Applied Physics Department for their technical assistance. We wish to acknowledge with thanks financial assistance from the Science Research Council. One of us (G.O.T.) has been supported by a grant from the English Electric Valve Co. Ltd., Chelmsford.

REFERENCES 1. Goetze, G. W. and Boerio, A. H., Proc. Inst. Elect. Electronics Engrs 52, 1007 (1964). 2. Filby, R. S., Mende, S. B., Rosenbloom, M. E. and Twiddy, N. D., Nature 201, 801 (1964). 3. Goetze, G. W., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 219. Academic Press, London (1966). 4. Boerio, A. H., Beyer, R. R. and Goetze, G. W., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 229. Academic Press, London (1966). 5. Filby, R. S.,Mende, S. B. and Twiddy, N. D., Int. J . Electronics 14,387 (1965). 6. McMullan, D. and Towler, G. O., Electronics Letter8 4, 360 (1968).

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D. MCMULLAN AND 0.0. T O W E R

7. Filby, R. S., Mende, S. B. and Twiddy, N. D., I n “Advances in Electronics

and Electron Physics”, ed. by J. D. McGee, D. McMullan and E . Kahan, Vol. 22A, p. 273. Academic Press, London (1968). 8. Butkevich, B. G. and Butslov, M. M., Radiotekhnika i Elektronika 3, 355 (1958). 9. Krittman, I. M., I.E.E.E. Tram. Electron Devices ED-10, No. 6, 404 (1963). 10. Beyer, R. R. and Goetze, G. W., I n “Advances in Electronics and Electron

Physics”, ed. by J. D. McGee, D. McMullan and E. Kahen, Vol. 22A, p. 241. Academic Press, London (1988).

ABSTRACT

Newly Developed Image Orthicon Tube with a MgO Target1 Y. KAJIYAMA, T. KAWAHARA and T. HIRAYAMA Electron Tube Division, Nippon Electric Company, Tamagawa Plant, Kawasaki, Japan

An image orthicon tube with a magnesium oxide target has been developed which has a high sensitivity and a long life. The difficult techniques of production of this type of tube, hitherto left unsolved, have been successfully established. It is the aim of this paper t o discuss the technical problems concerning the production of a MgO target and its assembly, including the behaviour of the mesh membrane at high temperature and a method of controlling the resistivity of the film. Theoretical considerations are discussed concerning the lag, amplitude response, and signal-to-noise ratio of the tubes. The performance of the resulting tube, type Zd-750, is as follows. I t s photosensitivity is 150 pA/lm; the illumination required on the photosurface at the knee point is 0.03 lm/m2; the amplitude response is 70% at 400 TV lines, and the signal-to-noise ratio is in the range of 45 : 1 to 50 : 1. Additionally, the tube has a sufficiently wide range of dynamic operation to cover from high to extremely low light-levels.

t

For full paper see N.E.C. Res. Developm. No. 11, p. 133 (1968). 180

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Electrostatically Scanned Image Orthicon S. MIYASHIRO and S . SHIROUZU

Toshiba Research and Development Centre, Tokyo Shibaura Electric GO.La., Kawasaki, Japan

INTRODUCTION An investigation has been carried out into the design of a small, high performance image orthicon tube, using electrostatic scanning, which eliminates the large and heavy coil assemblies from an image orthicon camera. Although a vidicon has been developed using electrostatic scanning, this problem had not been undertaken for the image orthicon because of the difficulty of designing the electron optics of the scanning section which has to satisfy the following two precise criteria for the scanning of the electron beam. 1. The primary electron beam must land normally on the target. 2. The return beam must be directed into an electron multiplier. Since our successful reproduction of images with an all-electrostatic image orthicon in December 1964 after three years of basic research,l further improvements have taken place which lead to the conclusion that the best combination is that of a short magnetic-focusing imagesection with an electrostatic s~anning-section.~-~ I n this paper, investigations into electrostatic low beam-velocity scanning are described and test results of an experimental image orthicon tube are presented. I n the mean time, it was reported4 in March 1965 that the General Electric Company had studied an all-electrostatic image orthicon for military purposes. Later, it was reported that this work was done by Dr. K. Schlesinger and his co-workers of the G.E.C.5

ELECTROSTATIC Low BEAM-VELOCITY SCANNING Collimator-lens Method From the electron-optical standpoint, two possible approaches were devised to ensure normal landing of the electron beam. The first is a double deffexion method, based on the principle that two pairs of 181

192

5 . MIYASHIRO AND S. SHIROUZU

deflexion plates placed along the tube axis, in which the fields are in opposite phase, will deflect and collimate an electron beam. However, experiments have shown that the deflexion efficiency is so low that this method is not practicable. Focusing lens

Target

Field -mesh

Wali-anode

Deflector

Electron gun

Electron multiplier

FIG.1. Low beam-velocity scanning by collimator lens method.

The second approach, using a collimator-lens method, which forms the subject of this paper, has been proved successful. The collimator lens (Fig. 1) consists of a field-mesh having the highest voltage of all the lens elements and a cylindrical wall anode. By adjusting the electrostatic accelerating field of the collimator lens, its focal point can be made to coincide with the deflexion centre.

FIQ.2. Trajectory of the electron beam near the target.

This results in the primary electron beam landing normally to the field-mesh, and hence normal landing on the target is ensured. Focusing of the primary beam is provided, for example, by an Einzel lens placed between the deflexion centre and the electron gun. Due to the initial velocity distribution of the returning electrons, the return beam inevitably diverges as shown in Fig. 2. However, this divergence

193

ELECTROSTATICALLY SCANNED IMAUE ORTHICON

can be so minimized by maintaining the field-mesh a t a relatively high voltage that the return beam can be directed t o the electron multiplier along the path of the primary electron beam.

Paraxial Ray Tracing Figure 3 shows the schematic arrangement of the collimator lens applied to the scanning section of an electrostatic low beam-velocity device. General characteristics of the collimator lens are obtainable by the computation of principal electron trajectories i.e., the path of an

/

\

Field- mesh electrode VI

i

Wall anode Vp

FIG.3. Electrostatic scanning section.

electron leaving the target with zero energy. The well known paraxial ray equation is d2V d2r d r dV 4V+2-+r-=O, dz2 dz2 dz dz where r is the distance of an electron from the x-axis and V is the potential distribution along the axis. The potential distribution can be expressed in the analytical form, V ( 0 , x )= V1 & ( V 2- Vl)[tanh {w ( z - S)} tanh {w ( z S)}J,(2) where V, and V2 are voltages of the field-mesh and the wall electrode, and S is the length of a skirt cylinder of the field-mesh electrode (cf. Appendix A). The paraxial electron trajectories were computed for the initial condition that electrons start perpendicularly t o the field-mesh with an energy corresponding to the field-mesh voltage. The relation thus obtained between the positions of the intersections of electron trajectories with the z-axis, the focal length fo of the collimator lens, and the

+

+

+

194

S. MIYASHIRO AND S . SHIROUZU

electrode voltage ratio V2/V1 is shown in Fig. 4. The skirt length 8 is taken as a parameter. Values of fo and X are normalized to the electrode radius p.

1.001 0

I

0.10

I

I

I

I

0.20 0.30 0.40 0.50 Electrode voltage ratio Cy/y)

I

0.60

040

FIQ.4. Focal length of the collimator lens as a function of the ratio of the potentials applied to the two electrodes.

Equation for Non-paraxial Ray An electron beam passing near the periphery of the collimator lens is affected by the spherical aberration. This is likely to cause corner shading or a reduction of the signal current. To analyse this situation, an equation for non-paraxial rays is required. The equation of motion for an electron in an axially symmetrical electrostatic field is given by

+

_ d2r - 1 (dr/dz)2{au dz2 -

2u

ar

dr au} dz az ’

(3)

where u is the potential of a point ( r , z) and is expressed as power series in r,

195

ELECTROSTATICALLY SCANNED IMAGE ORTHICON 00

u(r, z ) = 2 (- 1)nn= 0

(4)

(n!)

where v(z) is the potential distribution on the z-axis. u, v and r , z are normalized to the field-mesh voltage Vl and the electrode radius p respectively. By substituting Eq. (4) into Eq. (3), the equation of the motion of an electron is expanded into a series of terms involving v(z) and r . Since r and drldz can be considered to be much smaller than unity, terms involving higher powers of these variables may be neglected depending on the degree of approximation which is permissible. The following approximations may be obtained. 1. Up to the 1st power of r and drldz included, 4vr(2) 2 ~ ( 1 ) ~ +( V1()2 + = 0.

+

I

2. Up to the 2nd power, (8w - 2 ~ ( ~ ) r ~ )(r4( ~~ '-)~d 3) ) r 2 ) r ( l ) 2 { ( r ( 1 ) ) 2 1}d2)r = 0. 3.

+ ..........

+

+

6. Up to the 6th power, 2(2304v - 576d2)r2 36d4)r4- v(6)r6)r(2) (2304~'~) - 5 7 6 ~ ( ~ ) r36d5)r4 ~ - ~(')r~)((r(~))~+r(l)} - (1157d2)r- 144d4)r3 6 ~ ( ~ ) r ~ ) { ( r ( ll} ) ) ~= 0 where r = R/p = r(z), r ( l )= dr/dz, r ( 2 )= d2r/dz2, v = v(z), dl) = dv/dz, 4 ')= d2v/dz2. . . . . . . . .

+

+

+ +

(5)

+

...

It can be seen that the first power approximation results in Eq. (l), the paraxial ray equation. The distances, fo', between the field-mesh and points where the electron paths cross the z-axis are computed by each of the approximations of Eqs. (6) for electrons starting from points on the field-mesh which are a t distances from its centre equal to 0.1, 0.4, 0.6 and 0.8 times p. These distances, f,,',approach their true values with the addition of higher power terms of r and drldz to the equation, and the computed results are summarized in Fig. 5. This figure shows that the choice of a satisfactory approximation depends on the initial distance of an electron path from the z-axis. For example, an electron passing through the field-mesh within r = 0 . 6 ~is satisfactorily described by an equation containing powers of r and drldz up to the 6th. This approximation equation has been employed in the computation described in the rest of this paper.

196

.L? - 9.8 .? 9.6

9. M'IYASHIRO AND 5 . SHIROUZU

Q

n '

$ 9.4 C

9

9.2 E 'X 9.0

e

-5

a

L L

ij

8.8

8.6 8.4

-58.2 W 3

U

al 8.0

c

-

2" 0

f

7.8 7.6

W

? 7.4

0

e

Number of approximation equation used

Fra. 6. Convergence of the successive approximation equations.

Spherical Aberration Figure 6 shows the trajectories of the returning electrons for the same initial conditions as in Pig. 5 . As the electron starting point on the field-mesh approaches the periphery the influence of spherical aberration increases and the point where the trajectory croases the z-axis moves toward the target. The maximum spherical aberration of the collimator lens with a target radius of 0 . 6 ~is shown in Fig. 7 as

Distance along Z - a x i s

(PI

FIG.6. Electron trajectories in the collimator lens.

ELECTROSTATICALLY SCANNED IMAGE ORTHICON

-

00

2.0

4.0

6.0

8.0

197

10.0

Focal length f, for paraxial electron (p1

FIQ.7. Spherical aberration of the collimator lens.

a function of fo, where fo - fo’ is taken as a measure of the spherical aberration, It can be seen that the aberration is at a minimum for a certain value of S.

Landing of the Primary Beam The off-normal landing-error angle a is shown in Pig. 8 as the value of tan a (i.e., drldz) at the field-mesh plane. Since the value of tan a

Focal length for paraxial electron(p)

FIQ.8. Landing characteristic of the primary beam.

198

5. MIYASHIRO AND 9. SHIROUZU

can be made very small, it is possible to make the radial energy V 1 sin2a, corresponding to the shading potential, negligibly small compared with the target potential of a few volts. Electrostatic scanning optics of this kind may therefore have a marked advantage over conventional electromagnetic scanning optics in the orthogonality of the landing beam.

Broadening of the Return Beam The return beam must reach the first dynode of the electron multiplier without striking intervening electrodes. While the primary beam scans the target, the return beam also sweeps out a small raster owing Photocathode ( SbCs,)

\

IJniform

/

Colour filter

,,

m .4

WoII-anode

'Phosphor

screen

41

v;

t o the spherical aberration of the collimator lens. Further, the return beam is likely to diverge owing to chromatic aberration produced by the distribution of initial electron energies. This results in broadening of the return beam. If the broadening of the return beam a t the centre of the deflexion system is reasonably small, the beam will reach the first dynode, since it is converged by the Einzel lens after passing through the deflexion electrodes. Tracing the electron trajectories shows that the diameter of the return-beam raster may be made as small as lo%, or less, of the target diameter. I n addition t o the calculation, an experimental estimate of the broadening was made. The experimental tube used is shown in Fig. 9. A uniform, circular patch of light, the size of which

199

ELECTROSTATICALLY SCANNED IMAUE ORTHICON

corresponds to that of the target, was projected on to the photocathode. Photoelectrons liberated were converged by the collimator lens to a diameter di at the phosphor screen which corresponds to the broadening of the return raster. The brightness of the output image is increased by an accelerating field near the phosphor to make the observation easy. The value of @ is minimized by adjusting the electrode voltages. The focal point of the collimator lens is assumed to coincide with the position of the phosphor screen. The magnitude of the raster broadening as given by these measurements is shown by the solid curves in

4

I

0.5

I

1.0

I

I.5

I

2.0

I

2.5

I

3.0

Initial encrgy (eV)

FIG.10. The measured values of the broadening of the return beam.

Fig. 10 (cf. Appendix B). This figure shows that, when V , is approximately 1000 V, so that the target voltage and hence the initial energy of the electrons is not excessively high, the broadening of the return beam is not severe, and it can be expected t o strike the first dynode.

ALL-ELECTROSTATIC IMAGE ORTHICON Construction of Experimental Tubes Experimental all-electrostatic image orthicon tubes were constructed according to the principles described above as are shown in Fig. ll(a). The glass target, the electron gun and multiplier assemblies are the same as those of a conventional 3-in. image orthicon. An electrostatic beam deflector, of the box type with two pairs of plates, is employed and an einzel lens is mounted between the electron gun and the deflector.

tc

0 0

Landing correction ring

Einzel lens Photocathode (-600 V )

(+ 2 V )

(a) (+I000 V ) ( - 4 O O V ) ( + 2 V )

(t280Vl

P

3 v1

Mesh

(+I25

Landing correction electrode (+720VI

(b) (C) FIG.11. Experimental image orthicon tubes using electrostaticscanning: (a)all-electrostatictube, (b)improved all-electrostatictube, (c)electrostaticallyscanned tube with a magneticallyfocused image section.

ELECTROSTATICALLY SCANNED IMAQE ORTHICON

201

The spacing between the target and the field-mesh is kept fairly large in order to avoid spurious beat patterns. A ring-shaped electrode mounted between these two electrodes maintains normal landing of the beam near the periphery of the target. The electrode configuration of the image section is of a tetrode type and a good quality electron image can be obtained by applying the highest potential to the middle electrode. By changing this potential the image magnification can be varied from 1-5 to 2 times.

FIQ.12. Output of an early all-electrostatic image orthicon tube.

By employing a test pattern of the kind shown in Fig. 12 or by printing the pattern of Fig. 14 on the target by metal evaporation, the electron optical properties of the image section and the scanning section can be separately examined.

Experimental Results The picture obtained by the experimental all-electrostatic tube at an early stage of this work is shown in Fig. 12. As was predicted theoretically, the picture shading is fairly good as can be seen from the horizontal output waveform of a checker-board pattern shown in Fig. 13. Furthermore, it was shown that the shading does not occur even with an excess beam current. The “black-border” effect is found to be less noticeable as compared with that of the conventional image orthicon. A horizontal deflexion voltage of about several tens of volts per plate was required when the voltages of the field-mesh and the

202

9. MIYASHIRO AND 5. SHIROUZU

wall anode were 1250 V and 280 V respectively. Raster distortion was acceptable as can be seen in Fig. 14. The major problems in this tube are its large size and the imperfections

FIG.13. Horizontal output waveform.

of its picture geometry. These are mainly due to the electron-optics of the image section. I n order to solve these problems a new design of the electron-optics for the image section, as shown in Fig. l l ( b ) , was studied in a demountable vacuum system. A high-voltage

FIQ.14. Linearity of scamling: the referencepattern on the target on which s Fating signal is superimposed.

mesh electrode close to the photocathode leads to a considerable increase in edge resolution as well as a decrease in image distortion. A relatively short image orthicon tube using this electron-optic system has been realized and improved performance has been achieved.

ELECTROSTATICALLY SCANNED IMAGE ORTHICON

203

ELECTROSTATICALLY SCANNED IMAGE ORTHICON WITH MAGNETICALLY FOCUSED IMAGE SECTION In spite of many attempts t o improve the design of the electrostatically focused image section, the all-electrostatic type image orthicon tube tends to be large and its picture quality is marginal. Hence a compromise was adopted by the introduction of a magnetically focused image section which allows the tube t o be kept short and good picture resolution t o be achieved. Prior t o the trial, the following difficulties were anticipated. 1. Picture corner shading. 2. Rotation and distortion of the scanning raster due to the leakage of the magnetic field into the scanning section. 3. Increase in size and weight owing t o the employment of the magnetic focusing coils; thus reversing the desired design trend. These problems were examined and satisfactory results were obtained as follows.

Tube Construction The arrangement of electrodes in the image section as shown in Fig. l l ( c ) is similar to that of a type 6820 tube. A focusing magnetic field with divergent flux is maintained by a doughnut shaped coil fitted in front of the photocathode so that a useful image magnification may be easily achieved. Thus, a small optical image can be projected on the photocathode and the magnified electron image can be transferred t o a comparatively large target. Consequently, both the optical object lens and the focusing magnetic coil can be kept small. After studies with a few experimental tubes, a small experimental

FIG.15. Experimental image orthicon tubes with electrostatic-scanning:The upper is an all-electrostatic tube, the middle is a hybrid type and the lower is a conventional 3-in. image orthicon.

204

8. M’IYASHIRO AND 9. SHlROUZU

tube has been built, the external appearance of which is shown in Fig. 16 (middle tube). The maximum glass tube diameter is 2 in. and the overall length is about 14 in. The same electron gun and multiplier are employed in the conventional 2-in. image orthicon tube. I n spite of its smaller size this tube has the same size target as the conventional %in. tube.

Performance Characteristics Although corner shading becomes noticeable as the focusing coil current is increased, it was shown that the magnetic flux leakage through the target is not more than a few gauss and with the fieldI

I

I

I

I

I

I

-

120-

-

a?

-

--

40-

I2

20-

.-

-

0

0.

0

- -

-I

I

100

I

ZOO

I

300

I

400

I

500

I

600

I

700

800

Number of television lines

Pra. 16. Square-wave response of the hybrid tube.

mesh held at a potential of several hundred volts the rotation and distortion of the scanning raster due to the magnetic flux leakage is negligible. The picture magnification in the image section is about three, and may be varied electronically. The “black-border” effect is insignificant. The reason for this has not been rigorously analysed, but it can probably be ascribed to the weak magnetic and electric fields in front of the target, which allow secondary electrons from the target to spread over greater distances than normal. The “whiteedge” effect is not pronounced. This may be due to the reduction of beam-bending owing to the strong electric field between the target and the field-mesh. Figure 16 shows an example of the square-wave aperture response curve a t the picture centre. A relative modulation of about 60% is achieved by 400 TV lines. No appreciable

ELECTROSTATICALLY SCANNED IMAGE ORTHICON

205

deterioration in resolution capability is found even with high targetvoltage operation which is desirable for a large signal-to-noise ratio. Problems of picture corner shading and picture distortion are nearly solved. Work is proceeding to improve corner resolution and the results are promising. Figure 17 is a picture taken with this small hybrid type image orthicon. By employing this tube, an experimental television camera packed in a volume 6 x 6.2 x 16 in.3 was recently constructed. It weighs only 8 kg and consumes 110 W.

FIG.17. Output of a hybrid tube.

CONCLUSION The return beam method of electrostatic, low beam-velocity scanning has been studied theoretically and experimentally. On the basis of these studies, all-electrostatic image orthicons have been built, and the results reported. However, the usefulness of the all-electrostatic tube seems somewhat restricted when excellent picture quality is needed by limited resolution and distortion in the electrostatic image section. Further efforts have led to the electrostatically scanned image orthicon with a magnetically focused image section. This hybrid tube has been proved to have the following desirable features. 1. Miniaturization: a large and heavy coil assembly is unnecessary and the electric power consumption is reduced. A small optical lens can be used because of the large magnification in the image section. 2. Good picture quality: shading can be reduced to a negligible level. The “black-border” and

200

9. MIYASHIRO AND 9. SHIROUZU

“white-edge” effects are not excessive. A target with a relatively large diameter for the size of the tube envelope gives a good signal-to-noise ratio. It is hoped that the electrostatically scanned tube will overcome some of the remaining problems encountered with the image orthicon. The electron-optics described here may be applicable to other electronic image devices. ACKNOWLEDQMENTS The authors would like to thank Dr. T. Okabe and Mr. Y. Nakayama for their kind guidance, and Dr. K. Kakizaki under whose direction this work was done. Thanks are due also to Mr. M. Suzuki and Mr. K. Hata for their kind assistance in electron-path computation. Appreciation is also extended to many colleagues for their assistance in making experimental tubes and their helpful discussions, and especially to Mr. M. Iwasawa for his co-operation in evaluation of the tubes.

REFERENCES 1. Miyashiro, S. and Shirouzu, S., Paper submitted to the Research Committee of Electron Tubes for Television of Inst. of Television Engrs. of Japan, No. 263 (Nov. 1965). 2. Miyashiro, S. and Shirouzu, S., J . Inat. Te2evia. Engra J a w n 21, No. 11, 794 (Nov. 1967). 3. Miyashiro, S., Shirouzu, S. and Iwasawa, M., Paper presented a t the Third National Convention of the Inst. Televis. Engrs of Japan, No. 3-13 (Oct. 1967). 4. Electronic News, March 15 (1965). 5. Day, B. E., Rep. No. 14 (final), U.S.Government Publication DDC, No. AD-620547 (1965). 6. Bertram, s., Proc. lnatn Radio Engra 28, 418 (1940).

APPENDIX A Potential Distribution in Collimator Lens I n an axially symmetric electrostatic field, the analytical form of the axial potential distribution V ( 0 ,z ) can be found by summing, with respect to a particular value of z = 5, the influence of the boundary electrode potential V(1, z ) upon the point (0, z ) : ~

I

+ m

V ( 0 ,2) =

V(1,

5 ) sech2{ W

(z - ()}d(.

--OD

In accordance with this equation applied to the collimator lens shown in Fig. 4, the axial potential distribution V(0,z ) is given by integrating with respect to 5 from minus infinity of the imaginary part to plus infinity of the real part:

ELECTROSTATICALLY S C A N N E D IMAGE ORTHICON

V(0,z ) =

+ '2

-s

/jiVl

207

- V,) sech2 ( w ( z - 5))dC

sech2 (w (z - ())dl

-+

sech2 ( w ( z - ())d<,

APPENDIX B Measurement of the Return Beam Broadening The minimum values of @ are measured for different values of the field-mesh voltage Vl. Red and blue light are projected on the photocathode using colour filters. If the mono-energetic photoelectrons were available, the observed values of Q, could be converted to be a function of Vo because the broadening due to chromatic aberration can be regarded as proportional to 2/(vo/V1) as shown in Fig. 2. Since it is impossible to expect a single value of vo in an actual experiment, the relation between @ and vo is obtained as shown by solid curves in Fig. 10 by taking the following into consideration. 1. The value of Q, observed using the red filter should not be larger than that caused by the electrons with an initial energy of 0.21 eV, which is the difference between the threshold energy of the photocathode (1.82 eV) and that of the red filter (2.03 eV). 2. The value of Q, observed using the blue filter should not be smaller than that caused by the electrons with an initial energy of 0*63eV, which is the difference between the peak energy of the spectral distribution of the blue filter (2.66 eV) and the threshold energy of the photocathode (2.03 eV).

DISCUSSION Do you have a problem with screening stray magnetic fields and how is this done? s. MIYASHIRO: We employ a cylindrical permalloy case to screen the stray magnetic field caused by the ambient electric devices and the earth field. J. A. LODQE: What is the amplitude of the deflexion voltage applied to the tube? s. MIYASHIRO: In the hybrid type tube, we us0 about 180 V for the line scan deflexion plate. w. P. WEYLAND: I assume that the resolution given in Fig. 16 concerned the centre of the image. Do you have also results from measurements of the resolution in the corners of the image? s. MIYASRIRO: We have not yet made quantative measurements of it. As stated in the lecture, the corner resolution is somewhat inferior to the centre resolution which is shown in Fig. 16. J . D. MCQEE:

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The Development of Image Isocons for Low-light Applications P. D. NELSON English Electric Valve Co., Chelmsford, Essex, England

INTRODUCTION The recent revival of interest in camera tubes using the isocon form of read-out has resulted from the development of types intended primarily for use in low-light conditions. Such applications as the viewing of live X-ray images with low dose-rates require the production of television pictures with photocathode illuminations in the range to 10-61m/ft2, that is between 1 and 4 orders of magnitude below what would normally be necessary in television broadcasting. Isocon read-out is advantageous in meeting this need because the noise level is much lower than with conventional orthicon operation. The development of a practical tube, having as its object the optimization of performance and the elimination of various spurious effects, is discussed here. The tube type referred to throughout is the 4&in., English Electric Valve Co. type P860.

TUBECONFIGURATION The first requirement in designing a tube for low-light work is to make use of the available light as effectively as possible, and to this end the image section configuration of the type P825 image orthicon, previously used in the X-ray application, was retained. The arrangement of electrodes is shown in Fig. 1. The curved face-plate allowed a large area of photocathode to be used and was correctly shaped to match a commercially available mirror optical system. A multialkali type of photocathode having an S.20 spectral response enabled typical sensitivities of 200 pA/lm to be obtained. Because of the low photocathode currents to be expected when operating in near-dark conditions, a much lower target capacitance was desirable than in tubes intended for broadcast applications. The reason for this is that an appreciable voltage swing has to be achieved 209

210

P. D. NELSON

on the target if excessive lag is to be avoided, as will be seen later. The relatively wide target-to-mesh spacing adopted allowed the knee of the transfer characteristic to be attained at a photocathode illumination of about 3 x l o v 4lm/ft2. The lower signal current due t o operation a t low light-levels and

FIG,1. Cut-away view of the P850 type isoeon.

in the isocon scanning mode, was compensated by the inclusion of two additional dynodes in the electron multiplier.

ISOCON BEAMSEPARATION The principle of separating the return beam to achieve isocon operation has been described in detail elsewhere112 and only a brief outline is given here. Figure 2 shows diagrammatically the arrangement used and its mode of operation.

IMAGE ISOCONS FOR LOW-LIGHT APPLICATIONS

211

The forward electron beam is deflected laterally by the steering plates and made to follow a helical path in the predominantly axial magnetic field. It approaches the target a t an oblique angle, and in the interaction with the surface only part of the beam lands and neutralizes charge on the target. The part which returns is made up of two components: a specularly reflected beam leaving the target in a well-defined direction, and a diffusely scattered beam having a range of directions distributed about the normal. After some additional deflexion on returning through the steering plates, the reflected beam

FIG.2. Beam separation in an isocon. The effect of the scanning fields is omitted for clarity.

is intercepted by the separator and the output is derived almost entirely from the scattered part of the return beam. The low noise level of the isocon results from the fact that as no beam electrons land on the target in darkness there is no scattered component. The output current (and consequent beam noise) corresponding t o picture black is therefore very low, and depends on the efficiency with which the reffected beam can be removed in a practical tube. I n order that efficient beam separation may be achieved, it is important that the width of the reflected beam a t the plane of the separator should be small. The presence of a slight residual scan acting on the return beam, owing to incomplete oancellation of deflexions after the forward and return passage through the scanning

212

P. D. NELSON

field, can be minimized by suitable adjustment of the decelerator electrode potential. Improvement in the separation efficiency was found to be possible when an additional electrode (known as G2A) was incorporated in the electron gun, providing a greater degree of control over the initial beam. It resulted in a further reduction of noise, particularly in the dark parts of the picture. The efficiency of separation of the reflected beam from the scattered beam in a tube of this construction, set up correctly at the knee of the transfer characteristic, is typically between 98 and 99%. This corresponds to a beam modulation of about 94%, taking into account the fact that the scattered component making up the output is only about a quarter of the total beam.

BEAMPULLING EFFECTS Examination of the performance of early tubes confirmed their good low-light characteristics, but disclosed two unpredicted effects which impaired the image quality. One immediately apparent was a background of slowly moving noise, travelling continuously across the picture in a direction which was fixed for any particular set of operating conditions. Associated with it was the appearance of either single or multiple fringes following any image which was moving in approximately the same direction as the noise. This effect was present only during movement, and is illustrated for a rotating test object in Fig. 3.

FIG.3. Reprodtiction of a test object rotating anti-clockwise at 1 revlmin, showing fringes in the top left quadrant with an early tube.

213

IMAGE ISOCONS FOR LOW-LIGHT APPLICATIONS

Both moving noise and the fringe effect seem to have been due to beam pulling, i.e. to lateral deflexion of the scanning beam near potential boundaries on the target. I n particular, the explanation of the multiple fringes has been put forward by van de Polder3? in relation to a Plumbicon with a non-orthogonal scanning beam (the “stern wave” 4 is a simplified representation of effect) and also by B e ~ r l e .Figure ~ the situation near a potential image on the target corresponding t o a dark object on a light background. Because the scanning beam is made to land more nearly perpendicularly close to the right-hand potential boundary, the axis1 energy of the beam electrons is higher here, and the target potential after scanning will be more negative than elsewhere. Similarly, a t the left-hand boundary a rather more positive target potential will result. I n a static scene, these variations in target potential make no difference t o the amount of beam current landing, since in the dynamic Direction of image movement e

+

+

+

+

Picture white

-

-

-

-

-

Picture grey

-

+ + + +

Target

Picture white

Fra. 4. Approximate representation of beam-bending near potential boundaries on the target. The assumed image is that of a dark object on a light ground.

equilibrium the charge added to the target at any point must equal the charge removed. When movement of the image is involved, however, equilibrium conditions do not apply, and variations can occur in the amount of current landing (and therefore in the tube output) during the subsequent scan. Near the right-hand edge of the image the more negative target potential reduces the signal level in this region, while near the left-hand edge the signal level is enhanced. When the direction of image movement is opposite to that of the beam, as shown in Fig. 4, these effects result in white-after-black and black-after-white fringes, as are observed. The multiple fringing may occur because of further beam bending in the region of the potential variations. On the other hand, when the image moves in the same direction as the beam, the polarity of the anomalous end-potentials is such as to cause only a loss in sharpness of the edges, also agreeing with observation. The presence of random noise fluctuations may be expected to cause beam bending effects of the same kind, and could lead to the

t

Seep. 237.

214

P. D. NELSON

appearance of a moving noise phenomenon, such as was found, travelling in a direction opposite to that of beam scanning. Once beam pulling had been established as the cause of these spurious effects, it was clear that one possible remedy was to increase the plane decelerating field close to the target so that the lateral forces at charge boundaries would have correspondingly less effect. This was accomplished by considerably reducing the field-mesh-to-target spacing, and it was effective in reducing both moving noise and fringe effects to

FIG.5. Reproduction of a rotating teat object using e tube with close field-mesh-totarget spacing. The conditions are identicel to those of Fig. 3.

negligible proportions. Figure 5 illustrates the performance of such a tube using the same rotating test object and under the same conditions as for Fig. 3.

MICROPHONY The lower limit to the field-mesh spacing was determined, not by field-mesh detail becoming visible on the picture, as might have been expected, but by an increase in the likelihood of microphony. One or other of two forms occasionally appeared on tubes with very close spacing, arising apparently spontaneously. One was similar to microphony in image orthicons, consisting of horizontal black and white bars superimposed on the picture and moving with respect t o it. I n the other related form the bars were fixed in position. Both varieties were

IMAGE ISOCONS BOR LOW-LIGHT APPLICATIONS

215

thought to be due to self-sustained mechanical vibration in the system comprising the target-mesh, the target and the field-mesh, as observed with image orthicons. On investigation, the static microphony was found to be a direct result of applying electrical blanking pulses to the target mesh, causing vibration at a frequency synchronized to that of the field. This was eliminated by using an alternative method of beam blanking in which blanking pulses are applied to the first grid in the electron gun.

LAG I n common with most camera tubes which use a low velocity electron beam to discharge the storage capacitance, lag is present in the isocon, causing characteristic smearing and loss of detail on moving objects a t low incident illuminations. The reason for this lies in the composition of the scanning beam, which for conventional electron guns has an approximately exponential distribution of axial e n e r g i e ~ , ~when .~ landing against a retarding potential. If only a small potential swing takes place on the target, as in low-light conditions, little beam current is required for charge neutralization. For dynamic equilibrium therefore, the target potential must become sufficiently negative for operation on the low-current tail of the exponential distribution. Some time is required after any change in illumination in these conditions for a new equilibrium to be established, and when this time is large compared with the field repetition rate, lag becomes noticeable. Some calculations giving a f~illeraccount of lag are presented in the Appendix, Figure 6(a) illustrates the lag present in low-light conditions when a light source, consisting of a cathode-ray tube with short persistence phosphor, was switched on and off. The corresponding build-up and decay as calculated from equations in the Appendix are shown for comparison in Fig. 6 (b), demonstrating that the theory adequately accounts for the observed tube behaviour.

REDUCTION OF LAa Because the isocon has such a good low-noise performance, for applications involving image movement it may be lag rather than noise that determines the lowest light capable of providing a useful picture. It was therefore necessary to consider ways in which lag might be minimized without at the same time detracting from other aspects of tube performance. It is shown in the Appendix, Eq. (2),that the time required t o scan off a charge image on the target to a given level is proportional to V B / V c ,where V , represents the energy spread in the scanning beam and V c is the change in target potential during charging.

216

P. D. NELSON

0) FIG.6. Signal build-up and decay. (a) Photograph showing lag at a photocathode illumination of Im/fta. The spots represent the signal amplitude at successive fields. (b) Calculated curves for the same conditions.

Obviously there are two possible ways of reducing lag, (a) to increase Vc, or (b) to reduce VB. Each possibility is considered below.

Target Potential Factors determining the potential change on the target for a given light level are the area and sensitivity of the photocathode, the target gain due t o secondary emission, and the target storage capacitance. Both photocathode parameters were already as large as could conveniently be attained, and although the target gain might be increased somewhat by the use of other materials, it was not thought justifiable to change from the well-tried and good quality “Elcon” glass target.

217

IMAGE ISOCONS FOR LOW-LIGHT APPLICATIONS

FIG.7. Remanent images due to lag, showing part of the Marconi No. 2 chart a short time after capping the lens. (a) Tube having close target to target-mesh spacing. (b)Tube with wide target t o target-mesh spacing, with the light level reduced t o obtain about the same degree of lag. Note the enhanced edges due to the predominance of inter-element capacitance. P.E.1.D.-A

9

218

P. D. NELSON

The remaining possibility was to reduce the target storage capacitance, which was achieved by increasing the target to targetmesh spacing, as has been mentioned. A large reduction in lag was obtained in this way, but a limit was reached with the spacing finally used, beyond which little further improvement was found. Investigation suggested that the "parallel plate" capacitance had been reduced to such an extent that inter-element capacitances at charge boundaries were now the predominant factor. Lag from the edge of an image, as a result, was something like two or three times that at the centre. Figure 7 illustrates this by comparing the remanent image a short time after capping the lens for a tube (a) with a close-spaced target, and one (b) with the adopted wide spacing.

Axial Energy Spread Any substantial reduction in the spread of axial energies in the scanning beam, which results partly from the range of thermionic emission energies and partly from electron optical shaping of the beam, would demand considerable re-design of the electron gun. Consideration

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Fro. 8. Lag, expressed as the time for the tube output to reach 5% of the initial level after switching off the light source. It is shown plotted against illumination for a typical tube operated normally and as an orthicon. The broken lines represent calculated relationships for different beam energy spreads, derived from Eq. (2) in the Appendix.

IMAGE ISOCONS FOR LOW-LIGHT APPLICATIONS

219

was therefore given to the differences in lag between isocon and orthicon operation. The magnitude of the spread in energies can be found by plotting measured lag against photocathode illumination and comparing it with calculated relationships derived from equations presented in the Appendix. This was done for the centre of an evenly illuminated area, to avoid the effect of inter-element capacitances, and a typical result is shown in Fig. 8. The corresponding energy spread parameter, V B ,is seen to be about 0.4 V, and for the same tube operating in the orthicon

FIQ.9. LOSSof resolution on a moving object due to lag. The waveforms show a block of 76 lines/picture-height resolution bars, static (on the left) and moving at 10 eec/picturewidth (on the right). The photocathode illumination is 8 x lm/fta.

mode about 0.2 V. The latter figure is in good agreement with values determined from beam landing curves taken on image orthicon tubes with metal targets. I n the Appendix it is shown that the difference in energy spreads can be accounted €or by the effect of non-orthogonal beam landing in conjunction with the angle of divergence of the beam. Substitution of appropriate figures shows a spread of energies of 0-3 eV from this cause (assuming that the angular deflexion of the beam by the steering plates is a’), compared with 0.03 eV for a similar orthogonal beam. Although the energy spreads cannot be added directly, this is clearly of the right order.

220

P. D. NELSON

Since an oblique scanning beam has the effect of increasing lag by a small factor it is obvious that the angle given to the beam should not be unnecessarily large. Reducing the angle, for instance by using a smaller separator aperture, cannot be taken too far however, or the separation efficiency suffers, with consequent increase in noise.

TUBEPERFORMANCE One of the effects of lag is t o reduce the ability of the tube to resolve detail on moving objects, as is illustrated by Fig. 9. Figure 10 shows the limiting resolution for a static scene and also for an object moving

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at 10 sec per picture-width, and Fig. 11 gives the signal-to-noise ratio performance. Both Figs. 10 and 11 are for critical setting of the beam. Taking for illustration a photocathode illumination of lm/ft2, a typical tube would have a lag time to the 5 % level of about 2 sec, a limiting resolution of nearly 600 television lines, and a ratio of peak white signal to r.m.s. noise in picture blacks of 35 dB. The appearance of the picture a t the same illumination is illustrated by Fig. 12.

IMAUE ISOCONS FOR LOW-LIGHT APPLICATIONS

221

50

40

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.$ c

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Photocathode illumination ( l m / f 1')

Flu. 11. Signal-to-noiaeratio, expressed as peak-to-peak signal over r.m.8. noise for a bandwidth of 8 MHe, against illumination.

FIG.12. Reproduction of the Merconi No. 1 chart at a photocathode illumination of lm/fta. Exposure time for the photograph was 4 sec.

222

P. D. NELSON

ACKNOWLEDGMENTS The author wishes to thank the Ministry of Defence (CVD) and the Managing Director of English Electric Valve Company Limited for permission to publish this paper. He also thanks the Ministry of Defence for part support of this work.

REFERENUES 1. Cope, A. D. and Borkan, H., Appl. Optic8 2, 253 (1963). 2. Mouser, D. P., Ruggles, P. C. and Slark, N. A.,J . Roy. Telewb. Xoc. 11, 261 (1967). 3. van de Polder, L. J., Philip Res. Rep. 22, 178 (1967). 4. Beurle, R. L., Private communication. 5. Meltzer, B. and Holmes, P. L., Brit. J . Appl. Phys. 9, 139 (1958).

APPENDIX THEORYOF LAG Target Potential for a Continuous Discharge It is assumed that the beam current able t o land and remain on a plane target having a potential v with respect t o the cathode (neglecting contact potential) is given by i = I exp(v/V,), for v
Beam Current Landing for a Continuous Discharge The actual beam current which lands and discharges the target is given by C J B = - - v,,

tF

where v, is the change in target potential in one field time, t,.

It is

IMAQE ISOCONS FOR LOW-LIQHT APPLICATIONS

223

assumed that interlace is ineffective, as seems to be the case from the absence of field pairing in the photographed signal decay curves. Expressing t as a number ( n ) of field periods,

If it is required to represent only the low-light behaviour of the tube, the number of field periods, n, which would have been needed to reach the operating point from a zero-potential initial condition would be large. Especially if enough beam current is available, the above expression may be simplified approximately to

Lag Decay Time For an initial equilibrium state, for which n = nE say, and having I , equal to the target charging current I,, Eq. (1) gives

where V , is the change in target potential due to charging. The ratio R between the beam current landing at m field-periods after switching off the light source and that initially is given by

R

=

nE - 1 n,+m-

1’

from Eq. (1).

Hence

This allows calculation of the number of field periods required after switching off the light source for the tube output to decay to R times the initial level. Putting the equation in an alternative form, the time to reach this level is VBC (1 - R) t=-EASG R ’ where E is the photocathode illumination, A and S are the area and

224

P. D . NELSON

sensitivity of the photocathode respectively, and G is the overall gain at thc target due to secondary emission. General Build-up and Decay From the expression for the target potential

During a charging cycle, n is reduced from no initially to a value n, given by

where V c is the change in potcntial due to charging. Assuming, as before, that both values of n are large, this simplifies to n1 = no exp ( - Vc/V B ) .

The discharge cycles alternate with charge cycles and advance the value of n by unity, since effectively one field of continuous discharge takes place. Thus the value of n changes during each actual field period (consisting of one charge and one discharge cycle) from no to n2 = no exp

(-Vc/VB)

+ 1.

(3)

This expression, with Eq. ( I ) , enables the beam current landing, and therefore the relative output, to be calculated field by field for any general change in illumination. Initially, n is equal to the equilibrium value nE for conditions before the change. For each succeeding field the new value n2 is calculated from the previous value no, as above. Range of Energies Due to Beam Divergence Disregarding the distribution of energies in the beam due to emission, a range of axial energies is present resulting from the divergence of the beam. The potential on the target necessary for an electron just to land, assuming its path makes an angle 0 with the tube axis in a region of V volts potential, is given by3

v

=

V sin2 0.

Since electrons travelling parallel to the axis can land with zero target potential, this expression also represents the spread in axial energies of a beam diverging with a half-angle 0. Because the quantity Vsin20 is conserved as the electrons cross from one potential region t o another near the tube axis, the values corresponding to the beam

IMAGE ISOCONS FOR LOW-LIGHT APPLICATIONS

225

at the electron gun may be substituted. Putting V = 300 V and sin fl = 0.01 gives an energy spread of 0.03 eV. Range of Energies for an Oblique Beam In the region between the steering plates and the field mesh the potential is almost constant (= V,, say), and the half-angle of the beam 8, is given by V , sin2 8, = V sin2 8. Assuming the angle given t o the beam as a whole by the steering plates is 4, the spread in axial energies is now

V , [sin2(+ =

+ 8,) - sin2 (4- el)]

V1 sin 24 sin 28,.

(4)

Putting V , = 150 V and 4 = 2" for the same beam as above, the spread of energies is found to be 0.30 eV. DISCUSSION Can you separate the loss of resolution for a moving object due to (a) 1/25 sec integration or (b)genuine lag? How near is tlhe tube to limitation by shot noise of the primary photoelectrons? P. D . NELSON: The integration between successive scans sets an upper limit to the resolution for a moving object, and measurements were made a t relatively low rates of movement (e.g. 10 sec per picture-width) to avoid this being too severe a limitation. The variation in dynamic resolution with photocathode illumination shows the effect of lag. Investigations have not been made on how close the tube is to limitation by photoelectron shot noise, but in view of the good performance a t very low light-levels, it is thought that the tube cannot be very far from this. R. K , H. GEBEL: Have you mttde any tests on the behaviour of your tubes when high intensity point sources are present 1 P. D . NELSON: No assessment has been made of the behaviour of the tube with high intensity point sources, other than purely subjectively. R. K , H. QEBEL: The most attractive and most important feature of the isocon is that, in contrast to the image orthicon, it can detect in the same scene very low light-level details when also high intensity point sources such as searchlights are present. I n the image orthicon, if the beam is set for the low light-level section of the scene, the point source may reach a size on the target which covers most of the target plate or if the beam is set sufficiently high for the point source, the low light-level section of the scene will be lost in the beam noise. I n the isocon mode, if good beam separation is achieved, it is most suitable to handle such point-source situations and still not lose the low light-level section of the scene. P. D. NELSON: I agree that this is a very useful characteristic of the isocon. K. o. LUBSZYNSKI: To what extent is the resolution reduced by the non-normal incidence of the scanning beam? P. D , NELSON: Perhaps rather surprisingly, the limiting resolution obtained with the non-normal isocon scanning beam is not reduced compared with orthogonal scanning. Figure 10 shows that the limiting resolution at the knee J . D. MraEE:

226

P. D. NELSON

is about 800 lines/picture-height, comparable with the best image orthicons. Proportionately good results have also been obtained on 3411. tubes. J. LOWRANCE: What is the maximum voltage deviation on the target? Please explain the ‘‘dark” signal-to-noise curve. P. D . NELSON: The target potential is normally set to 3 V above cut-off, giving a maximum voltage deviation of about the same value. Because the noise in the dark parts of the picture is less than that in the whites, it is useful to quote signalto-noise ratios for each case. The signal amplitude taken as reference for both is the difference between peak white and black levels, in common with normal practice for camera tubes. R. KNIUHT: Is the microphony induced by target blanking serious a t all lightlevels? P. D. NELSON: The microphony induced by target blanking was observed over a wide range of light levels. However, as has been mentioned, it could be eliminated by the use of beam blanking. u. w. GOETZE: Would you please comment on the operational stability of your isocon. I understand this used to be a problem in the early days of the isocon. P. D. NELSON: There is no problem with stability in operation. It is probably more necessary to have stable electrode supply voltages than with the image orthicon, in the interest of accurate beam steering, but this is not a great difficulty. A. s. JENSEN: Krittman’s analysis (IEEE Trans. Electron Devices ED-lo, No. 6, 404, (1963))indicates that a wide-spaced camera tube should have very low resolution on the first reading frame after a single exposure of a new scene. This is so since the reading beam has not yet had the opportunity to build up the counter point charge pattern that accounts for the high resolution these tubes can attain for a stationary scene read repetitively. Have you compared your resolution measurements made under the latter conditions with a resolution measurement made on the first reading frame after exposure to a new scene, particularly at low light-level? P. D. NELSON: During the transient image build-up for a new scene the resolution is initially poor, due mainly to the necessity of charging the interelement capacitances at charge boundaries on the target. At low light-levels such factors as lag due to the reading beam have also to be considered. The situation is the reverse of that illustrated in Fig. 7(b), which shows the enhancement of edges during the transient decay after removal of a scene. Measurements have not been made as suggested, but the dynamic resolution results shown in Fig. 10 give an indication of the tube performance in the related situation of a moving scene. L. ALT: My question pertains to the suggested application of the isocon to X-ray image intensification. I n view of the fact that image intensification in radiology is used chiefly in fluoroscopy for observation of dynamic processes, do you feel that lag may well be a problem, bearing in mind that presently frame rates desired are 30, 60 and even 120 frames/sec? P. D. NELSON: I am not qualified to answer in detail on medical applications, which are referred to in another paper.? Since many processes observed in fluoroscopy involve only low rates of movement, lag does not seem to be a severe limitation and a large number of P.850 tubes are already in use in hospitals in this application. w. P. WEYLAND: How uniform is the image reproduced by this tube? Do you have any figure concerning the landing error?

t

Seep. 827.

IMAUE ISOCONS BOR LOW-LIGHT APPLICATIONS

227

P. D . NELSON: Investigation of the uniformity of the image has not been carried out yet. As can be seen from Fig. 12, the picture whit,esare characteristically brighter a t the contre than towards the corners, tho amount depending on tube operating set-up conditions. Tho picture blacks have extremely good uniformity because of the very low tube output current corresponding to these areas.

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Dynamic Imaging with Television Cameras H. ANDERTON and R. R. BEYER Westinghouse E&xtric Corporation, Electronic Tube Division, Elmira, New Yorb, U.S.A.

INTRODUCTION The recent increase in interest in low light-level television has focused attention on the limitations of television camera tubes when they are required to image moving objects a t low levels of illumination. It has been found that some types of camera tube which function well for static scene imaging frequently show a considerable loss of performance when the scene is in m0tion.l The major loss of performance is a reduction in low light-level sensitivity which becomes more severe as the image-motion rate is increased. For this reason we have decided to evaluate tubes under moving-scene conditions and to include lowlight-level dynamic imaging information in our camera-tube datasheets. I n the following paper, the results of some of these investigations will be shown and reasons for the observed performance will be discussed. The major sources of loss of sensitivity are signal mixing and discharge lag. Signal mixing is an effect common to all camera tubes a t all light levels. Discharge lag on the other hand is serious only at low light levels in certain tube types. To illustrate this, measurements have been made with ti thin-film image orthicon, an SEC camera-tube, and an intensifier in combination with an SEC camera-tube.

IMAGE DETERIORATION DUETO SCENEMOTION The major performance parameter of the camera tube which is affected by scene motion is the aperture response which in turn affects the sensitivity of the device. To illustrate this effect, the aperture response of a thin-film image orthicon (type WL22722) is shown in Fig. 1 for image-motion rates of 5 , 10, and 20 sec per raster width. Also included for comparison is the static scene aperture response. These lm/ft2) under conditions data were taken a t a high light-level 229

230

H. ANDERTON AND R. R. BEYER

that are normally considered good for imaging moving scenes. It can be seen that even under these favorable conditions, the aperture response is severely reduced at these rates of motion. The major reason for the reduction in the aperture response of the image orthicon at this high light-level is signal mixing.

TV lines per raster height

FIa. 1. Degradation of aperture response due to scene motion for a thin-film image orthicon. The time periods shown are seconds per raster width.

Signal mixing is caused by the motion of the image across the storage target during the integration period. A bar chart imaged on to the photocathode of the camera tube produces a spatially distributed square-wave signal on the storage target. If the image is in motion, the square-wave signal is caused to move across the target surface. Excursion at a point on the target Square wave input

m t 3

Fra. 2. The target-voltage excursion resulting from moving a bar chart. a, Wavelength of the test chart. 2, Distance moved by pattern across target.

The effect on the resulting voltage excursion at a particular point on the target for a square-wave input is shown in Fig. 2. If the pattern is moved through a distance x then the total voltage excursion has a value V E . If the integration period is such that the pattern moves a distance A x between scans, then the voltage excursion read by the beam will be A VE. The value of A VIEdepends on the position of A x relative to the test pattern. This is equivalent to looking at the storage

DYNAMIU IMAGING WITH TELEVISION CAMERAS

231

target at different points along the direction of motion. The output signal of the camera tube is then proportional to the variation of A V , in the direction of the scan. The output waveforms of the camera tube are shown in Fig. 3 for a square-wave input moving at various speeds in the direction of the horizontal scan. It can be seen that as A x is increased, the relative signal-level (contrast) is reduced. This process continues until Ax: equals one wavelength of the pattern. At this point the charge image contrast on the storage target is zero. For more rapid motion, or smaller line-widths (i.e. Ax>a) negative contrast occurs due to the periodic nature of the pattern. For a natural scene which does not generally A$

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FIG.3. The spatial variation of the voltage excursion on the target for various rates of motion.

exhibit a periodic structure, these negative contrast values are not apparent. They must, however, be considered when a camera tube is evaluated using a bar chart. To illustrate the effect of signal mixing, the loss in contrast of a bar chart has been calculated using the waveforms shown in Fig. 3. The contrast loss is shown in Fig. 4 for two rates of motion at a scanning rate of 60 fieldslsec. Even at the relatively slow speed of 10 sec per raster width, the resolution limit is only 900 TV lines per raster height and maximum response at 600 TV lines has been reduced to 50%. The general case is shown in Fig. 5 where the resolution limit due to signal mixing is related to the pattern traverse time. The other major influence on the loss in moving scene sensitivity is image retention on the signal storage surface in the camera tube. Image retention can be due to either semiconductor lag or discharge

232

H. ANDERTON AND R. R. BEYER

TV lines per roster height

Fro. 4. The contrast reduction of a bar chart due to signal mixing in a TV system operating at 60 fieldslsec.

lag. For low light-level imaging the major problem is discharge lag which increases rapidly as the flux into the camera tube is reduced. Semiconductor lag is prevalent only in vidicon-type camera tubes which are not used for low-light-level imaging and it will therefore be neglected in this paper.

Sec/raster width

FIG.6. The resolution limit due to signal mixing as a function of pattern traverse rate.

Discharge lag2 is the result of the inability of the scanning beam to remove all of the signal from the storage target during one read-out cycle. The energy spread in the reading beam causes the scanned surface of the storage target to be chasged negatively during the manning process. If the target surface potential is still negative after

DYNAMIC IMAGING WITH TELEVISION CAMERAS

233

the integration period, only a small portion of the beam will have sufficient energy to land. Thus the discharge efficiency of the beam is reduced. If there is a change in the primary flux density at any point on the target, it will take several read-out cycles before this point reaches its new equilibrium potential. Because discharge lag occurs only with small voltage excursions, it is most prominent at low light levels.

MEASUREMENT O F DYNAMIC IMAGING CHARACTERISTICS To obtain data for moving scenes, a system was arranged which moves a chart of vertical bars at constant speed across the field of view in a direction parallel with the horizontal scan of the camera tube. The speed of motion was made continuously variable so that any time period could be selected for the image to cro~sthe raster. The speeds of motion considered for obtaining data were 10 and 20 sec per raster width. These speeds give signal-mixing resolutionlimits of 900 and 1800 TV lines per raster height as shown in Fig. 5 . The tube characteristic which was recorded was the discernible resolution, as seen on the display monitor for various input light-levels. It is realized that this measurement has shortcomings due to its subjective nature. It was used, nevertheless, because of its convenience and simplicity and because at low light levels it is difficult to take more analytical measurements, such as aperture response, owing to the low signal-to-noiseratios encountered at these low light-levels. Furthermore, this measurement of discernible resolution takes into account all image processing in the camera tube, the camera system, and the observer. Moving scene data have been taken from three types of TV camera tube. They are the closely-spaced, thin-film image orthicon (WL22722), the SEC camera-tube (WX-30654), and the intensifier/SEC camera-tube (WL-32000).t It would have been useful to have taken data from an intensifier-image-orthicon but such a device was not available. All of the tubes used have a 40-mm input diagonal but differ in other respects. The SEC camera-tubes operate using direct beam read-out and their low-light-level sensitivity is limited by preamplifier noise. The image orthicon operates using return-beam read-out through an electron multiplier which alleviates the pre-amplifier noise problem. The SEC tube type WL-32000 has only a 0.6-in. diagonal storage target so that its limiting resolution is lower than those of the WX-30654 and the image orthicon which use targets of 1.0 and 1.6 in. diameter respectively. The data taken from these three camera tubes are shown in Figs. ti, 7, and 8. These figures present results for both static and dynamic

t See p. 159.

234

H. ANDERTON AND R. R. BEYER

scenes. It will be noticed that for the two SEC camera-tubes, the moving-scene sensitivity is within one order of magnitude of the light level required for static scenes. However, the image orthicon loses about two orders of magnitude in sensitivity when the image is in motion. This is a clear demonstration of the differencesin the effects of discharge lag between the camera tube types.

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FIa. 7. Static and dynamic sensitivity for an SEC camera-tube, type WX-30664.

Because the SEC camera-tube target gain to capacitance ratio is high,3 the tube is operated with a high target-voltage excursion. Thus the relative insensitivity of the direct beam read-out mode does not present a serious limitation. I n the image orthicon the target gain is lower than in the SEC camera-tube but the read-out mechanism is much more ~ e n s i t i v e thus , ~ making it possible for the image orthicon t o have a static scene sensitivity which far exceeds that of the SEC

DYNAMIC IMAGING WITH TELEVISION CAMERAS

235

tube alone. Because of the discharge lag associated with small voltage excursions, the highly sensitive read-out mechanism is not useful under moving scene conditions. Another way to overcome the pre-amplifier noise is to couple an image intensifier to the camera tube by means of fiber-optics. The gain prior to the read-out process is increased thus ensuring a high target-voltage excursion at low light levels. This is illustrated by the intensifier/SEC camera-tube. The overall gain of this camera tube is similar to that of the image orthicon so that the tubes have similar static scene sensitivity. However, the high target-voltage excursion of the intensifier/SEC camera-tube gives far better dynamic imaging characteristics. 500

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Face-plate illumination (Lm/ftz)

FIG.8. Static and dynamic sensitivity for an intensifier/SEC camera-tube,type WL-32000.

SUMMARY The major phenomena affecting the low-light-level imaging of moving scenes have been discussed and demonstrated. It has been noted that signal mixing causes a loss in aperture response at all light levels. This effect becomes dominant in all camera tubes at image-motion rates above about 5 sec per raster width for a 60-fields/sec scan frequency. At low light levels and slower motion rates discharge lag assumes major importance. It is particularly noticeable in tubes which operate with small target-voltage excursions such as the image orthicon. I n the SEC camera-tube, discharge lag is not so important because the tube has to be operated with a relatively large voltage excursion in order to overcome the amplifier noise. The large target-voltage excursion is made possible by the high gain-to-capacity ratio of the SEC target. Thus, although the image orthicon is considerably more sensitive than the SEC camera-tube for static scenes, for moving

236

H.ANDERTON AND R. R . BEYER

scenes the SEC camera-tube is as sensitive as the more complex image orthicon. Excellent dynamic sensitivity is obtained by coupling an image intensifier to a camera tube. This has been demonstrated with the intensifier/SEC camera-tube. ACKNOWLEDGMENT The authors gratefully acknowledge the aid and encouragement given by their colleagues during the preparation of this paper. I n particular we wish to thank Dr. G. W. Goetze and Mr. R. A. Shaffer for their continued support.

REFERENCES 1. Parton, J. S. and Moody, J. C., Proceedings of the Image Intensifier Symposium, Fort Belvoir, Virginia, N A S A S P - 2 , 86 (1961). 2. Meltzer, B. and Holmes, P. L., Brit. J . AppZ. Phya 9, 139 (1968). 3. Boerio, A. H., Beyer, R. R. and Goetze, G. W., I n “Advances in Electronics

and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 229. Academic Press, London (1966). 4. Rotow, A. A., I.R.E. Convention Rec. 4, Part 3, 41 (1966).

Beam-discharge Lag in a Television Pick-up Tube L.J. v.d. POLDER Philips Reaearch Laboratoriea, N . 8.Philips’ Qloeilampenfabrieken, Eindhoven, The Netherlande

INTRODUCTION In scenes including moving objects televised by cameras equipped with cathode-potential-stabilized pick-up tubes of the photoconductive type, lag phenomena can sometimes be perceived, resulting jn reduced sharpness at the leading or trailing edge of the moving object or streaking behind the object. There are two principal causes for these phenomena, namely photoconductive lag of the layer and beamdischarge lag. Lag of the layer is due t o the fact that the photoconductivity does not immediately follow the change in light intensity. So, during the first few scans after a change of the light intensity an amount of charge is presented by the layer which does not correspond with the amount of incident light. In the following, attention will be paid only to the beam-discharge lag, while the lag of the layer will be ignored. In general terms, beam-discharge lag is due to the fact that the scanning beam does not stabilize to the same potential all picture elements on the target. This stabilization potential is a function of both the intensity of the light incident on the particular picture element and the geometrical position of the element on the target. The ctmses for these different stabilization potentials are (i) the spread in the axial ~ (ii) the oblique incidence velocity of the electrons at the m e ~ h l -and of the beam on reaching the target.4 These two aspects will now be treated in greater detail.

AXIALVELOCITYSPREAD The stabilization of the target element in the case of a stationary scene is in fact, the process of replacing by the scanning beam that number of electrons which has been supplied by the layer between two successive scans. (Fig. 1). In dark parts of the scene only a few electrons will have been supplied by the layer. In that case a small number 287

238

L. J.

POLDER

V.D.

of electrons from the beam is sufficient for compensation and it is clear that these few electrons will be those with the higher velocities. The equilibrium potential of the scanned surface of the target will therefore become so low that electrons of lower velocity cannot reach it. At a higher level of illumination more electrons of lower velocities are necessary for compensation, so that the stabilization potential V 2will be higher, as shown in the example of Fig. 2(a), where V1 is the potential to which a picture element rises and V 2 is the target stabilization potential. ( V 2 is positive because of a coiltact-potential.)

p

Mesh1

(a)

I

Output current

;;=Av -$-

(C)

Fro. 1. (a) Schematic arrangement of pick-up tube. (b) Equivalent circuit of a picture element. ( c ) The target potential V of a picture element aa a function of time.

Now if the intensity of illumination of a picture element changes, the former equilibrium state has to change to the state corresponding to the new level of illumination. This requires a change of the surface potential and also of the charge on the capacitance of the picture element. I n other words, during the change t o the new equilibrium state an output current is obtained which does not correspond to the new level of illumination. Figure Z(b) shows the course of the target voltage variation if the light intensity on an element is switched from white to near-black, e.g. an intensity level of 5% of white. The decrease of the potential A V a t the moment of scanning is found by means of Fig. 2(a). The output current, as shown in Fig. 2(c), is proportional to the decrease of the

BEAM-DISCHARGE LAQ I N TELEVISION PICK-UP TUBE

239

potential A V . Figure 2(c) shows that immediately after the change of the light intensity the output signal is still large. This is what is meant by beam-discharge lag. The spread in axial velocity of the electrons is greater than that expected on the basis of the cathode temperature. The exact reason for this is not yet quite clear and is the subject of further research.

(c’ I

0

I T

t

? 3T

2T

*

I

4T

I

FIG.2. (a)The target potential V l before the moment of stabilization and the target stabilization potential Pa a function of the signal output d V. (b) The target potential of a picture element as a function of time if the light intensity is switched from white to a dark grey, at t = 0. (c) T h e signal output current at Successive scam.

BEAM BENDING Beam-discharge lag phenomena can also occur when the beam does not arrive normal to the target. This will, in general, be the case. The potential step on the surface of the layer will influence the beam direction in the vicinity of the layer. This effect, called beambending, can be explained by the following simplified calculation. Suppose that the whole target has a high level of illumination. I n that case the beam always lands exactly on a place where there is a considerable potential gradient caused by the beam itself (Fig. 3). With a scanning time T of 1/50 sec between two successive scans, a layer capacitance of 1 nF, and a signal output current of 200 nA the potential step AV is 4 V. If the spot diameter As is about 25pm (as in the case of the Plumbicon tube), then the field strength along the target in the region of the landing spot is about 1-6 x lo6 V/m. With a mesh voltage of 300 V and a mesh-to-layer distance d of 2 mm the axial field strength between mesh and layer is 1.5 x lo6 V/m. Thus the force on the

L. J. V.D. POLDER

240

electrons tangential to the target as they approach to land is of the same order of magnitude as the normal retarding force. The electron paths were calculated by computer for the case of a beam landing in a region of potential gradient having components both normal and tangential to the target surface. I n this calculation the spread in the electron velocities was ignored. The results can be summarized as follows. If the beam traverses the mesh perpendicularly in the centre area of the target, then the potential gradient on the target causes the beam to be slightly deflected towards the region of higher potential; this deflexion being greater as the potential step is greater. This deflexion increases the component of the electron velocity parallel with the layer. Now it is clear ---Scan

- - -1-

I

I I

direction

-r------1

1 1 p I 3 e a m

,

Id

-- Mesh

X-

FIQ.3. The trajectory of the electron beam between the mesh and the target and the potential of the target ea a function of position.

that this will result in a higher stabilization potential, because this potential will be reached at the moment when, although the electrons have a residual velocity parallel to the target, they have zero velocity normal to it and are thereafter turned back to the mesh. Thus, beambending causes a rise in stabilization potential at higher illumination levels in addition t o that produced by the initial velocity distribution. The calculation shows that the lag effect caused by beam-bending is not very great so long as the beam passes normally through the mesh. When the beam passes through the mesh at an angle to the normal two alternatives can be considered: either the beam, traversing the mesh-target distance, passes over a region that has already been scanned (Fig. 4(a)), or it passes over a region still to be scanned (Fig. 4(b)). I n the case of Fig. 4(a) the lateral velocity component already present is somewhat increased, so that the stabilization potential rises at

241

BEAM-DISCHARUE LAG IN TELEVISION PICK-UP TUBE Scan direction

\\\ \'

___-----

'',

'hi

ck,

/

\>-

'/

,!9

&'

\' \

/ ,

Target

Target

X-

X-

(bl

(a)

FIQ.4. (a) The beam passes over a target region that has already been scanned. (b) The beam passes over a target region that still waits to be scanned.

higher signal levels. This results in a certain increase in lag. The calculations show that this is not very serious. But if the beam passes over that part of the target which has not yet been scanned (Fig. 4(b)), the lag effect can be much greater. The effect of the potential gradient is now such that the beam direction becomes more nearly normal to the layer, so that the layer is stabilized at a lower potential. The greater the potential gradient the lower is this potential. The result of an

AV(V)

FIQ.6. The calculated stabilization potential V a and the potential Vl before the moment of stabilization if the beam passes the mesh at an angle of & 4".

242

L. J. V.D. POLDER

approximate calculation is given in Fig. 5. Here the spread in velocities, among other things, has been neglected. The calculation was based on a mesh voltage V , of 300 V and a beam direction at the ' to the normal. As shown in Fig. 5, the stabilizamesh at an angle of 4 tion potential decreases with higher signal current for positive values of a, which means a negative slope of V z as a function of dV. Figure 6 shows the variation in the signal output during the subsequent scanning of a certain picture element after a change in the illumination level from white to grey. The output signal exhibits damped oscillations. If the slope of V z is strongly negative as a function of A V , permanent oscillation in the form of rippling or interlaced flicker may occur (of. Appendix). I

I

r r

I

I

0

I T

2r

I 4

AVW)

.

I

I

I

I

I 3r

4~

(b)

t

I

I

I

FIQ. 6. The development of the signal output, as shown in Fig. 2, for the c a m that the illumination level is switched from white to grey and the stabilization potential is that of Fig. 5 (a = + 4').

A favourable circumstance for the occurrence of this peculiar effect is the situation in which the stabilization potential for black is relatively high, since in this case there exists a possibility of a decrease with respect to this potential. Now, as already mentioned above, the stabilization potential for black is relatively high if the beam traverses the mesh obliquely. This potential will be further enhanced in circumstances where the beam, passing from mesh to target, is deflected by the high potential on that part of the target that is not scanned at all. This part is in fact at the signal-plate voltage V , (Fig. 7(a)). Figure 7(b) shows the result of a measurement of the approximate target voltage for black as a function of the distance x to the edge of the scanned area. With respect to the horizontal scale it should be remembered that in the case of a standard Plumbicon the width of one television line is about 20 pm, i.e. 0.4 mm corresponds to a width of 20 television lines. Within the region of these 20 television

BEAM-DISCHARGE LAG I N TELEVISION PICK-UP TUBE

-______-__-

243

- - - - - Mesh

1

Id

Unsconned area

12, 10

3

-

I

I

e-

i

1

4-

2-

0

I

i

6-

1 .

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Scanned area

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I

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d = 2.5mm

I

-

-

I

lines the increase in voltage is still quite high in the given example, so that the potential image itself may well be able bo give rise to a reduction of the potential. The curve shown in Fig. 7(b) can be described approximately by the equation

From this equation it follows that the memures to be taken to minimize the effect are (i) lowest possible signal-plate voltage, (ii) smallest possible mesh-to-layer distance, and (iii) highest possible mesh voltage. I n addition one can try to make the scanned part of the target bigger than the part that is covered by the image. Finally, it may be noted that the influence of the potential image will be greater the steeper the gradient; in other words the better the beam foms, the greater the influence. I n general it can be stated that the better the tube, in respect of both lag and definition, the greater the chance that the effect will be encountered.

244

L. J. V.D. POLDER

Apart from the edge effect it is necessary to ensure that the beam passes the mesh as near to normal as possible, which thus imposes requirements with respect to the design of the deflexion and focusing systems and also with respect to the adjustment of the alignment.

REFERENCES 1. Heijne, L., Acta Electrofiica 2, 124 (1957-1958). 2. Meltzer, B. and Holmes, P. L., Brit. J . Appl. Phya. 9, 139 (1968).

3. Beurle, R. L., Proc. Inatlz. Etect. Engm. 110, 1736 (1963). L.J. van de, Philipa Rea. Rep. 22, 178 (1967).

4. Polder,

APPENDIX Calculation of the Lag for the Case that the Stabilization Potential Va is a Linear Function of the Output Signal A V Assume, as in Fig. 8, & linear relation between V 2 and d V , i.e. V2 = PAV V3, (1)

+

then w0 find

V1= V , + A V = ( 1 + p ) A V +

(2)

V3,

and therefore also

I

I

AVFIG.8. The target potential V 1and the stabilization potential signal output A V .

<

I V 2as a function of the

+

Let the output signal, with t 0, be given by dV = dVd d V W , where d V , represents the dark signal contribution and A V , represents the white signal contribution. A t t = 0 the signal contribution A V , is switched off (by changing the illumination level).

BEAM-DISCHARQE LAO I N TELEVISION PICK-UP TUBE

245

At t = 0, the stabilization potential Vi0 (Eq. (l)),is

+

At t

=

+

vlo = v d PA 73. T,immediately before stabilization, the potential is

v:,

=

vg0-kdvd=

~ I L J

( l + p ) d v d + + d v w +

(4)

v3.

Consequently the output signal, using Eq. (3), is

so that the deviation from the correct output is During this first scan the stabilization potential is

vi = v: - d p1 = P A v d + ” AV, 1+B

At t = 2T we have =

+

d v d

= (1

so that the deviation is

~

(1

+

p ) d v d

+ V,.

+” AV,+ 1+B

83,

:

8 ) l d V W .

and so on.

If

l1

- < 1, the stabilization potential finally becomes v z = pd

vd

+

v3a

From Eqs. ( 5 ) end ( 6 ) it follows that the deviation in the output 8). This leads to several signal per scan is amplified by a factor /?I( I different possibilities, depending on the value of B: B>O Monotonic decrease of the deviation, giving rise to the normal streak.

+

No lag.

p=0

0 >B

-3

> -3

>/3

> -1

Alternating decrease, giving rise to the “stern-wave” phenomenon. Alternating increase, resulting in rippling or interlaced flicker.

This Page Intentionally Left Blank

A 13-mm All-Electrostatic Vidicon J. WARDLEY and F. W. JACKSON Research Laboratories, Electric and Musical Industries Ltd., Hayes, Middleaex, England

INTRODUCTION Following the production of an improved electrostatically deflected and focused vidicon, 25 mm in diameter,l consideration was given to developing a miniature version of the tube. It was decided to use a similar envelope to that employed for the 13-mm magnetic vidicon2 developed in these Laboratories. This entailed engineering a precise electrode structure to fit into a glass envelope of 11 mm bore. TUBECONSTRUCTION The principles have been described in detail elsewhere,l but the main features can be summarized as follows,. Crossed planar lenses are used to focus the electron beam sequentially in the frame and line directions instead of the rotationally symmetrical lenses commonly used in electrostatic vidicons. The arrangement is equivalent to crossed cylindrical lenses in light optics. Each of the planar lenses is formed between two pairs of parallel plates at different potentials. The second pair of plates in each set is used for scanning. By introducing a fine mesh in the plane of symmetry of the twoelement lens, one pair of plates in the second section may be omitted, increasing the focusing power of the lens and making it shorter. The focusing system images the electron beam cross-over rather than a physical aperture in the electron gun. A cylindrical anode and a separate field mesh follow the plate system. A diagram of the design is shown in Fig. 1. The electron gun is similar in construction to that in the 13-mm magnetic vidicon. It omits the wall-anode flange and small changes are made in the electrode apertures and spacings. The gun utilizes titanium electrodes and precision ground, sintered alumina collars, vacuum 247

248

J . WARDLEY AND F. W. JAUKSON

brazed with the aid of copper shims. A cathode assembly of low heater power is used, requiring only 0.6 W. The focusing and deflexion plates are mounted in a cagc-like structure of sintered alumina rods and titanium discs vacuum brazed on a jig with copper-titanium alloy. The plates are welded into position using st jig having ground blocks accurately aligned a t right angles. The electron gun is mounted on the axis of the plate system. The intermediate mesh G3 is made of copper with 80 meshes/mm. By using careful plating and etching techniques, it has been possible to make and stretch taut meshes with a transparency of more than 55%. The final

FIQ.1. Electrode structure of tube.

anode G4,consists of an evaporated hard chromium layer on the inner wall of the precision bore glass envelope. The target mesh G5, of 60 meshes/mm, is mounted on a cold-press indium seal and is the same as that employed in'the small magnetic vidicon. The complete electrode structure, mounted on a miniature 15-wire glass base, is shown in Fig. 2. The photoconductive target, is the well known porous/solid Sb,S3 vidicon layer prefabricated on to an optical window prior to sealing to the tube with a second indium seal. The tube is pumped through the glass base tubulation and the high vacuum processing is quite conventional. The completed tube shown in Fig. 3, is 9 cm long, with an outside diameter of barrel of 1.32 cm and a target ring diameter of 1-43 om.

249

ALL-ELECTROSTATIC VIDICON

FIG.2. The complete gun structure.

FIQ.3. The completed tube.

TUBECHARACTERISTICS The operating voltages on the tube electrodes are similar t o those on the larger tube. An overall voltage of 1500V is applied t o the gun anode GZ, and the focusing voltages are derived from a resistance chain connected t o this supply. Typical peak t o peak scanning voltages required are: line scan 90 V and frame scan 80 V for a scanned area of 6.4 x 4.8mm2. The characteristics of the tube which are a function of the target layer and are not significantly affected by the method of deflecting and focusing the electron beam, will be identical t o those of the all-magnetic P.E.1.D.-A

10

250

J . WARDLEY AND F. W. JACKSON

tube. These characteristics: photosensitivity, dark resistance, light transfer curve, spectral sensitivity arid lag, have been described before.2

Resolution The variation of modulation with frequency for this tube is shown in Fig. 4. The measurements were made for a signal current of 0.15 PA, and a series of alternately opaque and transparent bars of equal width in front of a light box provided the various input frequencies. The percentage modulation was measured in the centre and in the corners of the field. The “corners” are defined as the points on the diagonals

0

I

I

I

I

I

I00

200

300

400

500

I

I

I

I

I

75

I50

225

300

375

Llneslpicture width

T V lines

FIG.4. Modulation &s a function of Bpatiel frequency at centre and corners.

of the scan raster 15% of their length from the geometrical corners. I n the centre of the field the modulation a t 250 TV lines is 40% and a t the “corners” is 31%. The resolution of this tube is rather better than would be expected from a comparison of the sizes of the scanned areas of this and the larger, 25-mm tube. Limiting resolution in the centre of the field is just over 450 TV lines, at’ the “corners” it is 380 and in the extreme corners it is still 350 TV lines. There is very little difference in the focus potentials between centre and corners.

Geometrical Fidelity and Beam Landing Errors The geometrical fidelity of the scanned rectangle and beam landing error at the edges of the scanned raster depend on the potential ratio of the target mesh G 5 and the anode G4. As this ratio increases, so do

ALL-ELECTROSTATICVIDICON

251

the geometrical errors, but the beam landing error is reduced. A reasonable compromise is found with a ratio of 1.6 : 1. The geometrical error appears at the corners of the displayed picture as pincushion distortion increasing the picture width and height by 0.5%. The orthogonality of the scans, which depends on the constructional accuracy, can be maintained to within an angle of less than 0.5”. The beam landing error is about 5 V at the extreme edge.

CONCLUSION A miniature all-electrostatic vidicon of an all-brazed rugged construction has been described. The tube could be operated in a camera of only 15 mm overall diameter, suitable for such applications as the inspection of pipes and for use in other confined spaces. ACKNO WLED a MENTS The Authors would like to thank the Directors of Electric and Musical Industries Limited for permission to publish, and Dr. H. G. Lubszynski under whose direction the development took place. Thanks are also due to colleagues, R. D. K. Gilden and Mrs. J. Perkins for assistance during the work.

REFERENCES 1. Lubszynski, H. G., Mayo, B. J., Wardley, J. and Barford, N. C. Proc. Instn Elect. Engrs 116, 339 (1969). 2. Wardley, J., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 211. Academic Press, London (1966).

DISCUSSION w. P. WEYLAND: Have you ever seen any spurious signal due to that part of the return beam which is reflected by the gun diaphragm containing the limiting aperture?In electrostatically focused and deflected camera tubes, theoretically, the return beam travels exactly along the path of the forward beam. In practice however, small deviations occur due to the landing errors and so the return beam “scans” the aperture-determining element of the electron gun of the tube. The return beam there is partially reflected and so some electrons may reach the target, giving rise to a spurious signal which is, as a rule, a weak image of the diaphragm. These effects are generally noticed at high beam currents and low signal currents. J. WARDLEY We have not seen a spurious signal such as you have described which is generated by the return beam, either at high beam currents or low signals. There are several reasons why this does not occur in our all-electrostatic tube compared with the other types of vidicon. In our tube, we form an image of the beam cross-over for scanning and since the beam-limiting aperture is 1.5 to 2.0 om in front of this cross-over, the return beam at this plane is a large defocused circular area. Most of this return beam will probably go through the relatively

252

J. WARDLEY AND F. W. JACKSON

large aperture (0.5mm diameter), and be lost, since the man amplitude of the return beam at this plane will probably be small. Any beam that is reflected back towards the target will not satisfy the existing conditions for refocusing in the target plane. Furthermore the return beam will have to traverse both the G5 and G:, meshes twice before again reaching the target and will be attenuated by a t least 90%.

An Infra-red Sensitive Vidicon With

a New Type of Target H. HORI, S. TSUJI and Y . KIUCHI Toshiba Research and Development Centre, Tokyo Shibaura Electric Co. Ltd., Komukai, Kawasaki, Japan

INTRODUCTION A number of photoconductive materials for the targets of infra-red vidicons have been investigated. Among these materials, PbO-PbS is sensitive to light of wavelengths up t o 2 pm a t room temperature and seems suitable as a target material for the infra-red vidicon. Infra-red sensitive PbO-PbS layers of high resistance were studied by Frank and Raithell and later many attempts were made to use this material for vidicon targets.2-6 Unfortunately, most of these devices showed a somewhat long time-lag in their photoresponse and they were not suitable for transmitting pictures of moving objects. The authors have tried to improve the time-lag of the PbO-PbS target. At an early stage of the work, the targets were made by the method used by Frank and Raithel. According to this method infrared sensitive layers are obtained by evaporating on to a substrate a porous layer of PbO which is then activated by suitable thermal treatment with sulphur vapour. The vidicons with targets produced by this method had good sensitivity in the infra-red region but their time-lag could not be reduced. Subsequently, a novel method of producing PbO-PbS layers has been successfully devised. The infra-red vidicon thus developed shows a very short time-lag, which is the same as, or less than, that of an ordinary vidicon, and is able to reproduce images of fast-moving objects without smearing.

MULTI-LAYERINFRA-RED SENSITIVE TARGET The photoconductor used for a vidicon target must have a dielectric relaxation time greater than 1/30 sec, and hence a resistivity of above 253

254

H . HORI, S. TSUJI AND Y . KIUCHI

1Ol1l2 cm, since the photoconductor is also used as a chargestorage medium. Consequently the energy gap AE of the photoconductor theoretically must be greater than about 1.8eV. On the other hand AE must be less than 1.55eV in order to make the tube sensitive t o infra-red light of wavelength 0.8 pm or longer. Thus it is desired to find a photoconductor of high resistance and narrow bandgap* To satisfy these conflicting requirements a novel method of target making has been developed. Figure 1 shows schematically the structure of the new photoconductive target. It is deposited on a transparent conducting film (Nesa) on a glass substrate which is the window of the tube. It consists of two kinds of materials. One is PbO which has Glass face-plate

//

Transparent signol electrode

PbS lavers

PbO

toyers

Frc. 1. Cross-sectional view of a portion of the target.

excellent sensitivity in the visible wavelength region and has very high resistivity. The other is PbS which has a good infra-red sensitivity but low resistivity. I n order to make a target suitable for infra-red sensitive vidicons, PbO layers and PbS layers are deposited one upon another alternately. The total thickness of this laminated photosensitive layer is from 5 to 20 pm. Figure 2(a) is a scanning electron micrograph of the surface and the cross-section of a portion of a vapourdeposited PbO-PbS multi-layer target is shown diagrammatically in Fig. 2(b). When the infra-red light quanta are absorbed in the PbS layers, electron-hole pairs are generated there and injected into the PbO layers by the high electric field existing in the PbO layers. Infra-red photoconduction thus takes place. As the PbS content increases the

INFRA-RED SENSITIVE VIDICON

255

wavelength threshold shifts toward the longer wavelengths. However the proportion of PbS should be limited because an excess results in low dark-resistance of the layer. In experimental tubes, the proportion by weight of PbS is, for instance, about 10%.

FIQ.2. Electron micrograph of tho surface and the cross-section of a portion of a vapour-deposited PbO-PbS multi-layer target. (a) Electron micrograph. (b) Sketch showing the portion recorded by the electron microscope.

PERFORMANCE CHARACTERISTICSOF THE EXPERIMENTAL TUBE The electrode arrangement and operation of this tube are identical with those of an ordinary I-in. vidicon. Measurements of the following characteristics were made with the standard scanned area of 9.5 x 12.7 mm2 using the scanning rate of 525 lines per frame and 60 fields per second. Spectral Sensitivity Distribution The spectral response of the PbO-PbS vidicon is shown in Fig. 3. For reference, the response curve of the PbO vidicon is given by a broken line. These spectral response curves were obtained using a monochromator. The intensity of the light a t each wavelength was adjusted to generate a constant signal output (0.1 pA) from the vidicon. A thermopile was used for absolute measurement of the light intensity. The peak of the sensitivity is in the visible range (0-5 to 0-6 pm) which corresponds t o that of the PbO vidicon, but the photoresponse extends well in to the infra-red region, decreasing slowly with wavelength. The long wavelength threshold of sensitivity lies a t about 2 pm and shifts a little with varying content of PbS in the target.

256

H. HORI, S. TSUJI AND Y. KIUCHI

T

Wavelength (+m)

Fro. 3. Spectral response.

Target Current versus Target Voltage Figure 4 shows the photocurrent, curves A and B, as well as a dark current C versus the applied voltage. Curve A was measured with white light of colour temperature 2870°K and an illumination of 10 lm/m2 on the tube target, and curve B with infra-red light obtained by filtering the above white light with a Toshiba IR-D1B filter which passes radiation of wavelengths longer than 1 pm. The target current of the PbO vidicon shows saturation with increasing target voltage, while that of the PbO-PbS vidicon appears to be spacecharge-limited, increasing as the square of the applied voltage.

Light Transfer Characteristics The sensitivity of the vidicon can be derived from the light transfer characteristics which show the variation of the photocurrent as a function of the light intensity on the photoconductive target. The light transfer characteristics at a target voltage of 40 V (a dark current of 0.01 FA) are shown in Fig. 5. The light source was a tungsten filament lamp whose colour temperature was 2870'K. I n the case of curve (B) an infra-red filter (IR-D1B) was inserted in the path of the light. The exponent, 7 , of the light transfer characteristic curves is about 0.85 at low light-levels and about 0.65 at high light-levels, that

267

INFRA-RED SENSITIVE VIDICON

Target voltage ( V )

FIO 4. Signal current (A, B) and dark current ( C ) versus target voltage.

0 4-

-a -i

020 I-

E 0 08-

-

-

I

01

0 2

I

I

0406

I I

I

2

I

4

I

6

l

l

I

10

20

_

Illumination on tube face (lrn/m2)

FIG.5. Light transfer characteristics.

is, the y decreases gradually with increasing light intensity. On average this infra-red vidicon gives a photocurrent of about 0.5 pA at a tube face illumination of 101m/m2 for white light and 0.2 pA with the infra-red filter in the path of the Barn0 light.

258

H. HORI, S. TSUJI AND Y. KIUCHI

Sensitivity to Radiation from Hot Bodies Since every body radiates infra-red light, the spectral distribution of which depends on its temperature, the infra-red vidicon is capable of taking pictures of hot bodies which radiate light of wavelengths below approximately 2pm. Experiments made in a dark-room with the infra-red vidicon, have shown that a weak response is obtainable from a body a t a temperature of 15OOC and an image of a hot body above about 200°C can be clearly reproduced.

FIG.6. Monitor display of a brass bar with temperature gradient.

Figure 6 shows the reproduced image of a brass bar in which a temperature gradient was maintained by heating one end with a nichrome wire and cooling the other end with water. The brass bar was coated with carbon black t o obtain high radiant emissivity. The distance between the brass bar and the vidicon camera was 60 cm. Figure 7(a) shows the monitor display of three copper blocks which are heated to different temperatures: 255OC, 260°C and 265°C from left to right. Figure 7(b) shows an oscilloscope display of one horizontal scanning line corresponding to Fig. 7(a). As a result, it is clear that a temperature difference of 5°C can be easily distinguished in this temperature range.

Speed of Response The speed of response was very slow in most of the PbO-PbS infrared vidicons previously investigated. For example, the signal output current rises to only 50 to 60 yoof its steady-state value a t the third field (50 msec after exposure to light) and the residual signal in the third field read-out is 30 to 50% after illumination is turned off and

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259

FIG. 7. Observation of temperature differences. (a) Monitor display of copper from left to right at 255, 260 and 265°C. (b) Oscillogram corresponding t o (a).

about 15% after 1 sec. This time-lag is mainly due to the conduction mechanism of the photoconductor and only a little improvement is possible by increasing the target voltage, or operating the tube a t a high incident light-level. Thus, the infre-red vidicons previously reported seemed to be unsuitable for transmitting moving objects because of their slow response. The new infra-red vidicon has a much improved speed of response;

260

H. HORI, 9. TSUJI AND Y. ISIUCHI

the signal output current rises to about 90% a t the third field after exposure to light and the residual signal in the third field read-out is approximately 10% after removal of illumination.

FIG 8. Time-lag characteristica (the dots are spaced at intervals of 1/60 sec). (a)Rise of photocurrent. (b) Decay of photocurrent.

Figure 8 shows oscilloscope displays of the photoresponses of the improved infra-red vidicon with a photocurrent of 0.2 pA and a dark current of 0.02 PA: (a) the rise characteristic, (b) the decay characteristic. The intervals between the dots correspond t o one field or 1/60 sec. The time-lag measured for infra-red light is almost equal t o that for visible light.

illumination

2nd field read-out

3rd field read-out

OL

I

0.05

I

0.1

I

0.2

Signal current ( P A )

Fra. 9. Dependence of lag Characteristics on the signal current.

Figure 9 shows the time-lag characteristics as a function of the signal current; broken lines refer to a target voltage of 20 V and solid lines to that of 40 V. As in the case of ordinary vidicons, the time-lag of this

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261

vidicon increases with decreasing signal current or light intensity. From these experimental results, it follows that the vidicon with the new type target is suitable for transmitting pictures of fast moving objects in the infra-red region.

Resolution The resolution is found to approach that of an ordinary vidicon. Measurements using the RETMA pattern illuminated with 60 W incandescent lamps have shown a resolution of 600-600 TV lines at the

Fro. 10. Test pattern reproduced by an infrrt-red vidicon with an IR-D1B filter interposed.

centre and 400-450 at the corners. It has also been shown that the resolution remained unchanged when an infra-red filter was placed in front of the tube. Figure 10 is an example of the monitor displays of the test pattern.

APPLICATIONS A useful application of the infra-red vidicon is the viewing of objects illuminated by infra-red light in the dark. Because of the high speed of response, this tube can be used for the observation of hospital patients, various forms of night surveillance and for the processing of photographic films, without any smearing of images. It has also proved useful for detecting infra-red light emitted from

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H. HORI. 9. TSUJI AND Y. KIUCHI

FIG.11. Image of infra-red radiation from an SCR. (a) Firing operation. (b) Steady state operation.

the junction of various semiconductors. Figure 11 shows the instantaneous change from (a) firing operation to (b) steady state operation of an SCR. The image of infra-red radiation from the junction of a conventional silicon rectifier which is forward biased is shown in Fig. 12. Other applications can be cited in the fields of ophthalmic diagnosis, forest fire detection, infra-red terrestrial and air-borne photography, the examination of materials and temperature measurement.

FIG.12. Image of infra-red radiation from a silicon rectifier.

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263

CONCLUSIONS A new infra-red vidicon of fast response has been successfully developed by employing a PbO-PbS multilayer target. The residual signal in the third field read-out is only about lo%, which makes it useful for observing moving objects. The long wavelength threshold is about 2 pm, which is long enough to take pictures of bodies at temperatures as low as 200°C. Besides the PbO-PbS target, multilayer targets of PbO-PbSe, PbO-Sb,S,, etc. have also been investigated and successful results have been obtained. The basic idea of the multilayer target is expected to be very usefuI in developing high performance vidicon targets. ACKNOWLEDGMENTS The authors wish to acknowledge their indebtedness to Dr. T. Okabe, Mr. Y . Nakayama and Dr. K. Kakizaki for many helpful suggestions during the development of the infra-red vidicon, and to thank Mr. T. Sugano for taking electronmicrographs of the targets.

REFERENCES Frank, K. and Raithel, K., 2. Phys. 126, 377 (1949). Morton, G. A. and Forgue, S. V., Proc. Instn Radio Engrs 47, 1607 (1959). Horii, T., Nishida, R. and Okamoto, S.,J. Inst. T V Engrr,Jupan 13,530 (1959). Heimann, W. and Kunze, C . , Infrared Phys. 2, 175 (1962). 5. Heimann, W. and Kunze, C., In “Advances in Eloctronics and Electron Physics”, ed. by J. D. McGee, W. L. Wilcock and L. Mandel, Vol. 16, p. 217. Academic Press, New York (1962). 6. Gaedke, W., Nachrichtentechnik 12, 256 (1962). 1. 2. 3. 4.

DISCUSSION a. w. BATES: Are you familiar with the work of Schottmiller et al. on selenium and bismuth mixtures which gives similar results t o your PbO-PbS mixtures? The numbers of 10I1 Q cm a t about 10% PbS compare quite well with Se-Bi mixtures of similar proportions. One striking difference appears to be that the Se-Bi mixtures contain no selenium-bismuth compounds. H. HORI: Thank you for your comments. The photosensitive layer studied by Schottmiller el al. is a complete mixture of selenium and bismuth, while ours has a multilayer structure of lead oxide and load sulphide. w. M. WREATHALL: Have you any information about the life of these tubes? H. HORI: We have not yet got the final data on useful life, but some experimental tubes have shown no deterioration after 2000 h. z. SZEPESI: How good is the reproducibility of the infra-red vidicon tube? H. HORI: It is quite good. z. SZEPESI: What is the yield? H. HORI: We have no data at the moment. E. F. LABUDA: Do you take any precautions to shield the target from stray light from the cathode? H. HORI: Y e s , we do, We use a low-power heater for the tube which helps to reduce the light from the cathode.

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RCcherche d’un Dispositif Nouveau de TClCvision Thermique F. LE CARVENNEC Cornpagnie BknLrale de TBlBgraphie Sam Fil, Paria, France

INTRODUCTION Des etudes r & ~ e n t e s ~ ont - ~ attire l’attention sup l’utilisation des substances pyroelectriques dans la realisation de detiecteurs thermiques. L’utilisation de telles substances sous forme de mosaique ou de retine devrait pouvoir permettre la rhalisation d’un tube de prise de vue pour t616vision thermique sensible dans tout l’infra-rouge.

Phdmmdnes Physiques UtiEisabEes Au voisinage de leur temperature de Curie, certaines des proprietes Olectriques des substances pyroelectriques: polarisation spontanee, constante dielectrique, dependent fortement de la temperature. Une image infra-rouge focalisee sur une retine pyroelectrique mince (recouverte si necessaire d’une couche opaque B l’infra-rouge incident) y produib un “relief de temperature”. La temperature moyenne de la retine &ant maintenue au voisinage de la temperature de Curie, ce relief de temperature se traduit donc par un relief de polarisation spontanee et/ou de constante didlectrique devant permettre, par une analyse dectronique de type t&levision, d’obbenir un signal video frequence image du rayonnement infra-rouge incident. CHOIX DE LA SUBSTANCE PYRO~LECTRIQUE La substance pyroelectrique plus perticulitmment consideree au cours de l’dtude, a 6th le sulfate de triglycine (TGS), sous forme monocristalline (NH,CH,COOH), .H,SO,. Cette substance a plusieurs avantages. 1. Elle possede une temperature de Curie Idgbrement sup6rieure b la temperature arnbiante (T,M 49’C), et donc particuli&rementifavorable b la realisation d’un thermostat. 2. Au voisinage de la temperature de Curie, elle est trds sensible aux variations de temperature. La phase cristallinerelative aux tempera,tures 265

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infkrieures b la temperature de Curie, est polarishe suivant une seule direction. La Pig. 1 donne les variations de polarisation spontanee en fonction de la temphrature. Au-dessus de la temperature de Curie, la polarisation spontanee disparaft et la substance devient paradectrique. La Fig. 2 donne, en fonction de la temperature, les variations de constante dielectrique relative initiale E r t mesurhes le long de I’axe polaire.

Temperature (“C)

FIG.1. Polarisation spontanbe en fonction de la tempbrature.

3. Elle peut &re obtenu sous forme de lames? monocristallines minces (100 pm), d’assez grandes dimensions (20 x 20 mm2) et it faces perpendiculaires it I’axe polaire. 4. Elle est un bon absorbant de l’infra-rouge. La transmission d’une lame d’kpaisseur 60 pm est pratiquement nulle it des longueurs d’onde superieure it 3.2 pm. La mesure, suivant l’axe polaire, de la valeur de la constante de temps EP du TGS a montr6 qu’au voisinage de la temperature de Curie, cette dernihre est de l’ordre de quelques centaines de secondes. Une telle valeur de la constante de temps n’autorise Bvidemment pas une simple transposition des systhmes d’analyse Blectronique classiques en t6lBvision. I1 importe de noter que la phenomenologie du TGS est semblable it celle de nombreux autres materiaux pyroelectriques et il en rdsulte Ces h e s monocristallines sont r6alis6es dana lea laboratoires du Professeur Hedni.

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donc qu’un systitme d’analyse approprie it une retine en TGS doit pouvojr 6tre adaptee a ces autres materiaux pyroblectriques. Parmi les systkmes d’analyse envisages, deux d’entre eux ont Bt6 plus particulikrement BtudiBs, theoriquement et experimentalement: l’un utilisant dans le domaine paraelectrique la variation de constante didectrique avec la tempdrature, l’autre utilisant dans le domaine pyroBlectrique la variation de polarisation spontanke avec la temperature.

Tempkroture ( “ C )

FIG.2. Constante diblectrique relative initiale q1 (it 1500 Hz) en fonction de 1s ternp6rature.

UTILISATION DE

LA

VARIATION DE CONSTANTE DI~LECTRIQUE

La systitme de lecture experiment6 necessitait de fortes densites de courant de lecture, lesquelles n’ont pu &re mises en oeuvre dans le dispositif experimental utilise et, en consequence, il n’a pas M6 possible d’obtenir des images thermiques de differences d’eclairement inferieures W/cm2. Ce systitnie a BtB, pour l’instant, abandonne au profit B de celui decrit ci-aprits. UTILISATIONDE

LA

VARIATIONDE POLARISATION SPONTAN~E

Structure de la Reline La retine representee sur la Fig. 3 est constituee d’une lame monocristalline mince en TGS, semi-m6tallis& & 1’or sur une de ses faces et collee par sa face semi-m&allis8e sur un support lui-meme depose sur un bloc mdtallique thermostat6 B la temperature To. L’axe polaire est perpendiculaire aux faces de la lame, et la temperature To eat sjustee de fagon & ce que toutes les temperatures du relief de tempdra-

268

F. LE CARVENNEC Thermostat

Support

Collecteur

t Rayonnement inf ra- rouge

Amplificateur

FIQ.3. Structure de le r6tine.

ture soient maintenues B des valeurs inf6rieures 21 la temperature de Curie.

Principe de la Lecture Un obturateur mecanique “hache” 1’6nergieincidente decoupant une succession de phases 6clairees et obscures. Une lecture Blectronique, B l’aide d’un faisceau concentre d’klectronsrapides (coefficient d’kmission secondaire > 1 ) intervient un certain temps a p r h le debut de chaque phase eti tend it ramener le potentiel des points analyses b un potentiel d’equilibre stable voisin de celui d’un collecteur. Les variations de tempdrature produites par le “hachage” de 1’6nergie incidente sont au cows de chaque phase de signes oppos6s. Elles provoquent des variations de polarisation spontanee, donc de charges, Bgalement de signes oppos6s. La lecture des reliefs de charge ainsi cr&s fournit aux bornes de la resistance placke en serie avec la rdtine des signaux vidBofrdquence dont la polarit6 eat donc inversee d’une image B la suivante. Performances Thkoriques L’etude theorique du cycle de lecture a montrB que les diffkrences d’eclairement minimales dbtectables avec un tel systhme Btaient d’autanb plus faibles que la temperature moyenne de la retine Btait voisine de la temperature de Curie. Pour une temperature moyenne de r6tine de 48°C’ un nombre d’Blbments de resolution de l’ordre de

DISPOSITIF NOWEAU DE T ~ L I ~ V I S I O THERMIQUE N

269

200 x 200 points et une cadence d’image rapide, elles pouvaient htre de l’ordre de W/cm2.

TUBED’ESSAI Le tube d’essai (Fig. 4) destine it l’experimentation des systkmes d’analyse e t des retines comporte essentiellement: (i) Deux canons it Blectrons (un canon d’arrosage de toute la surface de la retine, et un canon it faisceau concentre destine it la lecture de la cible). (ii) Une optique

FIG.4. Tube d’essai destine it l’expbrimentation des aystbmes d’analyse Blectronique et des rbtines.

Blectronique. (iii) Deux fenhtres infra-rouge (une fen6tre infra-rouge laterale, associee st un miroir permettant de projeter une image infra-rouge sur la face bombardee par les Blectrons, et une deuxihme fenetre situee sur la face avant du tube et pouvant servir Bventuellement de support de cristal).

Mise en Oeuvre du Dispositif d’Essai Le canon it faisceau concentre a Bt6 seul utilise au cours de ces essais. Le canon d’arrosage a servi it la mesure de l’emission secondaire (Fig. 5 ) et it l’experimentation du systbme d’analyse utilisant les variations de constante dihlectrique avec la temphrature.

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F. LE CARVENNEC I

I I I--- Crislal no. I

/:- -- - -- - -

I 100

,Cristol

I

I

200

300

no.2

inergie des blectrons primaires (eV1

FIG.5. Coefficient d’bmission secondaire du TGS, en fonction de 1’8nergie des Blectons primaires. OQturateur Filtre

Oscilloscope

TGS

Circuits de cornrnande du faisceau et de deftexion

FIU.6. Dispositif utilissnt les variations de polarisationspontanee avec la temp6rature.

Le bloc diagramme du dispositif d’essai est represent6 Fig. 6, Un oscilloscope sert B la fois de generateur de signaux de synchronisation et de rhcepteur. La base de temps de l’oscilloscope est synchronisee sur le mouvement de l’obturateur m6canique 8. l’aide du signal electrique fourni par une photodiode. Les signaux d6livres par lea bases de temps de l’oscilloscope permettent, aprAs mise en forme et

DISPOSITIF

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THERMIQUE

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amplification,la commande de l’ouverture et du balayage du faisceau de lecture. Le signal video-fr6quence preleve aux bornes de la resistance de charge de la retine est, aprbs amplification, applique une image sur deux, b la cathode du tube cathodique de l’oscilloscope permettant de visualiser soit les images de polarit6 positive, soit les images de polarit6 negative. Le dispositif experimental mis en oeuvre, permet une grande souplesse sur le choix des valeurs des differents parambtres: frequence image, frdquence ligne, etc.

RBSULTATS Les photographies (Fig. 7) sont celles de deux images de polarites differentes d’un diaphragme en forme de croix, eclair6 par une source

FIG.7. Images d’un diaphragme m6tallique (en forme de croix) Bclair6 par une source infra-rouge. (a) Polarit6 positive, (b) polarit6 negative. Les diff6rences d’6clairement entre les parties noires et blanches des images sont de l’ordre de W/cma.

infra-rouge. Les differences d’6clairementintervenant entre les branches de la croix et le fond de l’image (partie circulaire) sont de l’ordre de 10-3W/cm2. La frequence image Btait au cows de ces essais de quatre images de polarit6 positive et de quatre images de polarit6 negative par seconde. Le nombre de lignes par image Btait reduib b une cinquantaine, compte-tenu du manque de finesse du spot analyseur et I s faible surface de cristal alors balayde (8 mm en diamhtre). La sensibilite depend de la temperature moyenne de la retine, elle-meme fonction de la temperature du thermostat. NBanmoins, au cours des premiers essais, l’amelioration da sensibilite constat6e en rapprochant la temperature moyenne de la retine de la temperature de Curie n’a pas 6th celle prevue thdoriquement. I1 importe de preciser ces rdsultats. 11 n’est pas douteux que les performances indiquees doivent pouvoir

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&re ameliorees. La sensibilite de W/cm2, en parliculier, ne constitue certainement pas une limite du systeme experimental actuel. D’autre part, d’autres systdmes d’analyse ont 6th envisaghs. L’etude se poursuit.

R~F~RENCES 1. 2. 3. 4. 5.

Chynoweth, A. G., J . Appl. Phys. 27, 78 (1956). Hanel, R. A., J . Opt. Soo. Amer. 51, 220 (1961). Cooper, J., J . Sci. Instrum. 39, 467 (1962). Hadni, A. J. P h p . 24, 694 (1963). Hadni, A., Henninger, Y., Thomas, R., Vergnat, P. et Wyncke, B., J . Phys. 26, 346 (1965).

DISCUSSION z. SZEPESI: La sensibilite de W/cm2 Btait mesurhe B quelle longeur d’onde? Quelle est la longeur d’onde limite? W/cm2 est une sensibilite thkorique, F. LE CARVENNEC: La sensibilite de la sensibilite est independante do la longueur d’oride du rayonnement incident; elle ne depend que de 1’6nergie absorbbe (syst8me “thermique”). L’absorption des retines actuellement utilides est superieure B, 80% dans la bande 3-30 pm. N. J. RARRICK: What is the minimum detectable temperature differencetheoretical and experimental? F. LE CARVENNEC: La difference de temperature minimale detectable depend Bvidemment de 1’8missivit6des objets et des caractBristiques de l’optique utilisbe. Avec une optique d’ouverturef/l.2 et de facteur de transmission 0.5 la sensibilite theorique de W/cm2 correspond, pour un corps noir, 8, une difference de temperature minimale detectable de 0.03”C au voisinage de l’ambiante. Dans ces mhrnes conditions, la limite de sensibilite actuellement mesuree ( W/cm2) correspond B une difference de temperature minimale detectable de 20°C. R . a. LWSZYNSKI: 1. How does TGS behave in vacuo? 2. Were your tubes sealed-off or demountable? 3. You are using fast electrons in your reading beam. Do you find that the heating by the beam is comparable with the heating by the radiation you want to detect? 4. What is your chopping frequency? F. LE CARVENNEC: 1. Un TGS ne degaze pas s’il est maintenu B une tempBrature inferieure B 70°C. 2. Les expBrimentations actuelles sont effectuees en tube dhontable, ces expBrimentations Btaient pr6cBdemment effectuBes en tube scell6. 3. Pour les faibles Bclairements incidents, les Bnergies & dBtecter et les Bnergies transportees par le faisceau de lecture sont effectivement comparables. I1 convient oependant de noter que 1’6nergie apportee par le faisceau est uniformement distribuee sur la surface de la r6tine. 4. La frequence de “choppage” est actuellement de 8 Hz. Des frequences superieures (16 Hz) et inferieures (jusqu’h 1 Hz) ont Bgalement 6th utilisBes.

Un Tube de Prise de Vues Sensible aux Rayons X M. BLAMOUTIER Compagnie Franguke ~ h o ~ s o n - H o ~ tParis, o n , France

INTRODUCTION Les techniques d’utilisation des rayons X pour l’examen de structures de natures diverses ont 6t6 developpees intensivement ces dernihres annkes, notamment par l’usage de dispositifs de visualisation utilisant les tubes a image et la tel6vision. Dans le domaine medical, l’utilisation d’un intensificateur d’image radiologique et d’une cam6ra de television B vidicon eat extr6mement repandue et a presque supplant6 la radiographie classique. Un besoin identique est ressenti dans le domaine industriel: la complexit6 croissante des systhmes rend la notion de fiabilite des composants et des sous-ensembles imperative et necessite le renforcement des methodes de contrble non destructif parmi lesquelles l’examen aux rayons X occupe une place de choix. Dans ce domaine, le codt d’investissement et d’exploitation est un facteur extremement important et la radiographie qui ne permet pas un contrble instantan6 et dynamique se trouve, au dkpart, lourdement handicapbe par rapport Q la radioscopie t616vish Q laquelle l’enregistremenb des images sur magn6toscope fournit une solution d’archivage Bquivalente. La possibilit6 d’un contrble B distance avec des systhmes automatiques de manipulation d’6chantillons est enfin un facteur non negligeable dans la conception d’une installation complhte.

CONCEPTIONDU TUBE Les imp6ratifs impos6s B un tube de ce type par la nature de l’utilisation prevue sont. 1. La necessitd d’utiliser des Bquipements (cam6ra et voies) standards: en l’occurrence, les cameras vidicon de 26mm qui sont le plus largement r6pandues; elles ont en outre l’avantage de permettre la realisation d’installations de dimensions raisonnables. 2. Le tube doit 6tre robuste, de r6glage facile, fiable et de d u d e de vie de l’ordre de 6000 h. 3. Son pouvoir de resolution doit 275

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M. BLAMOUTIER

&re Blev6, compte-tenu de la dimension des pieces it examiner, notamment dans le domaine Blectronique. 4. Son rendu des contrastes doit 6tre excellent. I1 doit possBder une tr&s bonne uniformit6 d0 sensibilit6 et &re exempt de signaux parasites. 5 . Le signal de sortie qu’il dBlivre doit &re suffisamment 6levB pour permettre l’obtention d’un bon rapport signrtl/bruit. 6. La persistance d’image doit &re faible pour ne pas diminuer la cadence d’observation ou la perception des dBplacements. 7. Son champ utile doit 6tre au minimum de 20 mm. Les tubes du type vidicon permettent de respecter un grand nombre de ces impdratifs et l’objet principal des etudes a port6 sur la realisation de couches photosensibles aux rayons X.

VIDICONA RAYONSX Un premier tube a 6t6 realis6 sous la forme du vidicon classique de 26 mm avec fen6tre en verre. Cette fenstre, malgrB tout, presente une

FIG,1. Vidicons 8. rayons X.

absorption non nkgligeable de I’knergie (de l’ordre de 50% pour des rayons de 50 kV)et engendre un rayonnement secondaire prejudiciable 8. la nettete des images. Pour y remkdier, un certain nombre de tubes ont BtB realis& avec des fen6tres de beryllium de 1 mm d’dpaisseur, dont l’absorption eat de l’ordre de 4% pour la meme Bnergie. Des experimentations en laboratoire ont mis en valeur l’int6r6t que prksentait ce tube TH X817, mais ont r6velB l’insuffisance de grandeur du champ utile pour certaines applications (16 mm). C’est pourquoi une version du m6me tube avec un champ utile de 25mm a OtB entreprise tout en maintenant l’imphratif d’utilisation de cameras standards. La Fig. 1 permet une comparaison globale de ces deux tubes TH X817 et TH X832 dont le principe de fonctionnement est strictement identique it celui des vidicons classiques it post-acceldration. Dans le

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275

cas du tube TH X817, le faisceau d’6lectrons est focalise, deflect6 Blectromagnetiquement et collimate par la lentille it grille finale de fapon it obtenir une bonne uniformit6 du potentiel de stabilisation. La surface utile de cible eat de l’ordre de 150 mm2 (12 x 12 mm2 ou 10.2 x 13.6 mma). Dans ce dernier cas, la resolution limite de l’ensemble est de l’ordre de 30 pl/mm, ce qui correspond B une dimension du plus petit Blement detectable de 15 it 20 pm. Le tube TH X832 est constituh par le mbme ensemble canon, utilise les mbmes bobines mais comprend, en plus, un espace de glissement. Cette structure permet d’obtenir, en utilisant le tube dans une camera standard prevue pour lea vidicons de 26 mm it post-acceleration, un champ utile de 335mm2 (18 x 18mm2 ou 15 x 20mm2) avec un pouvoir de resolution de 20pl/mm (Fig. 2). L’espace de glissement

FIG.2. Schema du vidicon sensible aux rayons X.

permet, en outre, pour la puissance de balayage disponible, une augmentation des tensions appliquees sur les 6lectrodes g,, g, et g,, ce qui contribue it diminuer les aberrations et B conserver au tube un bon pouvoir de r6solution. L’utilisation d’une haute tension de 800 V permet d’obtenir une resolution de l’ordre de 25 pl/mm.

Couche Photoconductrice La couche photoconductrice utilisee sur ce type de tube est constituee par une couche mixte de selenium, arsenic et tellure de 20 pm d’epaisseur environ. Elle est deposee par Bvaporation sous vide sur la face interne de la fenetre en beryllium qui a BtB prealablement parfaitement polie avant de recevoir un d6p6t d’or ou de tellure d’environ 2000 A d’bpaisseur.

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M. BLAMOUTIER

L’6tude de telles couches a r6v616 que la distribution des centres de pi6geage et de recombinaison est fort complexe et t r b probablement uniforme dans toute la bande interdite. La conductibilit6 de ces couches est du type-p et la mobilit6 des trous est fortement influencke par les centres divers. Pour des distributions donnkes, une valeur optimale d’dpaisseur a kt6 trouvke; une grande 6paisseur (30 B 100p.m) permet en effet d’accroitre l’absorption des rayons X, mais le temps de transit est alors largement sup6rieur it. la dur6e de vie et la sensibilit6 globale diminue. L’optimalisation de ces couches, qui n’est pas terminke, permet h l’heure actuelle d’obtenir les caractkistiques suivantes. Pour un champ

100 -

-

50-

-

-2 0 ._ c b

D a +

e

s io=K V” 0

10

20

30

40

50

0 60

70

Tension de cible ( V )

FIG.3. Caracteristiquea du courant signal (A) et courant d’obscuritb (B).

de 2-5 x lo4 V/cm, c’est-&-dire une tension de 50V appliquke sur 1’6lectrode de signal, le courant d’obscurit6 est de l’ordre de 3 nA. Cette valeur est suffisamment faible pour que sa variation spatiale n’introduise pas d‘effets parasites nuisant it l’observation de faibles contrastes. La Fig. 3 repr6sente I’allure de la variation du courant d’obscurit6 et du courant de signal en fonction de la tension appliquke sur 1’6lectrode de signal. La Fig. 4 fournit la caract6ristique dii courant de signal en fonction de l’intensit6 d’excitation. La pente de ces courbes correspond B une valeur de gamma pratiquement kgale B 1. Cette caract6ristique montre qu’il eat possible d’obtenir une image de bonne qualit6 pour des doses transmises de 50 B 1000 R/min. La Fig. 5 constitue un exemple de r6seaux de courbes permettant de faciliter les r6glages dans le cas d’observation de pieces en acier.

UN TUBE DE PRISE DE VUES SENSIBLE AUX RAYONS X

10

I00

277

1000

Royonnement X ( 1 0 0 k V ) sur la face du tube (R/min)

FIG.4. Courbe caracteristique vidicon rayons X.

Ioc

0.1

I

10

Epoisseur d'acier (mm)

FIG.6.Courant de signal en fonction de 1'Qpaisseurdu matbriau absorbent (300 R/min).

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M. BLAMOUTIER

Enfin, la Fig. 6 represente l’allure de la variation du signal lorsde l’htablissement et de la cessation de l’excitation. La relativement faible valeur de la constante de temps permet d’effectuer des contr6les dynamiques sur des pieces en mouvement 8. faible vitesse.

-

Excitation rayons X(100kV) b

T r a m balayde

FIG.6. Courbe d’6volution du courant de signal (d6but et fin d’excitation).

R~SULTATS D’EXPLOITATION Des experiences ont 6th rkalisees en laboratoire, notamment avec le tube TH X832 pour tester les possibilites d’exploitation de ce tube. Ces essais ont 6th effectuQsavec un gQn6rateurde rayons X comportant un tube B anode tournante de foyer 1 mm. Le tube Qtait monte dans une camera standard 819 lignes modifiee par l’adjonction d’une haute tension de 800 V, et dont la largeur de bande passante des amplificateurs Qtait de 10 MHz. La face avant Btait blindee par une feuille d’aluminium de 0-5mm d’epaisseur et la distance moyenne des structures 8. la fen6tre de bdryllium etait de 3 mm. L’equipement utilisk permettait en outre d’obtenir le signal video? soit positif, soit negatif. Dans certaines occasions, cette possibilith s’est rQv6leeutile. D’autre part, une modification simple de la camera a permis de faire fonctionner le tube avec des doses reduites par augmentation de la periode d’intdgration, et en lisant par exemple une trame sur 5 ou sur 10, l’image &ant alors observhe sur un tube cathodique rQmanent. Ce mode de fonctionnement permet l’observation de structures tres Bpaisses en utilisant un gdnerateur de rayons X de puissance moyenne, la valeur du signal utile obtenu en fonctionnemenb normal &ant insuffisant pour l’obtention d’une image exploitable. Enfin, pour certains cas d’exploitation avec observation B distance,

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279

Fra. 7. Image de structure d’un transistor.

FIG.8. Image d’un pbn6trambtre.

des essais d’utilisation du tube en analyse lente ont 6tk effectuks et se sont r6vdlds trks satisfaisants pour une cadence de 1 imagelsec. Les Figs. 7 0.1; 8, montrent les rbsultats obtenus dans l’observation de structure d’un transistor, et d’un type de pdnktramktre utilisd pour 1’6valuation globale du systhme.

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M. BLAMOUTIER

DISCUSSION E. B. LABUDA: I n M. BLAMOUTIER:

what X-ray energy range is the tube sensitive? Les tubes d6crits sont sensibles dans u n e gamme d’6nergie d‘excitation de 10 rl200 keV. 11speuvent atre utilis6s dam les contrBles industriels classiques et Bgalement pour l’observation d’organismes vivants de petites dimensions. 11s permettent de visualiser des diagrammes de diffraction en rayons X. w. HEIMANN: What is the highest temperature at which you can operate the tube? M. BLAMOUTIER: Les tubes peuvent fonctionner entre 20°C et 60°C. Pourtant, leur temp6rature normale de fonctionnement est de l’ordre de 30°C. z. SZEPESI: Est-ce que vous pouvez d6tecter un trou de 1 T diamhtre sur un ph6trarnhtre de 2%? M. BLAMOUTIER: Les m6thodes d’apprhciation de pouvoir de d6tection d’un ensemble sensible aux rayons X d i f f b n t suivant les utilisateurs. Dana le dornrtine industriel, il est d’usage d’appr6cier la dimension d’un fil, d’or par exemple, sur un mat6riau de nature et d’6paisseur d6finies. Les essais effectues ont montr6 par ailleurs qu’il est possible de d6tecter un trou de 0.2 mm dans r plaque d’aluminium de une plaque d’aluminium de 0.16 mm d’6paisseur ~ u une 7.6 mm d’6paisseur avec des rayons X de 150 keV.

Adjustable Saturation in a Pick-up Tube with Linear Light Transfer Characteristic J. H. T.VAN ROOSMALEN Philips Research Laboratories, N . V . Philips’ Gloeilarnpenfabrieken, Eindhoven, The Netherlands

INTRODUCTION For television broadcasting, the television cameras make use of pickup tubes in which the signal-current I , has a near-linear dependence on the incident light flux L. This can be described by the equation I, = Ly, where y is roughly equal to 1. Although this linear dependence is very useful for colour reproduction it can sometimes be desirable to build in a signal-limiting mechanism to ensure that very bright lights cannot give unwanted signals. At the illumination level of a normally lighted scene, the maximum photocurrent is a few hundred nA. With a view to controlling slightly higher luminous intensities the beam current I, is given a value somewhat greater than the maximum photocurrent I,. At still higher luminous intensity, however, t.his beam current is insufficient. A built-in “knee” characteristic for these camera tubes would therefore be very welcome. In this paper some proposals are discussed, each with its own advantages, and finally an account i s given of the solution we prefer, together with some concluding notes. The investigations were made using an experimental hybrid PlumbiconT camera tube where the photoconductive layer consisted mainly of lead m0noxide.l The method developed can also be used in other designs of tube using different electric and magnetic fields and/or other photosensitive layers, for which the values of y are not unity. THE EXPERIMENTAL HYBRIDPLUMBICON CAMERATUBE The gun of this tube consists of an electron-emitting cathode, a control grid, an accelerating anode, a focusing lens and a correcting lens with separate mesh for landing correction (Fig. 1).2 A photoconducting layer is applied to a transparent conducting signal-plate.

t

Registered trade mark for colour television camera tubes.

P.E.1.D.-A

281

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J. H. T. VAN ROOSMALEN

282

Depending on the scene, which is imaged optically on to the layer, a positive charge-image is built up on the free surface of the layer. This charge-image is scanned by an electron beam, which is deflected by the coils. During each scan the slow electrons reduce the surface potential to that of the thermionic cathode. This is the so-called cathode potential stabilization. The picture signal appears as the voltage drop across the signal resistor produced by the charge current. I n order to display all details in this charge image with maximum resolution, it is necessary to focus the electron beam to a small spot. The lens action between the cathode, grid and anode focuses the electrons emitted by the cathode at the “cross-over”. By means of the focusing lens this cross-over is

+300V

+70V

FIQ.1. Schematic representation of ail experimental hybrid Plumbicon tube.

imaged on the layer. I n order to obtain a small spot, however, it is necessary to stop down the cathode current I, with the aid of a diaphragm, thus minimizing the effects of spherical aberration of the focusing lens. This also helps to maintain the beam focus at the corners of the picture by reducing the influence of the aberrations of the deflexion coils. The current I , through the diaphragm is then only a fraction, about 1%, of the cathode current I,. The photocurrent I , causes a potential difference between the free surface of the layer and the thermionic cathode. The higher the luminous intensity the greater will this potential difference be, and in exceptional situations it may rise t o as much as 10-15V, depending partly on the choice of the signal-plate voltage and the capacitance of the layer. The pre-set beam current I , will then require more than one scan to stabilize the free surface t o cathode potential and if the highlights in the scene are moving, they give rise to what are known as

ADJUSTABLE SATURATION IN A PIOK-UP TUBE

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“comet tails”. These unstabilized areas may even adversely affect the properties of the layer. Work has been in progress in our laboratories for some time with a view to solving this problem: efforts are being made to give the linear output characteristic a “knee”. This means that, irrespective of the luminous intensity of the scene, the primary electron beam I , will be capable of stabilizing the free surface of the layer to cathode potential upon every scan. If I , is capable of stabilizing a maximum layer potential of for instance 5 V positive to cathode potential, special measures are needed to ensure that either the layer potential cannot rise any higher than this, or that a higher potential is reduced to 5 V before the passage of the electron beam. This corresponds to saturation in the output characteristic. Some possible solutions will now be discussed in turn.

Lower Signal-plate Voltage The potential of the free surface at a given illumination depends only on the capacitance of the layer and on the pre-set signal-plate voltage. For good colour reproduction, this capacitance has to be fixed between relatively narrow limits. An obvious solution is to bias the signal-plate at a low voltage. If we wish to allow the potential of the free surface to rise to only 5 V positive, the signal-plate voltage would have to be only about 5 V instead of the normal 45 V. As a result, however, the field strength over the layer would drop by a factor of 9, completely changing the characteristic of the layer. The stringent studio requirements with regard to sensitivity, speed of response and “burn-in”, for which this layer was specially designed, can then no longer be satisfied. For this reason a low signal-plate voltage is not a satisfactory solution.

Signal Feedback Another possible method is to feed back the signal to the grid of the gun if it should exceed a value of but a few volts. The grid is then made less negative, so that more current becomes available locally and the excessively high potential can be stabilized. Although this system provides some degree of correction it has the drawbacks that (a) the correction comes a little too late, (b) the maximum current through the diaphragm is limited, and incidentally, the cathode is very heavily loaded.

Extra Cathode What is in fact needed is an extra electron source which can provide an uninterrupted stream of electrons over the free surface of the

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photoconductive layer. This extra electron source, which gives a flooding beam, must be able to reduce t o say 5 V all higher potentials that are likely to occur. This residual 5 V can then be stabilized by the primary beam. For this purpose the extra cathode must have a potential 5 V higher than that of the main cathode and a wide beam diameter a t the position of the layer. Since this extra cathode delivers current continuously, the extra signal contribution due t o a high-light will cause only a slight increase in level, being averaged out over a long period. I n practice, however, an extra cathode is found to have the following disadvantages. (a) If the extra cathode is introduced in front of the deflexion field, its beam is deflected and focused simultaneously with the primary beam, as a result of which the extra, unwanted signal is no longer distinguishable from the signal produced by the primary beam. (b) If the extra cathode is introduced in or behind the deflexion field, the electrostatic field is disturbed by the low potential of the extra cathode which can give rise to serious local distortions. Moreover, unavoidable light from the extra cathode causes irregular illumination of the layer, producing in it an extra photocurrent and hence spurious signals. (c) The construction of the tube becomes complicated.

Fly-back Switching To separate the high-light signals from the wanted signals, it is possible t o arrange for the high-light signals t o occur during the time when the primary beam is suppressed, i.e. during the line fly-back. The fly-back time is about 15% of the total line period. Since it remains desirable to use part of this time for clamping the black level, an unused time of about 70% of the line fly-back time remains available. If during this time the gun cathode is raised t o the required potential of about 5 V, then for normal illumination the scanning beam will not land, until the potential on the free surface reaches this same level. During the same time a pulse on the grid of the gun ensures that a higher current flows through the diaphragm. I n principle it is possible also to defocus the electron beam during the fly-back time so as to make the extra stabilization-henceforth referred to here as auxiliary stabilizationeffective well in advance of scanning by the signal generating beam. The disadvantage of this circuit is that the dwell-time of the beam on a picture element is only one-tenth of that for the passage of the normal scan, and that the maximum current through the diaphragm remains limited and necessitates a high cathode load. Although this arrangement could conceivably be combined with that described by the extra cathode method above, the technological objection mentioned still applies. As we have seen, only 1% of the total current normally forms the beam.

ADJUSTABLE SATURATION I N A PICK-UP TUBE

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If, during the fly-back period, a greater proportion of the available current is directed through the diaphragm, then the target, can be stabilized. This can be done by introducing an extra focusing electrode and a number of such experimental tubes have been made in our research laboratories.

The Extra Focusing Electrode The gun in these tubes contains an extra lens electrode which is situated between the cross-over and the diaphragm, as shown in Extra lens electrode +IOOV

Extra lens electrode-OV

FIG.2.(a) An experimental hybrid Plumbicon tube with extra lens electrode during the (forward) line period. The drawn lines show the beam when the extra lens electrode is at the anode potential 300V; the dashed lines when the beam is “prefocused” a t 100 V. (b) An experimental hybrid Plumbicon tube with extra lens electrode during the line fly-back period.

Fig. 2. With an appropriate focusing voltage on this extra lens electrode it is possible t o image the cross-over once again a t the position of the diaphragm. Although this extra focusing lens has considerable aberrations a t this voltage, nevertheless it has proved readily possible to focus about one-quarter of the cathode current through the aperture of the diaphragm. The beam diameter a t the target is so large that part of any high potential can already be reduced before the primary beam reaches it in the normal scan. The current I , flowing through the aperture is shown in Fig. 3 as a function of the voltage applied t o

J. H. T. VAN ROOSMALEN

286 160

1

1

1

1

1

1

1

1

1

1

1

1

1

I

1

1

1

1

l

1

Cathode current

Voltage on the extra lens electrode ( V )

FIQ.3. The current through the diaphragm as a function of the voltage on the extra lens electrode, with the cathode current as parameter. 360 320

-

240

< \c

200

c

F

"=

c

160

a 2

a c

120

80 40

0

2

4

6

8

10

12

14

16

I8

20

22

24

Illumination on the layer (lux)

FIG. 4. Output characteristics of a Plumbicon tube with extra lens electrode showing the adjustable saturation as a function of the cathode voltage during the fly-back time.

the extra lens electrode. For a range of values of the cathode current I , , the extra focusing voltage a t which the transmitted current is highest is seen to be about zero potential. It will now be clear that, if

ADJUSTABLE SATURATION IN A PICK-UP TUBE

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the cathode potential is made 6 V higher during the fly-back period, this transmitted current I , will be available for auxiliary stabilization. The tube is operated as follows: during the line fly-back time a positive pulse on the cathode determines the saturation level and a positive pulse on the control grid ensures the highest possible transmitted current. At the same time a negative pulse on the extra lens electrode ensures a proper image of the cross-over on the aperture in the diaphragm. Experimental tubes operated in this way have an output signal characteristic whose saturation can be adjusted as required. This adjustable “knee characteristic” is shown in Fig. 4 for four values of the cathode potential during the fly-back time.

CONCLUSIONS Prefocusing If the potential in the region of the extra lens electrode is a t 300 V and a focusing voltage of,roughly cathode potential is required on the extra lens electrode to obtain maximum current 1, through the diaphragm, the necessary amplitude of the switching pulse during the line fly-back is about 300 V. From the point of view of circuitry, it is desirable to use a smaller pulse amplitude, partly because of capacitive cross-talk. It has proved possible to construct an extra lens electrode which, depending on its configuration and location in relation to the cross-over, has only a small effect on the beam when its potential is varied between 300 and lOOV. Although, in theory, lowering the potential of the extra electrode to 100 V will give a slight increase in spot size, it is found that no deterioration in the total spot diameter of the primary beam can be observed. Because of this we can apply this “prefocusing” potential of 100 V during the forward line period without detriment to the sharpness of the picture. With this configuration a switching pulse of only 1OOV amplitude is needed, which is easily obtained. Edge Distortion Due to Beam Bending Due to the applied signal-plate voltage of 45V, the free surface of the photoconductive layer at those places outside the scanned raster which are not reached by the primary beam, will charge up to this potential. This high potential attracts the primary beam towards the edges of the rectangle, causing a landing error in the primary beam, which in turn can give rise to a variety of errors.? If the solution described above is adopted, the edges of the picture are overscanned by the wide beam during the fly-back time. Consequently, the surround-

t See p. 237.

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J. H. T. VAN ROOSMALEN

ings of the scanned rectangle on the free surface of the layer are kept a t a potential of 5 V positive. This low potential causes virtually no oblique landing of the primary beam. ACKNOWLEDGMENT I am greatly indebted to Mr. J. C. Vermulst of Philips’ Elcoma Division for his contribution to the discussions and for the enthusiasm with which he tackled the circuitry aspects of this problem.

REFERENCES 1. de Haan, E. F., van der Drift, A. and Schampers, P. P. M., Pltilipa Tech. Rev. 25, 133 (1963/64). 2. van Roosmalen, J. H. T., Philip8 Tech. Rev. 28, 60 (1967).

DISCUSSION J. A. LODGE:

Are any ill effects found from ions generated by the enhanced

beam during fly-back? J. H. T. VAN ROOSMALEN: We have not found any disadvantage from ions generated using this extra lens electrode method. A. S. JENSEN: Your demonstration clearly shows that the technique you describe is successful. However, I do not understand why the enlarged beam spot size during fly-back does not erase the yet unread signal charge ahead of the reading beam. What have you done to prevent this loss of signal? J. H. T. VAN ROOSMALEN: Simultaneous with the refocusing, the cathode is raised in voltage so that the electron beam cannot land on areas of the storage surface whose potentials are within the proper dynamic range of the camera tube. W. E. TURK: Is it possible to draw a new light-transfer characteristic for the experimental Plumbicon? J. R. T. VAN ROOSMALEN: As you can see from Fig. 4, the linear part of the light -transfer characteristic is unchanged and the saturation can be controlled as desired by adjusting the cathode potential during the fly-back time. J. WARDLEY: In the experimental Plumbicon tubes described, do you employ an image of the beam cross-over or of the limiting aperture in the gun for scanning the target? I f the former, is the resolution worse than in the normal Plumbicon? J. H. T. VAN ROOSMALEN: I n the experimental hybrid Plumbicon tubes the cross-over of the beam is focused on the target. The resolution in the centre of the picture is as good as that of the standard Plumbicon tube and better in the corners of the picture.

Measurement of TV Camera Noise A. S. J E N S E N and J. M. FAWCETT Westinghouse Defense and Space Center, Baltimore, Maryland, U . S . A .

INTRODUCTION The measurement of noise a t the output of a TV camera, or its camera tube, has always been a problem.1-5 The TV system requires a scanning waveform that has a maximum useful scan time and a minimum return time. Signals including noise usually appear different when read from the storage surface during return scan from what they do during the forward scan. This is particularly true if there is significant dark current, for then the average signal level is different. Hence, noise measurements cannot be made in the usual manner with a simple thermal-power meter. In fact, the video signal appearing on a kinescope is not a valid signal so that conventionally the return signal is blanked t o a constant black level. For a physical measurement of noise by a thermal-power meter these blanking pulses and all spurious signals during return must be gated out, but by a method which introduces a switching signal power which is insignificant compared with the noise power to be measured. Two methods of gating have been used t o date, both of which have been somewhat unsatisfactory. Ordinarily the video signal is displayed on an oscilloscope (A-scope); the experimenter estimates by eye the double amplitude of the visual envelope of the noise, excluding in his judgment shading, blemishes and other disturbance signals which form a fixed pattern stationary with time, and hence are not noise, unless it is his intention to include these. He may compare this measurement with the same type of measurement using a noise generator as ti source or he may simply divide his voltage measurement by six and call the quotient the r.m.s. noise voltage. The IEEE standarde for measuring camera noise is similar, but the eye of the experimenter is replaced by a spot photometer which measures the amplitude density distribution, including high frequency disturbances, in the small region selected by the eye of the experimenter. The r.m.s. noise voltage is calculated from this distribution. 288

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Though probably reliable and repeatable, these measurements are not as convincing or as satisfying as a thermal-power meter-measurement. The IEEE standard measurement is too lengthy and complex to be generally used in the laboratory. To date satisfactory video gating circuits have not been possible since the input to output capacitance of available tubes and transistors has been so high that undesirable spikes on pedestals were injected that contained too much power compared with the noise to be measured. Satisfactory transistors are now available so that camera noise measurements can be made by direct means. It is advisable that the standards for camera noise measurements be revised accordingly.

VIDEOGATINUCIRCUIT The gating circuit performs synchronous blanking of the video signal.

This blanking can be controlled so that noise from small intervals of +V

t Video in

Mixed blank

Gated video out

-

Multivibrator (delay width)

1

Multivibrator (sate width) I

FIG.1. Gating circuit for noise measurement.

the forward scan can be selected for measurement during different portions of the active scan time. These intervals can be rather small, and arbitrarily located within the raster, whilst still giving accurate information of the true r.m.8. noise of the system. However, to explain the technique used, the noise present in a vertical strip of the raster will be selected. Therefore during each horizontal line, a portion of the noise will be passed while all other signals are attenuated. The circuit for selecting and passing this strip of noise is illustrated in Pig. 1. The synchronizing signals of the camera system are used to trigger variable delay, one-shot multivibrators. These in turn drive a transistor switch. The transistor is normally saturated, therefore it attenuates all

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signals passing through the resistor R. The horizontal sync pulse triggers the delay multivibrator which drives the gate multivibrator which turns the transistor switch off, thus passing the video noise for the portion of horizontal time selected by the multivibrators. If the video signal at the time the transistor is turned off is a t the same d.c. level as the level when the transistor was turned on, then no stray signals would be introduced to the video by the blanking circuit. However, to achieve this same level, the video signal would have to be capacitance coupled, and a d.c. potentiometer used to adjust the d.c. level. This technique would require adjustment each time a new system was measured, and would be critical at low signal levels. Since this adjustment is undesirable the technique shown in Fig. 1 was used. The circuit is essentially a clamping or d.c. restoration circuit common in many television systems. The purpose of the circuit is to develop

FIG.2. Wave forms in gating circuit.

a d.c. charge across the capacitor shortly before the transistor switch is opened. Once the switch is opened the impedance seen by the capacitor is kept very high so that the d.c. potential is maintained across the capacitor. The effect of this procedure is illustrated in Fig. 2. The RC product has to be kept small so that the capacitor will reach full charge shortly after any transient signals. However, the time constant should not be too short. If the RC time constant is comparable with the reciprocal of the noise bandwidth, then the probability that the noise will have some d.c. component before the switch is turned off is high. This effect is sometimes referred t o as clamping noise, i.e. the d.c. level of the gated noise is not equal t o that of the ungated video signal, but varies randomly from gate to gate depending on the noise charge stored on the capacitor before the switch is opened. Therefore, the RC time constant has to be long enough to ensure that the noisecharge fluctuation on the capacitor averages to some small value of the original r.m.8. value. This time constant compromise was empirically

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J. M.

FAWCETT

determined to be about 10psec (T= 2.2RG) for 5 MHz noise and standard television scan rates. The transistor is selected for fast switching and low input to output capacitance, so that switching transients are small. Figure 3 is a photograph of a dual-trace oscillograph showing one line of video Aignal taken from 3 successive TV frames. The lower trace (b) is the output signal from a TV camera with the lens capped, showing noise and horizontal blanking pulses. The upper trace (a) is the same, but gated so that the output signal is attenuated except for a selected time duration in the middle of the line. These are the signals that appeared at the terminal labelled “gated video out” in Fig. 1. At this point the horizontal blanking pulse amplitude is 0.35V, and the noise

FIa. 3. Video signals from three successive frames with lens capped. (a) After gating. (b) Before gating.

FICA4. Video signals between two successive vertical blanking pulses. (a) After gating. (b) Before gating.

is 7 mV r.m.s. This corresponds to a noise current at the input of the pre-amplifier of 2 nAr.m.s. Figure 4 shows the same signals on the dual-trace oscilloscope displayed between two successive vertical blanking pulses. I n the lower trace the horizontal sync and blanking pulses run together to give the effect of the lower two apparently continuous lines with small dips during the vertical blanking. Note that in both figures switching transients and d.c. shifts are imperceptible.

MEASUREMENTS The noise power indicated on the meter must be corrected for the duty cycle of the gate. This can be done quite accurittely if the gate width is determined by a high speed counter that operates at a frequency near the top of the video passband. There are, of course, some small but finite switching spikes and low

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level leakage signals so that the most accurate measurements are made a t the output of a linear amplifier of high gain. To verify the accuracy of the measurements made using the gating circuit, white noise having a bandwidth of 5MHz from a General Radio Type 1390B random noise generator was gated with various duty cycles and measured. The results are plotted in Fig. 5 and constitute a set of calibration curves. For a large duty cycle and large noise input, the expected accuracy is achieved, but for a small duty cycle (less than 1%) a high amplifier gain is required, but this is limited by the dynamic range of the gate to 0.32 V r.m.8. of noise signal at the input.

FIQ.6. Calibration curves. Noise output as a function of noise input and duty cycle.

A thermal-power meter measures the total signal power so that it does not distinguish between noise and disturbance, i.e. the fixed pattern, stationary in time, at the output of the camera. This latter, the disturbance, includes shading, blemishes and other variations in the signal from the camera when its lens is capped. If the experimenter wishes to exclude the disturbance from his measurement and confine it entirely to time-varying noise, he must reduce the duty cycle to a small amount, probably with the gate operated to select a small area by being gated in both horizontal and vertical directions. He then locates the gate in an area of the raster that he has determined from a kinescope to be reasonably free from low frequency disturbance. Alternatively, he could search the raster with the gate to determine the lowest output power. This general procedure is not unlike that now

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followed by other methods. It does not exclude high frequency disturbance; probably only successive frame cancellation methods can succeed in doing this. Recent experiments with magnetic disks hold out hope of success in this respect. If they are successful, so that a separation of noise and disturbance may be achieved, then noise and camera-tube disturbance-pattern may both be measured objectively. This would, indeed, be a step forward in the art.

CONCLUSIONS Since there are available today transistors with sufficiently low input to output capacitance, it is quite possible to build a video gate which gates out the blanking pulses at the output of a TV camera or camera tube. This makes possible the direct measurement of noise by means of a thermal-power meter in the conventional manner. This is sufficiently simple to be standard equipment for every TV laboratory so that there should no longer be any need of estimating noise by eye on an A-scope. The various industrial standards for the measurement of camera noise should now be revised to make them similar to the standards for the measurement of noise in other circuits. REFERENCES 1. Holder, J. E., Television Engineering. Report of the International Conference of the Institution of Electrical Engineers 1962, p. 20 (1963). 2. Weaver, L. E., BBC Engineering Monograph No. 24 (1959). 3. Edwardson, J. M., BBC Engineering Monograph No. 37 (1961). 4. Broderick, P., Marconi Imtrumentation, 10, 18 (1965). 5. Rye, J. H., Electronic Equipment News (May 1967). 6. I.R.E., Standards in Electron Tube Methods of Testing, 62 IRE, 7.51, p. 128 (1962).

DISCUSSION M. ROME: Have you, using your method, been able to determine whether the conventional method of measuring S/N ratios (observation of peak-to-peaknoise) is most accurate using the usual factor of 6 or perhaps a smaller factor? Since your method is not subjective, does comparison with subjective determinations show much variation in the latter. A. s. JENSEN: We did make a comparison using two subjects who were skilled and experienced in the measurement of noise from cameras and camera tubes. Their measurements and those of the meter agreed within the rated accuracy of the thermal voltmeter used. We do not claim that the gated meter is necessarily any more reliable than a skilled observer, but it is certainly more satisfying than a measurement that depends upon human acuity. Our observers find the factor of 6.0 good; other observers report factors as low as 5.6. This variation depends upon the observer’s contrast acuity. P. H. BATEY: We have examined the output of one of the commercial TV noise measuring equipments which uses valve circuits to switch out the blanking

MEASUREMENT OF NOISE IN TELEVISION CAMERAS

295

pulses from the video signal. Although there were no obvious switching spikes in the noise waveform when it was viewed on an oscilloscopein the manner mentioned in the paper-a search through the spectrum from line frequency to 5MHz revealed power peaks at harmonics of the switching frequency up to 300 kHz. The equipment used for the search was of the “Weaver” type.2 This is a more sensitive test for switching transients than visual examination of the waveform with an oscilloscope. A. s. ZENSEN: In this paper we do not claim to be the first to measure TV camera noise by gating out the blanking. Rather we mean to emphasize that with today’s transistors any skilled electronic circuit engineer can design and build a highly satisfactory, simple, gating circuit that permits an accurate measurement by a thermal voltmeter. Hence our TV camera and camera-tube standards should be changed to exploit this, and laboratories should cease using the method that depends upon human acuity. The existence of prior literature only serves to support this view. We are indebted to Mr. P. H. Batey for these references to prior work which we now gladly acknowledge. 11. Q. FREEMAN: A technique based on this principle was described in a BBC monograph some 4 years ago. By using the thermal-power meter as a null device in conjunction with a calibrated osciIlator, it is possible to obtain p.t.p. signal/ r.m.8. noise ratios directly. How do your results compare with those of the BBC device? A. s. JENSEN: We were not aware of the BBC device so we have no comparison. We are indebted to you and Mr. Batey for calling our attention to these prior works.

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An Electromechanical Picture Signal Generating Device A. BOKSENBERG and A. C. NEWTON Mullard Space Science Laboratory, Physics Department, University College, London, England

INTRODUCTION This paper describes a new technique for reading information in the form of a spatial distribution of charge.'B2 The basis of operation is similar to that of the vibrating reed electrometer; a charge image previously produced on the surface of a dielectric layer by direct photoemission, photoconduction or other means is read out by closely scanning it with a rapidly vibrating conducting probe, the direction of vibration being normal to the surface. Alternating currents of equal magnitude are induced in both the probe and a conducting backing to the layer and are directly proportional to the charge density in the vicinity of the probe at any point in the scan. Phase-sensitive detection, operating on the induced current, in principle allows the system bandwidth to be reduced to the point where the limiting signal-to-noiseratio is predominantly due to the statistical fluctuations in the charge image itself. This is a basic advantage of the non-destructive reading process. Furthermore, images may be read repeatedly during or following exposure without detriment to the stored information. Another basic advantage of the technique follows from the total avoidance of electron optics. This allows the receptive area t o assume almost any required extent and contour and makes the system particularly appropriate for use in spectrographic instruments whose focal surfaces are often long and curved. A one-dimensional variation of image is assumed here, such as may be obtained in a spectrogram. A two-dimensional system is no different in principle but was excluded from this preliminary study because it requires greater mechanical sophistication.

SIGNAL GENERATION Referring to Fig. 1, consider a pattern of free charge having onedimensional variation residing on the surface of an insulator of uniform 297

298

A. BOKSENBERG AND A. 0.NEWTON

thickness backed by a conducting electrode. The free charge surface density at position x is defined as ~ ( x ) .C2 and C, (X, x, t) are the respective capacitances per unit area of surface at x to the backing electrode and to a semi-infinite conducting probe at X held above the surface and vibrated normal to it. A potential difference V exists between the backing electrode and the probe. The instantaneous charge on the elemental capacitance C, (X, x, t ) dx, being the appropriate I

dbl

I

I

I

I

FIG.1. Principles of signal generation.

proportion of the free and induced charges on a surface element at x of width dx and unit length normal to the x-direction, is

If dip and di, are the respective currents flowing from the surface element at x to the probe and the backing electrode, d dip = - di - - (dql(X, x, t ) ) . - dt Integrating over the whole of x, we have for the magnitude of the total current to the probe or backing electrode from a surface strip of unit width m

299

ELECTROMECHANICAL SIQNAL QENERATINQ DEVICE

Thus the signal current is periodic with the frequency of the probe vibration, its amplitude is proportional to the surface charge density and to the rate of change of C , ( X , x, t ) and it may be detected either at the probe or the backing electrode. Also, i t can be shown that the process of signal generation is most efficient when C2 and the mean value of C , ( X , x, t ) over one cycle arc approximately equal. V may be a contact potential difference, a potential difference deliberately applied t o back off the signal, or a combination of the two. In the work described here V was maintained at zero. SPATIAL RESOLUTION The spatial resolution of the system is related to the probe-tosurface capacitance and this depends on the probe configuration used. A simple type of probe is a thin conducting plane held normal to the surface terminating in a straight edge parallel to it and normal to the x-direction as assumed in the previous section. The resolution may be expreased in terms of the modulation transfer function of the system which is related to the spread function, i.e., the response to a line of charge parallel to the probe edge. Assuming such a line image contains K units of charge per unit length at x and zero elsewhere, we obtain from Eq. (3) (with V = 0)

This equation was used to calculate the resolution for various probeto-surface separations and amplitudes of vibration. The results of

-s

100

-c

0 -

U =I

8

50

-

I

0,

2

I

I

I

I

5

10

20

50

I 3

Spatial frequency (Ip/nrn)

FIG.2. Computed modulation transfer functions for f i s t and second harmonic signals and various probe-to-surfaceseparations.

300

A. BOKSENBERG AND A. C . NEWTON

some computations for the first and second harmonic signal components are reproduced in Fig. 2 for vibration peak amplitudes approaching the mean probe-to-surface separation. Prior computations had confirmed the intuitive result that for a given probe-to-surface separation the resolution improves as the vibration amplitude is increased. I n each case, the second harmonic component gives a, modulation transfer function that exceeds 100% for some line frequencies, and shows much improved resolution over that attainable with the first harmonic. The hump in the modulation transfer function arises from a negative region in the second harmonic spread function which begins approximately at the point of inflexion in the form of the spatial variation in probe-to-surface element capacitance, C,(X, x); the positive curvature in the region beyond this point leads to the negative region in the second harmonic component.

TRIALAPPARATUS An experimental system was built in order to prove the technique and investigate its characteristics.l Photoemission excited by ultraviolet radiation was the method primarily used to produce a sudace charge image since spectrometry in the vacuum ultra-violet was an application of particular interest. The image storage area, consisting of a dielectric spacer (an adhesive polyester film) separating a photoemissive surface layer and a backing electrode, was mounted on the front surface of a Perspex block which was fitted into a stainless steel plate. In operation, a mesh was held above the surface and maintained at a positive potential relative to it to collect the emitted photoelectrons and was subsequently drawn aside by a solenoid to allow the vibrating probe unit to scan across the charge image for reading. Scanning was achieved by a rack and pinion movement driven by a small electric motor and a reduction gearbox sealed in a canister to avoid outgassing, the whole apparatus being operated in vacuum. The probe unit, shown in Fig. 3, supported by three p.t.f.e. pads, was spring-loaded against the steel plate which was therefore the reference plane for the scanning motion. The probe was vibrated by means of a flexing cantilever made by cementing a strip of piezoelectric ceramic to a metal strip of comparable strength. Used near its mechanical resonance at approximately 400 Hz,this transducer had an amplitude response of 30 pm per 100 V r.m.s. applied. The probe itself was a gold film evaporated on to one face of a Perspex prism cemented to the transducer as shown. The images were erased by a simple electron gun, consisting of an indirectly heated cathode and an accelerator mesh, mounted on the scan carriage and operated in the cathode potential stabilization mode.

30 1

ELECTROMECHANICAL SIQNAL QENERATINQ DEVICE

The signal current produced by the action of the probe was passed to a sensitive current amplifier and then to a phase-sensitive detector system used to detect the first and second harmonic components, the phasing reference being provided by the transducer oscillator. The integration and storage characteristics of the device were measured for saturated photoemission using radiation of 1216 8. Various photoemitting layers were used, including magnesium fluoride and caesium iodide. The signal increased linearly with exposure over more than three orders of magnitude and image half-lives of 470 h and 1-5 h, respectively, were obtained for these two materials. Perspex prism \

Gold electrode

Piezoelectric ceramic

Spring steel (250 p m )

Section through XX

t

--- -- - - ---- --- - -_

wire

\

3r-

I4 m m

L

__ -@ __ - - -

I

I _

Platinum wire

(50pm)

A full study of the parameters affecting the sensitivity of the system has not yet been made. However, some preliminary and un-optimized first harmonic measurements have indicated the detection of 100 electronic charges, with a signal-to-noiseratio of 3, in an image element of area approximately cm2, using a vibration frequency of a few kHz and an effective bandwidth of 1 Hz. This is to be taken as an indication of performance only: an amplifier having lower noise than the one used, an increased vibration frequency, a decreased system bandwidth and an improved mechanical performance would all serve to lower the detection threshold.

302

A. BOKSENBERO AND A. C. NEWTON

RESOLUTION MEASUREMENTS Measurements of the resolution properties of the system were made using a charge image in the form of a step function, achieved by terminating the storage layer in a sharp, straight edge and charging it by means of a /3-ray source. The probe was aligned with the edge and repeated scans were made across it using different values of mean separation and vibration amplitude. The known input function and the resultant output functions were then used to construct the corresponding spread functions and modulation transfer functions using Fourier transforms.

5.0

IC 0

Spatial frequency (Ip/mm)

FIQ.4. Measured modulation transfer functions.

The measurements agreed well with the computed data for mean separations greater than 5 0 p or so, but, due t o the mechanical deficiencies of the trial equipment, become increasingly worse for values below this. The best resolution actually obtained is indicated by the curves in Fig. 4 pertaining to the smallest value of mean separation used, nominally 16 pm. As predicted by the computations, the resolution is improved (a) by increasing the vibration amplitude and (b) by taking the second harmonic component rather than the fist. There appears to be no fundamental difficulty in improving the resolution by more careful design and manufacture of the simple probe system described. I n addition, further improvement may be achieved by the use of guard electrodes on each side of the probe to

ELECTROMECHANICAL SIGNAL GENERATING DEVICE

303

narrow the form of C1(X,x) and by servo-control of the mean separation and vibration amplitude of the probe.

GUARDEDPROBE In order to investigate the effect of guard electrodes on spatial resolution, a simplified, scaled-up apparatus shown in Fig. 5 was built to measure the relative variation in capacitance of a representative practical probe, guarded (Case A) and unguarded (Case B), to a surface ,Conducting probe

With guard Insulator Without guard

Surface electrode

Guard electrode

----

Case C

-Insulator

r-7-7-

Thin probeconductingf$

th -d

I / &

Insulator

-I---

+d

FIQ.6. System geometry for guard electrode studies (dimensionsin mm).

electrode embedded in an insulating block over which the probe was scanned but not vibrated. A thin, plane unguarded probe was also included for comparison (Case C). All dimensions were in mm for the apparatus used. Figure 6 gives some results obtained for the three cases and Fig. 7 shows the variation of full-width half-height values with the distance of the probe to the surface electrode. Although the adopted geometry is

304

A. BOKSENBERB AND

A. C. NEWTON

Re1ative capacitance Drobe-surface electrode

Case C

I

I

I

-10

0

10

Probe position ( d )

Fro. 6. Measured relative variation in capacitance between probe and surface electrode.

I

2

Distance, probe- surface electrode (b)

Fro. 7. Measured variation of full-width half-heightcapacitance values with distance of probe from surface electrode.

somewhat arbitrary, the advantage of using guard electrodes and the unsuitability of the thin plane probe are well indicated by these results. STABILIZING RESOLUTION AND SENSITIVITY Servo-control of the mean separation and vibration amplitude of the probe has already been mentioned in connexion with resolution. It is also a means of stabilizing sensitivity, a feature which likewise becomes

ELECTROMECRANfCAL SIGNAL UENERATING DEVICE

305

increasingly necessary with increasing separation. Such servo action may be realized simply by applying a correcting bias to the piezoelectric transducer to stabilize the mean separation and controlling the transducer drive oscillator t o stabilize amplitude. One possible way of measuring the two parameters to be controlled is by the use of a conducting strip deposited along the edge of the storage surface with an overlaying subsidiary probe mounted on the transducer. By maintaining a potential difference between them and measuring the first and second harmonic components of the resulting signal current, the mean separation and vibration amplitude can be uniquely determined. In applications where the passive mechanical performance of the system is adequate for resolution but inadequate for maintaining a sufficiently constant sensitivity, the latter may be acceptably stabilized by means of negative feedback. Assuming, for the moment, that the free charge density is constant over the surface and I ( X , t ) is made zero in Eq. (3) by adjustment of V , then u = C,V.

(5)

Thus u is obtained from V directly and the system is relieved of all dependence on C , ( X , x, t ) . Although this special case will rarely occur in practice, the result remains approximately true when the spatial variation in u(x)is considerably less rapid than the variation defining the limiting resolution capability of the system.

CURRENTWORK In collaboration with the Atomic Weapons Research Establishment, the National Research Development Corporation and Messrs. Hilger and Watts Ltd. the device is being developed to fit a Hilger and Watts two-metre grazing incidence vacuum spectrograph. An illustration of the proposed apparatus is shown in Fig. 8; it is designed to operate over the wavelength range 5 to 250 A using a curved photocathode 10 in. long. As an additional function, the system will be capable of time resolution by gating the collecting grid from negative to positive potential relative to the photocathode layer in order to alternately suppress and saturate the photoemission as required. In collaboration with the Royal Greenwich Observatory, a form of the device is being developed for spectrometric measurements in groundbased astronomy. Photoemission is less convenient for application in the visible than in the vacuum ultra-violet. In the latter region, unlike the former, many insulators are to be found that are both useful photoemittcrs and unaffected by exposure to the air. Photocathodes sensitive t o visible light, being conductors, would have to take the form

306

A. BOXSENBERG AND A. C. NEWTON

of a mosaic deposit on an insulating substrate (as in the C.P.S. Emitron) and be kept continually evacuated, itself an operational inconvenience. For these reasons, a photoconductive layer, used in air, will take the place of the photoemitter. The material chosen for this purpose is vitreous selenium, which has a spectral response similar to that of the predominantly blue-sensitive photographic emulsions commonly used in astronomy. By applying an electric field through the layer during exposure a surface charge image is produced by hole migration. By

FIQ. 8. Illustration of the Hilger and Watts two-metre grazing incidence vacuum spectrograph with the new image detector in place of the photographic plateholder (by courtesy of A.W.R.E.).

cooling t o near O'C, an image half-life of about 20 h has been obtained, which is entirely adequate for this application. The quantum efficiency of the layer is dependent both on its temperature and the field a ~ p l i e d . ~ A field of about lo5 V/cm is required to give a peak quantum efficiency of unity at 0°C; methods of achieving this field without breakdown are a t present being investigated. REFERENCES 1. Boksenberg, A., Boyd, R. L. F. and Jones, J. C., Nature 220, 556 (1968). 2. Boyd, R. L. F. and Boksenberg, A., U.K. Pat. Appl. No. 17208/65 (1965). 3. Xerox Corp., Rochester, N.Y. (private communication).

ELECTROMECHANICAL SIUNAL UENERATING DEVICE

307

DISCUSSION H. O. LUBSZYNSKI: 1. Were the modulation transfer functions in Figs. 2 and 4 calculated or actually measured? 2. Have you any photographs of actually evaluated spectrograms? A . BOKSENBERO: 1. The modulation transfer functions in Fig. 2 were calculated; those in Fig. 4 were evaluated from measurements of a charge image in the form of a step function. 2. No. O . 0. TOWLER: The device described is sensitive in the ultra-violet. In view of the mechanical scanning, how do you intend to introduce high quantum efficiency surfaces for the visible end of the spectrum? Also could you please quote the length of the storage surface, for if the probe spacing is as little as 5 pm as quoted, an angular aligning accuracy of 20 seconds of arc would appear to be required for scanning a surface only 5 cm long? Is this precision easily obtainable? A . BOKSENBERU: I n the visible, we are investigating the use of vitreous selenium as a photoconductive detector, for which a vacuum system is not required. The required angular aligning accuracy depends only on the length of the probe. The probes we have used are about 1 cm long and have not been difficult to align to the required precision of 1 pm or so. The length of storage surface scanned need not be ri factor in this, since the probe moves on a carriage that can use the storage surface as a reference. R. w. AIREY: What do you consider to be the required mechanical precision in the motion of the probe across the surface of the charged plate for acceptable signal-to-noise ratio? A. BOKSENBERU: About 0.1 p m for 10 pm mean separation and an uncertainty in signal of 1%. As described in the text, if necessary this can be achieved by servo-action or alternatively the sensitivity may be stabilized with negative feedback. s. MAJUMDAR: 1. What is the minimum charge you can record? 2. What is the thickness of the "target"? A . BOKSENBERO: 1. This depends on various factors including the signal bandwidth, vibration frequency and the probe separation and amplitude. The figure quoted in the text is 100 electronic charges per image element for the defined conditions. I believe this could be decreased to about 10 electronic charges. 2. Typically 10 to 50 pm.

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Effects of Caesium Vapour upon the Target Glass of Image Orthicons M. HIRASHIMA and M. ASANO Department of Electronic Engineering, Univemity of Electro-Communicationa, Chofu City, Tokyo, Japan

INTRODUCTION The present writers have reported in a previous paper1 on an interesting phenomenon observed during the course of a study on the effects of caesium vapour upon the electrical resistance of one of the glasses (Corning type 0083) for image orthicon targets. Since the publication of that paper, new experimental results have been obtained but some problems remain to be solved before a full discussion of the mechanism of the e.m.f. generation can be given. This paper, therefore, is the second interim report, in which only some of the experimental results obtained so far are described. Throughout the present paper reference will be made to Corning 0083 glass as the target glass, except where otherwise stated. Other kinds of glasses also are now being investigated, and they will be compared with Corning 0083 glass in a future paper.

INFLUENCE OF THE SURROUNDING MEDIUMUPON THE SHORT-CIRCUIT CURRENT CHARACTERISTICS Throughout the experiments described previously the medium surrounding the experimental tubes was air. As is well known, sodium ions play a major role in the conduction of electricity through the glass under investigation. Therefore, the sodium ions that were produced either by electrolysis of the sodium oxide due to the current flowing through the glass or by the reduction of the sodium oxide by caesium, may be transported through the body of the glass from the inner to the outer electrode when the short-circuit current flows from the outer electrode towards the inner one through the external circuit. The concentration of sodium atoms in the glass in the vicinity of the outer electrode will thus increase with time. 300

310

M. HIRASHIMA AND M. ASANO

It is not at present known whether or not all the positive charges carried by the sodium ions under consideration are neutralized by the negative charges carried by the electrons that flow from the inner electrode to the outer one through the external circuit. The fraction of the positive charges to be neutralized by the electrons a t the outer electrode seems to depend upon the ionization potential of the sodium atoms in the glass and also upon the work function of the material constituting the outer electrode; a guiding rule t'o determine this fraction would be that the work function of the outer electrode material should be smaller than the ionization potential of the sodium atom in the glass in order that all the positive charges carried by sodium ions may be neutralized by the negative charges of the electrons at the outer e l e ~ t r o d e . ~ - ~ This work function would be modified in a very complicated way because of among other things, the adsorption of sodium ions. However, it seems probable that sodium ions and sodium atoms coexist near the outer electrode. The following experiments were carried out to investigate the effects that the medium surrounding the tube has upon the sodium ions and atoms that collect about the outer electrode and consequently on the short-circuit current of the cell. Construction of the Experimental Tubes For this purpose a few double-walled tubes were constructed. One of these is shown in Fig. l(a). I n this figure, only the hatched part of the tube was made of the target glass, Corning 0083, and the other part was made of conventional soda-lime glass. Caesium was admitted into the inner tube only. A film of caesium adsorbed on the inner surface of the inner tube was employed as the inner electrode, as described previously; for the outer electrodes, films of platinum and of silver were painted on the outer surface of the inner tube. The outer tube was first evacuated to a high vacuum, and after a set of short-circuit current measurements under this condition had been finished, air was introduced into the outer tube by breaking the exhaust tube, and the measurements were repeated. Mention should be made at this point of a detail of the structure of one of the outer electrodes, i.e. the Ag electrode, which is illustrated in Fig. l(b). This electrode was accidentally scratched a t two points, as shown, and part of it was electrically isolated from the rest; as a result, this part was a t a floating potential. This accident proved fortunate as will be briefly described later.

EFFECTS OF CAESIUM VAPOUR ON IMAGE ORTHICON TARGET GLASS

7 c

311

(b) Target glass

Pra. 1. Experimental tube (a) and structure of the Ag electrodo (b).

Short-circuit Current Characteristics An experimental tube, as described above, was put in a regulated electric furnace and heated to the desired temperature of about 200°C, and was kept at this temperature during the measurements of short-circuit currents. I n Figs. 2 and 3 are shown some results of measurements of this type. I n Fig. 2(a) typical curves showing the variation of short-circuit current with time are plotted, measured a t 198OC. This will henceforth be referred to as the discharge characteristic of the “cell” under investigation. I n Fig. 2(a) and in Fig. 3, the figures marked on the curves in parentheses denote values of the e.m.f. which were measured by means of a potentiometer. After obtaining the discharge characteristics of Fig. 2(a), the cell was charged using a 10.22-V storage battery, and the variation with time of the charging current was measured. The curves of Fig. 2(b) show

312

M. HIRASHIMA A N D M. ASANO

0.1

1

1

1

1

I

I

I

I

I I I l l

I

I

I

I I I l l

I

typical examples of the results obtained in this manner. This will henceforth be referred to as the charge characteristic. In the next experiment air was introduced into the outer tube, and similar measurements t o those described above were made. Figure 3 shows a typical example of such measurements and should be compared

313

EFFECTS OF CAESIUM VAPOUR ON IMAGE ORTHICON TARGET GLASS

with Fig. 2(a). A remarkable difference between the shapes of the curves as well as the magnitudes of the short-circuit currents may be seen in the two cases in which the surrounding media are vacuum and air, respectively. It seems to be almost certain that the oxygen and/or moisture contained in air were responsible for the latter result. I n the case of the vacuum, the charge and discharge process could be repeated a number of times whilst in air charging was not possible once the discharge operation, as shown in Fig. 3, had been made. Another important difference between the two cases was the apparent dissipation of the caesium contained in the inner tube.

Surrounding medium. air

x)

Time (mid

Time (h)

FIQ.3. Discharge characteristics in air.

Apparent Dissipation of Caesium during the Discharge Run During the above-mentioned experiments a peculiar feature was observed. While the discharge and charge runs were being done with the experimental tube whose outer tube was evacuated (Fig. 2), the caesium deposited on the inner surface of the inner tube looked as if it was not dissipated a t all a t the wall temperature of 198OC; the discharge and charge runs could be repeated as many times as desired without dissipation of the caesium, a8 far as could be detected by visual observation. I n contrast t o this, in the case where the surrounding medium was air, the caesium was found to disappear almost completely during a single discharge run, which lasted for about 70 h, as seen from Fig. 3. Having once decayed, the short-circuit current could never be restored P.B.1.D.-A

12

314

M. HIRASHIMA AND M. ASANO

by applying a charging voltage. This was the case with both the Pt and the Ag outer electrodes. At a tube temperature of 25OoC,however, results were quite different; the caesium in the inner tube disappeared a t nearly the same rate under both conditions. It is of interest to enquire where the caesium went. The target glass was found t o become a little coloured after the above experiment, so that it was thought most probable that the caesium might have penetrated into the glass. This possibility will be discussed more fully later. These experimental results suggest that the target glass under investigation might undergo some drastic change in its properties a t a temperature between 198°C and 250°C. This speculation has also been borne out by the measurements of conductivity and other properties. It was noticed that after the measurements of Figs. 2 and 3 had been conducted and the silver electrode had been scratched off the glass surface, the region of the glass lying just beneath the part of the silver film through which the electric conduction had taken place, was coloured faint yellowish-black, while the part of the glass lying just beneath the electrically isolated area of silver film, through which no electric conduction had taken place, was not noticeably coloured. It is beyond doubt that silver penetrating into the glass during the charging runs was responsible for this c ~ l o u r i n g . I~n view of this experimental finding, care should be taken not to use silver as the material of an electrode in direct contact with glass, t o which a positive voltage is t o be applied.

PENETRATION OF CAESIUM INTO

THE

TARGET GLASS

I n order to investigate the migration of caesium suggested by the above experiments measurements were made on an isotope of caesium, \yCs (half-life = 2.3 years),? using the following procedure. A few flat discs of the target glass, Corning 0083, were prepared, 20 mm in diameter and 0.66 mm in thickness and with both faces polished very flat and parallel so that the thickness of the discs could be measured accurately. One of these flat discs was sealed t o one end of a tube of commercial soda-lime glass in the form of an end-window and, near to it, a thin platinum wire was sealed through the glass tube wall, as shown in Fig. 4. Caesium was admitted into the tube in the usual way and a layer of it, adsorbed on the inner surface of the endwindow, served as the inner electrode, while the outer surface of the

t This experiment was carried out through the use of the nuclear reactor of Toshiba Atomic Power Company, Kawasaki City, by courtesy of Mr. M. Satoyama and Mr. K. Tsukui of The Central Research Laboratory of Tokyo-Shibaura Electric Company, Kawasaki City. The writers are most grateful for their taking trouble in conducting the measurements.

EFFECTS OF CAESIUM VAPOUR ON IMAQE ORTHICON TARQET ULASS

315

1 I I

I I I

L-------------,-,--J

No .I

No.2

..

650pm

lOpn

20prn

30prn J

\

Layers of inner surface dissolved with hydrofluoric acid

FIG.4. Preparation of samples for measurement with the caesium isotope '&%s.

end-window was coated with an Aquadag film which served as the outer electrode. Four such tubes were made, and each of them was subjected to a discharge run by short-circuiting the inner and the outer electrodes via a microammeter, as shown in Fig. 4, under the conditions given in Table I. TABLEI Conditions of discharge runs Tube No.

Discharge time (h)

1 2 3 4

400 180 110 140

Initial Total charge Tube Amount of weight of Cs carried by temperature Cs remaining Na+ in the tube* in Cs2Cr0, (mg) (coulombs) ("C) 36.28 15.78 14.90 12.48

0.670 1.056 3.121 0.205

little almost none ditto ditto

194 260 260 180 ~

~

EL

~

~~~

Observed by the naked eye.

After finishing the discharge runs, the end-windows were carefully cut off the four tubes. About four samples were prepared from each face-plate and their inner surfaces were dissolved off with hydrofluoric acid to depths ranging from 0 to about 30 pm, as illustrated in Fig. 4. The thickness of the samples was measured by means of a microscope at various parts of each sample and 8 mean value was

316

M. HIRASHIMA AND M. ASANO

obtained. The surface of the glass became rough on being dissolved by hydrofluoric acid, so that the thickness could not be measured t o great accuracy. The samples thus prepared were pulverized and put in the nuclear reactor referred to above, and the caesium remaining in each sample was made radioactive by neutron bombardment. Their y-ray activity (796 keV energy) was then measured. The results obtained in this manner are shown in Fig. 5, from which it can be seen that the caesium has penetrated into the glass in question

FIQ. 5. Caesium content

(yo)as

function of thickness of layer dissolved o f f by hydrofluoric acid.

to > 30 pm at 260°C while at temperatures lower than about 200°C it has penetrated to depths < 20 pm. There is reason to believe that the curve for tube No. 3 lies a little lower than it should, for the cutting of the sample in question was done near the periphery of the end-window and an attached piece of soda-lime glass was difficult to remove. As will be readily understood the curves shown in Fig. 5 are integral curves, and true curves of the distribution of caesium in the glass will be obtained readily by differentiation of each curve of Fig. 5. The values of the total amount of caesium that has penetrated into the glass during the discharge run for each tube is given by the values corresponding to zero thickness in Fig. 5 .

EFFECTS OF CAESIUM VAPOUR ON IMAUE ORTHICON TARGET ULASS

317

Comparison of the Number of Transported Sodium Ions with the Number of Penetrating Caesium Atoms during a Discharge Run It is of interest and importance to compare the number of the sodium ions that have been transported through the glass with that of the caesium atoms, or more probably ions, that have penetrated into the glass during a discharge run. The number of the sodium ions may be calculated from the total charge that was carried during the discharge run, assuming that all the charge was carried solely by the sodium ions. On the other hand, the number of the caesium atoms or ions that have penetrated into the glass may be calculated by using the following 2 cm2, the thickness data: the effective area of the outer electrode is of the end-window is 0.65 mm, and the density of the target glass is 2.46 g/cm3; hence the weight of the portion of the face-plate through which the discharge took place is calculated to be 0.32 g. The amount of caesium contained in 1 g of the target glass may be obtained from Fig. 5 (the values a t zero thickness) for each of the four tubes, and these values of caesium content must be multiplied by 0.32 to obtain the values of effective weight of caesium. The result of the calculation is given in Table 11. N

TABLEI1 Comparison of the number of transported sodium ions with the number of caesium atoms penetrating into glass during a discharge run Tube No.

1 2 3 4

Total charge carried by Na+* (coulombs) 0.670 1.056 3.121 0-206

Total Cs content of Net weight number of target glass? of Cs Na (%) (mg)

Total number of Cs atoms

0.11 0.306 0.12 0.048

1.6 x 10" 4-45 X lo1' 1.69 X 10" 7.0 X 1017

+

4.2 X 10" 6.6 x lo1' 1.95 x lolo 1.28 x lo*'

0.352 0,980 0.384 0.184

Taken from Table I.

t Taken from Fig. 5.

From Table I1 it can be seen that agreement between the two sets of values is fair, with the exception of Tube No. 3, measurement of which is believed t o be less accurate than the others, as mentioned previously. If errors in the measurements are taken into account, it may be said that the correspondence of sodium ions t o caesium atoms is one to one. An important conclusion to be drawn from this result is that all the sodium ions carrying charges through the target glass are probably produced by caesium atoms or ions that have penetrated

318

M. HIRASHIMA AND M. ASANO

into the glass. This suggests that the sodium oxide in the glass is reduced by the caesium that has penetrated into the glass in accordance with the chemical reaction: 2 Cs --f Cs,O 2 Na, Na,O Na + N a + e-. (1) Thus, our speculation that sodium oxide might be reduced by caesium is supported by experimental data.

+

+ +

ELECTRON-MICROSCOPE OBSERVATIONS OF TARGET GLASSSURFACES It was thought to be of interest to see if the surface of the target glass under investigation underwent any change when it was brought into contact with caesium vapour a t elevated temperatures. For this purpose observation of the glass surface by means of an electron microscope was attempted. Fjgure 6(a) shows an example of electron

FIG.6. (a) Untreated surface of target glass. (b) After baking in air at 400°C for 1 h. ( 0 ) After exposure t o caesium vapour at 240°C for 48 h.

micrographs of the outer surface of an untreated (i.e.? as drawn) target-glass tube. I n this photograph can be seen a number of small surface cracks (the so-called Griffith crack^^*^) distributed a t random. When the tube was subjected to baking in air a t 400°C for 1h, for instance, it was found that the cracks became a little smaller and orientated themselves in one direction, probably in the direction of drawing of the tube, as appears in Fig. 6(b). Their depth is unknown, but it would perhaps be of the same order of magnitude as their width. If a piece of the same kind of glass was heated a t 24OOC for 48 h, for instance, in an atmosphere of caesium vapour, the surface of the glass was found to look as though etched by the caesium, as seen from Fig. 6(c). Comparison of Figs. 6(b) and (c) reveals that some chemical reaction must certainly have taken place between the caesium vapour and the glass. The inner surfaces of the tube were also observed, and it was found that the general trend was the same as for the outer surfaces.

EFFECTS OF CAESIUM VAPOUR ON IMAGE ORTHICON TARGET GLASS

319

CONCLUSION The observation that, once the short-circuit current has decayed with air as the surrounding medium, it can not be restored by further charging, is not surprising since the inner electrode consisted of an adsorbed layer of caesium. When the caesium was all dissipated, the inner electrode must also have disappeared; this is equivalent to charging a conventional battery through a very large resistance. If the experimental tube had been provided with an inner electrode made of, for example, a platinum film, charging would have been possible, as was the case when the outer chamber (jacket) was evacuated, in which case the adsorbed layer of caesium was present, acting as the inner electrode. The curves in (a) and (b) of Fig. 2 can be interpreted as showing that, in the case when the outer tube was evacuated, a fair quantity of un-neutralized positive charge might have accumulated in the vicinity of the outer electrode, forming perhaps a positive space charge. This would account for the rapid decay of the charging current shown in Fig. 2(b) when a reverse voltage was applied between the outer and inner electrodes. If this is the case, the transportation of sodium ions towards the outer electrodes would be hampered by the presence of the space charge. As a result, the reduction of the sodium oxide in the glass by caesium will not be very active in view of the reaction formula Eq. (1))and so the dissipation of the caesium will in turn not be very conspicuous. I n fact, by comparing Fig. 2(a) with Fig. 3, we can estimate that the total charge carried by sodium ions in the case of Fig. 2(a) is about one-tenth of that in the case of Fig. 3. I n the case where the surrounding medium was air, especially moist air, as was actually the case, the transportation of sodium ions would be acceIerated according not only to the chemical reaction as in Eq. (1)) but also the following chemical reactions: 2Na 2 H,O + 2NaOH H,, (2) 2 H, O2 --t 2 H,O. Sodium oxide is known t o be extremely hygroscopic and thus it seems to be reasonable that the dissipation of caesium is much more rapid in moist air than in a vacuum. The experimental fact that at a temperature of -250°C the dissipation of caesium was also very rapid, even in a vacuum, remains t o be explained. As was described in a previous section, there is some evidence that some properties of the target glass change a t temperatures higher than about 2OO0C, and it is speculated that a change in the structure of the glass might be responsible for this result.

+

+

+

320

M. HIRASHIMA AND M. ASANO

ACKNOWLEDUMENTS The writers wish to express their hearty thanks to Mr. M. Satoyama and Mr. K. Tsukui of The Central Research Laboratory, Tokyo-Shibaura Electric Company, Kawasaki City, but for whose co-operation the paper would not have taken its present form. Thanks are also due to Mr. K. Watanabe of Research Laboratory, Japan Broadcasting Corporation, for his generosity in supplying the tubes of target glass used in the present experiments. The writers also wish to take this opportunity to express their sincere thanks to their colleagues and students who kindly assisted them in various phases of the experiments.

REFERENCES 1. Hirashima, M. and Asano, M., “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 651. Academic Press, London (1966). 2. Langmuir, I. and Kingdon, K. H., Proc. Roy. SOC.A107, 61 (1925). 3. Smith, R. W., Phy8. Rev. 97, 1625 (1955). 4. Kroger, F. A., Diemer, G. and Klassen, H. A., Phy8. Rev. 103, 279 (1956). 6. Kirby, P. L., Brit. J . Appl. Phys. 1, 193 (1950). 6. “Glass Handbook”, ed. by T. Moriya et al., p. 195. Asakura Shoten, Tokyo (1965) (in Japanese). 7. Holland, L., “The Properties of Glass Surfaces”, p. 132. Chapman and Hall, London (1964).

DISCUSSION L. L. ALT: Have you observed significant changes in mechanical properties of the caesium-treated glass? Such changes in mechanical properties are not surprising as it has been shown earlier that glass, possessing ion exchange properties will exchange Na+ ions for others. As long as the ionic radii are close, the properties do not change significantly. With increasing ionic size of the substituting ion however, the glass lattice becomes strained which eventually leads to shattering of such glass membranes. M. HIRASHIMA: Y e s , we have observed some change in mechanical properties. The glass in question has become brittle soon after it was caesium-treated. As you have just suggested, the larger radius of the caesium ion compared with that of the sodium ion seems likely to be responsible for this result. a. NAJUMDAR: 1. Have you performed similar experiments with sodium and potassium? 2. What reaction mechanism do you propose for these? M. HIRASRIMA: 1. Yes, we have performed similar experiments with sodium, potassium and rubidium, and experiments with the latter two alkali metals are still in progress. Interesting results are being obtained, which we hope to publish in the near future. 2. We have an idea concerning the reaction mechanism. However, in view of the fact that Corning glasses types 0083 and 0089, whose sodium-oxide contents are nearly equal, show different properties for caesium vapour, we think that we must be very careful not to draw a hasty conclusion from the results mentioned so far. N. J. HARRICK: I wish to point out that silicon nitride serves as a good barrier to sodium ions in silicon for transistor technology. Perhaps you will find it useful as a barrier to caesium a8 well. M. HIRASHIMA: We are now making some experiments on how to protect the surface of the glass in question against caesium vapour. Our principle is to cover

EFFECTS OF CAESIUM VAPOUR ON IMAGE ORTHICON TARGET GLASS

321

the surface of the glass with a very thin film of some oxide that cannot be de-oxidized by caesium vapour. We have not been aware of the fact that silicon nitride might be useful as a barrier to caesium vapour. We wish to try to test the utility of that substance. Thank you very much for the interesting information. J. D. MOGEE: Is there any critical temperature in the reaction of caesium with soda-lime glass? M. HIRASHIMA: Yes, there is. As far as Corning 0083 glass is concerned, the critical temperature is around 200°C. I f the conductivity of the glass, for instance is plotted as a function of the reciprocal of temperature, the slope of the curve is observed to change at about 200°C.

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Research on Photocathodes in Czechoslovakia M. JEDLIC'KA Tesla Vacuum Electronics Research Institute, Prague, Czechoalovakia

INTRODUCTION This paper describes the results of some experiments carried out during research on photoelectric emission at the Vacuum Electronics Research Institute in Prague. Information is given on Sb-Cs-Rb and Sb-Na-K photocathodes and some experimental results from research on alkali arsenides are given. Experiments with the photocathode composed of Sb-K-Rb-Cs, which has also been worked on at our institute, are discussed by Dvoi%k in this volume.?

THESb-Cs-Rb PHOTOCATHODE As has been described in various papers, there are different processing methods for producing the Sb-Rb--Cs photocathode. Kanev and Nanev,l and also Morrison,a prepare a layer of antimony and rubidium which is subsequently processed with caesium vapour. Our method is to prepare a caesiated antimony layer and t o introduce rubidium as a second alkali metal. A considerable rise of sensitivity can be achieved by a suitable oxidation of the photosensitive layer.3 Figure 1 shows the effect of the individual processing steps on the sensitivity. Curve A shows the spectral sensitivity after the antimony has reacted with caesium at 180°C and just before the introduction of rubidium. Curve B shows the spectral response of the Sb-Cs-Rb layer a t 180°C before oxidation, and curve C shows the spectral response of the photocathode after oxidation a t room temperature. As can be seen, the oxidation of the layer results in a considerable increase in the h a 1 sensitivity. Figure 2 shows the statistical distribution of the luminous sensitivity measured on photocathodes of this type in quite a considerable number of image orthicons. Photocathodes with a luminous sensitivity of 80 pA/lm were obtained most frequently.

t See p. 347. 328

324

M. J E D L I ~ K A

Figure 3 shows the statistical distribution of the maximum spectral sensitivity, the maximum sensitivity being most frequently at a wavelength of 4800 8. The spread of the position of the maximum spectral sensitivity is without doubt due to certain differences occurring during the processing of the photocathode. For instance the thickness of the substrate layer varies from cathode to cathode to a certain degree. From measuring a great number of tubes we could ascertain that the maximum spectral sensitivity is mainly in the region between 4400 8 and 5400 8.

80

70

I

d

60 -

Y

0

$

50-

p. 0

IL

40 -

30 -

Wavelength ( 4 )

FIG.1. Spectral response of the Sb-Cs-Rb photocathode during processing. A, Sb-Cs layer at 18OOC; B, Sb-Cs-Rb layer at 180°C; C, oxidized Sb-Cs-Rb layer at room temperature.

THESb-Na-K PHOTOCATHODE The Sb-Na-K photocathode, having a chemical composition which can be most probably described by the formula Na,KSb, was mentioned by Sommer4 in connection with the discovery of the multialkali photocathode. The basic photoelectric properties of this layer can be seen from the curves of spectral sensitivity (Fig. 4). The figure shows two curves: curve A is for illumination from the vacuum side and curve B for illumination through the glass substrate, the luminous sensitivities

RESEARCH ON PHOTOCATHODES I N CZECHOSLOVAKIA

20

-

18 I6 14 -

-z 5

a

12-

f

z

10-

L

m

n

6

z

325

86-

4-

id

0

L

Sensitivity (pA/lrn)

FIR.2. Statistical distribution of the 1UmhOUR sensitivity of the Sb-Cs-Rb photo. cathodes in the image orthicons.

Wavelength

(1,

FIG.3. Statistical distribution of the wavelength of the maximum spectral sensitivity of the Sb-Cs-Rb photocathodes in the image orthicone.

326

M. JEDLI~KA 30

10x 10-2

-za x +

9-

10

8-

I0

7-

70

6-

50

-s. -h

j0

._ In 'E

c

-

'5 .c .-

5

4

4-

10 I-

3-

30

2-

20

I

10

In

:

w

0 )O

FIG. 4. Spectral response of the Sb-Na-K photocathode (thickness 1700A): A, illuminated from vacuum side; B, illuminated through glass; C, transmission of glass.

I 1.8

I

22

I

I

2.6

I

I

28

I I 34

I

3.8

I

I

I

~

4.2

.

4.6

Radiation quantum energy h d e V )

FIQ.5. Quantum yield of the Sb-Na-K photocathode (luminous seiisitivity 89 pA/lm (2864°K)):A, quartz substrate; B, glasa substrate.

RESEARCH ON PHOTOCATHODES I N CZECHOSLOVAKIA

327

being 45 pA/lm and 51.5 pA/lm respectively. I n the short-wave region the sensitivity is limited by the absorption of the molybdenum gInss (thickness 3 mm), its absorption curve being shown in Fig. 4, curve C. Figure 5 shows the quantum yield of a layer having a luminous sensitivity of 89pA/lm. The long-wave threshold A, is in the region of 6800 8.Curve A applies to a layer on a quartz substrate, while curve B is for a layer on a glass substrate.

Wavelength

(A)

FIG.6. Reflexion R,absorption A and transmission T &B functions of wavelength for the Sb-Na-K photocathode (thickness 1700 A).

Some of the basic physical properties have been reported by Spicer.6 We have measured some of the optical properties. Figure 6 shows the absorption, reflexion and transmission curves, from which the thickness of the layer and its absorption coefficient can be calculated. The thickness of the layer was found to be 1700A; the curve of the absorption coefficient can be seen in Fig. 7. The excellent properties of this photocathode can be fully appreciated from investigations of its temperature dependence. Figure 8 shows the temperature dependence of the thermionic emission of some layers: only a slight rise can be noted over the measured range. The average A/cma, the value of the thermionic current density a t 20°C was

5x10

1 c

-k +

0) ._ ._ v

10

L

8 0 C ._ c

5: a n

5x10’

1

Ioa ruOO

6000

5000

7000

8000

Wavelength (8)

FIG.7. Dependence of the absorption coefficient on wavelength for the Sb-NeK photoca;thode (thickness 1700 A). 1

I

I

I

No

I

30

I

I

40 50 Temperature (“c)

I 60

.

FIU.8. Temperature dependence of the thermionic current density of several Sb-NeK photocathodea.

RESEARCH ON PHOTOCATHODES I N CZECHOSLOVAKIA

329

best result being 5 x 10-19A/cm2. The corresponding temperature dependence of the thermionic work function yT is shown in Fig. 9. This was calculated from the equation6 yT

= 1.984 X

T (2.079

+ 210gT - log JT),

where T is the absolute temperature and J, is the thermal emission current density, which can be derived from Richardson's equation. The work function is generally assumed t o vary linearly with the temperature (Fig. 9). The slope of the straight line gives the value drp,,ldT, which is about 0.005 eV/"K.

1.4

I

20

I

30

I 40

I 50

I 60

I

70

Temperature ("C)

PIO.9. Dependence of the thermionic work function on tho temperature for SbNa-K photocathodes.

We also investigated the temperature dependence of the conductivity. The results are shown in Fig. 10; the slope of the curves give the values of the activation energies over the temperature range as 1.3 eV, 0.43 eV and 0-07 eV. We also measured the influence of temperature on the spectral sensitivity of this photocathode and found several types of variation. Most frequently there was a rise of spectral serisitivity with temperature over the whole range of the spectrum (Fig. 11). The Sensitivity has been normalized to unity a t 25°C for every measured wavelength. I n other cases we found a decrease with temperature in the spectral sensitivity near the long-wave end and in some the sensitiviby had a tendency to decrease also near the blue end of the spectral response.

.23

i

I 0.' 2.5

3.0

3.5

45

40

~ O X I O - ~

(Ternperaturerl ( O K - ' )

FIQ.10. Temperature dependence of the electrical dark conductivity of the Sb-Na-K photocathode.

Wavelength

FIG. 11. Variation of the spectral sensitivity

(8)

8T/826~0 of

with temperature.

an S b N a - K photocathode

RESEARCH ON PHOTOCATHODES I N CZECHOSLOVAKIA

331

I n the middle region of the spectrum, that is from about 4500 t o 6000 A, the sensitivity always exhibited an increase with temperature. THINALKALI-ARS WNIDE FILMS Thin layers composed of As-Na, As-K, As-Rb and As-Cs were prepared in evacuated glass tubes by a suitable procedure. The deposition of thin layers of arsenic is difficult, but some information can be found in the l i t e r a t ~ r e .The ~ pressure of the saturated vapour, which consists almost entirely of As, molecules, is aboub four orders higher than that of antimony. The temperature of the evaporator must lie between 300 and 350°C. The temperature of the substrate for the deposition of the arsenic is critical; it must certainly be lower than O"C, Kansky' recommending less than -30°C. We expected our thin alkali-arsenide films to be semiconductors with a bandgap of about 2eV for an As-Cs layer and with higher values for the other cases. We investigated the temperature dependence of the dark conductivity for an As-K layer and found that i t was typical of a semiconductor. I n the temperature range of -70°C to 50°C the activation energy was 0.37 eV. The chemical composition of thin layers of evaporated alkaliarsenides has not been investigated yet. It is known, however, that compounds of arsenic and potassium can be described by the formulae K,As, KgAs4,U s , KAs,.O It is reasonable t o assume that the compound K,As is predominant in the layer but we have no experimental proof for this. Similar conditions can be expected in other compounds of alkali metals with arsenic. The assumption, that these films have maximum sensitivity in the ultra-violet rogion, was experimentally verified. Figure 12 shows the spectral response of a group of the layers studied; it can be seen that the long-wave threshold moves to shorter wavelengths with decreasing atomic number. The spectral response curves in Fig. 12 are distorted by the absorption of the glass used as the entrance window for the radiation; the relative spectral transmission of the glass is also shown in Fig. 12 and it can be clearly seen that the sensitivity maxima are determined mainly by the transmission of the glass. The real positions of these maxima have not been measured. The values of the ultra-violet quantum yields (calculated for the case of a quartz substrate instead of glass) of the above experimental photoemitters are shown in Table I. It can be seen that the quantum yields of our experimental layers are not as high as those of other types of photocathodes in this region of radiation, for instance compounds of tellurium or antimony with alkali metals.

332

M. J E D L I ~ K A

o.lp Wavelength ( b )

FIG.12. Normalized spectral response curves for the alkali-arsenide layers and spectral transmission T of the glass window.

TABLE I Quantum yield of alkali-arsenide layers

Layer

As-Na AS-K As-Rb AS-&

2600 i% 5

10

I

Quantum yield 2800A 3000A 1-9 4.9 1.9 4.5

0.5

2.0 0.9 1.9

(yo) 3200A

0.16 1.3 0.6 1.4

3400

A

0-03 1 0.4

1.25

Apart from simple compounds of arsenic with alkali metals we also investigated more complex structures, consisting of arsenic and two or three alkali metals. Figure 13 shows the spectral response of an As-K-Cs layer 2500-A thick deposited on glass, when illuminated

RESEARCH ON PHOTOCATHODES IN CZECHOSLOVAKIA

333

from the vacuum side (curve A) and through the glass substrate (curve B). The curves have been normalized, the luminous sensitivities, which are of course very low, being 1.25 yA/lm and 1 pA/lm respectively. As before, the short-wave response is affected by the transmission of the glass. The optical properties of these layers are of some interest. Figure 14 shows the typical curves of the spectral reflexion, transmission and

Wavelength

(dl

FIG.13. Spectral response of an As-K-Cs layer: A, illumination from the vacuum side (luminous sensitivity 1.25 pA/lm); B, illumination through the glass substrate (luminous sensitivity 1 pA/lm).

absorption for such a layer. The absorption curve has a marked peak in the region of 5800A which appeared also in other measured samples. Figure 15 shows the curve of the absorption coefficient of two As-K-Cs layers. Near the wavelength 4000 A the coefficient reaches the value lo5 cm-l, which is the smallest value consistent with efficient photoemission. It is interesting to note that alkali antimonides have an absorption coefficient of 3 to 5 x 106cm-l in this region. For comparison the figure shows the curve for a thin layer of Sb-Cs-Rb. By determining the sign of the thermoelectric potential which

44000

YR+

wx 5000

6000

7000

8000

Wavelength ( 8 )

FIQ.14. Reflexion R, absorption A and transmission T as functions of the wavelength for an As-K-Cs layer 2100 A thick. 5 x 10'

4000

I I 6000 7000 Wavelength ( 4 1

I 5000

8000

FIG. 16. Dependence of the absorption coefficient on wavelength for two Aa-K-Cs layers, and an Sb-Cs-Rb layer.

RESEARCH ON PHOTOCATHODES I N CZECHOSLOVAKIA

335

appeared a t the interface between a thin layer of As-K-Cs and an aluminium electrode, the semiconductor was found to be n-type. The results of our experiments show that the alkali arsenides have a significant photoemission only in the ultra-violet region. The efficiency in the visible region is very low, but the optical absorption coefficient of the As-K-Cs layer is not low enough to be the sole cause of the low efficiency. The conductivity of our alkali-arsenide layers is n-type, which evidently is not favourable for photoelectric emission. It is of course possible that certain modifications of the processing methods could lead to better results. It would be useful to carry out structural and chemical analysis of the layers since the results could help t o clarify the optical and photoemissive properties of these materials .

REFERENCES 1. Kanev, V. G. and Nanev, N. R., Bulgarian Pat. No. 10456 (1963). 2. Morrison, C. W., J. Appl. Phys. 37, 713 (1966). 3. JedliEka, M. and Vilim, P., 1%“Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 449. Academic Press, London (1966). 4. Sommer, A. H., Rev. Sci. Inatrum. 26, 725 (1955). 6. Spicer, W. E., Phya Rev. 112, 114 (1958). 6. JedliEka, M., I n “Transactions of Third Czechoslovak Conference on Electronics and Vacuum Physics”, ed. by L. Pat$, p. 337. Academia, Prague (1967). 7. Kansky, E., Rep. Inst. Electronics Automatics Ljubljana 4, 156 (1962-63). 8. Dorn, F. W., Klemm, W. and Lohmeyer, S., 2. Anorg. Allg. Chem. 309, 304 (1961).

DISCUSSION Concerning the temperature coefficient of the dark current of Sb-Na-K, do you find that it is substantially less than for Sb-Cs? Could you please comment on the values of the coefficient you have measured? M. JEDLIEKA: According to our measurements the thermionic current of the Sb-Cs photocathode rises in the range 20°C to 70°C by about 8 to 10 times, while that of the Sb-Na-K photocathode increases under the same conditions only by 1.2 to 2 times. The temperature coefficient of the thermionic work function of the Sb-Cs photocathode is about 0.003 eV/OC, while that of the Sb-Na-K photocathode is nearly 0.005 eV/”C. A. F. PEARCE: The dark current quoted for the Sb-Na-K photocathode, namely about 1 electron cm-asec-l, is about the same as that quoted by R. P. Randallt for a similar type of cathode. w. BAUMQARTNER: How do you introduce the arsenic in your arsenic/alkalimetal layers? M. J E D L I ~ K A :There are three possible methods. Evaporate arsenic first and then the alkali metals or vice-versa or evaporate all simultaneously. When evaporating arsenic, the temperature of the substrate is critical. The temperature for outgassing the experimental tube has to be chosen with regard to the evaporation temperature of arsenic. t See p. 713. M. ROME:

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Crystal Structure of Multialkali Photocathodes T. NINOMIYA, K. TAKETOSHI and H. TACHIYA NHA- Technical Reeeurch Luborutoriee, Setaguya, Tokyo, Japan

INTRODUCTION Research directed to an improvement in the sensitivity of image orthicon tubes for broadcasting has been conducted in our laboratories for several years. A very high sensitivity image orthicon tube having a multialkali photocathodel and an improved magnesium-oxide-film target2 was developed and used for the first time over the NHK networks in 1965. Since then, efforts have been made to improve the sensitivity and the picture quality of this tube. As a result, these image orthicon tubes are now being used extensively outside the studios because .they have the best sensitivity of all the pick-up tubes used for broadcasting. This paper is a report on one portion of the above development, namely, the method used to prepare a highly sensitive multialkali photocathode, and the results obtained by observing this photocathode through an electron microscope equipped with a special evaporation device.

METHODSFOR PREPARING THE PHOTOCATHODE Initially fairly good multirtlkali photocathodes resulted from the repeated evaporation of antimony and alkali metals in the following order: Sb, Na, Sb, K, Sb and Cs. However, because this method was time-consuming and difficult to control, improvements were made. These resulted in the development of a means for the simultaneous evaporation of Na and K. For the image orthicon, the following materials were used in making the photocathodes: Sb in globular form was fused to Pt-clad MO wire; for the Na and K sources, Na2Cr04 and K2Cr0,, together with reducing agents, were packed into a pair of metal channels, and for the Cs source, Cs2Cr04and a reducing agent were packed into another pair of metal channels. 337

338

T. NINOMIYA, K . TAKETOSHI AND H. TACHIYA

The activation process used at present for photocathodes of image orthicons is shown in Fig. 1. Before the activation, Sb is evaporated by one of two methods, depending on the spectral sensitivity required. Either, (a) the Sb isevaporated t o a thickness having about 5 5 % transparency to light, or (b) it is first evaporated to a thickness having about 30% transparency, is heated for about 10 min to a temperature of 200-220°C when it becomes almost completely transparent, is cooled and then more Sb is evaporated until the layer has 80 to 90% transparency. The activation process consists of evaporating the Na and K while the substrate is a t a temperature greater than 140°C (A to E in Fig. 1). Two points of maximum sensitivity, B and D are generally observed. 240

-5 a

210;

I

I

I

-140-240T4 cooling No-K

evoporotiod

180-

1

; e F Sb evoporotion

I

I

I

I90- 24OoC--l - cooling Cs, ISbI evoporntion+ f'

+-

,,0-6--

I

FIG.1. The photocathode activation process.

Between E and F the substrate is slowly cooled and the photosensitivity rises still higher. Sb is now evaporated with the substrate at room temperature until the photosensitivity becomes nearly zero (F to G). From G to H the photocathode is kept at a temperature of about 200°C and activated by heating the Cs source. During this process, a small quantity of Sb is also evaporated. On cooling (H to I) the maximum sensitivity is obtained. When measured with a tungsten-filament lamp having a colour temperature of 2864°K) the average photosensitivities of photocathodes obtained by the above activation process were 158 pA/lm (a maximum of 216 pA/lm) when the initial deposition of Sb was by method (a), and 212 pA/lm (a maximum of 285 pA/lm) with method (b). These results were obtained from 18 image orthicon tubes of which one half were made using method (a) and the other half by method (b). The spectral sensitivities of typical examples of the two methods are shown in Fig. 2.

URYSTAL STRUCTURE OF MULTTALKALI PHOTOCATHODES

339

PIG.2. Spectral sensitivities of the multialkali photocathodes prepared by methods (a)and (b).

PREPARATION OF SAMPLES FOR OBSERVATION Alkali metals such as Na, K and Cs which compose the photocathode easily become oxidized or contaminated. For this reason an evaporation chamber, as shown in Fig. 3, has been set up adjacent to the specimen chamber of an electron microscope, and samples made in the evaporation chamber are transferred to the specimen chamber of the microscope with the specimen manipulating rod. In order to maintain the vacuum when this rod is operated, it is encased in a pre-evacuation chamber. The specimen chamber of the electron microscope is evacuated to a pressure of lo-’ torr or Iess, and while it is in the microscope the specimen holder is cooled to -150°C for the purpose of preventing re-evaporation of the Na, K and Cs metals. Specimen multialkali photocathodes were made inside the evaporation chamber according to the processes described in the previous section. The photocathodes were prepared simultaneously on the monitor glass plate and on the carbon film (-200 d thick) at the bottom of the specimen holder. The photocurrent was measured with the collector electrode which was located under the monitor glass plate,

340

T. NINOMIYA, K . TAKETOSHI AND H. TACHIYA

However, in order to reproduce the process shown in Fig. 1, it was necessary that the conditions should be the same as in the image orthicon. These are: the thickness of the evaporated Sb, the temperature of the photocathode during the heating of the Na-K and Cs channels, the heating time of these channels, the conductance of the evacuation system, and the temperature difference between the wall of the evaporation chamber and the specimen. Moreover the wall material must be the same and for this reason a glass cylinder is placed inside the stainless steel chamber. Since the growth process of the photocathodes could not be observed continuously with the electron microscope, the Sb base and the layer W n ,dow i

for light projector

Specimen holder To specimen chamber of electron microscope

S&cirnen manipulating rod

Monitor gloss plate

/

'/ 411

Gloss cylinder

Pre-evacuation chamber Water-cooling pipe

1

1 W -l\al

rKccl

heater Collector Terminals for heating channels Window for receiving light

FIU.3. Cross-section of the evaporation ohamber for preparing samples.

were observed only at the maximum or minimum points A, B, C, D, and I of Fig. 1. After each observation the specimen was discarded and a new one prepared for examination at the next stage of the process.

EXPERIMENTAL RESULTS I n this section, the activation process is followed by means of electron micrographs and diffraction patterns, commencing with the Sb layer deposited using method (b). The difference between photocathodes made using methods (a) and (b) is discussed at the end of the paper. The transmission electron micrograph of a specimen of Sb which had been evaporated t o a thickness having 30% transparency to light at room temperature is shown in Fig. 4, and its electron diffraction pattern

CRYSTAL STRUCTURE OF MULTIALKALI PHOTOCATHODES

341

FIa. 4. Transmission electron micrograph of Sb layer which was evaporated to a thickness of 30% transparency to light.

Fra. 6. Diffraction pattern of the specimen shown in Fig. 4.

Fra. 6. Electron micrograph taken after heating the Sb layer shown in Fig. 4 and evaporating more Sb.

Fra. 7. Diffraction pattern of the specimen shown in Fig. 6.

in Fig. 5 . From these photographs it is evident that this Sb layer is composed of grains whose average size is 340A and that the Sb is preferentially orientated. Figure 6 shows that after the Sb layer has been heated and more Sb evaporated (to a thickness having 85yotransparency) the Sb grains are now slightly larger. The diffraction pattern (Fig. 7 ) shows a ring pattern in addition to the spot pattern and this indicates that some of the Sb is randomly orientated.

342

T. NINOMIYA, K. TAKETOSHI AND H. TACHIYA

The transmission electron micrograph and the diffraction pattern of the specimen at the maximum point B in Fig. 1 are shown in Figs. 8 and 9, respectively. The spot pattern in Fig. 9 is caused by K,Sb with its [210] axis standing in a direction nearly normal t o the base plane.

Fra. 8. Transmission electron micrograph of the specimen atthe maximum point B in Fig. 1.

FIO.9. Diffraction pattern of the specimen shown in Fig. 8.

The size of each K,Sb crystal is about 7 0 0 k It was found after analysis of the diffraction pattern that the structure of the crystal of this K,Sb is as shown in Fig. 10. Moreover, note that Na has not yet been observed, the probable reason being that the vapour pressure of potassium is much greater than sodium, and that the potassium evaporates prior t o the sodium.

0 K 0.5 i Sb 0.5 O K 1

FIa. 10. Structure of K,Sb.

CRYSTAL STRUCTURE O F MULTIALKALI PHOTOCATHODES

343

The transmission electron micrograph and the diffraction pattern of the specimen corresponding to the minimum point C in Fig. 1 are shown in Figs. 11 and 12. The spot pattern inlFig. 12 is again caused by K,Sb, and the ring pattern by Sb. The electron micrograph shows that

FIQ.11. Transmission electron micrograph of the specimen at the minimum point C in Fig. 1.

FIQ.12. Diffraction pattern of the specimen shown in Fig. 11.

FIQ.13. Transmission electron micrograph of the specimen at the maximum point D in Fig. 1.

FIQ.14. Diffraction pattern of the specimen shown in Fig. 13.

the size of K,Sb crystals is about 800-900 A, and that the size of the Sb grains is about 100 A. At the next maximum point D in Fig. 1 the electron micrograph (Fig. 13) is blurred because Na and K, which are in the liquid phase,

344

T. NINOMIYA, K. TAKETOSHI AND H. TACHIYA

cover the surface. The spot pattern in Fig. 14 is caused by Sb, some K,Sb and (Na,K),Sb. The specimen at the point I in Fig. 1, that is the finished multialkali photocathode, is obtained by adding more Sb between points F and G, and re-activating the photocathode with Cs and Sb. The transmission electron micrograph and the diffraction pattern are shown in Figs. 15 and 16. The spot pattern is caused by Cs,Sb which has the same structure as the K,Sb as shown in Fig. 10, but the lattice parameter is 8.72 A, which is 5% less than the lattice parameter (9.18 8 )

FIa. 15. Transmission olectron micrograph of the finished multialkdi photocathode (initial deposition of Sb by method (b)).

FIQ.16. Diffraction pattern of the specimen shown in Fig. 16.

of pure Cs,Sb. Hence, it is assumed that about 30% of the Cs atoms have been replaced by Na and K atoms. Figure 15 shows that the size of the Cs,Sb crystal is 1000-2000 A. As with K,Sb, the [a101 axis of Cs,Sb stands in a direction nearly normal to the substrate, and Cs liquid covers the surface. From the foregoing result, it is believed that the multialkali photocathode is formed through the process shown diagrammatically in Fig. 17. The structural difference between photocathodes having the initial layer of Sb deposited by method (a) and those where method (b) was used is visible in electron microgra,phs and diffraction patterns. An Sb layer which was formed after evaporation to a thickness of 50% transparency (method (a)) had a particle size of 170 8 with an amorphous structure at room temperature. The Sb crystallized by

345

CRYSTAL STRUCTURE OF MULTIALKALI PHOTOCATHODES

heating at 16OoC, but the structure was different from the Sb layers formed by method (b) (Figs. 4 and 5 ) : it was of random orientation. The activation process for this Sb layer was the same as that previously

Sb preferentially and
a-

-360

-460A-

Liquid

4

n

k- Carbon substrate

-

Cs

[Z lo] 1000 2000

,

I

Isubstrate

i

*

FIQ. 17. Diagram showing the nay in which the 'multialkali photocathode may be formed. (a) Sb base, and (b), ( 0 ) and (d), the structure at points B, D, and I respectively of the activation process (see Fig. 1 ) .

described, the only difference being the smaller sizes of the grains. These were 550 A at point B, 360 A at point D, and 700 A at point I. Figure 18 shows the transmission electron micrograph of the final multialkali photocathode in this case. P.E.1.D.--A

13

346

T . NINOMIYA, K . TAKETOSHI A N D H. TACHIYA

FIG.18. Transmission electron micrograph of the finished multialkali photocathode (initial deposition of Sb by method (a)).

ACXNOWLEDOMENTS The authors would like to thank the Directors of NHK for their permission to publish this paper, and also to thank Messrs. K. Watanabe, S. Takahashi and T. Hatakeyama for their technical assistance in this research.

REFERENCES 1. Tachiya, H., J . Inst. Television Engrs. Japan 20, 199 (1966). 2. Watanabe, K. and Maebara, A . , Tech. J . Japan Broadcasting Corpn. 17, 38 (1965).

DISCUSSION What is the thickness of the C s (liquid) layer on the crystallites of the photocathodes? T. NINOMIYA: The thickness of Cs layer could not be accurately estimated because it was in liquid form; however, we think that it is thicker than a monoatomic layer in view of the absorption contrast of the layer in the transmission electron micrograph. P. VERNIER: Do you observe any charging of the layer under the electron beam? T. NINOMIYA: We did not observe any charging phenomenon because we used very weak beams and conductive carbon substrates. C. H. A. SYMS: You indicated in one diagram that the individual crystallites extend from the substrate to the caesiated emission surface. Do you in fact know that there are no crystallite grain boundaries between these two surfaces? T. NINOMIYA: The dark field images of these films show that there are no boundaries between these two surfaces. s. MAJUMDAR: Can you explain from your crystal model the reason for the low sensitivity of 5-20photocathodes that is very commonly encountered? T. NINOMIYA: The fabrication of our high sensitivity photocathodes is different from that of commonly used S.20 photocathodes. Hence we do not know if our crystal model can be applied to the low sensitivity 8.20 photocathodes. We have not yet investigated this aspect and therefore cannot give a satisfactory answer to your question. J. D. MCOEE:

Some Properties of the Trialkali Sb-K-Rb-Cs Photocathode M.

DVO~AK

Tesla- V U V E T , Prague, Czechoslovakia

INTRODUCTION

The alkali antimonides are an important group of photocathodes having high sensitivity in the visible region of the spectrum. I n the last few years these photoemitters have been further developed following Sommer’s discovery of the multialkali ph0tocathode.l The trialkali Sb-Na-K-Cs photocathode has high luminous sensitivity and broad spectral response and the bialkali photocathodes also have special advantages: the Sb-Na-K photocathode which is very suitable when operated a t higher temperatures, the Sb-K-Cs photocathode with its very low thermionic emission, and the Sb-Rb-Cs photocathode with a spectral response comparable to the spectral sensitivity of the human eye. All these photocathodes have been investigated a t the Research Institute for Vacuum Electronics and are being used in Czechoslovak photoelectronic image tubes. It seems, however, that not all the possibilities offered by the combination of the elements of groups I (Li,Na, K, Rb, Cs) and V (Sb), which should have high photosensitivity, have been exploited. This paper discusses some properties of the Sb-K-Rb-Cs photocathode which has recently been investigated a t our institute. We had several reasons for studying this type of photocathode, the most important being the difficulty encountered when trying to use the trialkali Sb-Na-K-Cs photocathode in image orthicons. The glass target of a camera tube with this type of photocathode exhibits considerable image retention which renders it unusable for normal purposes. Our experiments have led, in agreement with other authors, t o the conclusion that image retention is due t o the glass target being attacked by sodium vapour during the processing of the photocathode. I n our photocathodes sodium has been replaced by rubidium. 347

348

M.

DVO~AK

EXPERIMENTAL TUBE The photocathode was processed in experimental phototubes consisting of the image sections of image orthicons (Figs. 1 and 2). This configuration enabled the luminous and spectral sensitivity of the photocathode t o be measured when illuminated from either side. The electrical conductivity could also be measured. The photocathode

Fro. 1. Experimental phototube.

\

c

E

FIQ. 2. Schematic view of experimental phototube. -4, Photocathode; B, anode; C, photocathode contact; D, alkali generator; E, antimony evaporator; F, pumping stem.

SOME PROPERTIES OF Sb-K-Rb-Cs

PHOTOCATHODES

349

was deposited on the face-plate with two leads to a seven-pin stem. Attached t o the anode was the antimony evaporator and the alkali generators containing a mixture of alkali chromates with titanium powder as the reducing agent.

Processing of the Photoemissive Layer Following a bake at 380°C for 5 h, the alkali generators were thoroughly outgassed. The processing of the photocathode followed a procedure described by the author for preparing composite photolayers.2 The first processing step was to evaporate a layer of antimony on the face-plate. The tube was then heated to a temperature of 180 to 200°C and potassium was introduced until the sensitivity was rtt a, maximum. After depositing a second layer of antimony, rubidium was added a t a temperature of 180 to 200°C until another maximum was reached. At this step the sensitivity rose three to four times and the colour of the photosensitive layer Sb-K changed from violet to light brown. A third layer of antimony was then deposited and the subsequent activation with caesium a t a temperature of 160°C increased

Wavelength ( 8)

FIG.3. Spectral response 8 and quantum yield 7 of the Sb-K-Rb-Cs photocathode illuminated through the glass substrate.

350

M.

DVO~AK

the sensitivity two to three times; the photoemissive layer was yellowbrown in colour. Finally, with the tube at room temperature, the layer was superficially oxidized at a pressure of torr. This resulted in increasing the sensitivity above the maximum attained during the caesium activation.

LUMINOUS AND SPECTRAL SENSITIVITY Photocathodes processed according t o the above procedure had sensitivities of 15 to 20 pA/lm when illuminated through the gla,sssubstrate and 40 t o 70pAIlm when illuminated from the vacuum side.

100

-

90 -

3

a

5 vI

%

80 7060-

0

a VI

e!

50-

-

-Be

40

-

v)

30 20 -

Wavelength

(d)

and quantum yield 9 of the Sb-K-Rb-Cs photocathode upon illumination from the vacuum side.

FIG.4. Spectral response

8

The temperature of the tungsten lamp used for the luminous sensitivity measurements was 2850°K. Figure 3 shows the spectral response curve in mA/W and the quantum yield for a photocathode illuminated through the glass side. Zigure 4 shows the same curves for the same sample when illuminated from the vacuum side. A comparison shows that the curves differ t o a great extent in shape and abBolute value. The spectral response of the photocathode

SOME PROPERTIES OF Sb-K-Rb-Cs

PHOTOCATHODES

351

illuminated through the glass (Fig. 3) is very peaked with a maximum a t 5400 A and a quantum yield of only 2%. The luminous sensitivity of the photocathode was in this case 14.8 pA/lm. The curve in Fig. 4 for illumination from the vacuum side, has a maximum in the ultraviolet region a t 3300a, where the high quantum yield of 39% is reached. The sensitivity measured from this side was 67-6 pA/lm. The long-wave threshold A. of the samples ranged between 6900 and 7200 A. Taking A. = 7000 A as an average value gives a photoelectric work function of 1.8 0V.

ELECTRICAL CONDUCTIVITY The dependence of the dark conductivity on temperature was measured in the range of -50" t o +90°C. The activation energies AW as determined from the slope of the straight lines (see Pig. 5 ) were I 0-5-

10-6 -

"?t+ \+ A W='.04eV \+ +

1

'c

z .>

\+

-

IO-~

c

V

TI

. I3 0 V

'; + +'

+'

r

~:=0-68ev 10-8

+'

-

\+\ +\

+\

t

(Temperaturer'

(OK-Ix

lo3)

Fro. 5 . Temperature dependence of the dark conductivity of the Sb-K-Rb-Cs photocathode,

362

M. D V O ~ ~ K

about 1 eV; a further group of energies ranging from 0.4 t o 0.7 eV were found; these were due to thermal electron excitation. Higher energy values, equal to the bandgap energy were not found in the measured temperature range. For still higher temperatures, above 90°C, changes on the surface and inside the photoemissive layer must be anticipated. From a measurement of the sign of the thermoelectric e.m.f., the photocathode was found to be a p-type semiconductor.

ENERGY DISTRIBUTION OF PHOTOELECTRONS The energy distribution of the emitted photoelectrons was determined using the retarding field in a spherical condenser as shown in Figs. 6 and 7. The photocathode was prepared on a small target outside the spherical measuring section which was isolated by a thin glass membrane. Thus the deposition of alkali metals on the spherical inner surface was avoided, this being in our experience a very important

FIG.6. Spherical condenser apparatus.

FIQ. 7. Schematic of spherical condenser. A, Photocathode; B, anode with Sb evaporator; C, alkali generator; D, glass membrane; E, spherical condenser; F, quartz window.

SOME PROPERTIES O F Sb-K-Rb-Cs

PHOTOCATHODES

353

condition for obtaining reliable measurements. Following the processing of the photocathode and the sealing-off of the tube from the pump system, the photocathode was moved into the spherical section after breaking the glass membrane. The photocathode was illuminated through a quartz window in the spherical section. Using this apparatus, current/voltage characteristics from zero values of the photocurrent up to saturation were measured (see Fig. 8), the

Collector potential ( V )

FIQ.8. Photoelectric current vs. collector potential characteristics measured using the spherical condenser with monochromat,ic illumination (photon energies: 3.4, 4, 4.3 and 5.15 eV).

photocathode being illuminated by monochromatic radiation. The energy distribution of the photoelectrons is given by the curves in Fig. 9 which were obtained by differentiation of the current/voltage characteristics. For a photon energy of 3.4eV most of the emitted photoelectrons have energies around 0.5 eV. With increasing photon energy a rise in the number of fast photoelectrons having energies from 0.8 to 1.6eV could be observed. The photoelectric work function derived from this measurement is 1-8eV, which is in good agreement with the value calculated from the long-wave threshold (Ao = 7000 A).

354

M. D V O ~ ~ K

Photoelectron energy ( e V )

FIG.9. The energy distribution of the photoelectrons for various photon energies.

CONCLUSION

A new alkali-antimonide photocathode composed of Sb-K-Rb-Cs was investigated and its properties determined. The discrepancies of the luminous and spectral sensitivities between illumination through the glass or from the vacuum side can be attributed to the fact that the photocathodes were thicker than is common with semi-transparent photoemissive layers. When illuminating the photocathode through the glass substrate, blue light is absorbed and electron excitation occurs near the substrate so that photoelectrons are unable to reach the surface. This absorption is thus photoelectrically ineffective as the layer behaves for these wavelengths as a filter and the spectral sensitivity is low in this region. Light of longer wavelength is absorbed t o a lesser extent and penetrates nearer to the surface of the layer and the photoelectrons can escape more easily. This results in increased sensitivity for the red region of the spectrum. For illumination from the vacuum side no filter-effect for the blue light occurs and the photocathode has maximum sensitivity in this region. We believe that a decrease in the thickness of the layers would result in thin semitransparent photocathodes having properties similar to those we have found for thicker layers illuminated from the vacuum side. Such photocathodes would be suitable for use in the blue and in the near ultra-violet region with an input window of uviol glass or quartz.

SOME PROPERTIES O F Sb-K-Rb-Cs

PHOTOCATHODES

355

REFERENCES 1. Sommer, A. H., Rev. Sci. Inetrum. 26, 726 (1955). 2. Dvo?ak, M., Slaboproudd Obzor 24, 377 (1963).

DISCUSSION Can you explain the fact that sensitivity in terms of pA/lm differs by a factor of 3 whilst in terms of spectral efficiency it differs by a factor of more than 20, considering the area of integration when illuminating from different sides? M. DVOBAK: The difference is due to the different position of the maximas of both curves. Because of the relative spectral energy distribution of the tungsten source (2850"K),the value of the luminous sensitivity will not be much influenced by the high quantum efficiency a t 3300A. The increase of the luminous sensitivity for the illumination from the vacuum side is caused by the higher quantum efficiency values of the photocathode in the region of 4000-5400 A. w. E. TURK: When this new photocathode is used in image orthicons, is the sticking effect eliminated? M. DVOBAK:This photocathode has not yet been used in image orthicons. R. w. AIREY: What were the optical transmissions of the layers of antimony evaporated during cathode activation? M. DVOBAK: Three layers of antimony were evaporated. The optical transmission of the fist layer was 75%. The second and third antimony layer were evaporated before sensitizing with rubidium and caesium respectively, their thickness being such as to cause the sensitivity of the photocathode to drop to zero in both cases. E. H. WAGNER:

This Page Intentionally Left Blank

Decay of S-20 Photocathode Sensitivity Due to Ambient Gases R.W. DECKER Westinghouse Aerospace Division, Baltimore, Maryland, U.S.A.

INTRODUCTION As described in another papert in this volume this laboratory is developing$ a large-image electronographic camera for recording highresolution images directly on photographic film with photoelectrons. The criterion for the life of the camera and indeed whether the camera will even operate initially is: can the partial pressures of contaminating gases be maintained a t such a low level that the photoresponse of the photocathode will remain high? I n a study program conducted over the last two years, the contaminating effects of pure gases on the response of the trialkali 5.20 photocathode were measured. These effects were then related to the outgassing of materials used in the camera. All photosensitive surfaces that are responsive t o low-energy photons in the visible spectrum are very reactive. The sensitivity of a photosurface is altered by two general forms of contamination. First, deposits of foreign material on the substrate on which the photosurface is formed will cause a poor photocathode. I n this study it was assumed that if a good photocathode is formed and remains stable, the effect of substrate contamination is negligible. Second, the sensitivity of a photosurface can be changed by gaseous materials depositing on the surface, these coming from the outgassing of materials in the device, the vapor pressure of materials, and leaks. We have measured the outgassing of materials, and in the design of the electronographic camera have reduced this by a suitable choice of materials and processing technique. The materials tested were processed in the manner appropriate to their operation in the camera, and the outgassing was measured with

t See p.

19.

2 Under contrmt from the U.S. Air Force. 367

358

R. W. DECKER

a residual gas analyzer. Stainless steel, Viton-A, Teflon, Mylar, and Kodak SO- 159 film were studied and the predominant gases found were hydrogen, nitrogen, methane, oxygen, carbon dioxide, carbon monoxide, and water vapor; those having the highest partial pressures being hydrogen and water vapor.

EXPERIMENTAL PROCEDURE

A special vacuum system, shown in Fig. 1, was constructed for measuring the residual gases. This system consists of three separate pumping stations, a gas bottle manifold,l n test chamber, and a residual gas analyzer, all interconnected by bakeable valves. A diagram of the arrangement is shown in Fig. 2. Each atation can be isolated from the

FIQ.1. Vacuum system for measuring residual gases.

DEUAY OP S.20 PHOTOCATHODE SENSITIVITY

359

others, the bakeable leak-valve between the gas manifold and the test torr against chamber being able to maintain a vacuum of atmospheric pressure. First the system was evacuated and baked t o obtain a low background pressure, typical total pressures obtained being in the range of

0 Valve Standard leaks

FIQ.2. Diagram of vacuum system.

to tom. At such pressures the most persistent gas was hydrogen. Second, a photosurface was formed in the photodiode attached t o the test chamber; a photodiode assembly is shown in Fig. 3. After the 5-20photocathode was formed, it was cooled and its stable response was measured using a calibrated light source. The sensi-

FIQ.3. Photodiode assembly.

360

R . W. DECKER

tivity was typically > 120pA/lm. I n theinitial studies only the luminous sensitivity was measured, but later the spectral response was also measured with a grating monochromator. The spectral response of the S.20 was found to be a more sensitive measurement of the contamination than the total response because the response in the red decreases more rapidly than that of the mid-range of the visible spectrum. The response of the photocathode was then monitored continuously as the partial pressure of the pure gas admitted was increased in steps. The gases used wcre hydrogen, nitrogen, methane, oxygen, carbon dioxide, carbon monoxide, chlorine, and water vapor. The purity of the gas that was leaked into the system from the gas manifold was measured by the residual gas analyzer. I n the first experiment the gases admitted as contaminants were hydrogen, nitrogen, and methane, the photoresponse of the photodiode was initially 190pA/lm, and the background pressure was below torr total pressure in the measuring system. The first gas admitted through the leak valve was hydrogen. The residual gas analyzer was used to measure the partial pressure as well as the purity of the gas: at a pressure of 2 x torr of hydrogen the system pressure was essentially due t o pure hydrogen. The pressure was then increased in steps by opening the leak valve, and a t each step 5 min was allowed for the gas to react with the photosurface. The response of the photocathode was monitored by recording the cathode current while it was being illuminated by a 0.1-lm calibrated light source. From two to four measuroments were made with each gas t o make sure that the results were reproducible.

RESULTS The reaction a t a surface is proportional to the number of gas molecules that strike it. The number of monolayers of gas striking a surface in a 5-min period as a function of pressure of the gas is represented in Fig. 4, and this indicates that a t pressures greater than 8 x lo-* torr, in 5 min a t least one monolayer of gas has struck the photosurface t o react with it.2 Since semi-transparent 5.20 photocathodes are very thin, any change in response due to contamination of the surface should be noticeable within 5 min. A composite graph of the reactions of hydrogen, nitrogen, and methane with the 190-pA/lm S.20 photocathode is shown in Fig. 5 . The hydrogen and nitrogen pressures were increased to 2 x torr without a permanent change in the luminous response, although the response was lower while the pressures were high. With methane the response decayed when the pressure reached 1 x torr (a contamination rate of 2000 monolayers/min).

DECAY O F 9.20 PHOTOCATHODE SENSITIVITY

36 1

A similar graph showing the reaction when oxygen, carbon monoxide, and water vapor were introduced to a 160-pA/lm S-20 phutotorr of cathode is shown in Fig. 6. At a partial pressure of 5 x oxygen the response decreased to 110 pA/lm in 10 min, but recovered

Pressure ( torr 1

4. Vacuum contamination rate as a function of pressure.

Fra. 5 . Photocathode contamination by hydrogen, nitrogen and methane. Continuous line partial pressure; broken line, photocathode sensitivity. Initial response 190 eA/Im.

to 128 pA/lm after the oxygen was removed. Contamination by carbon monoxide commenced at a pressure of 5 x 10 - 5 torr, the initial response of 182 ,uA/lm falling to 145 pA/lm. When water vapor was introduced, the contamination began a t a much lower partial pressure, and in order to obtain a more sensitive measure of the effect on the photo-

362

R. W. DECKER

response, each partial pressure was maintained for 1 h. Figure 6(c) shows that the decay in the response began at 2 x torr. More complete data on each gas contaminant is given in Table I, which lists the gas contaminants, the initial photoresponses, and the partial pressure at which a change in response occurred for each experiment. As the Table shows, the partial pressure a t which the response first begins to decrease is quite reproducible for each contaminant. In the study of vacuum materials it became obvious that all the gases in the electronographic camera except water vapor can easily be held

E

2

1

I

I

I

I

\

I

I

1

-- .

I

I

1-200;

t

a

"-*O

'

~

O

Time (min) Oxygen

'

~

' 0~

'O

~'

~

Time (min) Carbon monoxide

b ' ; 0

'

'

a'

6

Time ( h ) Water

FIG.6. Photocathode contamination by oxygen, carbon monoxide and water vapor. Continuous line, partial pressure of contaminant; broken line photocathode sensitivity.

to a much lower partial pressure than is required to prevent the photocathode response decaying. Of particular interest for the electronographic camera, the gases evolved from Mylar and Kodak SO159 film are only water vapor and hydrogen, and since the latter is not detrimental to the photocathode, the water vapor is the problem. The detrimental effect of this may be due to the OH radical because neither hydrogen nor oxygen affect the photocathode to the same extent. I n the electronographic camera water vapor is difficult to remove, but we have found that by vacuum-preprocessing SO-159 film, the partial pressure of water can be maintained at 5 x torr with small holding pumps. To determine more about the effects of gaseous contaminants on 5-20 photocathodes, we used a grating monochromator to measure the

363

DECAY O F 5.20 PHOTOCATHODE SENSITIVITY

spectral response of the photocathode a t each partial-pressure increment. The results obtained when a sensitive photocathode was exposed t o water vapor are shown in Fig. 7. Note that it is the longer wavelength response that is first affected by the contaminant. The response TABLEI Summary of contaminant tests

Contaminant

Initial photoresponse (pAllm)

Decay first noted (torr)

x

10-4

x

10-4

Remarks

100 190 63

2

Nitrogen

100 190 182

5 x 10-5 2 x 10-4 2 x 10-4

Response recovered

Methane

108 180 182

I x 10-4 1 x 10-4 1 x 10-5

Slight recovery

92 155

1 x 10-6 6x

Partial recovery

Carbon dioxide

96 165 130

2 x 10-8 2 x 10-8 5 x 10-8

No recovery

Carbon monoxide

165

i 30

5 x 10-6 5 x 10-5

164 140 125 87

8 7 5 5

165

2 5

Hydrogen

Oxygen

Chlorine

Water vapor

158

2 x 10-4 2

Response recovered

Slight recovery

x lo-‘ x 10-7 x 10-7 x 10-7

x

x

10-7 10-8

Slight recovery

No recovery

before contamination (labelled “pre-contamination” in the figure) was stabilized for 10 days a t a background pressure of less than 2 x torr. The life of an S.20 photocathode can be estimated from these data. At a partial pressure of water vapor of 1 x lo-’ torr, the fall in the peak response will not exceed 10% over a period of a t least 10h.

R. U’. DECKER

364

However, at the same partial pressure of water vapor the spectral response curve A (Fig. 7 ) shows that the red response a t 8500 A has changed from 3.8 mA/W to 2.4 mA/W in 45 min, a change of 33%. At this wavelength the decay rate is 1.8 mA W-’ h - l , and from Fig. 4 the contamination is 132 monolayers in 1 h a t lo-’ tom, so that the decay can be expressed as 1-4 x mA/W per monolayer. With this figure one can then compute an estimated life for the photocathode if one knows the partial pressure of water vapor. For example, let a response of 1 mA/W a t 8500 be the minimum response that will -

-

Partial Time(min1 pressure (torrl

Pre- contamination

-

A 45

IxIO-~ . _c

-

[r

L

l 4000 l

l 5000l

l 6000 l

l7000 l

8000 l l

9000 l

l

IC

I00

Wavelength

(8. )

FIG.7. Spectral response of photocathode after exposure to wattor vapor at various partial pressures and for various times. Luminous eensitivity: initial, 350 pA/lm; after 45 min at torr, 330 pA/lm.

give the sensitivity required and let the initial sensitivity of the photocathode be 4 mA/W a t 8500 A. If the partial pressure of the water vapor were 5 x l o d 8 torr, the contamination rate (from Fig. 4 ) would be 72 monolayera per hour. Then using the decay conversion figure previously derived (1.4 x mA/W per monolayer) the decay rate is 72 x 1-4 x M 1 mA/W per hour or a 3-h life for the photocathode. It is important t o note that the life of the photocathode depends on the spectral region of the incoming radiation. I n the above example the red response has been taken as the criterion; however as can be seen from Fig. 7 the decay in the remainder of the visible spectrum is much less and in fact the luminous sensitivity would still be 90% of the initial value.

DECAY OF 5-20PHOTOCATHODE SENSITIVITY

365

The most significant result of this study is that the sensitivity of a photocathode is selectively degraded by contaminants and is most degraded in the longer wavelengths. Hence contaminants may be only a small problem for images in a particular region of the visible spectrum but a much larger problem in other regions. ACKNOWLEDGMENTS The author wishes to thank Mr. J. S. Knoll and Mr. W. H. Beck I11 for their work on the fabrication of test photosurfaces and measurements of residual gases.

REFERENCES 1. Hastings-Raydists Inc., “Calibrated Gas Leaks”. Specification Sheet 904, June 1966. 2. Dushman, S. and Lafferty, J. M., “Scientific Foundations of Vacuum Technique”. Wiley, New York (1962).

DISCUSSION w. o. TRODDEN: The decay curves shown appear to represent a dynamic rather than a static equilibrium state, in that poisoning gases were pumped out while the photoemission was still decaying rapidly. It might be more realistic to show the steady otate corresponding to a gas partial pressure. Have you any such data? R. w. DECKER: The system is a dynamic system at equilibrium. That is, the gas flow in is equal to the gas being pumped out. The partial pressure is measured and maintained a t the constant values reported for the period of time of 5 min or 1 h as shown in Figs. 5 and 6. M. HIRASHIMA: Could you tell me about what kind of gas is most predominant among the residual gases in electronic image devices, apart from cesium vapor? If I remember correctly, it is believed that CO is most predominant in ordinary receiving tubes. R. w. DECKER: The type of gas present in a particular tube depends on the construction. In all stable tubes the partial pressure of the gas present must be far below the level that caused contamination as reported here. In metal tubes, hydrogen and helium are present with hydrogen predominating. In tubes with an electron gun, CO predominates. E. ZIEMER: In your work on photocathode decay as a result of the introduction of various gases, did you study the effects of materials such as Viton A and Teflon. I f so, what was the effect? R. w. DECKER: The degassing of Viton A and Teflon was studied. The details of the degassing are extensive and very dependent on the preparation of the materials prior to exposing to a photocathode. If properly prepared, Teflon will not affect an 5-20photocathode. Viton-A has always caused the photocathode to decay. H. BACIK: Do you know of any similar work being done on S.1 photocathodes? R. w. DECKER: Y e s , it is now under way but there are no results to report.

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A New Technology for Transferring Photocathodes P. DOLIZY and R. LEGOUX Laboratoirea d’&lectronique et de Phyeique Appliqude, L~med-Brduannea,Prance

INTRODUCTION The conventional method of preparing the photoemissive layer in a photoelectric tube is by the thermal evaporation of antimony and alkali metals from sources mounted inside the tube. Some of these sources must be located either directly in view of the photocathode, so as to achieve uniform evaporated layers, or in such a manner that the molecular diffusion rate is relatively high. It is easily understood that such sources prohibit the use of quite a number of electrode configurations, particularly those in which the electrodes are placed close to the photocathode. Moreover, with the conventional method, the alkali vapour permeates throughout the tube during processing, and very often interacts chemically with the surface of the electrodes. This can enhance undesirable effect,s such as leakage currents, field emission, and parasitic photoemission. It can be said that, as a general rule, the performance of the photocathode is dependent to a large extent on the internal structure of the tube. These troubles, often encountered with conventional tube processing methods, can be avoided by the so-called “transfer technique”. The main features of this technique are as follows. (1) The substrate, on which the photoemissive layer is to be formed, is isolated from all other component parts of the photoelectric tube, so that it alone is exposed to the materials evaporated during the processing of the photocathode, (2) The several evaporation sources are grouped together in an auxiliary enclosure, which is located in front of the substrate and is maintained in that position throughout the processing of the photocathode, after which it is removed. (3) The photoemissive layer is only transported to its final position in the tube, if, after stabilization and cooling, it has the required photoelsctrio properties. 367

368

P. DOLIZY AND R . LEGOUX

DESIGNO F THE “TRANSFER” TUBES The design of tubes to be made by the transfer process is strongly influenced by the way they are to be pumped and processed in the special chamber. The envelope of a tube to be made by this process is divided into two parts, each of them containing elements of the tube itself. One of these two parts is often the input window of the tube, and in many msea, this is used as the photocathode substrate. The two parts are eventually joined together by metal rings (Pig. 1). However, the tube is first pumped and the photocathode processed with the two parts separated. The two parts are then joined by an indium seal which is made by pressing the two rings together, one of these having a groove Glass window and Dholocathode substrate

Metal rings

FIG.1. Indium compression seal a t front of phototube. The electrode structure in the lower half of the tube is not shown.

to hold the indium and the other a tongue. Indium is particularly suitable for the purpose because of its great malleability, its low vapour pressure at high temperature, and its great resistance to oxidation. The seal-off tip, which is usual on convendional tubes, is of course no longer required.

THETRANSFER EQUIPMENT A photograph of transfer equipment capable of handling photocathodes up to 120 mm in diameter is shown in Fig. 2. A cross-section of the cylindrical transfer enclosure is shown in Fig. 3. This enclosure is made of stainless steel and the top, which is in the form of a bell-jar of glass or metal, is removable. The two halves of the tube to be processed can be seen in the figure. One half of the tube, pre€erably that including the photocathode substrate, is fastened to the upper part of

NEW TECHNOLOGY BOR TRANSFERRING PHOTOCATHODES

369

the enclosure with a clamping ring held by two columns. The other is set up in the lower part of the enclosure on a movable table guided by the columns and connected to a hydraulic press. At a later stage this lifts up the lower half of the tube for the seal to be made to the upper half. All the evaporation sources necessary for the preparation of the photocathode are grouped together in an auxiliary enclosure which is open at the top and can be moved sideways by a second mechanism.

FIG.2. The transfer apparatus.

The system is pumped first with a cryogenic pump (zeolite and liquid nitrogen) to about 6 x l o T 4torr and then either with an oil diffusion pump having two refrigerated baffles in series or with a Penning ion pump. The ultimate vacuum as measured by the ionization gauge shown in Fig. 3 is generally of the order of torr. The entire equipment is outgassed for 16 h during each pumpiiig cycle by heating coils at a temperature of 260°C, while the transfer bell-jar is brought to between 400°C and 460°C. When the bake-out is terminated, the photoemissive layer is processed in the conventional way but with modifications to allow for the large volume of the processing chamber. The pressure is then about torr.

370

P. DOLIZY AND R. LEQOUX

FIQ.3. Croae-section of the transfer encloaure.

NEW TECHNOLOGY FOR TRANSFERRING PHOTOCATHODES

37 1

During the processing, the sensitivity of the photocathode is monitored using the light from a tungsten filament lamp placed above the bell-j ar, As soon as the photocathode has cooled to a temperature of between 40°C and BO'C, and has stabilized, then, providing that the required photoelectric properties have been achieved, the tube is ready to be closed. The pressure at this stage is torr. In the closing process the sensitizing enclosure is first moved to a lateral position in the belljar leaving room for the tube to be lifted up (Fig. 4). The two parts of

Processing enclosure

1 I

FIG.4. Showing how the processing enclosure is moved to one side and the tube is closed.

the tube are then joined by compressing the indium seal. The photocathode is now in its operating position. Air is let into the bell-jar and the tube can now be removed from its mechanical supports and is ready for use. OF THE METHOD ADVANTAGES The advantages of the method are numerous. Apart from those that have already been noted, it avoids the poisoning of component parts of the tube by the physical and chemical action of alkali vapours during the photocathode processing. This applies in particular to the following; the fluorescent screens of image tubes, surfaces which must be highly insulating, and electrodes which must be free from field emission or photoemission. Photocathodes made using the method are comparable to those

372

P. DOLIZY AND R. LEQOUX

obtained by conventional methods. Trialkali photocathodes with sensitivities higher than 200 pA/lm have been achieved on glass and metal substrates. The uniformity of the photocathodes is also improved because the evaporation sources can be placed at greater distances from the substrate than are possible in conventional tubes.

FIQ.5 . High current photodiode. Anode t o cathode spacing 2 mm.

After a photocathode has been processed it can be tested before its introduction into a tube, and if inadequate it can be rejected, thus avoiding throwing away the entire tube. A most important aspect of the process is that it makes possible the construction of tubes in which the dimensions and shape are such that photocathode processing would not be practicable by conventional methods. As an example of such a tube, Fig. 5 shows a high current

FIR.6. High-speed shutter tubes.

NEW TECHNOLOGY FOR TRANSFERRING PHOTOCATHODES

373

photodiode in which the distance between the cathode and anode is 2 mm. Figure 6 shows a family of fast shutter tubes for which the photocathode diameters range from 40 mm to 120 mm, and the cathode to phosphor distances from 2 mm to 10mm. These are more fully described in another paper in this volume.?

DISCUSSION s. MAJUMDAR: 1. What current can you draw from the high current photodiodes? 2. In your image tubes, does the cathode performance deteriorate after many hours of operation? J . GRAF: 1. These diodes deliver a linear rosponse up to 10 A for an applied voltage of 3 kV and a pulse of 1 psec. The saturation current is 20 A. 2. We have not seen any change in cathode performance on a tube tested for 5 h continuous running. I t must be noticed that these types of tubes aro designed only for pulse operation. M. ROME: What improvements are found in dark current by the use of the transfer technique? Would you please compare the dark current of tubes with the same photocathode, (e.g. type 5-20),of similar sensitivity, which differ only that some are conventionally prepared and others by the transfer method? J. GRAF: For the same 5.20 sensitivity in two photomultiplier tubes, one conventionally prepared and the other by the transfer method, the transfer tube has a dark current nearly hundred times lower than the conventional tube. R. DECKER: 1. Is the tube isolated or just shielded from the photocathode processing chamber? 2. Is it possible to process more than one cathode a t one time? 3. How far do the bellows have to deflect to make a seal? J . GRAR: 1. The body of the tube is only separated from tho sensitizing enclosure containing the alkali dispensers. There is not a tight separation between the dispensers and the body of the tube. 2. Yes, it is possible to process simultaneously several photoemissive layers in the transfer equipment. 3. The deflexion of the bellows depends upon the height of the sensitizing enclosure which is itself a function of the diameter of the cathode to be processed. The maximum dellexion is 40 cm. R. AIREY: Have you attempted to effect an indium seal by bringing the parts together in the presence of the molten metal, thus eliminating the need for a high pressure hydraulic ram? J . GRAF: The sealing of tubes by means of a molten metal joint can be done provided that the gases desorbed by the joint inside the tube do not spoil the characteristics of the photoemissive layer and of the tube structure.

t See p. 989.

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Improvements to Photocathodes for Pulse Operation €3. R. C. GARFIELD and J. R. FOLKES

Engliah Electric Valve Co. Ltd., Chelmeford, England and

B. T. LIDDYt Department of Pure and Applied Phyeics, Queens University, Belfast, Northern Ireland

INTRODUCTION For pulse-operated image tubes it is necessary to deposit the photocathode on a transparent, low resistance substrate to prevent image distortion due to saturation effects.$ Because of its high overall sensitivity the trialkali photocathode is a desirable one to use. Unfortunately the usual substrate material, tin oxide (Nesa),is unsuitable for use with the trialkali photocathode as it is rendered highly resistive by reaction with sodium. I n addition, the resulting photosensitivities are usually lower than normal. As an alternative to Nesa as a conducting substrate a fine mesh embedded in the glass face-plate has been investigated. The resistance, sensitivity, spectral response and pulse performance have been measured for cathodes deposited on such substrates. TRIALKALI PHOTOCATHODE NESASUBSTRATES An investigation was made to determine at what stage during the processing the Nesa was attacked. The results, which are given in Table I, show that the substrate resistance was relatively unaffected by potassium, but that after the introduction of sodium it had increased by several orders of magnitude. The photo-cathode sensitivities were low and the deep orange colour of transmitted light suggested that the antimony component in the Nesa had been attacked.

t Temporarily at the English Electric Valve Co. Ltd. $ See p. 999.

376

B . R. C . OARFIELD, J. R . FOLKES AND B. T. LIDDY

376

TABLEI Variation of resistivity during trialkali processing on an antimony-doped Nesa substrate Initial Resistance

Resistance after K

Resistance after Na

a/0

Q/ 0

a/0

6

68

72

3.2 x 106

10

45

50

1-6

Tube No.

x lo6

Final resistance Q/

0

4 x 105 1.8

x 106

Photocathodes were also processed on Nesa substrates prepared from a formulation containing no antimony. The results, which are listed in Table 11, show that a major resistance change again occurred during the sodium stage, indicating that the tin oxide itself was attacked. TABLEI1 Variation of resistivity during trialkali processing on an undoped Nesa substrate Initial resistance

Resistance after K

Resistance after Na

Final resistance

a/0

Tube No.

a/0

14

140

150

7.6 x 104

2 x 106

15

150

164

2.1 x 104

4 x 104

16

140

140

2 x 105

5.4 x 105

0

Q/O

MESH SUBSTRATE For this substrate a fine metallic mesh is embedded in a Pyrex glass disc -3 em in diameter and the photocathode is formed on this. Copper meshes of 750 mesh/in. and 55% transmission, having square apertures of length -25 pm and bar thickness of -9 pm are used. The glass disc is prepared by sandwiching it with a stretched mesh between two flat carbon blocks and applying a pressure of approximately 20g/cma. The assembly is heated in a reducing atmosphere (90% N,, 10% H,) t o a temperature of between 750 and 800°C and is then cooled slowly ( 3 to 4 h) to room temperature. Scanning electron microscope studies indicate that during the forming process the glass flows up through the interstices thus firmly embedding the mesh. The surface resistivity of this substrate is less than 0.1 Q/o.

377

PHOTOCATHODES FOR PULSE OPERATION

Trialkali photocathodes were prepared on these substrates by standard processing techniques. Sensitivities were in the range 60 to 120 pA/lm, which corresponds to 100 to 200 pA/lm for the cathodes in the clear apertures, after allowance is made for the light obscured by the mesh. The spectral response was normal.

PULSE PERFORMANCE OF A TRIALKALI PHOTOCATHODE ON A MESH To investigate the pulse performance of these mesh photocathodes, a number of small test diodes of a co-axial design were prepared. I n these, the substrate disc was mounted close to the glass window and after photocathode processing a flat metal anode disc was moved into position a few millimetres from the photocathode surface and secured. For pulse evaluation the photocathode was illuminated by the light from a ruby laser (having a pulse half-width of about 50 nsec).

/+

I I I

i

I

I 005

I 0.10

I 0.15

I 0.2c

Relative light intensity

FIQ.1. Pulse performance of trialkali photocathode on glass substrate.

Uniform illumination was ensured by placing a diffusing screen a t an appropriate distance from the photocathode while the light intensity was varied by inserting suitable neutral-density filters. I n order t o correct for variations in laser output between pulses, a fraction of the output was arranged t o be incident on a second reference diode (E.M.I. Type 9648B with an S.10 photocathode on a metal substrate),this being operated a t a current density of less than 1 mA/cm2 t o ensure linear operation. The output signals from both diodes were fed simultaneously into a Tektronix 556 double-beam oscilloscope via 5042 terminations. The peak current obtained from the test diode was correlated with the P.E.1.D.-A

14

378

B. R. C. QARFIELD, J . R. FOLKES AND B . T. LIDDY

corresponding incident light intensity. Figures 1 and 2 show the performance of two such diodes. It can be seen that in the case of the trialkali on glass, saturation effects are evident a t current densities of less than 1 mA/cm2,whereas for the trialkali on a mesh these appear a t a current density of about 300 mA/cm2. Calculations using the ChildLangmuir equation indicate that the saturation evident a t the highest current densities was probably due to space-charge limitations.

c

m

t

u

Relative light intensity

FIQ.2. Pulse performance of trialkali photocathode on mesh substrate.

ANALYSISOF MEsIr PHOTOCATHODE Assuming a circular cathode configuration, a uniform emission current density and a uniform surface resistivity, analysis shows that the potential drop V a t the centre of the photocathode is given by the formula:

v=- p l4R2’ where p is the surface resistivity of the photocathode, I is the emission current density and R is the radius of the photocathode. Thus for p = lo7 Q/o, R = 10 pm and I = 1A/cm2, the potential drop a t the centre of each photocathode element is N 2.5 V.

PHOTOCATHODES FOR PULSE OPERATION

379

MESH PHOTOCATHODE IN A PRACTICAL DEVICE A mesh trialkali photocathode of 100 pA/lm sensitivity has been prepared in an English Electric Valve Company shutter tube, type P 8 5 6 . I This has been operated under pulse conditions in a Hadland ‘Imacon” camera. No detailed tests have as yet been made but preliminary observations are favourable.

CONCLUSIONS There are now two substrates available for processing semi-transparent photocathodes with saturation thresholds at high current densities: the conventional low resistance Nesa substrate, and the mesh as has been described. For use with trialkali photocathodes, the mesh substrate is necessary as the resistivity of the Nesa substrate is seriously affected by reaction with the alkali metals used in the photocathode processing. ACKNOWLEDGMENTS The authors would like to thank Mr. R. A. Chippendale of English Electric Valve Company and Prof. D. J. Bradley of the Queen’s University of Belfast for encouragement during the course of this work, and tho Managing Director of the English Electric Valve Company for permission to publish this paper. One of us, B. T. Liddy, is supported by a Postgraduate Studentship from the Northern Ireland Ministry of Education.

REFERENCE 1. Huston, A. E. and Walters, F., In “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, W. L. Wilcock and L. Mandel, Vol. 16, p. 249. Academic Press, New York (1962).

DISCUSSION J. D . MCCIEE: 1. Has the

use of the mesh to improve conductivity of the trialkali cathode improved the stability of the cathode? 2. Have you observed electrolytic effects in photocathodes? B. R. C . GARFIELD: 1. It is known that the trialkaliphotocathode,in common with most other types of photoemitter, fatigues during continuous operation at high current densities ( > N 1 pA/cma). The probable mechanism of this is electrolysis (due to the voltage drop across the layer produced by the photocurrent) of the constituent alkali metals through the layer, leading to departure from stoichiometry in the bulk, and loss of caesium from the surface (Miyazawa, H. and Fukuhara, S., J . Phys. SOC. Japan 7,645 (1962);Garfield, B. R. C. and Thumwood, R. F., Brit. J . Appl. Phye. 17, 1005 (1966)). Thus fatigue is substantially reduced when photocathodes are formed on conducting substrates (Nesa or solid metal) (Linden, B. R., I n “Advances in Electronics and Electron Physics”, Vol. 16, p. 311. (1962)). The mesh substrate is expected to be equally effective in this respect. 2. In the case of pulse opcration a t current densities of the order of 1 A/cm2, it seems likely that i t is the total charge drawn in any given time

380

B. R . 0. OARFIELD, J. R. FOLKES AND B . T. LIDDY

(hours or days) which will determine the extent of fatigue. In the present work the cathodes had only minimal pulse usage, and therefore electrolytic effects and fatigue were not expected to be appreciable. This appears to be the case as sensitivities have remained stable to within about 10%. M. K. KEY: Is it possible that the non-linear behaviour observed with the trialkali photocathode that was not deposited on a low resistance substrate is due not only to the voltage drop associated with the surface resistivity, but also to a fundamental solid-state property of the photocathode? This suggestion arises from observations with image converter cameras in our 1aboratory.t A. s . JENSEN: As a partial answer to the question by M. H. Key, in our laboratory about four years ago, we were working with a photostorage tube having a grating storage target (Jensen, A. S. et al., I n “Advances in Electronics and Electron Physics”, Vol. 22A, p. 155. (1966)) which is essentially a transmissive storage grid. When we illuminated the photocathode with a small spot of light, the current passing through the storage grid was such a function of the light intensities as could be explained by assuming that the photocathode surface potential pattern was a positive peak. Thus while this does not say that there is no solid-state effect, at least the positive swing of the photocathode voltage as a result of the voltage drop across its surface is a part if not all the explanation of the behaviour. B. R . C. GARFIELD: Our ideas concerning the non-linear behaviour observed with the trialkali photocathode on a high resistance substrate, are in agreement with those of A. S. Jensen, i.e. the current is space-charge limited due to a reduction of the effective applied potential between anode and cathode, resulting from the transverse voltage drop across the surface. More recent measurements of saturation current as a function of applied voltage, tend to confirm this. This does not, however, rule out the possibility that internal saturation effects may occur in photocathodes a t extreme levels of illumination and very fast pulse conditions.

t See p. 999.

Some Getter Materials for Caesium Vapour M. HIRASHIMA and M. ASANO Department of Electronic Engineering, University of Electro-Communicatione, Chofu City, Tokyo,Japan

INTRODUCTION Following the discovery that gold can be used as a getter material for caesium vapour,l efforts have been made t o find other suitable materials. I n general, materials to be used as getters for caesium vapour should be either metals that can react readily with the vapour or compounds containing elements that can so react. I n this connexion, Kienast and Verma2 have reported on the results of their exhaustive experiments with compounds of alkali metals and either copper, silver, or gold. As for caesium compounds, however, only AuCs is mentioned in their paper. Among the many compounds, oxides seem to be the most promising for gettering caesium. Not all oxides can, however, be used asgetters for caesium vapour, because the first requirement is that they must be reduced readily by caesium a t a temperature which is relatively low, but is not lower than the baking temperature. Furthermore, the final compound of caesium should preferably not be a good photoemitter. The value of the free energy of formation is a measure of the usability of oxides as getter materials for caesium vapour. To be precise, the absolute value of the free energy of formation of any oxide must be smaller than that of caesium oxide, and in fact, the smaller, the better. For this reason the useful oxides seem likely to be limited t o those tabulated in Table I.3 Of these, the oxides of tin and lead were probably used by Zworykin and his co-workers for gettering excess caesium when processing the 1conoscope.* It is also known among tube engineers that an Aquadag coating can adsorb a large quantity of caesium, but this has some drawbacks as a getter. I n the present paper the results of some experiments carried out with the oxides of nickel and iron will be described briefly. Copper oxide 381

382

M. IIIRASHIMA AND M. ASANO

TABLE I Free energy of formation of oxidest Element

Oxide

CAESIUM Tin Iron Nickel Cobalt Lead Copper Carbon Palladium Silver Gold

cs20 SnO FeO NiO coo PbO cu,o

co

PdO Ag2O Au203

25OC

Free energy 500°C

koal -79 - 61 - 59 - 52 - 51 - 45 - 35 - 33 - 20 -2 19

koal - 74 - 49 -51 -43 -43 - 34 - 28 -43 -18 +5 $31

+

1ooo"c kcal - 69

-36 - 45 - 32 - 34 - 22 -21 -54 -15 12 44

+ +

t After E ~ s t e i n . ~ was also studied without obtaining a satisfactory result. The possibility of using carbon monoxide as a caesium getter will also be briefly discussed, for this is one of the most common residual gases found in electronic tubes using oxide-coated cathode^.^

EXPERIMENTAL TECHNIQUE Oxides of Nickel and Iron As was the case with gold,l throughout the present experiments a silica spring balance was used t o measure the change in weight of each specimen during oxidation in an atmosphere of oxygen and also during the reduction of the oxide by caesium vapour. The sensitivity of the silica spring balance used was 0.2 mg per mm elongation. The silica spring was hung in the tubular quartz reaction chamber which was 30 mm in diameter and about 600 mm in length, as shown in Fig. 1. To the bottom end of the spring was hooked a strip of thin plate, in the case of the nickel, and a piece of thin wire, in the case of the iron. The quartz tube was connected to a conventional hard glass tube by a graded seal. It was heated from outside by means of a long electric furnace consisting of four sections, one of which was provided with a longitudinal slit through which the elongation of the spring could be measured using a telescope. The only differences from the experiments with gold were that a vessel containing potassium permanganate was connected to the reaction chamber via a tap so that oxygen could be introduced into the

GETTER MATERIALS FOR CAESIUX VAPOUR

383

reaction chamber in the oxidation stage, and pure hydrogen could also be fed into the reaction chamber through a small palladium tube provided with an electric heater. The hydrogen was necessary for removing the oxides that were almost always formed on the surface of the iron specimen during annealing in vacuum, even though the vacuum torr. pressure was only N

Electric furnaces

c

./ Sealed off after Cs admission

-

To H c linder 2

Y

To rotary pump

-

FIG.1. Schematic diagram of equipment for measuring the oxidation of nickel and iron, and the reduction of their oxides.

The preparation and treatment of the specimens were carried out in the following manner. As mentioned above, the nickel specimen was a strip. This wag 0.053 mm thick, 2 mm wide, and 15 to 20 mm long; it weighed about 17 mg, and the purity of the nickel was 99.99%. The specimen of iron was a wire 0.199 mm in diameter and -50 mm in length, weighing about 12 mg, and the purity was as high as 99.9987%. N

384

M. HIRASHIMA A N D M. ASANO

The ends of the iron wire were welded together to form a ring, and it was thus possible to heat it by induction in an atmosphere of hydrogen in order t o remove surface contamination prior t o oxidation. After degreasing, all the specimens were annealed for 30 min in a vacuum of torr a t the respective recrystallization temperatures, 760°C for nickel and 720°C for iron. The specimen of nickel was then oxidized a t 680 to 700°C in an atmosphere of oxygen a t a pressure of 63 torre%When the desired thickness of nickel oxide was attained, the oxygen was pumped out, and the pressure reduced t o about lo-? tom; meanwhile the electric furnace was kept at the same temperature in order t o bake the tube and was then lowered t o room temperature. Caesium was admitted into the reaction chamber from an appendage in which there was a nickel capsule containing a measured quantity of caesium chromate mixed with silicon as a reducing agent. The reaction chamber was then sealed off from the vacuum system. The temperature of the electric furnace was again raised t o a temperature of around 200"C, and was kept a t a constant value during the reduction period. I n the case of iron, the specimen was heated by induction from outside the quartz tube t o a temperature of about 780°C for 16 min in hydrogen a t a pressure of 3 torr. This removed surface contamination. The hydrogen pressure was then reduced to torr; oxygen was admitted into the reaction chamber a t a pressure of 63 torr, and the specimen was oxidized a t 540°C.' After the oxidation the oxygen was pumped out, and the temperature of the furnace was lowered t o room temperature; caesium was admitted into the reaction chamber, which was then sealed off from the vacuum system, as in the case of nickel. The temperature of the furnace was then raised t o the desired temperature for the reduction period.

Carbon Monoxide An ionization gauge? was used as the reaction chamber to see whether carbon monoxide could be used as a getter for caesium. As shown in Pig. 2 the ionization gauge was connected to a sealed ampoule containing caesium, and to an appendage which could be cooled by running water; an exhaust tube led to a carbon monoxide reservoir and t o a vacuum pump via suitable taps. The ionization gauge was evacuated to a good vacuum, and the envelope and electrodes thoroughly degassed. Then a small quantity of carbon monoxide was introduced into the gauge while the appendage was cooled with running water. When the pressure of carbon monoxide (measured by the gauge) was of the order of torr, the ionization gauge, the caesium ampoule and the appendage were sealed off, as indicated in Fig. 2. After the

t Toshibe, W-1.

GETTER MATERIALS FOR CAESIUM VAPOUR

385

caesium had been admitted into the gauge from the ampoule (by breaking the small glass tip with the iron ball), the whole apparatus was heated in the electric furnace to about 50°C, and was kept at this temperature for 10min. The gauge was kept at this temperature and the appendage was then cooled with running water, thus letting the caesium vapour in the reaction chamber condense into the appendage. After a further 10min the pressure of the carbon monoxide was measured, and it was found that the pressure had decreased as a result of the chemical reaction between the carbon

Iron

A

To

CO reservoir

FIG.2. Apparatus for measuring the reaction of caesium with carbon monoxide.

monoxide and caesium. The same procedure was repeated four times, the total reaction time being about 40min. I n conducting this experiment, however, it was difficult to keep the tube temperature constant a t a value as low as 50°C, because the tube temperature rose owing to the heat radiated from the filament of the ionization gauge while the pressure of carbon monoxide was being measured.

EXPERIMENTAL RESULTS Nickel Oxide ( N i O ) A typical example of the oxidation of a specimen of nickel and its reduction in an atmosphere of caesium is shown in Fig. 3; the weight

386

M. HIRASHIMA AND M. ASANO

gain of the specimen was 0.593 mg/cm2 after oxidation for 250 min. The number n, of oxygen atoms that have combined with the nickel per unit area of the specimen t o form nickel oxide can be calculated from the gain in weight: n, M 2.24 x 1019. On the other hand, the weight gain of the same specimen during reduction by caesium for 205 min was 5.8 mg/cm2, and the calculated number of caesium atoms reacting with the nickel oxide to form

0

1

2

3

4

0

1

2

3

Time ( h )

FIG.3. Typical curves of oxidation and reduction runs on a specimen of nickel.

caesium oxide and nickel is n2 M 2.63 x 1019 per unit area of the specimen : NiO 2 Cs + Cs20 Ni. (1)

+

+

Since two atoms of caesium combine with one atom of oxygen to form a molecule of caesium oxide, Cs,O, the number of the oxygen atoms that have been effectively used is equal to n2/2, assuming that all the caesium atoms have been converted into caesium oxide. Thus we can see that the availability of the oxygen for gettering caesium in this case is given by n 2 / 2 n , w 59%. I n Fig. 4, there are shown the curves for five specimens of nickel, the reduction by caesium being carried out a t different temperatures. I n the case of specimen No. 5 the oxide layer became detached after the reduction run had started. The experimental data obtained with the five specimens of nickel out of 15 specimens prepared a t the start are tabulated in Table 11. The fraction of the oxygen atoms reacting with caesium atoms to form caesium oxide (the valuea in the last

387

Nickel

----------N0.12

(

193OC)

Time ( h )

FIG.4. Reduction by caesium of specimens of niclrel oxide at various temperatures,

column of Table 11)is plotted as a function of the reaction temperature in Fig. 5 . It is seen from this figure that, as expected, the higher the reaction temperature the larger this fracbion becomes, but if the temperature is too high the oxide layer may become detached as was the case with the specimen a t 252°C (see Fig. 4). TABLEI1 Experimental data for five specimens of nickel FracWeight tion Thickgain Oxida- Reducof Surface nesst after Speci- Weight tion tion oxygen after of oxide dearea temp. temp. effect(mg) men (cm? OxIdation layer oxida("C) ively ("C) (pm) tion used (mg/cm2) (mg/cm2) Weight gain

(YO)

NO.3 NO.5 NO.6 No. 8 NO. 12

16.54 17.65 17-45 17.25 16.79

0.7294 0,7746 0.7642 0.7428 0.7227

0.593 0.384 0.186 0.279 0.317

t Calculated using the value:

3729 2410 1169 1750 1990

5.800 0.694 1.198 2.566

690 690 690 677 677

200 252 170 180 190

58.9 22.5 25.9 48.3

1 pg/cma = 62.9 A (Gulbransen and Andrews).

388

M. HIRASHIMA AND M. ASANO

p

zc

6ot

;I -5

Nickel

60-

4040

0

C

.t

2020

t

0

,x

Iron

-Xd-

I

1 I70

I0

I

I

190

210

Reaction temperature

("C)

FIG.5. Fraction of oxygen atoms reacting with caesium to form Cs,O, as function of reaction temperature.

Oxides of Iron (FeO and Fe,O,) A typical example of the oxidation and reduction of a specimen of iron is shown in Fig. 6. A comparison of the reduction curve of Fig. 6 with those in Figs. 3 and 4, shows that there is a marked difference in the shape of the curves. This difference may be partly attributed to the different form of the specimens of the two materials, viz. a strip of thin plate in the case of nickel, and a thin wire in the case of iron. However, the reason why the reduction curve for iron has two different slopes may be explained by the fact that two kinds of iron oxide have been formed8s0during the oxidation run, namely, FeO and Peso,; the first part of the reduction curve corresponds to the reduction of the TABLE111 Experimental data for four specimens of iron

Specimen

No. 2 No. 3 No. 8 NO. 10

Surface area

Weight Oxidation gain after temperaoxidation ture

(em2)

(mg/cma)

("C)

0-313 0.311 0.330 0.335

0.903 0.820 0.491 0.658

540 540 540 539

Fraction Weight Reduction gain7 of oxygen tempera- effectively after ture reduction used (mglcm") ("C) (%I 1.391 0.831 0.510 0.580

t After reduction for about 130 min.

174 155 145(?) 148

9.3 6.1 6.3 5.3

389

GETTER MATERIALS F OR CAESIUM VAPOUR I

I

Iron

I

I

I

Oxidation run at 700°C in 63torr 0

c

I

I

2

If

-

Reduction run at 17OoC in 2 x lo-, torr Cs

0

I

2

3

4

Time ( h )

FIa. 6. Typical curves of oxidation and reduction runs on a specimen of iron.

Fe,O, layer and the second part of the curve to the reduction of the FeO layer, which lies beneath the former. When the reaction temperature was less than 170°C, this effect could not be observed so clearly (see Fig. 7 ) .

Time ( h )

Fro. 7. Reduction by caesium of specimens of oxides of iron at various temperatures.

390

M. HIRASHIMA AND M. ASANO

The experimental data are summarized in Table I11 for four successful specimens out of the nine initially prepared. The fraction of the oxygen atoms comprising the oxides of iron which is effectively used t o form caesium oxide is plotted as a function of the reaction temperature in Fig. 5 . The chemical reactions are:

+ 8 Cs

Fe,O, and FeO

--f

+ 2 Cs

+ 3 Fe, + Fe.

4 Cs,O

+ Cs,O

(2)

(3)

Carbon Monoxide? I n Fig. 8, there is shown an example of the results of the reaction of carbon monoxide with caesium vapour. The pressure of the carbon monoxide just after the seal-off of the reaction chamber was 3 x lo-, torr, and on cooling the appendage with running water the I

after seAl-off

‘

I

I

I

I

the appendage was cooled

.. 00

30 50

$0

2 ?? ? .

f

W

t

?O 0

10

20

30

40

50

60

70

80

p: 9

10 I

Time(min1

FIG.8. Change in the pressure of carbon monoxide following reaction with caasium vapour a t about 50°C.

t Just after the present paper was read on Sept. 18, 1968, Dr. S. J. Hellier of S.A.E.S. Getters, Milan, Italy, kindly brought to our notice a paper entitled: “Alkali Metal Generation and Gas Evolution from Alkali Metal Dispensers” by P. della Porta, C. Emili and S. J. Hellier, which was presented at the Conference on Tube Techniques held in New York on Sept. 17-18. 1968. 111 this paper similar results to ours are described.

GETTER MATERIALS FOR CAESIUM VAPOUR

391

pressure decreased to 1 x tom. After the carbon monoxide had reacted with the caesium vapour,

co + 2 cs -+ cs,o + c,

(4)

the pressure of the carbon monoxide decreased t o torr. And after the four successive reactions mentioned above, the total reaction time being about 40 min, the pressure of the carbon monoxide was found to decrease finally to 9 x torr a t about 50°C. I n the above experiment, it should be understood that the caesium vapour was being used as a getter material for carbon monoxide, but from the other viewpoint the carbon monoxide could also be used as the getter material for the caesium vapour. The latter usage seems to be more advantageous than the former, since two atoms of caesium react with one molecule of carbon monoxide to form a molecule of caesium oxide, leaving an atom of free carbon which is thought t o be active as a getter for gases other than caesium vapour.

CONCLUSION I n view of the fact that different forms of specimen were used in the present experiments for the two cases of nickel and iron, it seems t o be a little dangerous to draw a conclusion concerning the relative merits of nickel oxide, the oxides of iron, and also gold,l as caesium getters. To be precise, the comparison should be made with specimens of similar form and similar size. If specimens in the form of wire are to be used, for instance, they should be wires of the same diameter. I n fact', although the nickel oxide appears from Fig. 5 t o be better than the oxides of iron as a getter material for caesium vapour, a comparison of the curves shown in Fig. 4 with those in Fig. 7 shows that the absolute value of the weight gain due to the absorption of caesium is larger in the case of the oxides of iron for the same reaction temperature (see curve No. 6 in Fig. 4 and curve No. 2 in Fig. 7). I n order to decide whether these materials might be used with advantage as caesium getters in the production of photocathodes, i t seems to be necessary first to obtain information concerning the effects of various gases upon the sensitivity of photocathodes. ACKNOWLEDGMENTS The writers wish to take this opportunity to express their hearty thanks to Professor S. Sakui and Dr. K. Satoh of the Department of Mechanical Engineering for their generosity in supplying tho specimens of pure nickel and iron used in the present experiments. The writers are also very grateful for the ever-willing assistance in various phases of the present research given by T. Yoshino, K. Utagawa, Y. Takegawa, N. Takasaki and others.

392

M. HIRASHIMA AND M. ASANO

REFERENCES 1. Hirashima, M. and Asano, M., In. “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 643. Academic Press, London (1966). 2. Kienast, G. and Verma, J., 2. Anorg. Allg. Chem. 310, 143 (1961). 3. Epstein, L. F., Ceramic Age, p. 37 (April, 1954). 4. Zworykin, V. K. and Morton, G. A., “Television”, p. 272. Wiley, New York (1948). 5. Thompson, B. J. and North, D. O., RCA Rev., p. 373 (Jan. 1941). 6. Gulbransen, E. A. and Andrew, K. F., J . Electrochem. SOC.101, 128 (1954). 7. Gulbransen, E. A., Tram. Electrochem.SOC.81, 327 (1942). 8. Paidassi, J., J . Metals 4, 536 (1952). 9. Kubaschewski, 0. and Hopkins, B. E., “Oxidation of Metals and Alloys”, p. 4. Butterworths, London (1963).

DISCUSSION B. R.

c. GARFIELD: Did you observe any changes in the weight measurements

during oxidation due to thermomolecular flow effects around the silica spring microbalance? M. HIRABHIMA: No, we did not observe such thermomolecular flow effects. We admitted oxygen from the vessel containing KMnO, into the reaction chamber gradually; it took about 1.5 min to attain a pressure of 63 tom.

New Approaches to Photoemission at Long Wavelengths P. SCHAGEN and A. A. TURNBULL Mullard Reeearoh Laboratories, Redhill, Surrey, England

INTRODUCTION Gradual improvements in processing techniques during recent years have made it possible to produce photocathodes of the alkali-antimonide type with high quantum yields to visible radiation. A requirement still exists, however, for highly efficient photocathodes with thresholds at longer wavelengths. An important example of the need for such cathodes is in image intensifier tubes used for visual observation at night, where, in the absence of moonlight, the only illumination is provided by the night sky. Extensive measurements of the intensity and spectral distribution of this radiation, illustrated in Fig. 1, have indicated strong emission components in the near infra-red, originating in the airglow.

$i 10"

I

I

I

I

1

I

1

5

1.8

Wavelength (pin)

FIG.1. Typical example of distribution of radiation in the night-sky. 393

i 0

394

P. SCHAQEN AND A . A. TURNBULL

An image intensifier tube, using a photocathode with a high quantum yield in the infra-red, would thus in principle achieve a considerably improved performance. A limiting factor in this respect is, however, the increase in background current which accompanies such a shift of photocathode threshold into the infra-red. This is caused not only by thermionic emission but also, and even more important, by theincreasing fraction of the black-body radiation from the scene and equipment to which the photocathode will thus respond. Such currents reduce the apparent contrast in the image, and calculations have shown that most of the possible improvement in performance would already be obtained if a photocathode could be employed with high quantum yield out to a threshold of about 1.25 pm. Very little would be gained by increasing the threshold to 1.6 pm, whereas from then on a rapid deterioration in performance would take place under typical nocturnal conditions. For this important application it therefore seems that a photocathode which is highly efficient out to a threshold of 1.25 pm would suffice.

APPROACHES TO THE DESIGN OF PHOTOCATHODES Figure 2 shows schematically the simplified energy band diagram of a p-type semiconductor ph0tocathode.l As a result of absorbing a quantum of the incident radiation, electrons from the valence band are

FIG. 2. Simplified energy band diagram of p-type semiconductor photocathode.

lifted into the conduction band, from where they must be able to escape into the vacuum. It is essential for any efficient photocathode with a certain desired threshold that there should be strong absorption of light out to that wavelength. This is ensured by selecting a semiconductor with a bandgap that is smaller than that corresponding to this threshold, the actual threshold in this case being determined by the surface barrier. The conditions for a high quantum yield are: firstly, the escape depth of the excited electrons must be large compared with the absorption length; secondly, a large fraction of the absorbed quanta must excite

PHOTOEMISSION

Ar LONG WAVELENGTHS

395

electrons to energy levels above that of the vacuum; thirdly, the probability of escape of electrons from the surface must be high. These three requirements for the efficient extraction of excited electrons into vacuum lead to two distinctly different approaches. 1. Reduced Surface Barrier

The first approach is to lower the surface barrier by providing the semiconductor with a suitable surface coating of low work function. Some examples of this approach are illustrated in Figs. 3 and 4.

FIQ.3. Surface barrier reduction by Cs monolayer coverage: (a) the general case, (b) the special case GaAs-Cs.

Ec

t

I-4eV

t-fc

LEF E"

E" (a)

(b)

FIG.4. Surface barrier reduction by application of Cs-0 layer: (a) GaAs-Cs-0: C s and 0 coverage by few monolayers, (b) GaAs-Cs-0; coverage of GaAs by Cs-oxide.

One of the most effective methods of reducing surface barriers is to apply a coating of approximately one monolayer of caesium to the atomically clean surface. This reduces the effective surface barrier, indicated as a drop in the vacuum level in Fig. 3 (a),to a value approximately 1.4 eV above the Fermi level. Figure 3 (b) shows the special case where a p-type semiconductor has been chosen with an energy gap equal to the reduced work function. An example of this is the wellknown system, gallium arsenide and caesium, first reported by Scheer and van Laar2 in 1965, and now a subject of study in many research laboratories. Because the vacuum level coincides with the bottom of

396

P. SCHAOEN AND A. A. TURNBULL

the conduction band, excited electrons still have sufficient energy for escape, even when they have thermalized. For this reason the escape depth may be as long as the diffusion recombination length. An even lower work function is possible by the application of both oxygen and caesium. This principle, applied to tungsten as a substrate, was first described by Kingdon3 more than 40 years ago. Applied to GaAs, this results in a negative electron affinity, illustrated in Fig. 4(a), due to a work function of about 1-1 eV. As expected, the principle can be applied to materials of smaller bandgap than GaAs to obtain high yields at even longer wavelengths.* The lack of stability with time of such thin surface coatings creates a serious problem in a practical device. As one of the authors has r e p ~ r t e d this , ~ can be improved by using caesium oxide in the form of a layer, a few tens of Angstroms thick, which appears t o leave the escape probability unaffected. Such a layer also reduces the effect of remaining contaminations on a semiconductor surface which is not atomically clean, because use is now made of the inherently low work function of caesium oxide itself (about 1.3 eV), as illustrated in Fig. 40)). It appears quite likely that other coatings will be found in the near future which reduce the effective work function to a value as low as about 1.0 eV. If this could be applied in practice to a suitable semiconductor with a bandgap energy of also 1.0 eV, a photocathode could result with its spectral response extending to 1.24 pm. Such a cathode, as has already been indicated, would be very attractive for image intensifiers used for nocturnal observation. 2. Applied Internal Field

The second approach to the problem of enabling the escape of the excited electrons, is the application of a controlled internal field. The electrons, which are drawn across this region, should gain enough energy from the field to exceed the work function. If they do not lose too much of this energy again on their further way to the surface, they will be emitted. Depending on the construction of the field layer, one can distinguish between photoemission based on p-n junctions, and on the tunnel effect. Some examples of field-assisted emission are shown in Fig. 5. I n diagram (a) a p-n junction is biased in the reverse direction. Excited electrons are thus accelerated by the field across the space-charge region towards the surface. Figure 5 (b) illustrates how, at least in principle, a hetero-junction could be employed, such as for example zinc selenide on a germanium substrate. The large bandgap of ZnSe would tend to minimize the internal reverse current. A further example,

397

PHOTOEMISSION AT LONG WAVELENQTHS

shown in Fig. 5 (c), is of a low bandgap semiconductor separated from a thin metal coating by a thin insulating layer. I n this case, as in the previous examples, the photoexcited electrons would be accelerated by the controlled internal field in the intermediate region, but tunnelling may here be one of the processes involved. I n all three cases the main problem likely to be met will be the losses in electron energy due to collisions. These will reduce the effective escape probability and lead to a low quantum yield, thus making this approach less promising than the first one.

(a I

(bl

(cl

FIG.6. Examples of the concept of field-assisted photoemission: (a) biased Si p-n junction, (b) biased Ge-ZnSe hetero-junction, and (c) biased Si-insulator-metal sandwich structure.

Other possibilities for obtaining photoemission at longer wavelengths that could also be considered, are conventional photoemitters in series with biased photoconductorsa or in combination with wavelength c o n v e r ~ i o n ,and ~ * ~with these the threshold could in principle be moved further still into the infra-red. For the important application considered here, however, the approaches already discussed appear to be more promising.

EXPERIMENTAL RESULTS I n surveying possible future developments in the field of long wavelength photoemission, several approaches have been discussed. In our laboratories, work has been proceeding for some yeam on one of these approaches, that of reducing the surface barrier of a semiconductor, namely GaAs, by the use of caesium, and caesium and oxygen. Considerable progress has been made in this work which is aimed, initially at least, at the development of a practical photocathode based on GaAs. Stable high-yield photoemission has been observed not only

398

P. SCHAOEN AND A . A. TURNBULL

from vacuum-cleaved samples of GaAs, but also from air-cleaved and vapour-deposited epitaxial samples. Figure 6 shows the spectral response of photoemission from GaAs doped with zinc carriers/cm3) and coated with caesium oxide. The GaAs was in two forms: a vacuum-cleaved crystal (curve A), and a vapour-deposited epitaxial layer (curve B).

p-

'"4!0

115

i.0

i.5 3:o Photon energy ( e V )

3!5

3

FIG.6. Examples of spectral dependence yield of GaAs-Cs-0. A, Vacuum-cleaved crystal of GaAs, Zn doped with 2 x 10lB carriers/cni3. B, 24-pm-thick layer of GaAs, Zn doped, with carriers/cm3, grown epitarially on GaAs substrate by vapour transport.

Further work is in progress on the study of photoemission from thin polycrystalline layers of GaAs on transparent substrates. Photoemissive yields are still a factor of some five times lower than those from good examples of single-crystal GaAs. In conclusion it can be said that, although such layers show promise for application in devices, it is not yet possible at this stage to estimate their ultimate potential.

REFERENCES Spicer, W. E., J . Appl. Phya. 31, 2077 (1960). Scheer, J. J. and van Laar, J., Solid State Commun. 3, 189 (1965). Kingdon, K. H., Phy.9. Rev. 24, 510 (1924). Bell, R. L. and Uebbing, J. J., Appl. Phye. Letters 12, 76 (1968). Turnbull, A. A. and Evans, G. B., Brit. J . AppZ. Phye. 1, 155 (1968). Auphan, M., Boutry, G. A., Brissot, J. J., Dormont, H., Perilhou, J. and Pietri, G., Injra-red Phys. 3, 117 (1963). 7. Kruse, P. W., Pribble, F. C. and Schulze, R. G.,J. Appl. Phya. 38, 1718 (1967). 8. Phelan, R . J., Proc. Inat. Elect. Electronics Engrs. 55, 1501 (19B7).

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

Gallium Arsenide Thin-film Photocathodes C. H. A. SYMS Services Electronics Research Laboratory, Baldock, Hertfordshire, England

INTRODUCTION Many experimental GaAs-Cs photocathodes have been prepared in recent years by cleaving, under vacuum, apiece of acceptor-doped singlecrystal gallium arsenide. A little caesium is then allowed t o condense on the freshly exposed face, which, when illuminated, yields a very high photocurrent. Values between 500 pA/lm and 1000 pA/lm have been reported.1-4 Such a photocathodeis thusmany timesmore efficient than the multialkali type with, moreover, a sensit,ivity extending into the longer wavelength region of the spectrum with high efficiency. The long wavelength threshold for GaAs is approximately 0.9 pm (1-4eV) but sensitivity can be further extended towards 1 pm by the use of semiconducting compounds with slightly smaller energy bandgap, for example In,Gal-,As. The high conversion efficiency and the infra-red sensitivity are of great importance in device development. Photocathodes formed from cleaved single crystals are not, however, very suitable for incorporation into photomultipliers and are virtually excluded from use in image tubes by the difficulties that would be encountered in the design of a practical folded optical and electron optical system. Therefore, a programme has been undertaken a t SERL to determine whether satisfactory photocathodes can be formed from thin films of GaAs deposited on t o transparent substrates. I n the course of the experimental work it has been shown that GaAs layers can be deposited on polished sapphire substrates. Some of these layers have then been caesiated t o provide photocathode emission efficiencies comparable with present commercial devices.

EXPERIMENTAL ARRANGEMENT Figure 1 shows the experimental arrangement and the disposition of the zinc metal used t o provide the p-type doping, the gallium metaI source, and the substrate position within the heated resction tube. 3QQ

400

C . H. A . SYMS

The gallium is held a t 950°C and the substrate a t approximately 675°C. Variation of the position o f the zinc changes the resistivity of the condensed layer, in a controlled way, in the range o f 0.01 t o 1000 Rcm. Deposition on the substrate commences when the hydrogen is bubbled through the AsCI, t o react with the hot gallium. The volatile components then condense in the cooler region of the reaction tube near the substrate to provide a layer growth rate of 1 pmlmin.

To fume cupboard exhaust

FIQ.1. Vapour-phase reaction tube and gas flow system.

After chemical etching, the layers are located on a mounting bebween two angled stainless steel mirrors in a vacuum chamber, as indicated in Fig. 2. Reflexion and transmission spectral response measurements may thus be made by reflecting the incident light on to the specimen with either of these mirrors, one mirror acting as an electron collector. The whole system is vacuum-baked overnight, and the caesiation process is then initiated by crushing a capsule of caesium metal in the copper-tube side-arm. The arm is warmed to increase the caesium vapour flow while monitoring the photoemission current. When the current has been substantially enhanced, the specimen chamber is separated from the main system t o allow spectral response studies. A small ion pump a t the rear of the system makes tfhe specimen chamber self-contained and portable. Further quantities o f caesium, and then oxygen, are applied t o the specimen during observation of the spectral sensitivity, until

GALLIUM ARSENIDE THIN-FILM PHOTOCATHODES

401

maximum emission is achieved. Normally, only a small (1 mm diameter) area of the photocathode is examined at a time in this way. Quantum efficiency curves are then evaluated for the specimen by reference t o the response of a calibrated silicon photodetector over the same spectral range. The luminous efficiencies are then computed. Some of these results are shown in Fig. 3. The upper curve is characteristic of that to be expected from a cleaved single-crystal specimen

FIG.2. Specimen vacuum chamber.

and is included for reference. The lower group has been obtained from several polycrystalline layers examined as reflexion photocathodes. The long wavelength threshold of emission in each case is near the optical absorption edge of GaAs a t 1.4 eV, indicating thc effectiveness of surface caesium in lowering the surface work function. The numbers here represent the pA/lm values of the respective specimens. These values are approximately the same as bhose obtained a t SERL from chemically cleaned and subsequently caesiated single-crystal GaAs specimens. This indicates that the photoemission is limited by surface

402

C. H. A. SYMS

/ Cleaved single crystal

/

1.3 1.4 1.5 1.6

Incident 1.8 2radiation 0 2.2(eV)

24

2.6

3

FIQ.3. Spectral resporise (reflexion) of five caeskted polycrystalline GaAs layers on sapphire substrates and of a caesiated cleaved single-crystal of GaAs.

effects and that the internal properties of the layers are reasonably satisfactory.

TEANSMISSION PHOTOCATHODES The problem with transmission photocathodes is the preparation of very thin, uniform samples. Chemical etching techniques have been used to prepare layers with thicknesses of only a few microns, but it is difficult t o achieve a uniform thickness over an area of 1 cm2, the peripheral area usually being considerably thinner than the central area. Figure 4 indicates some preliminary measurements of the spectral response of thinner layers operated as transmission photocathodes. The diagram shows the results for two such samples. Both the transmission and reflexion spectral response for the same small area of each sample is indicated. The long wavelength limit is again a t the optical absorption cdge of bulk GaAs. Tho overall yield is, however, much

GALLIUM ARSENIDE THIN-FfLM PHOTOCATHODES

403

lower. Note that a t the longer wavelengths the emission is higher for the transmission curve, which is possibly the result of the better optical impedance match from vacuum into the GaAs through the sapphire substrate. I

I

l

l

I

I

I

I

I

w, Transmission

lo-' B

Incident radiation ( e V )

Fra. 4. Spect'ralreaponse (reflexion and transmission) of two caesiated polycrystelline GaAs layers on sapphire substrates (A and B) and of a caesiated cleaved single crystal of GaAs (reflexion only).

THEORETICAL MODEL As the semiconducting properties of GaAs are relatively well understood it is possible to consider the photoelectric yield of a photocathode of this material from a fundamental theoretical standpoint. A simple model for the processes of photon absorption and subsequent electron emission has been examined. Incident light generates electrons throughout the GaAs. Some will diffuse to the caesiated surface and escape, others will recombine within the layer. The photoexcited electron density in the GaAs can bo

404

C. H. A. SYMS

calculated as a function of depth into the material from the GaAs surface on which the light is incident. Consider a layer of GaAs of thickness dx a t a distance x below the illuminated surface, under conditions of steady illumination. The electron density n(x) then has the equilibrium value given by the continuity condition,

where j ( x ) is the net electron current density in the thin layer, g(x) is the electron generation rate in the layer and e is the electronic charge. The three terms in Eq. (1) then represent, respectively, the electron diffusion (as there is no appreciable electric field in the layer the current is diffusion limited), electron generation and finally electron recombination (T is the electron lifetime). Now

and g(x) = (1 - R)Au exp(-ux) ,

(3)

where D is the electron diffusion coefficient, R is the optical reflexion coefficient, A is the incident light flux in photons, and u is the absorption constant of the GaAs. Substitution of Eqs. (2) and (3) in Eq. ( 1 ) yields a second order differential equation which may be solved for the electron density n(x). For a complete solution appropriate to a GaAs layer of finite thickness the boundary conditions a t the surfaces must be determined. I n the simple model this has been done by allowing the surface electron densities to recombine a t acceptor surface states a t a certain rate which is represented by appropriate values of the surface recombinationvelocity coefficient. Thus, a t the illuminated surface, j(0)= en(O)S,, and a t the electron emitting surface, j ( t ) = en(t)S,, where t is the total thickness of the layer and S , and S , are surface recombination-velocities. The solution to the equation is then ?%(X) =

(1 -B)AaT ( U Z L 2 -1 )

(8,+ 4 ( S , + D / L ) e - ' " ( S , +D/L)(S,-D/L)etiL

-k

(8,+uD) (8,--D/L)etiL -(S,-D/L) (8,+aD)e-at - (8,+ D / L ) (8, -D/L)etiL

[@,--D/L) (8,+D/L)e-i'L

UALLIUM ARSENIDE THIN-FILM PHOTOCATHODES

405

This expression has been evaluated for n(t)wherc t = 2-5 pm and using for the parameters the representative values for GaAs in Table I. TABLEI

R, reflexion coefficient A , photon flux from a black body source a t 2850°K between co and 1.350%' giving 5.95 x lo3 lm/cma a, optical absorption coefficient T, minority carrier lifetime D , diffusion coefficient L = (DT)'/', diffusion length S1= Sp,surface recombination-velocities5

=

0.16

= 4.9 x 1020photons cm-2sec-1 = 2 x 104cm-1 = 10-9sec = 150 cma/sec =

3.87 x cm x lo6 cm/sec

=2

The value of the photocurrent can then be calculated by using an effective surface recombination-velocity due t o emission, Si, with an assumed value of 2 x 104cm/sec. This value has been estimated from measurements of the photoemissive quantum yield near 2.0 eV for reflexion photocathodes (Fig. 3) and with the above value of S,, assuming that the photogenerated electrons either recombine through Average crystal li le

/

FIG.5 . Idealized polyorystalline layer.

the surface states or are emitted. Then j ( t ) = en(t)S, = en(t)(8; +&) where Si represents the actual recombination rate through the surface states. As &/S; 4 1 the emission current, j e ( t ) ,M en(t)Sh which, after evaluation, gives a photocathode efficiency of 1184 pA/lm. Figure 5 shows how the effects of polycrystallinity can be included by formulating an expression derived from the geometrical aspects of an average crystallite. In this very simple model the electrons are considered to be generated a t the centre of the base of the cylindrical crystallite. Solid angles subtended from this point then give the probability of the electron reaching the emitting surface. Recom-

406

C. H. A . SYMS

bination, and therefore, loss, of the electron a t the orystallite boundary is assumed. The function, t , f(b)= 1 (ba t 2 ) l i 2 is the required probability and the product j ( e ) f ( b )gives an indication of the yield expected from a polycrystalline thin film. Table I1 gives the yields for practical values, near unity, of the ratio blt.

+

TABLEI1

0.5 1 3 6 10

130 343 810 990 1070

The conclusion, therefore, to be drawn from the experimental work is that a thin film GaAs-Cs photocathode is possible, the present results giving efficiencies of about 1 pA/lm in transmission and 100 pA/lm in reflexion. Theoretical calculations indicate that there is no reason why a transmission photocathode of this material should not have a greater yield and a sensitivity to longer wavelengths than the S.20 multialkali type. ACKNOWLEDGMENTS This paper is published by permission of the Ministry of Defence.

REFERENCES 1. Scheer, J. J. and van Lam, J., Solid State Cornrnun. 8, 189 (1965). 2. Turnbull, A. A. and Evans, G. B., Brit. J . Ap p l. Phys. 1, 155 (1968). 3. Eden, R. C., Stanford Electronics Laboratory Technical Report No. 5221-1 (May 1967). 4. Uebbing, J. J., Bell, R. L. and Spicer, W. E., private communications. 5. Vilms, J., Stanford Electronics Laboratory Technical Report No. 5107-1 (Nov. 1964).

DISCUSSION w. E. TURK: 1. What is the reason for choosing sapphire as a substrate? 2. Can the material be prepared on glass? 3. What is the purpose of the chemical etch before caesiation? C. H. A . S Y M S : 1. Sapphire was chosen as the substrate material as it has good optical transmissionoverthe wavelength rangeof interest, it is inert a t thesubstrate temperatures used for deposition of the layer and may be readily obtained prepared to optical qualities. There is the possibility of epitaxial deposition on to the material. 2. The material can be prepared on glass. 3. Chemical etching is used to remove the surface layer of &As which is likely to be heavily oxidized

GALLIUM ARSENIDE THIN-FILM PHOTOCATHODES

407

in the period between layer preparation and caesiation. Samples aro also chemically thinned for transmission photocathode studies. c . w. BATES, JR: What chomical etcher did you use on your GaAs? c . H. A. SYMS: Generally a solution of 0.2% Rr in methanol. E. EBERHARDT: Were your films exposed to air before caesiation and, if so, for how long? c . H. A . SYMS: All the films are exposed t o air between preparation and caesiation; some for several weeks. All the samples that are to be caesiated are etched immediately beforehand, but there is still an effective exposure t o air for at least several minutes before the pressure in the vacuum system is reduced. B . R. GARFIELD: The semiconductor band model for OaAs on which the high photoemissive yield is usually explained has zero effective electron affinity a t the surface. This would suggest that the thermionic emission will be high compared with conventional alkali-antimonide photoemitters which have a finite electron affiity. Have you made measurements of the dark current? c . H. A. SYMS: Our routine current measurements are limited by the current meter sensitivity which is about 10-13A. We have not been able to measure any dark current a t this sensitivity. It is not yet clear from where the major contribution to the dark current will originate and the zero effective electron affiity model is not appropriate for the evaluation of the influence of localized surface states and other defects of the physical structure. I n principle, it is possible to adjust the energy bandgap of a compound like InGaAs so that a finite electron affinity remains for the structure as a whole. Without doubt, it is important that reliable measurements of the dark current of a GaAs-Cs photoemitter be made.

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6tude de I%mission Photoklectrique des Structures M6tal-Isolant-M6tal P. VERNIER, P. HARTMANN, G. NIQUET et M. TEPINIER

Laboratoires de Photodlectricik! des Facultds des Sciences de Dijon et de Besanpon, Prance

INTRODUCTION Nous avons eu l’occasion, lors du troisiltme Symposium,’ de montrer que la camBra Blectronique est un instrument de choix pour l’etude des faibles courants Blectroniques. Lifshitz et Musatova ont montrB que le courant Bmis par des structures m&al-isolant-m6taI, convenablement polarisBes, pouvait &re renforcB par un Bclairement de la structure. Nous avons entrepris d’htudier systBmatiquement ce phBnom&neB l’aide de la camera Blectronique. La sensibilite des plaques photographiques et la sBcuritB apportde par la connaissance des points d’oh sont Bmis les Blectrons procurent des QlBmentsnouveaux. PREPARATION DES STRUCTURES

EMISSIVES

torr, une Nous Bvaporons, en premier lieu, sous un vide de bande d’aluminium de 2 mm x 60 mm et dont 1’6paisseurest de l’ordre de 1000 d. Nous formons ensuite une couche d’alumine par oxydation, d’abord par mise zt l’air et transport dans une Btuve portke zt 100” C. Lea rBsultats ainsi obtenus &ant irrbguliers, par la suite, nous avons operB cette oxydation en faisant une rentrBe d’oxygltne sec et en chauffant zt 150°C la couche d’aluminium dans la cloche oh Btait eEectuBe 1’6vaporation de l’aluminium. L’Bpaisseur de la couche d’alumine obtenue varie de 60 d zt 160 d suivant la tempdrature et le temps d’oxydation. Nous Bvaporons ensuite une contre-electrode d’or sur l’aluminium oxydB. Nous formons ainsi un condensateur. Pour limiter les risques de courts-circuits et pour Btudier I’influence de 1’6paisseurde la contre-Blectrode, nous avons rBalisB sur une meme bande d’aluminium oxydh une aerie de couches d’or d’un millimlttre P.E.1.D.-A

409

15

410

P.

VERNIER, P. HARTMANN, a. NIQUET

ET M. TEPINIER

de largeur, isolees les unes des autres (Fig. l(a)). Les rksultats de ces experiences feront l’objet d’une autre publication. Pour les Btudes photo6lectriques’ nous nous sommes au contraire efforcds de realiser des structures aussi grandes que possible (Fig, l ( b ) ) .

-

/ “ 0 ,-

2

A

Fro. 1. Structures M-I-M: (a)avec six contre-6lectrodeset (b) avec une grande contreBlectro de.

GTUDEDE LA

CONDUCTION DES STRUCTURES

Une mesure de capacitB nous fournit d’abord une Bvaluation grossibre de 1’6paisseur de l’alumine, en admettant pour celle-ci une constante didlectrique relative E~ = 8. La mesure de la caracteristique courant-tension entre 1’6lectrode d’aluminium et la contre-Blectrode d’or a BtB effectuke A l’aide d’un oscillographe cathodique en utilisant le montage schematise sur la Fig. 2. ris !rice

Adoptoteur d‘imp&dance‘-

I

I

Oscillogrophe

.

* *

I

H 0

II

<

’Structure M-I-M

I 1

Fro. 2. Montage utilis6 pour tracer lea caract6ristiques des sandwich8 B l’oscillographe.

~ T U D EDE L’I~MISSIONPHOTO~LECTRIQUE

411

La Fig. 3 reproduit la photographie d’une caractdristique relevee ti l’oscillographe qui, dans le domaine de tension BtudiB, est parfaitement exponentielle, ce qui indique un passage de courant par effet tunnel. ** s5

FIG. 3. Caract6ristique courant-tension d‘un sandwich Al-Al,O,-Au. gpaisseur d’slumine: 60 A. Cette caract6ristique est trac6e B 20 Hz. La tension varie de 3-2 V 8. - 1 . 8 V. Nous rernctrquons, sup ce clich6, une dLsym6trie dans lea conductions suivant la polarit6 de la tension. Le cycle qui apparait est dQ essentiellement $, un effet de capacitb. (Echelle: H = 1 V/cm, V = 0.2 A/cm.)

+

MESUREDU COURANT$MIS La structure ti Btudier, convenablement polariske, constitue la photocathode d’une camera Blectronique (Fig. 4). Les Blectrons B m i s sont accQlBrQset focalisBs sur une plaque G6. Nous avons dBjti eu l’occasion de signaler que la relation densit6-nombre d’6lectrons Bmis par unit6 de surface est pratiquement 1inBaire. Une exploitation systematique des cliches par densitomdtrie nous fournit une mesure du courant Qmis.

Courant d’obscuritk La camBra dlectroniquenous permet de mesurer le courant d’obscuritk et sa rdpartition sur une structure. Nous avons constate qu’aux bords des contre-Blectrodes d’or correspond un renforcement de 1’6mission, lid ti la diminution de 1’6paisseur de la contre-dlectrode. L’existence de ce renforcement a dBjti Bt6 mise en Qvidencepar Albaut,6 et Savoye et Anderson.’ L’image Blectronique reproduite dans la Fig. 5 est obtenue en l’absence de lumikre avec une polarisation de 3 V; ti chaque extr6mite now voyons un renforcement de 1’Bmission qui correspond aux effets de bord. La Fig. 6 reproduit le microdensitogramme d’un cliche obtenu ti partir d’une autre structure.

412

P. VERNIER,

P. HARTMANN, 0. NIQUET ET

M. TEPINIER

Structure M-I-M

Roies du soecire du rnercure

FIG.4. SchBma d‘une oamOra Blectrnnique pour 1’6tude de 1’6mission photo6lectrique.

Avec la camera Blectronique, nous pouvons Btudier 1’Qmission normale et 1’Qmissionsur les bords du sandwich. Nous avons pu notamment montrer que I’importance de cet effet de bord dQpend,non seulement de 1’8paisseur de ]’or, mais aussi de la fapon dont l’alumine a Qt8 produite. La surface de l’alumine influe probablement 8ur la structure de la cont’re-dlectrode.

FIG.6. fimission Blectrnnique d’un sandwich AI-A1,O ,-Au polaris6 par une tension de 3 V.

I ~ T U D EDE L’~MISSIONPHOTO~LECTRIQUE

413

Diplacement suivant I‘axa de la structure

FIG. 6. Densitogramme de 1’6mission Blectronique d’un sandwich polarise par une tension de 3 V. La, contre-8lectrode d’or a une 8paisseur de 150 if.

l h i s s i o n PhotoLlectrique Pour Btudier 1’Bmission photoBlectrique, nous projetons le spectre d’une lampe vapeur de mercure sup une structure. Nous utilisons de prBfBrence une structure unique de grandes dimensions pour Bviter les effets de b o d . La partie des raies spectrales qui tombe en dehors de la

FIG.7. ]Emission photoBlectrique pour des raies du spectre du mercure proches du seuil de la contre-Blectrode d’or.

FIQ.8. lhnission photoBlectrique pour des raies de plus grandes longueurs d’onde.

414

P. VERNIER, P. HARTMANN, 0.NIQUET ET M. TEPINIER

structure, mais sur la contre-Blectrode, permet de mesurer 1’Bmission propre de celle-ci (Figs. 7 et 8). On peut remarquer que le seuil se situe entre 2654 A et 2804 d.Les Figs. 9 It 11 representent les densitogrammes des clichds des Figs. 7 et 8. Nous voyons sur ces enregistrements apparattre les raies sur le sandwich alors qu’elles n’existent plus pour un dBp6t d’or sur du verre. La lumibre ultraviolette diffuske provoque une Bmission photoBlectrique de l’ensemble de la structure et de la

2 .C VI

8

w Diplocement du cliche

FIQ.9. Densitogramme de l’bmission photoblectrique de la structure Al-A1,O 3-Au polarisbe par une tension de 3 V pour des raies proches du seuil photo6lectrique.

contre-Blectrode, mais 1’Bmission correspondant aux raies spectrales peut titre attribude 8, une longueur d’onde bien dBfinie. On peut observer que, pour une polarisation de 3 V, on dBtecte la raie 5790 d. L’Bmission photoBlectrique peut se produire suivant un ou plusieurs des quatre mdcanismes qui sont schBmatisBs sur la Fig. 12. (a) Le mBcanisme A correspond It 1’8mission photoBlectrique normale du metal M, qui se produit lorsque 1’6nergie hv des photons incidents est supdrieure au seuil photodlectrique p, du mBtal. I1 est Bvidemment exclu pour les grandes longueurs d’onde. (b) Dans le mBcanisme B, un Blectron du m&al M, absorbe un photon

I~TUDEDE L ~ ~ M I S S I OPHOTO~LECTRIQUE N

415

dont 1’8nergie hv est suffisante pour qu’il passe normalement dans la bande de conduction de l’isolant, puis traverse le metal M,. Pour cela hv doit &re superieur b rp et (rp, - eV). Ici, la tension n’intervient que pour modifier la repartition des bandes Blectroniques dans la structure. (c) Dans le mecanisme C, un electron excite dans le metal M, traverse une partie de l’isolant par effet tunnel. (d) Dans le m6canisme D, un electron passe du metal M, au metal M, par effet tunnel et reqoit 1& 1’6nergie d’un photon. A priori, ce

Deplocement du clichh

FIG.10. Densitogramrne de 1’6mission photo6lectrique de I’or depose direotement sur le verre. Seule la reie A = 2654 A apparaft.

mecanisme semble peu efficace, car l’existence d’une population importante d’electrons chauds dans le metal M, eat interdite par la charge d’espace qui en resulterait et par les phknomhes de ddsexcitation. La probabilite d’absorption d’un photon par un electron chaud sera donc trbs faible devant la probabilite d’absorption par un electron normal. Les raies d’dmission observees au-delb du seuil photodlectrique de l’or peuvent donc correspondre aux mdcanismes B ou C. C’est-&-dire que 1’QlectronBmis est excite dans le metal M, et la difference de potentiel appliquee aux bornes de la structure permet sa sortie avec ou sans effet tunnel.

416

P. VERNIER, P. HARTMANN, U . NIQUET ET M. TEPINIER

*a 0 la 0

7

x

l

l

i

1

Odplocernent du cliche

FIG.11. Densitogramme de 1’6mission photo6lectrique de la structure AI-AI,O,-Au polaris6e par une tension de 3 V pour des raies 6loign6es du senil photo6lectrique. (Ces raies ne produisent pas d’6mission photo6lectrique sur l’or depose directement sur le verre.) Niveou du vide

FIG.12. Schema des niveaux d’6nergie et de 1’6mission photo6lectrique.

I~TUDEDE L’I~MISSIONPHOTO~LECTRIQUE

417

CONCLUSION En polarisant la structure Al-Al,O,-Au par une tension de 3 V, nous avons pu dkplacer le seuil photoelectrique de 4.6 eV 2.2 eV. Le rendement photoklectrique de nos structures (quelques ne permet pas pour le moment d’envisager d’applications pratiques, mais il n’est pas exclu qu’en perfectionnant; le mode de preparation et surtout en utilisant d’autres matkriaux, on puisse envisager la rkalisation par ce prockdk de photocathodes ii seuil variable.

RBFBRENCES 1. Vernier, P. et Hartmann, P., Dana “Advances in Electronics and Electron Physics”, Bd par J. D. McGee, D. McMullan et E. Kahan, Vol. 22A, p. 519. Academic Press, London (1966). 2. Lifshitz, T. M. et Musatov, A. L., Zh. Eksper. Teor. Fiz. Pia’Ma 4, 295, (1966); Traduction anglaise: J.E.T.P. Letter8 4, 199 (1966). 3. Simmons, J. G., J . Appl. Phy.9. 34, 1793 (1963). 4. Sommerfeld, A. et Bethe, H., Dan8 “Handbuch der Physik”, Geiger und Scheel, Vol 24/2, p. 450. Springer-Verlag, Berlin (1933). 5. Holm, R.,J . AppZ. Phya. 22, 569 (1951). 6. Albaut, M., These 3Ame Cycle, FacultB des Sciences de Dijon (1967). 7. Savoye, E. D. et Anderson, D. E., J . Appl. Phye. 36, 3265 (1967).

DISCUSSION The photoemission from a metal into vacuum or into a semiconductor can be described by the Fowler theory, which gives the photocurrent as a function of the wavelength. Comparison of the theory with experiment is a valuable means to determine the threshold wavelength. Did you check your experiments in this way, and what is the threshold wavelength? P. VERNIER: The Fowler theory is often used to obtain the threshold from the variation of the photoelectric yield with photon energy. The success of such a computation cannot be considered as an evidence of the validity of all assnmptions on which the Fowler theory is based. I would point out in particular that the surface photoexcitation is generally negligible compared with the Fowler assumption. In the case of a polarized sandwich structure a further objection against the Fowler formula may be that the electronic band structure process of photoemission is quite different from what it would be in a free electron metal. To give an idea of the threshold I can only say now that the longest wavelength for which photoemission has been registered is 5790 A. Q. P. WRIGHT: Is there any current flowing in the polarizing circuit and, if so, how does it vary with bias voltage? Does the dark current increase rapidly at around 5-V bias? P. VERNIER: Quite a large current flows in the polarizing circuit. Its variation with bias voltage is exponential. Around 5 V, destruction of the sandwiches occurs and it is difficult to obtain reproducible results. H. J. G I . MEYER: By studying the voltage dependence of the photoemission a t some long waveIength, one should be able to distinguish between photon-aided tunnelling and other processes possible. Did you do that? P. H. BROERSE:

418

P. VERNIER, P . RARTMANN, 0 . NIQIJET ET M. TEPINIER

P. VERNIER: We shall of course study the voltage dependence of photoemission, but it is not yet done. I am not so sure that this study will determine the emission mechanism without ambiguity. D. R. CHARLES: Avez vous observe un effet photoBlectrique interne? Nos propres Btudes ne nous ont pas permis de detecter l’effet externe avec deplacement du seuil, mais nous avons trouve un effet interne. P. VERNIER: Nous n’avons pas encore effectue de mesure de l’effet photoBlectrique interne. GrBce 8, la sensibilite de notre montage, le moindre effet externe doit apparrtitre. Ces avantages ne se retrouveront pas pour l’effet interne qui se superposera au courant d’obscurite de la totalit6 du sandwich.

Interference Photocathodes D. KOSSEL, K. DEUTSCHER and K. HIRSCHBERG

Ernat Leitz G.m.b.H. Optical Worka, Wetzlar, Weat Germany

INTRODUCTION Highly sensitive photocathodes should comply with two requirements. 1. Photons should enter the cathode without reflexion losses. 2. Photons should be completely absorbed in a depth equal to or smaller than the attenuation length of the electrons. These two requirements may be summarized by saying that the incident light waves should be completely absorbed within the receiver, the thin photoelectric layer, the thickness of which is less than the attenuation length of the electrons. The adjustment of a receiver for maximum absorption is a problem which often occurs in physics. It is always solved in the same way: the receiver is matched to the signal source and is tuned to specific frequencies. Thin films are resonators for light waves. The two boundaries of the film partially transmit and reflect the waves, depending on the optical constants of the film. The boundaries define a cavity which can oscillate when excited by light waves. If the spacing of the cavity is just half a wavelength the standing-wave amplitude in the cavity is high, the film being tuned to the incoming wave. There is no reflected wave. Since the film is unsupported and non-absorbing, the amplitude condition for matching is satisfied, and all the energy is radiated in the forward direction. The amplitude of the oscillation is shown in Fig. l(a). A quarter-wave film (Fig. l ( b ) )suppresses the standing wave in the film. The large standing wave in front of the film indicates its high reflectance. Figure 2 shows the wave fields in front of, in, and behind an unsupported weakly absorbing film, as a function of thickness. The highest amplitude in the film occurs with a thickness of A/2. The film is tuned so that the phase condition is satisfied, but the amplitude condition for matching of a single film is not realized and there is a standing wave in front of the film. 410

420

D. KOSSEL, K. DEUTSCHER AND K. HIRSCHBERO

FIG. 1. Wave fields in, and in front of thin non-absorbing dielectric films of different thicknesses t. (a) t = 4 2 (resonance). (b) t = h / 4 (dissonance).

FIQ.2. Wave fields in front of, in, and behind a n absorbing film as a function of thickness. The film (refractive index nl = 3.6, extinction coefficient k = 0.3) is unsupported.

The absorption in a photoemissive layer is proportional to the square of the electric vector and the excitation of electrons is proportional to the absorption. The E2-curve therefore represents the rate a t which electrons are excited in each lamella of the layer.2 Figure 3 shows the local distribution of the excited electrons for two layers of different thicknesses. I n general, transmissive layers of a

INTERBERENCE PHOTOCATHODES

42 1

thickness up to h/4 always produce more excited electrons in the lamellas a t the back boundary than a t the front, and in a half-wave layer many electrons are excited in the lamellas a t the boundaries but very few in the middle of the layer. The attenuation length of the photoelectrons is small, e.g. 150 A for caesi~m-antimony,~-~ and therefore the E2 curves show only the production rate of the excited electrons. By combining the production rate with the photoelectron transport function $ ( x ) which contains the attentuation length, the

(a 1

(b)

FIG.3. Local production rate of excited electrons ( E z ( z ) )and their contribution t o photocurrent ( E z ( z )f(z)) for two layers of different thicknesses t , (a)t = 400 A ( x h/C) and (b)t = 800 A ( % A/2) for A = 6200 8.(no= 1, nl = 3.2, k = 0.44, n, = 1.5).

contribution of each lamella t o the photoemission is obtained. The dotted curves in Fig. 3 represent the contribution of each lamella t o the emitted current.

TRANSMISSIVE INTERFERENCE CATHODE (TIC) The principle of matching and tuning may be applied to a conventional photocathode of high quantum efficiency, for instance a Cs3Sb photocathodeg. I t s geometrical thickness may be typically 250 A and its optical thickness between h/8 and X/4 for visible light, so that it is in dissonance for the incident waves. This will also be true for multialkali photocathodes. A transmissive cathode may be tuned by adding t o the photoelectric layer a dielectric layer so that the total thickness is h / 2 or a multiple of it (see Fig. 4(a)). The refractive index and the thickness of the non-absorbing layer have to be properly chosen; for the Cs,Sb cathode a single layer of TiO, is suitable. The phase condition and the amplitude condition can be met more easily by two

422

D. KOSSEL, K . DEUTSCHER AND K . HIRSCHBERG

layers (Fig. 4(b));less light is reflected and the absorption is enhanced (as well as the transmitted light which is unfortunately lost). The maximum enhancement of the absorption A in a TIC is given by 1 A

A,

-

1 -R,'

where A , and R, denote the absorption and reflectance of the single photoelectric layer. It must be emphasized that the incident light cannot be completely absorbed in a TIC.

t

t

Emitted ebctrons

I

'I

"I

2 '

"2 '2

Emitted electrons

'I

"3 '3

4 Substrate f l g &4

t

Incident light (a)

I

Incident light

FIG.4. Basic arrangement of a TIC with (a)one, and (b) two matching films.

The reflectance of the Cs,Sb layer is about 30% and the absorption can be increased by a factor 1.5. The photocurrent is exactly proportional to the absorption, so that the photocurrent is also enhanced by the factor 1.5. In Figs. 5 and 6 the reflectance, photocurrent and the gain factor of two TICS tuned to 1st and 2nd orders are plotted against wavelength. TOTALLY REFLECTIVE INTERFERENCE CATHODE(TRIC) The increased absorption in the TIC is only obtained at the expense of energy lost by the increased transmission. This loss can be prevented by inserting an ideal reflector, which closes the cavity for the light waves but is still transparent to electrons. The totally internally reflecting surface of the layer at the vacuum boundary can act

INTERFERENCE PHOTOCATHODES

(0)

gain, of a 1st order TIC with

423

FIQ.5. Spectral dependance of (a)reflectance, (b) photocurrent, and Ag-Cs-0 photoemissive layer.

424

n

-

I

I I

e8

D . KOSSEL, K. DEUTSCHER AND K . HIRSCHBERU

I

INTERFERENCE PHOTOCATHODES

425

FIo. 7. Basic arrangement of a TRIC.

as this ideal reflector.’ Figure 7 demonstrates the basic arrangement of a TRIC. It can be described as a transmissive interference cathode illuminated a t an angle of incidence greater than the angle for total internal reflexion. The reflectance of the amplitude matching layer must be equal to that of the photoelectric layer t o accomplish the amplitude condition; this means that the reflectance of the photoelectric layer as a function

4 (degrees) FIa. 8. Reflectance and transmittance of a 250-A Cs3Sblayer plotted against angle of incidence 4 ( A = 6460 A).

426

D. KOSSEL, K . DEUTSCHER AND K. HIRSCHBERB

of the angle of incidence has to be known. Measurements of the reflectance and absorption of a Cs,Sb layer on glass are shown in Figs. 8 and 9.% The results of the measurements can be summarized as follows. 1. The photocurrent roughly corresponds to the absorption. 2. The reflectance of the component polarized parallel to the plane of incidence has a sharp minimum at an angle of 75", i.e. the energy is nearly all absorbed. Since the single layer has an absorption of about 50% near the angle of

+T

9(degrees) FIQ.9. Absorption and photocurrent of a 260-A Cs,Sb layer plotted against angle of incidence (A = 5460 A).

total reflectance the absorption can be pushed up in the TRIC by a factor of about 2. The performance of this cathode is shown in Fig. 10. The phase matching layer has an appreciable thickness and produces a close sequence of interference orders. The maximum of absorption and the corresponding maximum of photoemission is observed at 43" for the perpendicularly polarized component. Many investigations still have to be done with this type of interference cathode but one result can be stated: complete absorption of light of one polarization component and correspondingly high photoemission can be achieved with the TRIC.

427

INTERFERENCE PHOTOCATHODES

n u)

c

'5

4 4 (degrees) FIG-.10. Absorption and photocurrent of a TRIC with CssSb film plotted against angle of incidence 4 (perpendicular component).

REFLECTIVE INTERFERENCE CATHODE(RIC) The oblique incidence of light in a TRIC severely restricts its application. If the totally internally reflecting surface is replaced by a highly reflecting mirror, the optical cavity is closed and operation a t normal incidence is possible. The emission of electrons will of course be blocked in the forward direction; electrons can escape only out of the surface at which light enters the photoelectric layer. The principle arrangement

FIG-. 11. Basic arrangement of an RIC.

428

D. KOSSEL, K. DEUTSCHER AND K. HIRSCHBERQ

Flu. 12. Wave fields in front and in the interior of an RIC (no= 1.0, nl = 3.20, bl = 0.44, tl = 500 A, nz = 1.50, ideal mirror with ra2 = 1).

of the reflective interference cathode (RIC)gm10is shown in Fig. 11. A photoelectric layer is backed by a mirror and a phase matching film. I n order to illustrate its performance the E2-curves are plotted in Fig. 12. In this diagram the thickneaa of the photoelectric layer is assumed to be h / 4 , which is close to what it would be in practice. The tuning layer is wedge-shaped. Most electrons are produced when the

-

0

100

200

300

400

11

41 FIQ.13. Wave field in a single photoemissive layer (optical thickness h / 4 ) and in the mme layer incorporated into an RIC.

429

INTERFERENCE PHOTOCATHODES

total thickness of the photoelectric and dielectric layers is h/4 or an odd multiple of it; this is the phase condition for an RIC. Compared with a single layer the local production rate has changed completely (see Fig. 13), and the emission of electrons is favoured by an RIC.g I n Fig. 14 the photocurrent of a multialkali RIC is compared with that of a multialkali photocathode in the conventional transmissive arrangement. The ratio of the currents is called the gain. The cathode 4 0 - l ~I

Interference

3.0

cathode

Li

E5

'

' '

7000

6000

April

-

-

Normal cothode 5000

I

-

3

0.5

'

Boo0

A

PI

FIG.14. Photocurrents from (a)an RIC with multialkali photocathode tuned to 1st order and the corresponding photocathode, and (b) the gain of the RIC plotted against wavelength.

is tuned by an SiO, layer and by a silver reflector to 7300 8. The maximum gain of 3.7 reaches the theoretical value. Multialkali photocathodes of normal efficiency (180 pA/lm) increase in sensitivity to 450 pA/lm as has previously been shown.ll The bandwidth of the current enhancement depends on the order of tuning. An RIC tuned to the 1st order a t 6800 d has a bandwidth of 4000 d (Fig. 16); the same photocathode tuned to the 2nd order works

FIQ.16. Gain of a 1st order RIC with E multialkali photocathode plotted against wavelength.

more selectively with a l 2 0 0 d bandwidth (Fig. 16). The amplitude condition for zero reflectance cannot be realized in the RIC. The reflectance of the mirror has to be as high as possible to prevent the loss of light by transmission or absorption. So the residual reflectance of an S.20 RIC with a silver backing is still 15% (see Fig. 17).

430

D . KOSSEL, K . DEUTSCHER AND K. HIRSCHBERQ

90-

- 10

Bo-

- 20

SUMMARY

The main object of interference photocathodes is the enhancement of photoemission of conventional photocathodes by purely optical means. The transmissive interference cathode (TIC) does not absorb completely and some light is lost by transmission. The photoemission is enhanced by a factor of 1.5. The totally reflective interference cathode (TRIC) completely absorbs light of a given wavelength and polarization. Up to now highest photoemission has been achieved with the reflective interference cathode (RIC) in spite of the fact that in general complete absorption of light cannot be achieved. The photoemission is raised by factors of up to 8 depending on the initial absorption of the single layer. A luminous sensitivity of 450pAIlm can be obtained in an RIC with a multialkali photocathode.

INTERFERENOE PHOTOCATHODES

431

REFERENCES 1. 2. 3. 4. 5.

0. 7. 8. 9. 10. 11.

Kossel, D., West German Patent No. 910,570; U.S. Patent No. 2,972,691. Deutscher, K., 2 . Phy8. 151, 536 (1958). Burton, J., Phg.9. Rev. 7 2 , 531 (1947). Dyatlowitzkaya, B., Zh. Tekh. Fiz. 22, 84 (1962). Hirschberg, K. and Deutscher, K., Phya. f%Ztu8Solidi 26, 527 (1968). Deutscher, K., Phy8. Verhandl. 10, 131 (1959). Kossel, D., West German Patent No. 1,055,710; U.S. Patent NO. 3,043,976. Rusch, D., Thesis, Giessen University (1965). Deutscher, K., Naturwi88en8cbften 44, 486 (1957). Love, J. and Sizelove, J., Appl. Optic8 7 , 11 (1968). Weber, S., Telefunken Report on Reflective Interference Cathodes (1968).

DISCUSSION N. J. HARRICIC: We have been experimenting with structures identical to those described for the totally reflective interference cathodes (TRIC). Our structures have shown absorption enhancement for weak absorbers of an order of magnitude compared to the absorption losses observed in the absence of the matching alms. Our optical cavities are described in detail in “Internal Reflection Spectroscopy” by N. J. Harrick (Wiley, New York, 1967). K . DEUTSCHER: Internal reflexion spectroscopy is based on the same considerations of absorption enhancement in films as the TRIC. It is a new and important approach in the spectroscopy of weakly absorbing materials. L. L. ALT: A further optimization appears to be possible by applying an antireflexion coating, similar to the one described by you, also to the outside of the glass wall. K. D E u T s c H E n : I have demonstrated the improvement h the light energy transfer from the adjacent medium to the photoelectric film. Of course all glass surfaces should be coated with the classical anti-reflexion fYms to prevent reflexion losses. E . A. RICHARDS: With the reflective interference photocathode, would it be possible to rearrange the order of the films so that the photocathode is deposited on a transparent substrate (in the normal manner) within the vacuum, but the interference “tuning film” and mirror are located outside the vacuum on the other side of the substrate? If this could be done, one should be able to change the optimum wavelength of enhancement to suit the particular application, as well as separating the technologies of photocathode deposition and filter fabrication. K . DEUTSCHER: The proposed system will work in a very high interference order mode. Therefore it can be applied to highly coherent radiation of small aperture, such as from a laser. We are working on this special kind of photocathode.

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The Development and Application of Interference Photocathodes for Image Tubes W. P. RAFFAN and A. W. CORDON 20th Centuv Electronics Ltd., Croydon, England

INTRODUCTION The authors of the previous papert have given a detailed treatment of several types of interference photocathode; we shall discuss only the reflective interference type. Light incident on the interface between a conducting medium and a dielectric interferes with the reflected light to form a standing wave. The maximum energy density available at the antinodes of this standing wave can be as much as four times that available in the incident light. For an aluminium mirror in Z ~ ~ C Uthe O spacing of the first antinode from the mirror surface varies from 8 6 0 R at 4000-A wavelength to 1850 A at 8000-A wavelength. If one wishes t o place the first antinode of a standing wave of 8000-A wavelength just below a photocathode vacuum interface, so as to obtain maximum photoemission, one is faced with the problem of making very thick photocathodes. Figure 1 shows that the optimum placing of a cathodevacuum interface can be achieved with a thin photocathode, if a transparent dielectric is used as an intermediate substrate. As can be seen from the path of one ray shown here, this system gives rise to appreciabIe multiple reflexions. The relative intensity of the ray at each region of the path is indicated by the adjacent number, and the multiple passes give rise to a further increase in the absorption of useful photon energy. Figure 2 shows the enhancement of photosensitivity predicted by Novice and Vine1 for a 300 d thick 5-20 photocathode deposited on an aluminium mirror, with silicon monoxide as an intermediate substrate. A relatively panchromatic enhancement is obtained with the thinner dielectric and it is worth noting that enhancement factors greater than four are predicted only for wavelengths greater than 6000 A. t

See p. 419. 433

434

W. P. RAFFAN AND A. W. UORDON Vacuum

Photocathode

A1

SiO

FIQ.1. Amplitude attenuation of a 7000-A ray incident on a reflective interference photocathode.

Dielectric thickness

ti)

FIQ. 2. Theoretical enhancement of the photosensitivity of an S.20 photocathode deposited on a reflective interference substrate for various wavelengths of illumination. AD is the wavelength in the dielectric. (Photocathode 300 A thick, on a SiO/Al mirror substrate.)

REFLECTIVE INTERFERENCE PHOTOCATHODES Our main investigation? has been into trialkali, or S.20, reflective interference photocathodes. The normal 150-pA/lm, 5.20 photocathode has approximate sensitivities of 2 and 0.3 mA/W at 8000 A and 8500 A respectively. Our aim was to achieve SSO-pA/lrn white-light sensitivity with sensitivities of 6-0 and 1.0 mA/W a t 8000 A and 8500 A.

t Sponsored by Signals Research and Development Establishment, Christchurch, Hants.

INTERFERENCE PHOTOCATHODES FOR IMAGE TUBES

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435

436

W. P. RAFFAN AND A. W. GORDON

We found that an anodized aluminium mirror formed an efficient, readily adjustable, interference substrate that was compatible with the S-20 photocathode, and we used this throughout our investigation. Figure 3 shows the characteristics of an S.20 photocathode with a 200-pA/lm sensitivity enhanced t o 400 pAI1m on the interference substrate. It should be noted that the sensitivity of the photocathode is higher when the light is incident through the glass rather than on the

Al, 250pA/Lm Gloss (trans), 7 6 p A / L m Glass (vac), 5 3 p A / L m

I -

-

0

4000 5000 6000 7000 8000 9000

x (8, (a

FIQ.4. (a) Spectral response, and (b) enhancement curves of an S.20 photocathode, with thicker initid Sb layer and post-oxidation, on glass and A1 substrates.

vacuum interface. This effect has been reported by Ramberg.2 The figure for the enhancement is in terms of the vacuum interface sensitivity. Unfortunately the sensitivity at 8000 was only about 0.1 mA/W. To improve the red response of this photocathode we increased the thickness of the initial antimony film by a factor of four (from 12% to 60% light absorption) and post-oxidized. This produced photocathodes with the type of spectral response shown in Fig. 4. There were five different substrates for the photocathode in this tube

INTERFERENCE PHOTOCATHODES FOR IMAQE TUBES

437

and for clarity only the response for the glass and aluminium (with 5.0 8 of natural oxide) substrates are shown. From the middle curve it

can be seen that the spectral response of the basic photocathode has changed appreciably. I n this case, for an integral sensitivity of 76pAIlm the responses at SO008 and 85008 are 4.0 and 2*0mA/W respectively. The aluminium substrate enhances these values to 26O-pA/lm sensitivity and to 10.0 and 4*0mA/W a t 8 0 0 0 8 and a t 8500 8. So far we have obtained spectral enhancement factors of up t o 6 but, as is shown in Fig. 5 , Novice and Vine' have obtained an enhancement of 10 at 8000 8 for an 5.20 cathode on an SiO/Al substrate.

I

3000 4000

5000

x (a)

6000

7000 8000

(a 1

FIG.6. (a) Spectral response, and (b) enhancement curves of an 5.20 photocathode, on glass and SiO/A1 (1800 thick) substrates (Novice and Vine').

I n the reflective interference photocathode the enhancement of the photosensitivity is dependent on the thickness of the photocathode as well as that of the dielectric substrate. Since we were uncertain of the thickness of our original photocathodes and of the effect on this of the processing changes we had introduced, we set up a facility to measure the thickness of our photocathodes. We based our method on work reported by Kondrashov and Shefov3 who claimed an accuracy of & 10% for the 5.20 cathodes they had measured. For most of our cathodes we obtained values using three different wavelengths that fell within this tolerance range. To our surprise we found the majority of our cathodes had thicknesses in the range 330 to 420 8 without any

438

W. P. RAFFAN AND

A. W. GORDON

obvious grouping associated with the processing changes. It is interesting to note that this thickness range falls very close to the 320 to 360 A recommended by Kondrashov and Shefov for optimum sensitivity 5-20 cathodes. From the values of the absorption index obtained in measuring the cathode thickness, we were able to show that the increase in sensitivity at 8000 A, achieved with our later cathodes, was due more to an improvement in the efficiency of the photoemission rather than t o an increase in the absorption of the incident light. We conclude that the improved emission is probably due to a reduction in the work function of the cathode surface by the post-oxidation. We have also deposited Sb-K-Cs bialkali cathodes on Al,O,/Al substrates to form reflective interference photocathodes. With these cathodes we have had 2 to 2.5 times enhancement of the white light sensitivity and enhancements of up t o six in spectral sensitivity usually in the region of 6000 8. Recently we have carried out some preliminary experiments with the S-1,Ag-0-Cs cathode, also on an Al,O,/Al substrate but, so far, we have not achieved a particularly encouraging enhancement in the 8000 to 10,000 A region.

APPLICATIONTO IMAGE TUBES In the application of the reflective interference photocathode to image tubes, the major problem that arises is the opacity of the reflective substrate. If a convenient “in-line” device is to be obtained the opaque substrate necessitates folding either the optical path of the light forming the input image or the path of the photoelectrons generated inside the image tube. I n tube designs using a folded input optical path the curvature of the field of the optical image is usually opposite in sign to that required for purely electrostatic focusing systems. Also for refractive optics of reasonable aperture the back focal-length available for folding is very limited. Figure 6 shows the basic design of a tube we are developing, in which we have chosen to fold the paths of the photoelectrons. The tube is basically a planar triode and the input optical image is projected through a biasing electrode, of high optical transmission, on to the photocathode which is deposited cn an apertured metal plate. The photoelectrons emitted from the photocathode are reflected back towards the cathode by the retarding field between the bias grid and the apertured cathode support plate and are extracted through these apertures by the accelerating field between the apertured plate and the planar phosphor viewing screen. The electron lens formed at each aperture by the retarding and accelerating fields is of inferior quality,

INTERFERENCE PHOTOCATHODES FOR IMAUE TUBES

439

Fro. 6. Basic design of an electrostatically focused image converter tube using a reflective interference photocathode.

but, if each lens is small enough, an image with acceptable resolution can be obtained. Figure 7 shows an experimental tube of this design. The length is approximately 40mm and the 70mm overall diameter can accom-

FIQ.7. An experimental planar image converter tube with an interference photocathode.

440

W. P. RAFFAN AND A.

W. GORDON

modate a 35 mm diameter circular viewing area. With a cathode plate having 20 holes/mm we have obtained 8 lp/mm limiting resolution across the field of view, with negligible geometric distortion.

FIQ.8. Basic design of a n interference planar image tube with an a3ditional mesh for operation as an optical shutter.

A modification of this design is shown in Fig. 8. Here a fourth electrode is introduced between the cathode support plate and the viewing screen. This is a mesh with a high electron transmission that, by application of a suitable potential, can be used t o shutter the photo-

Shutter mesh /cathode potentiol (V)

FIG.9. Switching characteristic of a shutter planar image converter tube with the electrode configuration shown in Fig. 8. Screen-to-cathode potential 6 kV.

electrons moving towards the viewing screen. Figure 9 shows the switching characteristic for this type of tube, a 300-V pulse giving full brightness with 6 kV between the cathode and output screen. These two tube designs have several features that are worthy of

441

INTERFERENCE PHOTOCATHODES FORIMAGE TUBES

note. (1) The photocathode sensitivity can be enhanced by reflective interference. (2) The whole of the photocathode is in intimate contact with a low resistivity substrate. This permits large photoemission currents t o be drawn without the distortion of the electric field that occurs if the photocathode has high resistivity. (3) Gas ions created in the tube volume between the cathode support plate and the viewing screen, bombard the rear of the cathode support and do not damage the cathode, as often happens in simple planar-diode image converters. (4) Electron-optical shuttering can be carried out with pulses of only a few hundred volts. ( 5 ) The full tube operating voltage can be applied over a longer period than the exposure time required. ACENOWLEDQMENT

The authors wish to thank the Directors of 20th Century Electronics Ltd. for permission to publish this paper.

REFERENCES 1. Novice, M. A. and Vine, J., Appl. Optic8 6, 1171 (1967). 2. Ramberg, G. E., A p p l . Optic8 6, 2163 (1967). 3. Kondrashov, V. E. and Shefov, A. S., Izv. Akad. Nauk SSSR Ser. Fiz. 28, 1444 (1964).

DISCUSSION 1. What is the spacing between your photocathode grid and the shutter grid? 2. Have you any results on gating properties of the tuber w. P. RAFFAN: 1. The spacing between the photocathode grid and the shutter grid in the example given, was approximately 0,030 in. 2. These tubes are in the early stages of development and a t present we only have the d.c. switching characteristic shown in Fig. 9. J . D. M ~ C E E :Must the apertured gating electrode be aligned? w . P. RAFFAN: We have not attempted to align the gating electrode with the cathode support electrode. The loss of 10- 20% of the photoelectrons traversing the cathode to viewing screen space is not, at present, a major problem. P. VERNIER: What proportion of photoelectrons reach the screen in the planar tube? w. P. RAFFAN: With a properly designed grid over 95% of the photoelectrons are extracted through the photocathode support grid. When a fourth electrode is introduced for shuttering 1 0 - 2 0 ~ 0of these electrons can be lost. E. EBERHARDT: A certain portion of the flux falling on your mesh photocathode passes through the tubes. Would you comment on the resulting average photocathode sensitivity for flooding radiation. Can this figure (of 80%) light collection be obtained at the high electron collection efficiency which you quoted to the previous questioner ? w. P. RAFFAN: Preliminary experiments have shown that forming a photocathode on a grid substrate does not impair the ability of the substrate to enhance the phot'osensitivity of the photocathode in the manner described in our paper. The effective photosensitivity of a photocathode deposited on a grid type subs.

MAJUMDAR:

P.E.I.D.-A

16

442

W. P. RABBAN AND A. W. GORDON

strate will be directly proportional to the product of the optical stopping power of the grid, the enhancement factor typical of the surface of the grid, and the efficiency with which the emitted photoelectrons are extracted. Extraction efficiencies of 96% have been obtained with cathode substrate grids having 60% optical transmission. The extraction of a grid type structure, as a function of the extracting electric field strength, is very dependent on the geometry of the grid structure. We believe a grid with 20% optical transmission can be made that will give 90-96y0 extraction efficiency at reasonable electric field strengths; but, as far as we are aware, such grids are not currently available commercially.

Image Intensifier System Using Reflective Photocathode J. H. M. DELTRAP and A. H. HANNA Aerqjet Delft Corporation, Melville, New York, U.S.A.

INTRODUCTION An image intensifier system using the interference principle to provide a more effective match between incident radiation and the photocathode has been constructed. The cathode is used in the reflective mode for maximum interference enhancement of the cathode sensitivity. Another reason for the use ofareflectiveorspacedreflectivephotocathode may be a desire for improved electrical or thermal conductivity. High electrical conductivity will enable fast gating of electrostatically focused image intensifier tubes and good thermal conductivity is useful if cooling is required. Both properties could be of interest in tubes designed for high cathode-loading. In general, however, the potential increase in overall sensitivity and the possibility of tuning the response to a particular wavelength are of greatest practical interest in image intensifier systems, the latter possibility being of particular advantage when image intensifier systems are used in conjunction with laser light sources. A folded optical system using mainly refractive components has been designed for use with an electrostatically focused tube. A diagram of the system is shown in Fig. 1. The light entering the system is refracted by two doublets in front of the tube, then enters the tube through the front window and is subsequently reflected by the mangin mirror inside the tube. The image is formed on the 25-mm diameter reflective photocathode located on the inside of the front window. The electron image is focused in the usual way by means of an electronoptical arrangement having unity magnification.

IMAGE INTENSIFIER SYSTEM The use of a reflective cathode requires that the tube and optics are designed as an integral unit. For electron-optical reasons the photocathode must be curved and therefore the optical system must form a 443

444

J. H. M. DELTRAP AND A. H. HANNA

curved image. A major design problem is the matching of the optical image curvature to the electron-optical cathode curvature. I n this case, good off-axis resolution demands a cathode radius of approximately 50 mm. With a focal length of 150 mm, an image curvature of one third the focal length is required. This is difficult to obtain. I n the present design, a compromise of 58-mm cathode radius has been selected. The relative aperture of the optics is f/l-5 and the field of view is 10". There are constructional problems associated with the alignment of the optical components of the tube, namely, the front window and

FIQ.1. Image intensifier system.

mangin mirror with the rest of the optical system and photocathode, and the elimination of distortion in the front window due t o sealing on a metal ring. Alignment is achieved by a carefully machined back-end of the tube which provides an accurate seat for the mirror. The tube is then positioned in a precision-made telescope housing which aligns it with the rest of the optical system. Distortion in the front window has been eliminated by carefully sealing a metal ring between the inside and outside radii of the glass, and then after annealing giving the front window its final grinding and polishing. Good optical quality windows have been obtained this way. A small amount of distortion is introduced when the tube is evacuated but since the tube front window is a weak optical element, the image is not materially degraded.

IMAGE INTENSIFIER USING REFLECTIVE PHOTOCATHODE

445

An improvement in contrast is obtained by darkening the electrodes inside the tube, and by positioning baffles to intercept and suppress unwanted stray light reflexions. A combined resolution of tube and optics of 551p/mm has been obtained on-axis and 28 Ip/mm at 8 mm off-axis.

INTERFERENCE PHOTOCATHODES Photocathode experiments, using interference techniques, to obtain maximum sensitivity enhancement a t wavelengths between 8000 A and 8500 d have been carried out concurrently with the tube develop-

Wavelength (&

Bra. 2. Spectral response of 5.20 photocathode in transmission and reflexion, and enhancement curve with peak at 8100 A.

ment program. The interference cathode consists of a silver mirror, a silicon monoxide spacer of approximately 1500 A thickness, and an 5-20 photocathode with an extended red response. Some of the results are indicated in Figs. 2, 3 and 4. Figure 2, shows the spectral response of a reflective photocathode with a sensitivity of 25 mA/W a t 8000 A and 10 mA/W a t 8800 d. The response of the cathode in transmission, which is also shown, was obtained by measuring the sensitivity of a small area which was not silvered. The enhancement curve is also plotted and shows a peak at 8100 A. Non-uniformities in the cathodes

446

J. H. M. DELTRAP AND A. H. H A ” A

were not always eliminated and the comparison between transmissive and reflective cathode may not be exact in all cases. Nevertheless enhancement factors of 5 times were measured regulmly . The major problem in producing these interference cathodes is the control of the cathode thickness. This is not always completely successful. Figure 3 shows the characteristics of a cathode with an enhancement peak a t 7000 8;note that the sensitivity at this wavelength is 80 mA/W which corresponds to an efficiency of 15%.

I

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4000

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5000

I 6000

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7000

Wavelength

8000

9000

(A)

FIG.3. Spectral response of S.20 photocathode in transmission and reflexion, and enhancement curve with peak at 7000 A.

Figure 4 shows the calculated total light absorption in the cathode layer of an interference configuration, as a function of the spacer optical thickness. In order to arrive at this, a knowledge of the optical constants and thickness of the cathode layer is necessary. Reflexion and transmission data of 5-20 photocathodes were collected and have been combined with assumed values of the thickness of the cathode layer. These measurements were initially carried out on relatively thin cathodes. If a thickness of 500 d is assumed, the resulting values for the refractive index n and extinction coefficient k are as shown in Table I.

IMAGE INTENSIFIER USINQ REFLECTIVE PHOTOCATHODE

447

0.1 I

0

l

l

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l

l

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l

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400 800 1200 1600 xxx)2400 2800 3200 3600 40004400 4 00

Optical thickness of dielectric spacer (8)

FIG.4. Total light absorption in the photoemissive layer (5.20) of an interfereace photocathode as a function of spacer optical thickness end wavelength.

These values, which agree with those given by Kondrashov and Shefov,l were used to derive the graph presented in Fig. 4. It appears that maximum enhancement at 80008 is obtained for a spacer of 4 0 0 0 8 optical thickness. This together with a cathode having a thickness of 500 d and a refractive index of 2.6 would have a total optical thickness of 5300 A for the combination. TABLEI Optical constants of S.20 photocathode Wavelength

n

k

BOO0

A

39 1 0.75

6000 A

7000 A

8000 A

2.9

2.8 0.40

2.6 0.27

0.50

Superior red response has only been achieved in thicker cathodes, and it was experimentally found that a silicon monoxide layer of 2700-A optical thickness combined with these thicker cathodes resulted in peak enhancement at around 8000 8. In order to arrive at the optical thickness of 5 3 0 0 8 with a 1500-A silicon monoxide layer a cathode thickness of approximately 900 8 is required. Thicknesses of this order of magnitude were actually measured using the Tolanski2 technique applied in vacuum. Figure 5 shows an interferogram of an 5.20 cathode layer obtained using this technique.

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J. H . M. DELTRAP AND A. H. HANNA

I n a further study of the optical constants of 5-20 cathode layers, it was found that if a single homogeneous layer were assumed, no combination of refractive index and absorption coefficient for layers of greater than 600A thickness would result in the measured reflexion and transmission coefficients. This indicates that the actual composition of the thicker cathode layers may not be that of a singie homogeneous layer.

FIG.5 . Interferogram of an S.20 cathode.

I n the actual tube the values of sensitivity attained in the cathode experimental program have not yet been achieved. However, it is believed tha,t there is no intrinsic limitation in making such cathodes in tubes and that values of 30mA/W and higher at 8000A can be obtained. ACKNOWLEDGMENT The financial support of the Night Vision Laboratories a t Ft. Belvoir, Virginia is gratefully acknowledged.

REFERENCES 1. Kondrashov, V. E. and Shefov, A. S., Bull. Acad. Sci., U.B.S.R. Phys. Ser. 28, 1444 (1964). 2. Tolanski, S., “Multiple Beam Interferometry of Surfaces and Films”, Oxford University Press, London (1948).

IMAGE INTENSIFIER USWG REFLECTIVE PHOTOCATHODE

449

DISCUSSION w. M. WREATHALL: 1. Is it necessary to have a conducting layer on the inside of the window in order to stabilize the electric field? 2. What is the transmission efficiency of the lens? J . H. M. DELTRAP: 1. No conducting layer on the inside of the window has been applied; however, there are indications that this may be necessary a t higher current densities. 2. The transmission of the lens is approximately 50%. This results in a T-stop value of 2.1. P. SCHAOEN: 1. What is the ratio of the outside tube diameter to the useful cathode diameter? 2. How efficiently can you manufacture this type of photocathode, in view of the fact that if the photocathode sensitivity is too low some of the expensive tube components will have to be written off? J. H. M. DELTRAP: 1. The outside tube diameter is 4.85 in. and the useful cathode diameter is 1 in. 2. The sample tubes have been made under a development program and we have no meaningful yield figures; however there is no reason why the manufacturing yield should be less than for other high performance image intensifier tubes. Also, the tube components are less expensive than the fiber-optic face-plates of certain other tubes. M. H. KEY: It appears that such a tube would be very valuable in high-speed photography if it were designed to include streak deflexion electrodes. The use of the reflecting photocathode would make it possible to use metal-backed photocathodes capable of giving the high current densities that are necessary for time resolution in the picosecond range which is of considerable interest in laser physics at the present time. J . H. M. DELTRAP: we are very interested in the use of this tube in high-speed photography. Deflexion electrodes could always be incorporated. N. J. HARRICK: One can imagine that broad-band response for interference photocathodes might be obtained by achromatizing, as is done in making broadband transmission filters. Do you know whether any work has been done in this direction? J. H. M. DELTRAP: We are not aware of any experimental work with multiple dielectric layers for interference photocathodes. In our case we were particularly interested in selective enhancement a t 8000 and 8500A. A broad overall enhancement of the photoresponse can be obtained using a much thinner space or depositing the photocathode directly on to the mirror surface. R. L. VERMA: What dielectric material was used as a spacer? J. H. M. DELTRAP: The dielectric material was SiO.

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Scintillation Processes in Thin Films of CsI(Na) and CsI(T1) due to Low Energy X-rays, Electrons and Protons C. W. BATES, Jr.

Varian Associates, Palo Alto, Calijornia, U.S.A.

INTRODUCTION The use of CsI(Na) and CsI(T1) as scintillator materials of high efficiency for charged and uncharged particles has been reported by a number of authors. To the best of our knowledge, this work has been performed on bulk samples. We report for the first time measurements made on thin films of these materials varying in thickness from 5000 +& to 0.25 mm, deposited on glass, aluminum, and plastic substrates. We have demonstrated that the use of these thin films in practical devices, such as X-ray image intensifiers and multiple particle detectors is quite feasible.

MEASUREMENTS OF SCINTILLATION EFFICIENCY The spectrum of each emission band of most inorganic solids is independent of whether the primary excitation is from ultra-violet radiation, X-rays, or cathode-rays. The reason for this in the case of extrinsic luminescence is that the activator in the emitting electronic state relaxes with respect to the lattice coordinates to the same equilibrium configuration, regardless of the nature of the excitation. We have found this to be true for both CsI(T1) and CsI(Na) even though the luminescent efficiencies are different for different excitations, as one would expect. The host crystal (CsI) is transparent to the impurity emission for both the thallium-activated material (emission band maximum in the range 4100 to 5800 8)and the sodium-activated material (emission band maximum at 4200 A), having an absorption edge at 2000 A so that self-absorption is small and thus the luminescent efficiencies are determined solely by the processes of excitation and emission. Figure 1 is a block diagram of the apparatus used to measure the X-ray and cathodoluminescent emission spectra of the films which were 461

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C. W. BATES, JR.

vapor deposited in vacuum at a pressure of l o d etorr. The X-ray gun was especially made for this experiment so that the tungsten target could be removed and the electron beam used for cathodoluminescent measurements. A Sloane deposit-thickness monitor gauge was used in the vacuum system t o measure the thickness of the films deposited. The source materials were obtained from the Harshaw Chemical Company, Cleveland, Ohio, and contained optimum amounts of Na and T1 for mole TI).? maximum scintillation efficiency ( l o - * mole Na and Films varying in thickness from 0.5 t o 120pm were deposited on glass substrates in order to check their structure under a variety of

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FIG.1. Block diagram of X-ray emission spectra apparatus.

deposition conditions (degree of supersaturation and substrate temperatures). It was found that in order t o produce highly efficient films, substrate heating during evaporation followed by slow cooling to room temperature (2 t o 4"C/min) was essential. Substrate temperatures from 100 to 300°C were found to produce the best results. The structure of the films is shown in Fig. 2. The cracked areas in Fig. 2(a) average about 0.5 pm in size and in Fig. 2(b) they are 0.2 pm. These cracks are due to strains resulting from evaporation a t too high a rate. Subsequent films produced by evaporation a t rates considerably lower (-0.005 pmlsec) had few or no cracks at all. The streaks and small blotches are on the aluminum substrate, which can be seen quite distinctly through the clear films. I n appearance the films are like the These figures have been quoted in the literature aa being optimum, but it is unlikely that this is true for all types of primary excitations. The problem is presently being .studied at this laboratory.

SCINTILLATIONS IN THIN FILMS O F CsI(Na) AND CsI(T1)

453

bulk material. These measurements and the ones t o be subsequently described were made on films which were deposited on aluminum substrates. One very interesting and important difference between the sodium-activated and thallium-activated thin films should be pointed out. After evaporating several films, it was found that the fluorescent efficiency and the amount of strain of the sodium-activated films were much more sensitive to substrate heating and evaporation rates than those for the thallium-activated ones. It might be suspected that the answer to this difference lies in the process of evaporation. If it is assumed that during evaporation the sodium and thallium exist in the

FIQ.2. Photograph of 100-pn-thickfiIms of CsI(Na) ( x 200 magnification). (a] Evaporation rate 0.08 pm/sec, (b) evaporation rate 2 pn/sec.

form of the iodidest a check can be made, from vapor pressure data, of the degree of segregation which occurs because of a difference in the vapor pressures between the sodium and thallium iodides and the host crystal, cesium iodide. Figure 3 is a plot of vapor pressure versus temperature for CsI, N a I and TII. These data were obtained from the Landolt-Bornstein vapor pressure data tables. It is obvious from these curves that NaI and CsI

t We feel that the formation of the compounds TI1 and NaI from the doped CsI during evaporation is a reasonable assumption to make because T1I and NaI are the most stable compounds which could possibly result from the decompositios of the activated CsI. We have also found it possible to produce CsI(Na) and CsI(T1) by oo-evaporating CsI with NaI and CsI with TK, respectively. It is also known1 that TI enters the CsI lattice substitutionally for the Cs and we assume the same is true for Na.

454

C. W. BATES, JR.

have vapor pressures which are very close to one another over a wide temperature range compared with TI1 which has a vapor pressure more than four orders of magnitude greater than either CsI or NaI over the same range. Hence, it appears that T1I would segregate in the host CsI crystal to a far greater degree than NaI, resulting in a considerable loss in fluorescent efficiency when the thallium is substituted for the cesium. We feel that it is the latter process, i.e. the substitution of Na or T1 for Cs, which explains why the fluorescent efficiency of

Temperature ( O K)

FIG.3. Vapor pressure versus temperature for CsI, NaI and TII.

CsI(Na) is more critically dependent upon evaporation rates and substrate heating than CsI(T1). The diameters of Cs+I, N a f l , and T1+2ions are 1-698, 0.95 A, and 1-6 8, respectively. From this we conclude that the thallium ion is certainly a much better fit than the sodium ion in substituting for the cesium ion. This, in our estimation, could more than compensate for vapor pressure effects. Figure 4 shows the optical emission spectra of CsI(Na) and CsI(T1). CsI(Na) has a peak output a t 42008 with a half-width of 1 2 0 0 8 , while CsI(T1) has a peak output a t 5800 A and a half-width of 2000 8. The measurements of the response of CsI(Na) and CsI(T1) to low energy electrons and protons were made on the 300-keV accelerator a t

SCINTILLATIONS IN THIN FILMS OF CsI(Na) AND CsI(T1)

455

the Space Science Laboratories of the University of California a t Berkeley by Professor F. Mozer and Mr. F. Bogott. The accelerator was equipped with an r.f. source and a magnetic beam-analyzer. I n these measurements with electrons and protons, it was found for both CsI(Na) and CsI(T1) that the light output as a function of incident particle energy is near linear down to 20 keV. For X-rays the linearity extends down to about 30 keV as is shown in Fig. 5, where we have plotted light output as a function of X-ray energy for a 100-pm-thick film of CsI(Na). Similar results are obtained for CsI(T1). The results of the measurements on these films indicate that the light output for

1

Wavelength (8)

FIG.4. Emission spectra of CsI(Na)and CsI(T1).

X-rays and protons of the same energy are about equal for a given material, but the output reeponse of CsI(Na) is twice that of CsI(T1). For electrons the light output at a given energy is about three to four times the light output of X-rays and protons a t the same energy for both CsI(Na) and CsI(T1). Again the output of the sodium-activated material was twice that of the thallium-activated material. All measurements were made with a photomultiplier having an S.11 photocathode. CsI(Na) had a pulse height of about 70%, and CsI(T1) about 30%, of that for NaI(T1). The decay constants of CsI(Na) and CsI(T1) were measured with an oscilloscope and found to be 0.8 and 2 - 5 p e c respectively. It was found possible to store these films for several weeks under normal conditions. They are soluble in water and slightly hygroscopic. A mild air-bake (100°C) for about an hour usually

C. W. BATES, JR.

456

restored the fluorescent efficiency of a film which had picked up water vapor during exposure to the atmosphere. The vapor pressure is 1 mm a t 738°C and hence they are excellent high vacuum materials.

1

15

I

20

I

25

I

30

I

I

?6

40

45

X-Royenergy (keVV)

FIG.5. Light output versus X-ray energy for a 100-pm-thickfilm of CsI(Na).

THE SCINTILLATION PROCESS IN CSI(NA)AND CSI(TL) The scintillation mechanism in CsI(T1) appears to be well understood.2 The primary excitation creates, per unit path length, a number of electrons and holes; the holes may then be trapped near an impurity site (Tl+),with a lifetime in the trapped state which is a function of temperature. This hole is said t o be self-trapped. Free electrons may be captured a t T1+ sites, or may recombine directly with a trapped hole. The probability that a given electron will recombine with a trapped hole rather than suffer capture a t a T1+ will be an increasing function of the density of trapped holes. This concept treats luminescence from the T1+ (5800-11band) as arising from a process in which the Tl+ first captures an electron. A hole which is initially self-trapped can be thermally excited t o the valence band and can migrate t o the T1° center in a time depending on the temperature and depth of the hole. Capture of the hole a t the T1° center completes the cycle and permits the 5800-11 band luminescence. Hence, the light output as a function of temperature (for a fixed primary excitation energy) in this picture, should increase aa the temperature increases from liquid nitrogen temperatures towards room temperatures (indicating thermal excitation of trapped holes and subsequent capture a t T1° centers,

SCINTILLATIONS IN THIN FILMS O F CsI(Na) AND CsI(T1)

457

resulting in luminescence). As the temperature increases, however, the probability for free electron capture a t T1+ sites (or any sites) decreases. Thus the light output can be expected t o increase up to a certain temperature and then to decrease thereafter. This is, in fact, found to be the case in several T1-activated alkali halides.a We have found this type of behavior to be true for CsI(Na), though the exact role of the sodium is not well understood. Figure 6 gives the results for a 100-p.mthick layer of CsI(Na). The temperature for maximum light output is 85°C.

Temperature ("C)

FIQ.6. Light output intensity versus temperature for a 100-pm-thicklayer of CsI(Na) irradiated with 28-keV X-rays.

APPLICATIONS We have constructed two devices employing thin films of CsI(Na) and CsI(T1). The first is a double scintillator device, using CsI(T1) evaporated on to a plast'ic scintillator material, for unambiguously distinguishing electrons and protons in different energy ranges. This device has been described elsewhere3 and will not be considered here. The second device is an X-ray image intensifier using an evaporated film of CsI(Na) as the input phosphor. A cross-section of the complete tube is shown in Fig. 7. The tube body is constructed of glass. The input diameter is 22 cm and that of the output is 1.9 cm. The tube is constructed in two sections which are joined together by welding the two Kovar flanges shown in the figure. The front half contains the 500-pm-thick aluminum dish upon which the CsI(Na) film is deposited. Typical film thickness is 100 pm. After the film is vapor

458

C. W. BATES, JR.

deposited on the aluminum dish as described earlier, the dish is mounted in a glass dome assembly made of Corning 7052 glass which has high X-ray transmission. The two sections are joined together and the tube is then exhausted to l o F 8torr and baked for 6 h a t 300°C. This baking appears to have no deleterious effect upon the CsI(Na) efficiency. An S.9 photocathode is then made directly on the CsI(Na) surface by means of the trough shown in the figure, in the usual way. The fact that one can process the photocathode directly on the CsI(Na)

FIG.7. X-ray image intensifier.

without any harmful effect results in a! considerable saving of time and increases the simplicity of the device over other such devices which require some sort of protective coating to mask the phosphor from the usually harmful effects of the photocathode processing. The tube which is 35cm long is operated a t an output-screenpotential of 23 kV relative to the photocathode. Typical gains measured relative to a Patterson CB-2 fluoroscopic screen are 8000 to 12,000. The resolution, referred to the input, and measured with a brass mesh that has an open area of 25%, is 60 to 60 mesheslin. over 100% of the image diameter. The average background is 0.002 Im/ft2. Such a tube is finding wide applications in clinical X-ray diagnosis.

SCINTILLATIONS IN THIN FILMS O F C d ( N & ) AND CsI(TI)

459

CONCLUSION

It has been possible to produce thin fiIms of CeI(Na) and CsI(T1) which fluoresce with the same efficiency as the bulk materials. The ease with which these materials may be prepared in thin film form and their ability to perform satisfactorily in various environments suggests a wide range of applications in photoelectronic devices. REFERENCES 1. Teegarden, K., In “Luminescence of Inorganic Solids”, ed. by P. Goldberg, chap. 2. Academic Press, New York (1966). 2. Gwin, R. and Murray, R., Phya. Rev. 131, 508 (1963). 3. Bates, C. W. Jr., Varian Associates Central Research Memorandum No. 200, June, 1967.

DISCUSSION w.

KURL:

1. Could you comment on the function of Na in CsI(Na) with

respect t o the mechanism of fluorescence? (The role of T1 in CsI(T1)is rather well understood.) 2. You said the fluorescence yield of evaporated CsI(T1) layers is much the same as that of CsI(T1) crystals; what is it for evaporated CsI(Na) layers in comparison to CsI(Na) crystals? C. w. BATES: 1. From the figure showing light output as a function of temperature for CsI(Na), it appears that the mechanism of fluorescence is the same as for CsI(Tl), i.e., self-trapped holes being captured at a T1° site resulting in luminescence. However,more work is needed to determine if this is the only possibility. 2. Evaporated CsI(Na) layers give the same fluorescence yield as the bulk material. D. THEODOROU: I n practical X-ray image intensifier tubes, what improvements in performance have you obtained using doped CsI screens over conventional X-ray-sensitive phosphor screens? c. w. BA’rEs: Doped CsI appears to be more insensitive to tube processing than other X-ray phosphor screens. N. ROME: What is the sensitivity of the thin CsI(T1)evaporated scintillators as compared to the bulk material? Can you give the efficiency of the scintillators in absolute units for direct comparison? C. w. BATES: The CsI(T1) evaporated scintillators have the same sensitivity as the bulk material. Unfortunately, our measurements were made relative to NaI(Tl), so we do not give the efficiency of the scintillators in absolute units.

This Page Intentionally Left Blank

Quelques Aspects des Essais de DCpGt de Photocathodes S.20 et d’lhrans Fluorescents sur Fibres Optiques S . VERON Compagnie QdnBrale de Tdldgraphie Sans Pil, Paria, France

INTRODUCTION Dans le cadre d’6tudes realisees pour le compte de la Section d’Etudes et Fabrication des TBl&communications,tun certain nombre de travaux faisait appel l’emploi de plaques de fibres optiques. L’Btude et la realisation de ces plaques ont BtQ assurees par la Compagnie Saint Gobain et la Compagnie Sovis. S’agissant d’applications a des tubes A image, il Btait necessaire que 1’6laboration de ce materiel soit faite en liaison avec les utilisateurs fabricants de tubes afin que le produit obtenu donne satisfaction sur le plan des performances. Certains aspects de l’emploi de ces plaques ont donc BtB BtudiBs, notamment en ce qui concerne la possibilite de depot de photocathodes S.20, et la realisation d’gcrans A grand pouvoir separateur. Quelques uns des travaux realis& sont relates dans cet expose.

PHOTOCATHODES 5.20 STANDARD ET PLAQUES DE FIBRES OPTIQUES Matkiel Utilist? Pour realiser ces essais et permettre des comparaisons entre diff6rents types de supports, les Blements suivants ont BtB utilises: 1. Des cellules photoelectriques simplifiees dans lesquelles les photocathodes peuvent &re d6posdes soit sur du verre, soit sur des plaques de fibres optiques, soit sur ces deux types de support simultanhment. Suivant les besoins, les ghnerateurs d’alcalins et 1’6vaporateur d’antimoine peuvent &re incorpores ou exterieurs a l’enceinte de la cellule. 2. Des tubes A focalisation Blectrostatique (Figs. 1 et 2). La formation des photocathodes a Bt6 obtenue par la methode classique avec, Bventuellemeiit, quelques variantes.

t Fort d’Issy, 92 Issy-lea-Moulineaux, France. 461

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FIQ.1. Tube transformateur d’image D 18.

FIo. 2. Tubes diodes avec faces en verre et en fibres optiques.

RLsultats Obtenus Sur des supports en verre-f et dans des cellules simples, lea photocathodes sont relativement faciles & rdaliser et, frbquemment, lea sensibilitds ddpassent 170 pA/lm avec lea gdndrateurs incorpords ou extdrieurs. Lorsque la structure est plus complexe, comme dans le cas des tubes B focalisation Blectrostatique, la dispersion des rBsultats eat

t Sovirel747.01.

D I ~ P ~DE T PHOTOCATHODES ET

D’BCRANS BUR FIBRES OPTIQUES

463

plus grande, cependant une sensibilitd de 150pA/lm au moins peut &re obtenue sans trop de difficult&. Ces rephres permettent de qualifier les rdsultats des essais rbalisbs sur des plaques de fibres optiques. En ce qui concerne les fibres optiques, de nombreux essais ont 6th effectues sur des Bchantillons de plaques aux diffbrentes Btapes de 1’6tude. Tout d’abord, sur les verres entrant dans la fabrication des fibres. Puis, sur des plaques de fibres completes ne comportant pas de deuxieme couche absorbante. Et, enfin, sur des fibres munies de cette couche absorbante obtenue l’aide de differents matbriaux. Une premiere sBrie de plaques constitube de deux lots utilisant des verres diffdrents a fourni des rdsultats compris entre 30 et 110 pA/lm dans un cas et entre 100 et 160pA/lm dans l’autre cas, cela sur des cellules simples. Une autre serie de plaques montBes cette fois-ci sur des tubes transformateur d’images Qlectrostatiques a permis d’obtenir de bons rbultats, puisque 40% des tubes prhsentaient des sensibiliths comprises entre 120 et 150 pA/lm et 25% des sensibilitks comprises entre 150 et 200pA/lm. La comparaison avec des photocathodes r6alisBes sur des supports en verre fait apparaitre que les rdsultats obtenus sont du meme ordre. On peut donc conclure que les couples de verre choisis pour la fabrication des plaques de fibres sont compatibles avec les photocathodes 5.20 standard. Ces verres ont BtB mis au point par le fabricant spbcialement pour cet usage. I1 s’agit de verre 8. base d’oxyde de lanthame dont les indices de refraction sont compris entre 1.75 et 1-85 pour le verre de coeur et 1.46 et 1.52 pour le verre de revetement. Des contrbles effectuBs 3 et 6 mois apres la fabrication de ces tubes montrent que les sensibilitks sont stables. L’Btape suivante consistait it munir lea fibres d’une deuxieme couche absorbante. Tres souvent, cette adjonction a eu pour effet d’empoisonner lea photocathodes B des degrBs divers, de sorte que les sensibilitbs Btaient infBrieures B 100 pA/lm. Finalement, une couche d’enrobage a permis de rBaliser des sensibilitds supdrieures A 150 pA/lm.

GdnLrateurs Secondaires d’i-llcalins Au cours de cette Btude, nous avons BtB amen& a examiner le probleme des gBnBrateurs d’alcalins. La liberation des alcalins s’accompagne toujours d’une IibBration simultanee de gaz, qui peuvent perturber le processus de formation des photocathodes. Nous avons tent6 d’bliminer ce phBnomAne par l’emploi de ghnkrateurs ne libBrant pas de gaz. On a utilisB pour cela la proprihtB de certains corps de fixer les alcalins entre leurs mailles cristallines, l’btain par exemple.

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5 . VERON

La Pig. 3 presente le dispositif utilisB prht it l’emploi. On distingue la cupule du generateur primaire, un filament chauffant recouvert du fixateur constitue par de l’etain; les getters, la membrane mince qui, par sa rupture, permet, aprbs mise sur pompe, la mise en communication du rdservoir avec le tube. La liste des operations est la suivante. (1) Mise sur un bati de pompage auxiliaire, Btuvage. (2) Plash du genhrateur d’alcalin. (3) DBgazage du filament porteur de 1’6tain (par effet joule). (4) Activation des getters non flashables. ( 5 ) Etuvage avec moiltee lente jusqu’it 4OO0C environ: pendant cette operation, une partie de l’alcalin se fixe sur 1’6tain. ( 6 ) Scellement et stockage.

t

Vers le tube photoblectrique b former

I1 est possible, pour assurer un meilleur degazage de l’dcalin, d’effectuer les operations suivantes avant scellement : rkactivation des getters; liberation de l’alcalin par chauffage de 1’8tain it l’aide du filament (cette liberation s’effectue progressivement avec 1’818vation de tempBrature) ; scellement ; Btuvage de l’ensemble pour fixer it nouveau le metal alcalin sur 1’Btain. Le gBn6rateur peut ensuite 6tre monte le moment venu sur un tube pour fournir l’alcalin necessaire A la formation d’une photocathode. Malheureusement, pour une photocathode S.20, il faut 3 gdn8rateurs distincts, ce qui conduit it un ensemble encombrant et fragile. Aussi cette methode qui avait donne de bons resultats avec une cellule S.11, n’a-t-elle pas BtB exploree it fond prbsentement.

D B P ~ TDE PHOTOCATHODES

ET D’I~CRANSSUR FIBRES OPTIQUES

465

~ C R A N SFLUORESCENTS

Dans le but de realiser des Bcrans fluorescents it grand pouvoir separateur utilisables aussi bien sur verre que sur fibres optiques, un certain nombre d’investigation ont Bt6 faites dans le domaine des methodes de preparation. La m6thode la plus classique de fabrication des Bcrans, le depot par sedimentation avec ses nombreuses variantes, est bien connue des sp6cialistes. E n triant les grains par prkklimentation, il est possible d’atteindre avec une couche non ahminisee un pouvoir separateur d’environ 100 pl/mm contr616 en lumibre ultra-violette. Cela conduit avec des Bcrans aluminis& it un pouvoir separateur ne depassant guere 60 it 70 pl/mm. L’utilisation de grains trbs h s presente certaines difficult&. Les depots obtenus par ce procede montrent souvent des ddfauts, tels que: dispersion dans les Bpaisseurs, amas de grains, recouvrement insuffisant, micro-trous, et rugosite trop importante. Nous avons experiment6 le depot par projection Blectrostatique et la methode dite PVA: sedimentation dans l’alcool polyvinylique sensibilise au bichromate d’ammonium et fixation par insolation en ultraviolet.

Projection Blectrostatique Pour mettre en oeuvre la premiere de ces methodes, nous avons utilise un Bquipement pour poudrage 6lectrostatique Stajet de la Sames,? present6 sur la Fig. 4. Cet ensemble comprend un gdnerateur de 90 kV rkglable, un pistolet de projection special, un reservoir de poudre, un tableau de distribution d’air, et les cables haute et basse tensions de raccordement. Les grains de poudre sont charges, diriges par un jet d’air et se deposent sur les objets situes dans la zone d’influence du champ Blectrique. Des modifications de detail ont 6th apportees it l’appareil pour l’adapter it ce genre d’application. En variant les conditions d’utilisation, il a Bt6 possible d’obtenir des couches denses constitubes par des grains de poudre accoles les uns aux autres. Divers types de poudres ont Bt6 utilisds: P-20 et Pel1 de granulometrie variable (type B/SE FeinkbrningS et type P.207 composee de grains de 0.5 pm, mais aussi de grains plus gros). Ce mode de depot exige que la surface it recouvrir soit legerement conductrice de 1’6lectricit8, une resistance de 2 it 3 x lo5 !2/0 est satisfaisante.

t Sames, 21 rue Jean Mac& Grenoble, France.

3 Riedel de H e n AG., Seelze, Hannover, Allemagne. Dr. Stamm, Ebel Hausen, BaviBre, Allemagne.

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FIG.4. Equipement pour poudrage Bleotrostatique (Stajet de Semes).

La Fig. 5 montre la diffkrence de structure en examen par transmission entre un Bcran sBdimentB definissant en ultra-violet 100 pl/mm et un Qcranprojete avec de la poudre type B/SE. La Fig. 6 montre la s6lection des grains obtenue en agissant sur les diff6rents rBglages de l'appareil. Enfin, la Fig. 7 montre en lumibre refleehie l'aspect d'un Bcran

FIG. 6. Structure d'ecrans examines par transmission ( x 60, poudre B/SE Riedel de Haen). (a)fioran sBdiment6 (100 pl/mm en ultra-violet). (b) &ran projete.

D I ~ P I ~ TDE PHOTOCATHODES

ET D’I~CRANSSUR FIBRES OPTIQUES

467

sediment6 definissant 100 pl/mm en ultra-violet et celui d’un Bcran projet6 definissant 160 pl/mm. Le depot s’effectue rapidement en 10 ti 20sec; un dispositif de contrde de la transparence en cours d’opdration permet d’arreter celle-ci au moment voulu.

FIQ.0. SBlection des grains ( x 80, poudre P.20 du Dr. Stamm). (a) Les gros grains sont BliminBs. (b) Tom les grains sont projet&.

FIQ.7. Structure d’ecrans examines en lumiere r6flBohie ( X 80, poudre P.20 du Dr. Stamm). (a) &cran sediment6 (100 pl/mm en ultra-violet). (b) Goran projet6 (160 pl/mm en ultra-violet).

La poudre deposee sur une surface par pulv6risation 6lectrostatique reste plusieurs jours en place pourvu qu’on Bvite les chocs et les courants d’air violents. Son adherence est cependant insuffisante pour un 6cran de tube. I1 est donc ndcessaire de la fixer par un moyen agissant aussi peu que possible sur l’arrangement des grains. Deux voies paraissent possibles pour obtenir ce rksultat: (i) depot d’un liant sur le support

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avant projection de la poudre, et (ii) fixation aprbs projection de la poudre. Dans le premier cas, quel que soit le liant utilisB (acide phosphorique scBtone, ou PVA), I’adhBrence obtenue s’est rBvBlBe trop faible. La raison est que les grains de poudre ne sont pas lies entre eux, mais seulement avec le support. La seconde mBthode de fixation implique un mouillage de la couche par le liant choisi. Pour respecter la structure de la couche, nous avons utilisB des Grosols form& B partir de la solution de fixation. Cette dernihre Btait constitude par du silicate de potassium et de l’acdtate de baryum de fapon B former un gel de silice. Le mouillage peut se faire soit en une seule fois l’aide de cette solution, soit en

+

FIQ. 8. @cran projet6 ( x 60, poudre P.20 du Dr. Stamm). (a) Avant fixation (b) Aprbs fixation.

deux fois en utilisant successivement la solution du silicate, puis la solution d’acdtate. La composition des solutions est voisine de celle utilisBe pour la skdimentation des Bcrans. Une bonne reproductibilite a BtB obtenue en contr8lant les parametres influant sur les rBsultats de I’opBration. La Fig. 8 montre l’aspect d’un Bcran avant et aprbs fixation. Plusieurs Bcrans de ce type, aluminis& et montes sur un tube, ont permis d’atteindre un pouvoir sdparateur de 80 pl/mm.

Prdparation des &runs b 1’Alcool Polpinylique Ce type de liant est utilisB dans la fabrication des Bcrans de tubes de t6lBvision en couleurs et peut 6tre employ6 pour prkparer des Bcrans fins. Ce procBdB a BtB dBcrit ici-m6me il y a trois ans.l Cette mBthode a BtB exphimentee sur des tubes transformateurs d’image infra-rouges type D 16 avec succhs dans le centre de fabrication CSF de Saint Egreve.

DGPGT DE

PHOTOCATHODES ET D’&CRANS SUR FIBRES OPTIQUES

469

Le pouvoir sBparateur d’un tel Bcran termin6 atteint assez facilement 100pl/mm en moyenne. Par contre, son rendement est diminuB de 15 b 20% par rapport B un Beran sBdimentB classique definissant 50 B 60 pl/mm. TABLEAU I Caract6ristiques des divers types d’dcrans aluminis& &wens s6diment6s

gcrans PVA

7&8 4&8

Epaisseur moyenne (pm) Rugosit6 (pm) Efficacit6 lumineuse (lm/W) Pouvoir s6parateur (pl/mm)

6 28,3 45 80 & 120

55 40 8, 65

Gcrans projetes 58,9 2.5

45 70 8,100

Le Tableau I recapitule quelques chiffres relatifs aux divers types d’Bcran. Les Bcrans sediment& sont rBalisBs avec de la poudre P-20 Riedel de Haen; les Bcrans PVA et projetBs avec la m6me poudre ou la poudre du Dr. Stamm B grains de 0 . 5 ~ ” . L’Bpaisseur moyenne des Bcrans et leur rugositB sont mesurBes au microscope b coupe optique avant mBtallisation.

\

0

I

I

20

40

\

60

Fre’quence spatiaie (cycles/mrn)

FIG.9. Fonctions de transfert de modulation de tube D 16 avec Bcrans sBdimentBs e t 6crans PVA.

470

9. VERON

CoNCLUsIoNs

Les rdsultats obtenus en ce qui concerne les photocathodes et les Bcrans fluorescents peuvent &re exploitee pour r6aliser ou ameliorer divers types de tubes ZL image, comme ceux qui ont 6tB present& au debut de I’expos6. A titre d’exemple, la Fig. 9 permet de comparer Ies fonctions de transfert de modulation relevees sur des tubes D 16 avec &ran sediment6 et Beran PVA, tels gue ceux figurant au Tableau I. R~F~RENCE 1. Stone, H. D., Dana “Advances in Electronics and Electron Physics”, Bd. par J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 666. Academic Press, London (1966).

Channel Multiplier Plates for Imaging Applications R. W. MANLEY, A. GUEST and R. T. HOLMSHAW Mullard Research Laboratorim, Redhill, Surrey, England

INTRODUCTION Single channel electron rnultiplier~l-~ have been used for some time in space exploration for the detection of low energy electrons. This paper deals with the exploitation of the device t o produce twodimensional arrays of multipliers which offer great possibilities in image detection and intensification. The channel electron multiplier (Fig. 1) is a distributed dynode multiplier which combines the functions of the dynode structure of the Secondary electrons

Resistive

'I

Primary radiation

2500V

FIQ.1. Channel electron multiplier.

conventional photomultiplier and the resistor chain which divides the potential among the separate dynodes. It consists of a cylindrical glass tube with a length equal to about 60 times its diameter. The inside surface is coated with a semi-insulating layer adjusted to have a resistance between the electrodes a t each end of the tube in the range lo8 Q to lOI4 Q, depending upon the current output to be drawn from the channel. The multiplier operates in vacuum with a potential applied between 471

472

B.

W. MANLEY,

A. GUEST AND R. T. HOLMSHAW

the electrodes. Electrons enter the low-potential end and strike the wall to produce secondary electrons which are accelerated axially by the applied electric field. Their transverse energy of emission causes them to traverse the channel, so that they, in turn, strike the wall after gaining considerable energy and produce further secondary electrons. This process is repeated many times along the channel, and many

FIG.2. Photomicrograph of part of a channel plate composed of 40-pm channels. The distance between channel centres is 50 pm.

electrons emerge from its high-potential end. The gain depends upon the applied potential and upon the ratio of length t o diameter of the channel, as well as the secondary emission characteristics of the channel wall. With 1000 V applied, the current gain will typically be a few thousand, while a t 3000 V, the gain may reach lo*. Since the gain does not depend upon the absolute size of the channel, the dimensions may be scaled without affecting the performance, and honeycomb arrays of parallel channel multipliers, called channel plates,

473

CHANNEL MULTIPLIER PLATES

may be constructed.* The channels are usually made from special glasses which may be made electronically conducting. The techniques for the manufacture of channel plates may be similar t o those used for fibre-optics6*e.Tubing is drawn down to the required diameter either in one drawing operation, or in two stages, in which many channels of an intermediate diameter are assembled and the bundle of channels drawn until the constituent, channels are of the right diameter. These multiple units are then arranged together to make up the required area. The total bundle is sliced and polished into discs t o give the necessary ratio of channel length t o diameter. The separate multipliers are connected in parallel by evaporating a t an oblique angle a thin metallic coating of nichrome over the two polished faces of the plate. The film connects the interstices of the channels but leaves the channels open. Electrical connection is made t o the channels by a peripheral ring electrode pressed against each face of the plate. Figure 2 shows a microscope view of part of a plate composed of 40-pm-diameter channels. The thickness of the plate is 2.4 mm and the open area is 62%.

COMPUTERMODELOF A CHANNEL To assist in the analysis of channel plate performance, a computer model has been produced which, when taken in conjunction with experimental results, allows some measure of generalization t o be applied to the data. The present model is designed to minimize the number of simplifying assumptions and t o include as much experimental evidence as possible within its structure. Although a gain-limiting process is observed in practical multipliers when the gain is very high3-’.* this model applies only t o a channel operated in conditions in which space charge and wall charging do not modify the channel performance significantly. No evidence has been seen of space charge effects a t gains below lo6; thus the model is applicable t o most imaging applications, where electron gains in excess of lo5 are seldom required. A random number generating procedure is used in the computer programme to calculate the result of a primary electron collision with the channel wall. The mean yield, 6, as a function of electron energy in eV and angle of incidence B is described by the following function, derived in the Appendix,

6’ = (v‘ d/co8)Bexp [ a ( ~ cos 8 )

+ /3 (1 - v d c o s @)I, ~

where 6’ and V r are normalized t o the maximum value of 6 at normal incidence. ,8 is a constant which controls the form of the expression. P E.1.D.-A

17

474

B. W. MANLEY,

A. GUEST AND

R. T. HOLMSHAW

Whenever a collision is simulated, the appropriate form of the secondary emission function is used to determine the mean yield of electrons from the particular impact energy and angle of inclination. This value is used as the mean of a Poisson distribution and the actual number of secondaries generated by the collision is a random sample chosen from the distribution. Similarly the angles are chosen from a cosine distribution and the energies from a Rayleigh distribution with a modal energy which may be controlled in the programme. The trajectory of each secondary electron produced in this way is calculated from the ballistic equations and so the position, energy and angle of the subsequent collision with the channel wall are determined in three dimensions. The result of each collision is calculated as before and the process is repeated for every secondary electron generated. The length of channel available to each secondary for multiplication is stored in the computer and this information is up-dated a t every collision. The total number of electrons which have left the output of the channel is accumulated continually, so that when it is calculated that all the electrons have emerged from the channel, the total yield is known. The process is repeated for many individual input electrons t o produce a series of output pulses. By keeping the energy of the input electrons and the secondary electron characteristics constant for a complete set of output pulses, the effects of varying the length-to-diameter ratio and the applied voltage can be studied. The following properties can then be determined. (i) The mean gain C and the variance u2 can be derived directly from the series of pulses. (ii) The noise factor P,which can be expressed as Input signal-to-noise ratio F = ( - Output signal-to-noise ratio

=I+,,,

02

can be derived. (iii) If sufficient pulses are obtained for a single set of conditions it is possible to plot a pulse amplitude distribution. I n addition it is possible to study the effect of the primary electron energy, and of variations in the secondary emission characteristics a t the first collision, on the behaviour of a channel multiplier. The energy and directional distributions of the electrons leaving the end of the channel may also be determined. The approximate values of the parameters which control the form of the secondary emission function in the programme can be deduced from published experimental r e s ~ l t s . ~ - l l It has been found that different channels of the same material behave in slightly different ways; this variation may result from small differences in the processing of the material, or in the operational environment. I n order t o determine precisely the parameters appropriate for the simulation of a

CHANNEL MULTIPLIER PLATES

475

practical channel, it is necessary t o calibrate the programme with a typical set of experimental results; for example, the gain of the channel with a specific applied voltage and length-to-diameter ratio and controlled input conditions. The form parameters of the secondary emission characteristic must then be adjusted until the results from the computer agree with the experimental values. It is then possible to simulate various operating conditions with the secondary emission parameters set t o the constant calibrated values. Conversely, the effects of variations in the form of the secondary emission function with constant operating conditions can be simulated. Extensions of this programme permit the transit time and transit time spread to be determined, but a t present no experimental evidence is available to compare with the computer results.

PERFORMANCE OF CHANNELPLATES Measurements on plates composed of 40-pm-diameter channels have been made in sealed-off envelopes containing 5.20 photocathodes. This has permitted relatively simple control and measurement of low input currents, and precise control of the input energy of electrons to the channel plate. For measurements of noise factor and pulse amplitude distribution, experimental image intensifiers were used, and the channel plate output was monitored with a photomultiplier.

Gain The measured current gain of a channel plate composed of 40-pmdiameter channels, with a length-to-diameter ratio y of 60, is shown as a function of voltage in Fig. 3. The gain from a channel plate will depend upon the value of y , so it is necessary to consider what factors influence the choice of this parameter. By using the measured gain of Fig. 3 to calibrate the computer model, a universal gain curve can be derived (Fig. 4). From this it can be seen that a t a constant voltage there is a region in which the gain varies least with variations in y . This is a desirable operating point since (i),the gain is a maximum a t a particular applied voltage, and (ii), the gain variation from channel t o channel will be least dependent on differences in channel diameter. The optimum value of y occurs when the total applied potential is about 2 2 y V . Thus as the plate voltage is changed, the operating point will depart from the optimum. The value of y = 60 was chosen as a suitable compromise within the likely range of gain required for imaging applications. The gain measurements were made a t an input current of A. The maximum output current from the plate of area 1 in.2 was thus substantially less than the conduction current in the plate, determined

,

106

-

I

I

I

I

I

j

y.60 Primary electron energy=5000eV

Potential ( V )

FIG.3. Gain as a function of voltage with y

=

60.

Length/diameter ( 7 )

FIG.4. Universal gain curve for channel ( W

=

V/y).

CHANNEL MULTELIER PLATES

477

by its resistance of 10°R. The saturation effect of drawing output currents approaching the value of the conduction current may be seen from the current transfer characteristics (Fig. 5 ) . The operation of the channel plate is linear for output currents less than 5 % of the conduction current.

Input current ( A )

FIG.5. Transfer characteristics of a channel plate.

These measured gain values were used to calibrate the computer model in all subsequent simulations of the noise factor of the channel plate. The Noise Factor The noise factor of a channel plate is a measure of the information loss resulting from its use. The measured value of the noise factor depends upon the input energy of the primary electrons, and upon the potential applied t o the plate. The best measured value is about 4. The information loss is due to the following factors: (i) loss of electrons at the input of the plate, the open area of which is about 60%. (ii) a loss when primary electrons fail to produce secondary electrons, or secondary electron cascades die out after a few stages. (iii) the varia-

B. W. MANLEY, A . GUEST AND R. T. HOLMSHAW

478

tion in gain among the output electron pulses resulting from the statistical variation of the secondary emission yield. The noise factor has been measured by incorporating the channel plate in an experimental image intensifier, and observing the signal-to"

I

I

b

c

5 I 0.001

I

I

0.01

0.I

1.0

Integrating time (sec)

FIG.6. Measurod noise factor as a function of voltmeter integrating time (P.20 phosphor).

noise ratio a t the fluorescent screen with a photomultiplier and r.m.9. voltmeter having a variable integration time. With increasing integration time, the noise factor increases to an asymptotic value where it is unaffected by the phosphor decay time (Fig. 6).

._ 0 z

-

-

I

I

I

I

I

The asymptotic value of the noise factor has been measured as a function of input electron energy; these values are compared with the computer simulation results in Fig. 7. This variation in the noise factor results from the change in secondary emission yield with the energy of the primary electrons. The optimum is not well defined but occurs around 1000 eV.

479

CHANNEL MULTIPLIER PLATES

The noise factor is shown as a function of channel-plate voltage in Fig. 8. The high noise factor a t low voltages results from the low energy gained by electrons between collisions and the consequent significant probability of zero secondary emission yield in the early stages of pulse build-up. I

(

-

0

c

-

+Experimental points

-

6-

4-

+ 6 b O '

'

800

'

'

1000

'

I

1200

,

I 1400

I

l

l

1600

A principal factor contributing t o the loss in information is the variation in output electron pulse amplitude from pulse to pulse, i.e. the pulse-height distribution. Pulse-Height Distribution The pulse-height distribution has proved difficult to measure directly because of the problem of detecting the very small pulses in the distribution. Measurements in an experimental image intensifier incorporating a P.16 phosphor have been made by observing the output with a photomultiplier and recording the pulse-height distribution on a multichannel analyser. The results follow a negative exponential form down to very low values of output pulse height. Inaccuracies in the measurements a t these values are due to the still significant decay of the P.16 phosphor causing spurious pulses to be recorded by the analyser during the decay period. Figure 9 shows the experimental results compared with a computer-simulated histogram of pulse heights. It should be noted that the negative exponential distribution produced in the computer simulation is a result of the assumption of a Poisson distribution of the secondary emission yield about a mean which varies according to the collision energy and angle.

480

B. W. MANLEY, A. GUEST AND R. T. HOLMSRAW

The occurrence of a pulse-height distribution of negative exponential form may be shown t o be consistent with the measured noise factors. The measured fluctuation in the output signal from a channel plate results from the contribution of the input signal fluctuation and the added statistical fluctuation introduced by the secondary emission in the channel plate. Assuming the rate of arrival of input electrons a t the plate t o fluctuate about the mean n according t o a Poisson distribution, the r.m.s. deviation from the mean will be n1/2,which is the input noise N , . This will result in an r.m.8. deviation a t the output of n1lZGwhere G is the mean gain of the plate.

Relative pulse amplitude

FIG.9. Example of the pulse-height distribution.

The gain process will result in a yield for each input electron which is distributed about the mean with a standard deviation u. The total variance for a sequence of n electrons will be nu2, the sum of the variance for each. Hence the r.m.9. deviation introduced by the channel plate a t the output is n%. If the two noise contributions are uncorrelated we may add them in quadrature to obtain the total r.m.s. deviation, which is the measured noise : N o = (nQ2 nu2)1/2.

+

The negative exponential pulse-height distribution may be described by a Furry distribution12 for which the variance is given by: uz = G(l C), hence N o = n1/2(2G'2 G)ll2.

+

+

CHANNEL MULTIPLIER PLATES

481

Since G is large, this reduces to

N o = (2n)1'2G. The noise factor of the channel plate may be written

F

=

(g)',

from which we find the noise factor for the channel plate t o be 2. Because the channel plate open area is about 60%, the noise factor becomes 3.3 and this will be further increased in proportion to any loss of electrons a t the first collision, and by any channel-to-channel nonuniformity in performance. This latter spatial noise will depend very largely upon the dimensional accuracy with which the channels are fabricated.

Channel-Plate Uniformity

By controlling the diameter of the separate channels within about 5%, uniform operation can be obtained over the area of the plate.

This is demonstrated by the photograph (Fig. 10) taken from the screen of an experimental image intensifier containing a channel plate composed of 40-11." channels.

FIG.10. Photograph from screen of experimental image intensifier containing a channel plate composed of 40.pm-diameter channels.

482

B. W. MANLEY, A. GUEST AND R. T. HOLMSHAW

Application of Channel Plates Channel plates are physically rugged and stable in air, so no special care need be taken with their storage. They are thus well suited to applications in electron-optical imaging systems, both in sealed-off devices like the image intensifier described by Eschardf and in demountable experiments requiring frequent exposure t o air. Figure 11 shows a diagram of an experimental X-ray image intensifier containing a channel plate. The proximity of the X-ray-sensitive photocathode and screen to the channel plate avoids the use of electron-optical lenses,

Phosphor

/

Fro. 11. Diagram of experimental X-ray channel intensifier.

thus offering the possibility of a compact “panel” intensifier operating a t considerably lower voltages than conventional X-ray intensifiers. Further possibilities exist for the use of channel plates in space exploration. They will operate satisfactorily in pressures below torr, and thus the environmental vacuum of space is adequate. In this way no input window is necessary, and the detection efficiency of a channel plate is high to radiation in the range 1 to 10 nm (1240 eV to 124 eV) in which much stellar radiation fallsa4

CONCLUSIONS The extension of the channel electron multiplier principle to two dimensional arrays offers new possibilities in imaging applications. Very high electron gain can be obtained from a channel plate in a compact length and a t a low voltage compared with that required in

t See p. 499.

CHANNEL MULTIPLIER PLATES

483

other techniques of image intensification. I n addition, the device is stable in air and relatively robust, thus lending itself to use in demountable systems as well as in sealed-off tubes. The loss of information in a channel plate is a significant factor which may limit its usefulness for single-electron detection since preservation of the input signal-to-noise ratio is then of ultimate importance. However, there are many applications where this loss is not important or where it is offset by the advantages which the channel plate has over other techniques.

REFERENCES 1. Wiley, W. C. and Hendee, C. F., IEEE TTane. Nucl. Sci. NS-9, No. 3, 103 (1962). 2. Adams, J. and Manley, B. W., Electronic Engng 37, 180 (1965). 3. Adams, J. and Manley, B. W., IEEE Trans. Nucl. Sci. NS-13, No. 3, 88 (1966). 4. Adams, J. and Manley, B. W., P h i l i p Technical Rev. 28, 156 (1967). 5. Kapany, N. S., “Fibre Optics, Principles and Applications”. Academic Press, New York (1967). 6. Mullard Ltd., British Pat. No. 1,064,072 (1963). 7. Evans, D. S., Rev. Sci. Instrum. 36, 376 (1965). 8. Schmidt, K. C. and Hendee, C. F., IEEE Trans. Nucl. Sci. NS-13, No. 3, 100 (1966). 9. Hachenberg, 0. and Brauer, W., I n “Advances in Electronics and Electron Physics” ed. by L. Marton,Vol. 11, p. 413. Academicpress, New York (l#69). 10. Chuiko, G. A. and Yakobson, A. M., Radiotechnika i Electronika 11, 1471 (1966). 11. Bronshtein, I. M. and Denisov, S. S., Soviet Phy8.-SoZidh’tate 7 , 1484 (1965). 12. Baldwin, G . C. and Friedman, S. I., Rev. Sci. Instrum. 36, 16 (1965). 13. Yakobson, A. M., Radiotechnika i Electronika 11, 1590 (1966). 14. Bruining, H. “Physics and Applications of Secondary Electron Emission”. Pergamon Press, London (1954).

APPENDIX The Secondary Emission Function Used in the Computer Model The function chosen to represent the secondary emission coefficient as a function of energy V and angle of incidence 8 must satisfy the following results which have been determined experimentally. (i) The curve for normal incidence should be a close approximation to the published experimental curves. (ii) As stated by Yacobson13 and Bruining14

(iii) As stated by Yacobson13

6,(8) = 6, (0) exp [a (1 - cos e)],

484

B. W. MANLEY, A. GUEST AND R. T. HOLMSHAW

where V m is the collision energy in eV which is required to produce the maximum secondary emission yield 6,, and a is a constant of the material. 6 V Let V‘ = _ _ and 6’ = --. V m (0)

am

(0)

Now 6‘ = 6’ (V’, 0 ) and it is assunied that 6’ may be taken as the product of two functions, one o f which is a function of 0 alone, i.e. 6’ = f (V’, 8) F (8). Differentiating with respect to V’,

As the observed secondary emission curves have a single maximum, assume that a possible representation of the function f is ~.

f = A ( v ’ ) exp ~ [- ,fI 8’d c o s el, with A , /?and n constants. Differentiating with respect to V’, (3)

At the maximum of the secondary emission curve

So that to satisfy Eq. (1) n

=

p,

and ~~

6‘

=A

V @exp [- ,8 V’ dCos 0117 (el.

(4)

At the maximum, from Eq. 2, exp (- /3) P (8) = exp [a (1 - cos e)],

so that

F ( 8 ) = exp [a ( 1

-

cos 0 )

+ p] (dC0se)fi A ’ -_____----

The value of p is chosen to fit the published secondary emission curves

CHANNEL MULTIPLIER PLATES

485

a t normal incidence. Unfortunately, it was not possible to match the curve over its entire range with the same value of 8. For V’ 1, 8 lies in the region of 0.55 to 0.65. For V‘ 1, /3 is approximately 0.25. The two forms of the function are approximately equal when

<

>

v’ = 1.5.

The value of the constant a has been determined by experiment,1° and the value a = 0.62 was used in the programme. It must be understood that the particular form of the function used to simulate the secondary emission characteristics was chosen because its shape was similar t o the experimental curves. The gain of the channel is determined by the precise value of the low energy constant /3. This constant was varied within the region stated above, and the value which best simulated an actual gain measurement was chosen. As the gains of several channels may differ because of slight variations in the manufacturing process, the value of /3 can be varied slightly to give the best fit for each case. One value of the low energy has been used in all the simulations described in this paper, and this has been adequate for these cases.

DISCUSSION J. F. LINDER:

Are you able to build your channel arrays entirely of one type of

glass? B. w. MANLEY: Yes. The channel plates contain only one typs of glass. The plate is chemically processed after manufacture to form a conducting layer on the inside surface of each channel. N. s . PAPANY: May I ask what your experience has been on channel-to-channel variation in a given micro-channel plate and also the degree of fixed pattern noise, i.e. inter-multiple boundaries? B. w. MANLEY: We have no quantitative measurements on gain variations within a plate but the photograph (Fig. 10) gives a qualitative indication of this type of noise. J. D . MCGEE: Is the secondary emission exponential or Poissonian? What is the 6 of the secondary emitting surface? B . w. MANLEY: I n the computer model the secondary emission yield is chosen to have a Poissonian distribution about B value which is a function of impact angle and energy. The computed output pulse-height distribution from a charinel plate is quasi-exponential and so is consistent with the experimentally measured distribution. The 6 of the secondary emitting surface of these multipliers has yet to be measured exactly, Similar material is reported to have a maximum 6 of approximately 3 at normal incidence for 300 to 400 eV primary electrons. The computer model uses similar values. The collisions in the channel multiplier take place a t considerable angles to the normal to the surface, and tho calculated median impact energy is typically 110 eV. Tho predicted value of 6 in this case is approximately 2.3. M. ROME: Do you have experimental data on the exit energy distribution of

486

B. W. MANLEY, A. UUEST AND R. T. HOLMSHAW

the electrons from a channel plate? What is the field gradient at the output for the distribution shown? B. w. MANLEY: The computer model produces a histogram of the exit energy distribution of the electrons, using 10 eV energy intervals. This is nearly level for the first two intervals and then shows a continuously decreasing form. The precise form of the distribution is governed by the operating conditions. A typical case gives a median exit energy of 38 eV and a mean energy of 62 eV. Preliminary experimental measurements show a similar form, although the measured median energy is slightly less than the computed value for similar operating conditions. The field gradient a t the output is 500V/mm for the computed case. w. WILCOCK: If, as you show, the output pulse amplitude distribution is exponential, a2 should equal G2,and your noise factor F should be 2. Can you explain why your measured value is never less than 4? Does it mean that the distribution really has a delta function a t zero pulse amplitude? B. w. MANLEY: The pulse height distribution diverges from an exponential distribution for very small pulses and has a delta function a t zero pulse amplitude. The distribution in this ragion is dependent on the conditions a t the input end of the plate, especially on the angle and energy of the initial electron. Under optimum conditions about 90% of the electrons that enter a channel produce an output pulse. However, the open area of a plate is 60% and so about 50% of the electrons produce a pulse of zero amplitude. This increases the noise factor from 2 to 4.

An Analysis of the Low-level Performance of Channel Multiplier Arrays W. M. SACKINGER and J. M. JOHNSON Reseurch and Development Laboratories, Corning Glass Works, Corning, New York, U.8. A.

INTRODUCTION

A simulation has been made of a channel electron multiplier, using an IBM-360 Model 40 computer to investigate effects of operational parameters and surface properties. Two models have been studied: planar and cylindrical. The simplified two-dimensional planar model always assumed the trajectory to lie across the channel diameter in the transverse plane and was used to study effects not dependent on the third dimension. A full three-dimensional study of a cylindrical multiplier was also made for purposes of comparison with experiments and to study parameters affected by the third dimension. COMPUTING PROGRAM PROCEDURE Initial values of primary energy and angle are given to an electron colliding with the interior surface of the channel at the entrance plane. A random number is selected and compared to a Poisson distribution, having the first moment dependent on primary energy and angle, to determine the occurrence of either elastic reflexion or absorption. If absorption occurs, a random number applied to another distribution, the first moment of which is also dependent on primary energy and angle, selects the number of secondaries. This distribution is Poisson for moat runs. Data published by Goff and Hendeel are used for determining the variation of the first moments. Secondary energies are assigned by random numbers applied to an energy distribution. The energy distributions used were : the Maxwellian, one determined experimentally by Goff and Hendee,l and modifications of each. Care was taken not to allow the sum of the secondary energies to exceed the primary energy. The angle (or angles in the threedimensional case) of a departing secondary is selected by random numbers applied to a cosine, or modified cosine, distribution. 487

488

W . M. SACKINCER AND J. M. JOHNSON

Calculations are made t o find, for each electron, the length of the trajectory, arrival energy, arrival angle, and transit time. These quantities are needed as initial conditions for the next multiplying event. After the trajectory calculation, all secondaries are allowed t o strike the wall and produce tertiaries. A strike sequence is followed by storage of data, keeping only the last electron to initiate another strike sequence. This series is repeated until all electrons under current consideration pass the exit plane. Then data on one electron a t a time are retrieved from storage and allowed to continue until all secondaries have been followed past the exit plane. For each secondary electron departing from the three-dimensional multiplier, calculations are made to find the exit position and velocity components in all three directions. The esit electron density per radius ring is thus formed. This entire process is repeated for many initial electrons to obtain a distribution of gain and a value of detection efficiency. Many distribution variations have been tried as well as different choices of emission energy, secondary yield, channel voltage, and primary electron energy. Some selected results will be presented here. All figures have been made using an off-line Benson-Lehner incremental plotter.

RESULTS Figure 1 shows the effects of various surface materials on multiplier gain. Runs 4 and 5, which apply to multiplier surfaces with yields similar to amorphous MgO and amorphous KCl, respectively, illustrate the important effect of a low first cross-over. I n yield curve 4 the first cross-over occurs a t 60 eV, and in No. 5 it occurs a t 15 eV. Although yield curve 5 is much lower than No. 4 for primary energies exceeding SOeV, the gain is greater for No. 5 . Consistent yield of a t least one secondary contributes more to total gain than greater yield a t higher primary energies, combined with a large probability of absorption a t low primary energies. Effects of channel voltage, varied from 800 to 2000 V, are shown in Fig. 2. As channel voltage increases, trajectories become longer and strike angles become more grazing. Both effects increase gain significantly, and the number of primary electrons completely lost is definitely reduced. The negative trajectory lengths correspond t o electrons emitted with high velocities in a direction opposite t o the applied field. Figure 3 shows the effects of the emission energy distribution peak shifting from 1 eV to 4 eV, for a channel field of '20 Vldiameter. When

u !4u J--t4;

Goin distribution

Gain distribution

'v:

Gain distribution

-

.x .-

.*

$ 2

$2

U

$6 00

200

._ 2

k:

Gain distribution

~4~

.-

E

h 2

{$ 0

200

0

Incident energy (eV)

b 2

$2

K 0 l

0 0

s

0

$0

Incident energy (eV)

0

Goin distribution

0

ZOO

Incident energy (eV)

$0 0

Incident energy (eV)

FIG.1. Effects of yield curve on gain distribution.

200

$0

0

200

Incident energy (NJ

I

1 emvl

Goin distributon

Goin distribution

Goin distribution

I5WVl

Gain distributon

K I,pIm31 1500 V

0 30 Tmjectory length

Angle of impact

0

Trajectory length

0

Angle of impoct

Tmjectary length

90

0

Angle of impact

Trajectmy length

90

FIQ.2. Effects of channel voltage.

0

Angle of impoct

90

Trojectory length

F

m Peak at I V

0

Energy distribution 20

R1 PJ pwk ot

0

Gain distribution

0

Tmjectory length

30

Angle of ‘mpact

90

2.5V

Energy distribution 20

Gain distribution

0 30 Trajectory length

M o t 4v

0

0

Energy distributioo 20

Gain distribution

0 Trojectory length

FIG.3. Effects of electron emission energy.

30

0

Angleof impoct

90

m

0

90

Angle of impact

492

'I

W. M. SACKINQER AND J. M. JOHNSON

I

493

PERFORMANCE OF CHANNEL MULTIPLIER ARRAYS

emission energy is low trajectory lengths increase, producing collisions at high energy but also fewer strikes. When the energy distribution peak increases to 2.5 eV, distances are shorter and arrival angles are less grazing, producing fewer secondaries but more multiplying events. When the emission energy distribution peak is a t 4 eV, electrons cross the channel quickly, picking up very little energy from the channel

0

Energy distribution

Energy distri bution

0

10

.-

Energy distribution lo

Goin distribution

Trojectory length

Gain distribution

0 Trajectory length

Goin distribution

Tra@ctory length

Angle of impact

x ) o

Angle of impact

Angk of impact

FIG.5. Effects of electron emission energy distribution shape,

field. They strike the wall with low energy, resulting in a poor yield. Therefore, the intermediate distribution tends to balance the advantages of higher strike energy and a greater number of collisions. Effects of primary electron energy on detection efficiency and gain distribution are shown in Fig. 4. Yield curve No. 5 was used for all of these runs, varying primary energy from 50 eV to 1000 eV. The secondary emission yield of a standard glass electron-multiplier surface, such as was used by Sharber2 and Frank,3 is definitely less than

90

494

W. M. SACKINOER AND J . M . JOHNSON

yield curve 6 . Therefore, the theoretical detection efficiency is much higher than reported in either experiment. With a primary energy of 300 eV, yield curve 5 produces a detection efficiency of 0.82 while yield curve 1 has a detection efficiency of 0.24. The improvement trend in gain distribution as primary energy increases is independent of the yield curve, and is caused by the large increase in yield at the first strike. I n Fig. 5, the effects of a change jn secondary electron energy distribution-width are shown. The two-dimensional model was used in this case, since we suspected that the additional statistical effects associated with the third dimension would obscure any trends present.

Emissbn angle distribution

Emission angle distribution

0

Gain distribution

Trajectory length

30

0

Angle of impact

90

mmoa Gain distribution

Trajectory length

Angle of impact

FIG.6. Effects of emission angle distribution shape. Upper. Cosine emission angle distribution. Lower. Emission angle distribution more sharply peaked than a cosine distribution.

As the energy distribution broadens, more electrons travel shorter distances, and the impact-angle distribution broadens on both sides due to the emission of a larger number of both low-energy and highenergy secondaries. The gain distribution shape seems little affected, although there is a trend towards higher average gain as the energy distributions become narrower. I n the three-dimensional cylindrical multiplier, electrons are emitted with some transverse velocity. Experimental data on the angular distribution of secondary electrons in the plane perpendicular to the plane of arrival have never been published. Its shape undoubtedly is a function of primary energy and angle. In almost all calculations,

PERFORMANCE OF CHANNEL MULTIPLIER ARRAYS

495

we have used a cosine distribution in this transverse plane, but in order to examine this assumption, a comparison run was made with a more peaked distribution. Results are illustrated in Fig. 6. With the cosine distribution, a larger number of electrons escape at large angles and travel only a short distance before they strike the wall. They acquire little energy and add little to the gain process, but do strike with grazing impact-angles. Grazing impact-angle enhances yield only if the energy is high. The gain distribution shape is very different in the two cases, with the peaked distribution yielding much higher gain and B higher detection efficiency. Super -Poisson

Gain distribution

Yield distribution Mean = 3

Y teld distribution Mean = 3

Yield distribution Mean = 3

FIG.7. Variations in shape of yield distribution.

There is uncertainty in the literature regarding the number statistics of secondary electron yield. Measurements made on metallic dynode^^-^ indicate that a Poisson distribution is commonly obeyed. Measurements on insulating such as MgO and KCl, have shown distributions broader than Poisson. Point-to-point random fluctuations in surface properties would certainly tend to broaden the distribution.1° An indication of the effects of yield distribution-width on multiplier gain is given in Fig. 7. In order to see possible trends, unobscured by three-dimensional statistics, the variation of distribution-width was studied in two dimensions. The same trends should appear in the three-dimensional case. A distribution function due to Bardwell and Crow1' was used, which reduces to the Poisson distribution as a special case. The mean

496

W. M. SACKINOER AND J. M. JOHNSON

of the distribution was allowed to be the usual function of primary energy and angle in each case. For comparison purposes, Fig. 7 compares a Poisson distribution with a broader and a narrower one, all plotted here with a mean of three. The changes in the multiplier gain distribution are quite significant. The detection efficiency is higher as the yield distribution is narrower, which should be expected since the probability of zero emission for the first few events is less. The average gain and the gain distribution, shift to larger values for the broader yield distribution, since there is a larger probability of many electrons being emitted at each event along the channel. One would expect the detection efficiency for a single channel and for an array of channels to be the same if the yield curves and operating conditions are identical. The image degradation a t low input levels due to finite photocathode quantum efficiency is well-known. For intensifiers using channel multiplier arrays, one can simply multiply the photocathode quantum efficiency by the detection efficiency of the channel array to get the net quantum efficiency of photon detection. However, image intensifiers using channel arrays will also contribute additional image noise because of their very broad, exponential-like, skewed pulse-height distribution. If, in a given sampling time, one part of such an intensifier receives a small number of input events while another part receives many, it is more probable that the gain in the first area will appear in the vicinity of the peak of the distribution, whereas for the second area the gain will average out a t the mean of the distribution, a value greater than the peak. Thus, there is a probability of a contrast enhancement effect. Because of the very broad distribution, however, there is also a finite probability of the small-sample area having a wide variety of gain values. Generally, such a broad statistical distribution leads to more uncertainty in determining contrast at low input levels. Averaged over several hundred input events, these uncertainties will tend to average out, leading to reasonable grey-scale rendition at such input levels.

REFERENCES 1. Goff, R. F. and Hendee, C. F., “Studies of the secondary electron emission

2.

3. 4. 5. 0.

yield, energy and angular distribution from high resistance targets a t grazing angles of incidence”, The 27th Annual Conference on Physical Electronics, M.I.T.(1967). Sharber, J. R., Winningham, J. D. and Sheldon, W. R., DASS-68-1, Southwest Center for Advanced Studies (1968). Frank, L. A., Univ. of Iowa Report 65-22, The University of Iowa (1965). Simon, K. H., Herrmann, M. and Schackert, P., 2. Phye. 184, 347 (1965). Hliussler, P., 2. Phya. 179, 276 (1964). Barnett, C. F., private communication.

PERFORMANCE OF CHANNEL MULTIPLIER ARRAYS

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7. Delaney, C. F. G. and Walton, P. W., IEEE Trans. Nucl. Sci. NS-13, 742 (1966). 8. Smith, H. M., Ruedy, J. E. and Morton, G. A., IEEE Trans. A'ucl. Sci. NS-13,77 (1966). 9. Murray, C. B., Jr., Ph.D. Thesis, University of Minnesota (1966). 10. Prescott, J. R., Nucl. Inatrum. and Methods 39, 173 (1966). 11. Bardweli, G. E. and Crow, E. L., Amer. Statistical Ass. J . 59, 133 (1964).

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Quelques Problkmes Concernant les Multiplicateurs Canalishs pour Intensificateur d’Image G. ESCHARD e t J . GRAF Laboratoires d’gleetronique et de Physique Applipuke, 94 Limed-Bre‘vannes, Prance

L’Btude des multiplicateurs d’dlectrons Istructure tubulaire, entreprise depuis bientbt 5 ans dans nos laboratoires, permet de rBaliser I present des faisceaux B structure BlBmentaire trAs fine presentant un gain Blectronique Blevt5 pour de faibles dimensions.

PRINCIPE DU MULTIPLICATEUR TUBULAIRE Le principe du multiplicateur tubulaire est schBmatisB sur la Fig. 1: un Blectron pBnBtrant dans le canal heurte la paroi et crBe, si celle-ci est bonne Bmettrice secondaire, plusieurs Blectrons. Ceux-ci sont accBlBrBs par le champ Blectrique crBB B I’intBrieur du tube et heurtent B leur tour la paroi en donnant naissance B de nouveaux Blectrons. Ce processus en avalanche se poursuit tout au long du tube et, suivant la tension appliquBe au tube et la forme de celui-ci, on peut atteindre des gains compris entre lo3 et lo8. Si l’on associe cbte B cbte quelques los canaux BlBmentaires de ce type, on obtient un faisceau multiplicateur de quelques millim&tres d’kpaisseur. Ce faisceau, place entre une photocathode et un Bcran luminescent, permet de rdaliser un tube intensificateur d’image Ihaut gain, dont les performances sont comparables B un tube cascade B trois Btages. Dans le tube ainsi obtenu, l’image Blectronique en provenance de la photocathode doit stre reproduite sur 1’entrBe de la “galette” de multiplicateurs. De meme, l’image Blectronique plus intense fournie par la galette doit &re transmise B 1’6cran. Trois possibilitds se prdsentent pour rBaliser ce transport d’image; ce sont la focalisation magndtique, la focalisation par optique Blectrostatique et la focalisation par proximitd. Ce dernier type de focalisation, tr&ssBduisant par sa simplicith, a BtB particulierement BtudiB dans nos 409

G . ESCHARD ET J. GRAF

500 1

Electron Drirnaire Electrons secondaires

I FIG.1. Principe clu multiplicateur d’6lectrons tubulaire.

laboratoires pour la mise au point d’obturateurs Blectroniques ultrarapides. Ceux-ci sont dBcrits par Eschard et Po1aert.t Si l’on Bcarte la focalisation magnBtique pour les trop grandes contingences d’alimentation et de poids qu’elle implique, il reste A Qtudier les combinaisons possibles des deux focalisations par optique dlectrostahique et par proximit6. Ceci conduit pratiquement it comparer trois solutions.

FIG.2. Lea trois structures de focalisations possibles.

t Voir p. 989.

MULTIPLICATEURS C A N A L I S ~ SPOUR INTENSIFICATEUR D’IMAGE

501

La Fig. 2 montre les trois structures envisageables, reprBsentBes it la mbme Bchelle, pour une m6me surface utile de photocathode et un grandissement Bgal t i unit&

FOCALISATION DE PROXIMIT~ La focalisation de proximitt? entre la sortie de la galette et 1’6cran semble 6tre la solution qui permette d’obtenir la meilleure qualit6 d’image. L’emploi d’une optique Blectronique it cet endroit se heurte aux difficultBs crBBes par la planBit6 de la surface 6missive et par la grande dispersion des vitesses des Blectrons it la sortie de la galette. Entre la photocathode e t l’entrBe de la galette, l’une ou l’autre des deux gBomBtries peut &re envisagee du point de vue de la qualit6 de l’image. La solution de double focalisation de proximite est la plus sdduisante car elle conduit 8. une structure trbs simple dans laquelle les trois 616ments plans Bliminent toute distorsion entre centre et bord. Le problbme primordial consiste d’une part it possBder une technologie de sensibilisation de la photocathode qui autorise it venir placer celle-ci 8. quelques dixibmes de milIimAtres seulement de 1’6lectrode suivante; d’autre part, il faut &re stir que le fait de placer au voisinage immBdiat les uns des autres une photocathode, un multiplicateur et un Bcran ne va pas entrainer un processus de reaction qui perturberait compli?tement le fonctionnement du tube. E n ce qui concerne le positionnement de la photocathode, la technique de transfert sous vide de la photocathode,t a BtB mise au point dans nos Laboratoires. Elle permet d’obtenir les distances trbs faibles entre la surface photosensible et I’entrBe de la galette qui sont nkcessaires it l’obtention d’une rBsolution satisfaisante de l’image. Les reactions que l’on peut craindre peuvent btre dues soit it une remontBe ionique vers la cathode donnant lieu it un processus d’emballement par Bmission secondaire d’klectrons sous l’impact des ions, soit it un couplage optique au travers de la galette entre 1’6cran et la photocathode.

La Rdaction Ionique fitant donnB les faibles dimensions du multiplicateur e t son gain 6lev6, la densit6 Blectronique est importante dans les derniers paliers de multiplication des canaux et dans I’espace compris entre le multiplicateur e t 1’6cran. Les molBcules de gaz prBsentes dans cette rBgion sont donc susceptibles d’btre ionisBes et les ions ainsi form&, accBl6rBs par le champ Blectrique, viennent frapper l’entr6e des canaux ou la photocathode. On obtient alors, par Bmission secondaire, des Blectrons qui peuvent &re it I’origine de nouveaux processus de multiplication. Voir p. 989.

502

0. ESCHARD ET J . QRAF

On peut pallier ce ddfaut dans les canaux uniques en leur donnant une courbure suffisante pour que les ions ne puissent remonter j u s q u ’ i l’entree. Cet expedient ne peut atre employe dans le cas d’un faisceau ou d’une galette; aussi, avons-nous Bt6 conduits B Btudier le phenomhne de reaction, d’abord dans un multiplicateur unique rectiligne de grande dimension, puis dans un faisceau de tubes de 200 pm de diamhtre, enfin dans une galette de microcanaux dont les canaux avaient un diamhtre de 40 pm.

A : anode G: grille

M.EC. helicoidal

_

-

Fro. 3. Dispositifs pour 1’8tude dea r6actions dans les inultiplicateurs d’8lectrons B flux csnalis6.

Le dispositif experimental est decrit it la Fig. 3. C’est un tube B vide, monte sur un bMi de pompage, equip6 d’une fuite rdglable permettant de faire varier la pression. Le multiplicateur unique que l’on Btudie a une longueur de 50 mm pour un diamhtre interieur de 1 mm. Pour detecter les ions crdes par ce multiplicateur, qui remontent vers l’entree et den Bchappent, un second multiplicateur incurve est place dans le prolongement pour fonctionner comme detecteur de particules. Deux plaques dhflectrices, placBes sur le trajet, permettent de s’assurer que l’on a bien affaire ides particules chargees. On pouvait craindre, en effet, que le bombardement klectronique des parois du multiplicateur crBe des photons ultra-violets qui ne seraient Bvidemment pas d6viBs par le champ applique entre les plaques, mais

MULTIPLICATEURS CANALIS~SPOUR INTENSIFICATEUR D’IMAGE

503

qui seraient ddtectds par le canal courbe ou, dans le cas d’un tube rdel, par la photocathode. Les impulsions dlectriques obtenues sur le multiplicateur et sur le dBtecteur sont observdes a l’oscilloscope, avec possibilitd de synchronisation des ddclenchements de balayage. Une lampe ultra-violette fournit l’excitation I’entrde du canal a dtudier. Nous rtvons tout d’abord relev6 la courbe donnant la pression d’apparition du regime d’autoentretien en fonction de la tension appliqude au canal (Fig. 4). Cette courbe prdsente une ddcroissance t o n , au-dessous duquel aucun autorapide avec un seuil 5 x entretien ne peut etre ddceld. Pour des pressions supdrieures 8.5 x

2

3

4

Tension ( k V )

FIU. 4. Seuil d’apparition du regime de regheration en fonction de la tension appliqu6e.

torr, les impulsions ddlivrdes par le ddtecteur montrent un front de montde trhs irrdgulier et une largeur de plusieurs microsecondes; ces impulsions disparaissent si le canal droit cesse d’ktre aliment&. L’application d’un champ dlectrique de 300 a 500V/cm suffit pour rdduire l’amplitude des impulsions d’un facteur voisin de 100. L’origine de la rdgdndration est donc bien une remontde d’ions vers l’entrde du canal. Les impulsions qui apparaissent sup le ddtecteur, malgrd l’application d’un champ de ddflexion, sont li6es i la pression dans l’enceinte. Leur origine est probablement due a une diffusion des ions sup les moldcules de gaz rdsiduel. On peut aussi penser & la creation de photons par bombardement des parois du tube. Les mesures ont Btd renouveldes avec un faisceau de 1000 canaux de 200 pm de diamhtre et de rapport Lld = 50. Le faisceau a un diamhtre

504

0.ESCHARD ET J. CRAF

de 6.5 mm et est place face au ddtecteur selon un montage identique torr, le faisceau au prdcBdent. Pour des pressions supdrieures b entre de lui-meme en rBgBnBration et pour une pression d’autant plus faible que la tension est plus BlevBe, c’est-&-direque le gain est plus grand. LA encore, l’application d’un champ Blectrique de 500 V/cm sur les plaques deflectrices Blimine pratiquement les impulsions dBlivrBes par le detecteur, ce qui montre que ce sont bien des ions qui sont it l’origine des impulsions parasites. Enfin, des essais ont BtB faits sur un faisceau de microcanaux dont le diamhtre BlBmentaire Btait de 40 pm. Ce faisceau est place dans un tube ddmontable, relid iL un bBti de pompage. La photocathode est

Pression ( t o r i 1

FIQ. 6. Variation relative du gain d‘un Bisceau de microcanaux en fonction de la pression.

constitube par une couche d’or BvaporBe sup une glace en corindon BclairBe en ultra-violet. Une anode collectrice est placBe en face du faisceau; la gBomBtrie est donc celle d u n tube it double focalisation de proximitb. Les mesures ont BtB faites dans une gamme de pressions allant de b tom, pour des gains pouvant atteindre lo5. Nous avons mesure l’accroissement relatif du gain AG/G en fonction de la pression pour une tension donnee appliqude b la galette (Fig. 5). Lorsque le courant d’anode est trhs infbrieur au courant de conduction dans le faisceau, ACjG eat infdrieur 33 10% pour une pression infbrieure b l o b 6torr. Au-delb de torr, AG/G augmente trbs vite et peut atteindre 100% pour une pression de lo-* torr. Les mesures, rBpBtBes pour d’autres valeurs de gain, ont montrB que l’accroissement relatif

505

MULTIPLICATEURS CANALIS& POUR INTENSIFICATEUR D’IMAOE

AG/G Btait le meme pour un valeur constant du produit pression P x gain Q. Pour un courant d’anode voisin du courant de conduction, une 616vation de pression jusqu’i torr n’a pas d’effet mesurable sur le gain en courant.

La RLaction Optique La realisation d’un tube trhs compact peut faire craindre un retour de la lumihre 6mise par 1’6cran vers la photocathode. Deux obstacles s’opposent it cet effet de r6g6n6ration: la presence du faisceau multiplicateur et la membrane metallique d6pos6e sur 1’6cran. Chaque point de 1’6cran est en vue directe du point correspondant de la photocathode, mais l’angle solide sous lequel il peut &re vu est, sterad. Dhs que l’on n’est plus en fait, trhs faible, 1.5 it 3 x exactement dans l’axe, plusieurs r6flexions interviennent sur les parois des canaux qui att6nuent fortement la lumihre transmise, ceci d’autant plus rapidement que l’angle de reflexion diminue, ce qui r6duit le facteur de r6flexion. Par un calcul rapide, on montre qu’au-deli du troisihme tube moins de 5% de la 1umiBre est transmise. Une niesure directe de la transmission optique du faisceau en lumihre diffuse a permis d’obtenir une valeur de 2-8 x Par ailleurs, la membrane mdtallique d6pos6e derriAre 1’6cran peut permettre d’atteindre une opacit6 supdrieure it 3 x lo3. On peut donc atteindre des gains trhs 6lev6s sans que cet effet de reaction optique soit sensible.

TUBEA DOUBLEFOCALISATION DE PROXIMIT~ Nous avons r6alisd un tube it double focalisation de proximite d’aprhs les principes ddcrits ci-dessus. La Fig. 6 montre ce tube dans

Fro. 6. Tube B double focalisation de proximit6. P.E.1.D.-A

18

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G . ESCHARD ET J. QRAF

son enrobage isolant. Le tube a un diamktre de 40mm, le diambtre utile de photocathode et d’6cran Qtant de 25 mm. Une premidre maquette BquipBe d’une galette dont les microcanaux ont un diamittre BlQmentairede 40 pm a atteint une rQsolutionsur l’image de 10 pl/mm. Les Btudes se poursuivent pour amhliorer les performances de cet Bquipement qui prBsente des points intBressants: (1 absence de distorsion sur l’image, permettant un emploi de toute la surface utile de l’Bcran, (2) alimentation totale du tube infBrieure B 10 kV,rBduisant les problkmes de tenue en tension et d’encombrement de l’alimentation, et, (3) compacitB et faible volume du tube, permettant de l’incorporer dans des Bquipements lBgers et facilement portables.

DIscossIoN Dans le tube B galette amplificatrice, utilisez-vous la galette comme element de commutation? Incorporez-vous, au verre supportant la photocathode, un Bcran metallique B mailles fines pour augmenter la conductivite? La constante de temps du phosphore joue-t-elk un r61e? J. QRAF: Ce tube est conpn pour fonctionner en regime continu. M. Polaert a pr6sent6, dans sa communicationt, les premiers rt?sultats obtenus sur tin obturateur B galette de microcanaux, de principe analogue, dans lequel la fonction obturation est asswee en agissant sur la tension de polarisation de la photocathode. La conductivitt? de la photocathode est arnelioree par une couche m6tallique semi-transparente. D. THEODOROW: Could you give us some further information on your “wafer” tubes particularly as regards resolution, gain and life-time? Where are the “galettes” manufactured? J. QRAF: The first results obtained with this tube give a resolution of 10 lp/mm and a gain of lo4. The channel plates are manufactured in our laboratory. J . F. YNDER:

t

See p. 989.

Effects of Vacuum Space Charge in Channel Multipliers W. M. SACKINGER and J. M. JOHNSON Rmearch and Development La60ratories, Corning aha Worka, Corning, New York, U.S.A.

INTRODUCTION Our preceding paper? has described an analysis of the operation of a channel electron multiplier for low-level inputs. Using the results of that analysis as a foundation, we wish to examine the region of multiplier operation in which the performance is affected by vacuum space charge. This saturation mechanism should be distinguished from the other possible cause of saturation, charge depletion at the semiconductive wall. I n the following analysis, wall charging is ignored, and an accumulation of space charge, described by p ( r ) , is assumed to exist near the output end of the multiplier. RESULTS Figure 1 shows the results of a computer study of the variation of space charge density with radius. The function is predominantly constant, the fluctuations near r = 0 being caused by the small area, and hence the small sample of output electrons. Figure 1 was obtained from the computer simulation of channel operation, at a field strength of 20 V/diameter and a length-to-diameter ratio of 5O:l. The radial position of each output electron was calculated, and incremented one of the 100 annular segments provided by the program. Simulations for other choices of operational parameters also yielded a constant value of space charge density with radius. I n order to relate the charge density to the current, one must know the electron drift velocity. In fact, at any value of radius r , the three velocity components v,, v,, and v, are not unique, but rather there is a Statistical distribution for each component. All of this information has been obtained from the computer simulation study, and may be

t See p. 487. 607

508

W. M. SACKINGER AND J . M. JOHNSON

used in calculations of focusing effects at the end of, or beyond, the multiplier. For purposes of space charge analysis, it is sufficient to examine the variation of axial velocity v, versus channel radius, as shown in Fig. 2 in which we have plotted the number of electrons per unit area in a given velocity class versus radius. Most electrons travel less than two diameters of axial length, acquiring energies of less than 40 eV. Many electrons in the 0-10 eV range have just emerged from the wall, whereas electrons in the 10-40 eV range have progressed towards the center of the multiplier. In the 30-40 eV range, there is an increase in the relative number of electrons emerging near the wall, a trend which becomes more pronounced at higher exit energies.

Radius

I

FIG.1. Variation of charge density with radius.

The relative number in each velocity category is different, but, as will be shown below, the space charge calculations are not very sensitive to this effect. The electron motion r (t) in the transverse plane in the presence of space charge is described by Eq. 1. This equation has been derived by other authors.1*2

We have examined variations of the current I per channel, the channel diameter a,the electron drift velocity v, and the angle of the secondary electron emission in the transverse plane Bez. The emission energy eVo was assumed to be 2 eV, which is near the peak of the energy distribution, The qualitative effects of the radially directed space charge force are that most electrons are emitted with small values of Bs2, and therefore

SPACE CHARUE IN CHANNEL MULTIPLIERS

m

E

.-a

Y

.-w

509

510

W. M. SACKINUER AND J . M. JOHNSON

pass near the center of the channel. The space charge force decelerates them until they pass the center, and then accelerates them radially. The net effect is a longer transverse transit time, enabling them to acquire more energy from the channel field and give higher gain. The few electrons emitted at large angles will be deflected, striking the walls sooner and hence lowering the overall gain. As output current increases, one therefore expects a space charge enhancement of gain, followed by a drop in gain at higher output currents. In Fig. 3, the change of transit time with space charge is quantitatively illustrated for four emission angles: lo, 22.5", 45", 67.5'. The apace A per channel. At that level the transit charge effect begins at I = 2.0

I

I

I

I

I

I

I

I

10

-

1.5-

3

-

22.5'

b -

;1.0-

-

'=

45"

._ C t n

*

0.5-

67.5"

0

I 10-9

1 10-8

I

I

10-7

10-6

I 10-5

10-4

10-3

10-2

10-1

time of electrons passing near the center is definitely increased, whereas the transit time of those emitted at large OS2 is slightly decreased. Since there are more electrons emitted in the former category, this current level should correspond to gain enhancement by space charge. At I> A, space charge acts to lower gain and to saturate the output current. The normalized transit time change due to space charge is dependent upon the emission energy and angle, and upon the potential difference due to the space charge between channel center and edge. The actual channel diameter should have no effect. A series of calculations was to lO-'cm, and the made for channel diameters in the range results are plotted in Fig. 4. The normalized transit time change due to a current of I = l o v 4A per channel is plotted vertically. No variations are seen.

511

SPACE CHARGE IN CHANNEL MULTIPLIERS

1-

?Ole L <

-

o?2'50

,450 67.5' -0.10: 10-5

1

I

10-3

10-4

I

50

10

I

I00

0

Log (Drift voltage)

FIG.6. Space charge effect versus drift voltage.

In Fig. 5 the variable is the voltage producing electron drift. Only minor changes are seen in the range from 10 to 200V, implying that the saturation current level of I = lO-*A is a reasonable estimate for all expected drift velocities in the channel.

CONCLUSIONS The results presented above are similar to those reported in the literature. It is important to stress that space charge saturation is independent of channel diameter. Furthermore, the analysis for a single channel should be valid for each channel of an array. This implies that an array of lo3 channels, for example, should not be

512

W. M. SACKINOER A N D J. M. JOHNSON

saturated by vacuum space charge effects until an output current of 10-IA is reached. Goodrich3 has recently reported a peak current of 10-3A for such an array operated under single-pulse conditions. Wall charging may still be a factor under short-pulse conditions depending on the charge stored in the distributed capacitance of the channel wall. If the pulse width is decreased, and the channel length-todiameter ratio is large enough, the peak output current should ultimately be limited to 10-4A per channel by vacuum space charge. However, if the pulse width approaches 1O-O sec, comparable to the transit time of the electrons through the multiplier, then the assumption of a uniform space charge cloud would no longer be valid. The implications of these results for channel multiplier arrays in image intensifiers are that under continuous operation, space charge saturation of arrays of many channels cannot be achieved, but rather the output current will be limited at or below the conduction current, which in turn is limited by power dissipation to about the 10-3A/~m2 region. If phosphor screen brightness is to be held constant, a 10-3A/cm2 output current density is a drastic improvement over normal photocathode current density, and much lower voltages can be used from the array to a phosphor screen. This is especially helpful in reducing extraneous noise and breakdown if proximity focusing from the array t o a phosphor screen is used. If normal voltage is used, the high output current density from the array would increase image brightness, yielding shorter exposure times for film-recording of the image. More significant, perhaps, is that very brief exposures-of microsecond duration-are possible. By choosinga phosphor of long decay-time, brief events, such as flash X-ray pictures, can be viewed directly at low dosage levels. As has been s ~ g g e s t e dsuch , ~ an array could also drive a many-segmented collector or a charge-storage target, to give an electrical output signal rather than an image. The essential advantage in all cases is that an electron image with steady-state intensity as high as 10-3A/cm2, and much higher peak pulse intensity, can be delivered by an array of channel multipliers.

REFERENCES 1. Schmidt, K. C. and Hendee, C. F., IEEE Trans. NucZ. Sci. NS-13, 100 (1966). 2. A d a m , J. and Manley, B. W., IEEE Tran8. Nucl. S c i . NS-13, 88 (1966). 3. Goodrich, G. W. and Love, J. L., IEEE Trans. NucZ. Sci. NS-15, 190 (1968).

Statistics of Transmitted Secondary Electron Emission W. L. WILCOCKandD. E. MILLER Physics Department, University College of North Wales, Bangor, Wales

INTRODUCTION This paper is concerned with the refinement and elaboration of an experiment, first reported by one of us a t the Third Symposium on Photoelectronic Image Devices,l which had as its object the measurement of the probability distribution P ( n )for the number n of secondary electrons emitted when a primary electron strikes a transmission-type electron-multiplying dynode. The method employed is direct. Each group of secondary electrons arising from a single primary is accelerated and focused on t o a semiconductor barrier-layer detector, where they are absorbed within the sensitive layer effectively simultaneously. The resulting pulse of charge a t the detector output, which is proportional to the total energy absorbed, is therefore proportional to the number n of secondaries in the group. Thus when the dynode is bombarded by a succession of single primary electrons, the spectrum of pulse amplitudes from the detector reproduces the discrete probability distribution P ( n ) , n 2 1, modified by the effects of noise in the detector and associated amplifiers. The pulse amplitude spectrum from the detector cannot, of course, by itself give information about P(O), the chance that a primary electron will not give rise to any secondaries at all. This parameter can be determined by comparing the total pulse count rate when the dynode is present with the total pulse count rate when the dynode is removed from the beam and the primary electrons themselves fall on the detector. Alternatively, when P ( n ) ,n 2 1, is known, P ( 0 ) can be inferred from a measurement of the ratio of secondary and primary currents, which is simply the mean E of n. Although the experiment is in principle straightforward, a number of requirements have to be met before consistent and reliable results can be obtained. I n particular, the primary beam current must be stable at the low levels ( -lO-laA) needed for undistorted spectral analysis, over periods long enough for measurements with and without the dynode in position; the position of the focal spot on the detector has 613

514

W. L. WILCOCK AND

D. E. MILLER

to remain the same when the dynode is withdrawn as when it is in the beam, over the required range of primary energy; the rate of background pulses at the detector must remain low in the presence of the accelerating voltage of 40 kV or so which is needed to obtain satisfactory resolution from the detecting system; and the vacuum environment must be clean enough t o preserve the emission characteristics of the dynode for the time required to make a full set of observations.

FIG. 1. Diagram of experimental tube used to examine statistics of transmitted secondary electron emission.

By suitable attention to design, all these requirements have been satisfactorily met in our present apparatus, which is illustrated diagrammatically in Fig. 1. The apparatus is basically a tube made up of cylindrical sections joined together with demountable gasket seals, and mounted vertically. The uppermost section A is of glass and contains the primary electron gun, which is a high-voltage gun of commercial manufacture, modified by the substitution of a tungsten wire filament in place of the normal oxide-coated cathode. At its lower end A is joined to a short metal section B which closes the tube except for a small central hole 0.1 mm in diameter. Beneath this hole,

STATISTICS OF T.S.E. EMISSION

515

which acts as a field stop, are parallel ways in which the dynode holder slides. Translational motion of the dynode holder is introduced from outside the tube through a metal bellows which can be compressed by a highly insulated external screw. By this means the dynode can be positioned beneath the hole, or withdrawn, whilst it is at high potential. Below B is a glass section C which encloses the accelerating lens. The cathode of this lens is formed by the dynode itself together with a surrounding spherical field-forming electrode attached to the lower side of B. The focus electrode is also suspended from the lower side of B, but by insulating supports, and the lead to it is taken through the glass wall of C. The pierced anode of the lens is mounted on the upper end of a metal section D, which is followed by a gate valve and a final metal section E. This contains a set of deflexion plates, with the aid of which the focused beam from the lens can be directed a t will to the semiconductor detector, a Faraday cage, or a luminescent screen. Apart from O-rings of Viton A in the gate valve, all the gasket seals are of lead wire, and the electrode connexions through the tube walls are either tungsten-Pyrex wire seals, or metal-ceramic feedthroughs. The purpose of the gate valve is to allow the detector to be brought to atmospheric pressure when it is not in use without losing vacuum in the remainder of the tube. Each side of the gate valve is therefore connected to a sorption pump and getter-ion pump. The pumps are torr in the tube when the able to maintain a pressure of about 5 x filament is heated. Under these conditions the multiplication characteristics of the types of dynode we have examined remain substantially unchanged over long periods of time. A convenient test of this is provided by measuring the mean multiplication Z,which is found as the ratio of currents collected in the Paraday cage with the dynode in, and out of, the beam. Observations over the course of several days have shown that ii falls by less than 2% per day, which corresponds to a negligible change during the time required for a complete set of observations. For counting purposes the primary beam current passing the aperture stop is typically set at about lo3 electrons[ sec, and then exhibits slow drift of the order of 10% per hour, which is also negligibly small in the context of the measurement,s. The accelerating lens gives a magnification of approximately 4 between the dynode and the detector, and with the field stop in position above the dynode the focal spot on the detector is about 0.5 mm in diameter. When the dynode is withdrawn, the focal spot is reduced in size, and a t high primary energies it is shifted laterally, but this shift can be compensated by applying appropriate voltages to the deflexion plates, and the beam is thereby restored to its original position at the centre of the detector.

W. L. WILCOCK AND D . E. MILLER

516

Most of our observations have been made with a lithium drifted silicon detector, of 25-mm2 active area and 0.5-mm depletion depth, supplied by the Nuclear Equipment Corporation, San Carlos, California. The detector is cooled to a temperature near to that of liquid nitrogen and then makes only a small contribution to the noise level of the system. This latter is equivalent to about 5 keV FWHM, and arises mainly from the charge sensitive amplifier which follows the detector. With the high voltage off, the rate of background pulses is about 3 per 1000

AC2O3(5O0

4.8 keV:

+AL(200;)

-

t KCC(500i)

n

= 4.90

P(0)

= 0 25

n=l

Pulse amplitude

FIG.2. Pulse amplitude spectrum from dynode of bulk-density KCI 600A thick. Energy of primary electrons 4.8 keV: mean secondary yield 4.9.

second, rising t o about 10 per second with 40 kV applied across the accelerating lens. Most of these pulses are then of a size corresponding to the arrival of a 40 keV electron a t the detector, and so are presumably due to single electrons emitted from the dynode. The remaining pulses appear to be distributed more or less uniformly over all measurable sizes. A typical example of the spectra obtained is shown in Fig. 2. It relates to a conventional dynode of bulk-density potassium chloride 500 A thick, bombarded with primary electrons of energy 4-8 keV, which is near the energy for maximum transmitted secondary yield.

STATISTICS O F T.S.E. EMISSION

517

The mean number of secondary electrons per primary is 4.9,and the probability that a primary electron produces no secondaries is 0.25. The spectrum shows clearly resolved peaks corresponding to the emission of groups of secondary electrons with all integer values of n from 1 to 17. The obvious asymmetry of the peaks, and the relatively high background on which they appear superposed, are consequences of the back-scattering of electrons at the detector. One electron in about every seven incident is scattered out of the detector before it has lost all its energy, with the result that the response of the detector to single electrons shows a low energy tail in addition to the sharp peak characteristic of full energy loss. The response of the detector to a group of n electrons is the n-fold convolution of this single-electron response; and the observed distribution is a sum of such multi-electron responses, weighted in proportion to the true probability distribution P(n),and smeared out by amplifier noise. Delaney and Walton2 have described a method of reducing the effects of back-scattering by using a strong magnetic field to return the back-scattered electrons to the detector, but we have not judged the improvement likely to be gained in this way to be worth the technical complication. Instead we have made careful measurements of the single electron response of our detector, and of the noise characteristics of our amplifier, which we believe are of sufficient precision to allow the true probability distributions to be extracted from our data. Unfortunately, the computational work needed for this is not yet complete, so that we must a t present rely on approximate analysis. This leads to the conclusion that the true probability distribution underlying the spectrum of Fig. 2 is nearly geometric, i.e. n = 0, P(n) = P(O), = kn-lP(l),n 2 1, where k is a constant. This form of distribution appears to be characteristic of transmission-type dynodes, although to some this comes as a surprise because of the widespread expectation that emission phenomena ought to obey Poisson statistics. Figures 3 and 4 show spectra from the same dynode as in Fig. 2, but for different values of the primary energy. Figure 3 relates to a primary energy only just above the threshold a t which transmitted secondary emission begins : over three-quarters of the primaries produce no secondaries, and the mean yield is not much above unity. By contrast, Fig. 4 relates to primaries with energy more than twice that corresponding to maximum secondary yield, whose range is much greater than the film thickness. Evidently the form of the spectrum is quite insensitive to the primary energy. I n all cases

100

n

1-38

>

E f

10

.3 e

-

B

I

Pulse amplitude

FIG.3. Spectrum from same dynode as in Fig. 2, but with primary electron energy 2.7 keV. 1000

-

I

Al2O3(5001)+ A L ( 2 0 0 % ) + K C l (500;) 10.0 keV:

-

n

= 2.19

Pulse amplitude FIG.4. Spectrum from Name dynode a8 in Fig. 2, but with primary electron energy 10.0 keV.

519

STATISTICS OF T.S.E. EMISSION

Pulse amplitude

FIG.6. Spectrum from dynode of bulk-densityKCI 2600 d thick. Energy of primary electrons 6.1 keV: mean secondary yield 9.9.

large pulses are present, corresponding to the emission of electron groups of high multiplicity. We are able to resolve these puIses as peaks up to n = 24, but the distribution continues beyond this in exponential form as far as we have been able to observe (n M 60). The occurrence of events involving such large values of secondary multiplication is not, 100

5.1 keV:

-

n

11.5

P ( 0 ) =0.22

0 x

0

10 > .+ 0 a4

LT

I

Pulse amplitude

FIG.6. Spectrum from dynode of bulk-density CsI, 600 d thick. Energy of primary electrons 6.1 keV: mean secondary yield 11.5.

520

W . L. WILCOCK AND D. E. MILLER

as has been suggested, in some way associated with space charge, because a freshly prepared dynode gives the equilibrium proportion of large pulses as soon as the primary bombardment begins. Figure 5 illustrates the effect, or rather the lack of effect, on the distribution which results from an increase in the thickness of the potassium chloride layer. Similarly, Figs. 6 and 7 are sample spectra from dynodes I

AL20,(5008)t

A L ( Z O O 8 ) t C s I (2000%)

6.0keV:

._ n

= 13.0

P ( 0 ) ~0.27

n= I

fB

6

g0

a

Pulse amplitude

FIG.7. Spectrum from dynode of bulk-density CsI, 2000 A thick. Energy of primary electrons 6.0 keV; mean secondary yield 13.0.

in which the emitting material is caesium iodide. These examples, all of which relate to primary energies near that for maximum secondary yield, have mean values of n much higher than are obtained from potassium chloride dynodes of the usual structure (cf. Fig. 2); but the form of the distribution is essentially the same in all cases, and remains so over the whole range of primary energies we can explore. To sum up, as far as our observations go we find the extraordinary

52 1

STATISTICS OF T.S.E. EMISSION

result that the most likely outcome of a primary encounter, if there is any emission a t all, is always the emission of a single secondary; and the chances of the emission of larger numbers of secondaries then follow in descending, and approximately geometrical, progression. This result does not fit easily into the framework of the customary model of the secondary emission process. This model, which can be made to explain well enough the observed dependence of yield on primary energy, assumes that the dissipation of this energy leads to the production of secondary electrons in the interior of the dynode, each of which diffuses and may escape from the surface. Fluctuations of the number of secondaries emitted then arise through the number v of internal secondaries produced, and the probabilities pl,.. .pv that these secondaries will escape. Our observations appear to require an approximately geometric probability distribution for either v or the average of the p’s. There are undoubtedly fluctuations of v, but such data as are available on total energy losses in the passage of electrons through thin films does not suggest that the distribution of v has the required insensitivity to primary energy and dynode thickness. Similarly, an explanation in terms of fluctuations of the mean p seems to call for severe, and implausible, inhomogeneity of the dynode. I n short, we are unable to offer a convincing explanation of these observations, and if, as seems likely, they are evidence of some basic underlying physical process, we are not yet in a position to identify it.

REFERENCES 1. Wilcock, W. L., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22, p. 629. Academic Press, London (1966). 2. Delaney, C. F. G. and Walton, P. W., IEEE Trans. Nticl. Sci. NS-13, No. 1, 742 (1966).

DISCUSSION G. W. GIOETZE: Have you made any measurement on “low-density” TSE films? We know that these films give much higher average yields (50 to 100) and one might suspect that in those cases where the average yield is much closer to the maximum number of secondaries generated by the primary electron the distribution is much narrower, or should at least approach more and more closely the distribution of the “generating” process. W. L. WILCOCH: We have not made measurements with low-density films, and i t would certainly be interesting to do so. However, judging from the results reported by Dietz, Hanrahan and Hance (Rev. Sci. Inatrum. 38, 176 (1967)), I would be surprised if we did not find a nearly geometric distribution from this ilm also. I wonder, too, if it is reasonable to expect the distribution in type of f this case t o approach that of the generating process. It is true that the average yield of low-density films is closer to the maximum number of secondaries generated, but not, I believe, significantly so. My estimate is that these numbers P.E.1.D.-A

19

522

W. L. WILCOCK AND D . E. MILLER

differ by a factor of order 10 for low-density films, compared with a factor of order 100 for bulk-density films. G . T. REYNOLDS: This is very interesting work, for its own sake, and also very important for all types of multiplying structures, including channel devices. Professor Wilcock is to be congratulated for a very beautiful piece of work. J. D. M ~ Q E E : Would the author comment on the origin of the large values of TSE gain of order 10 and on the relation of these results to the noise characteristics of the TSE image tube? w. L. WILCOCK: I think it is now quite widely known that, by proper selection of thickness and material, it is possible to prepare transmission-type dynodes of average yield appreciably higher than the value of about 5 obtained with 500 A thickness of bulk-density KC1, which was the recipe used in TSE image tubes. Unfortunately our results show that no improvement in noise characteristics is to be expected from the use of such dynodes: the single-electron response of the tube would still be quasi-exponential.

Two Methods for the Determination of the Imaging Properties of Electron-optical Systems with a Photocathode V. JARES and B. NOVOTNY Vacuum Electronic8 Reaearch Imtitute, Prague, Czechoslovakia

INTRODUCTION The determination of the imaging properties of electron-optical systems for transferring the electron image from the photocathode t o the target of a TV camera tube or to the luminescent screen of an image intensifier is a rather complicated problem. Conventional methods, based on the distribution of the electrostatic field in the system of electrodes to be investigated, are very tedious and they do not yield results of the desired accuracy. Measurements on complete experimental samples of tubes give the required values. This method, however, is tiresome and expensive. The present paper briefly describes two methods for determining the imaging properties of the electron-optical system of an X-ray image intensifier with variable magnification. These are the experimental method, based on the use of a demountable model of the tube t o be investigated, and the computational method, utilizing programmes prepared in advance for the determination of imaging properties of the given electron-optical system with the aid of a National-Elliott 503 computer.

MEASUREMENTOF

IMAGING PROPERTIES OF AN X-RAY IMAGE INTENSIFIER

THE

The Demountable Model Method To determine the imaging properties of the electron-optical system of the X-ray image intensifier, a n experimental demountable model of the intensifier was constructed, the input section of which (the fluorescent screen and the photocathode) was replaced by a concave alnminium disc with several apertures (see Fig. 1). Tungsten cathodes were mounted behind the apertures which were covered with metal grids having a variable pitch. The cathodes were spot-welded t o 623

524

v. JARES AND

B. NOVOTNI?

metallic holders embedded in ceramic supports. To eliminate the scattering of the emitted electrons through adjacent apertures and to prevent the imaging of the tungsten filaments, the ceramic supports with the thermionic cathodes were inserted in metallic cylinders which were provided with additional metal grids (see Fig. 1). The metallic cylinders and grids were fixed under the aluminium disc. The electrical

FIG.1. Diagrammatic section of aluminium disc D showing one of the apertures with variable pitch metal grid MI, metal grid Mz,and tungsten filament K.

connexions to the thermionic cathodes were by metal pins sealed on the flank of the glass envelope. A general view of the aluminium disc and the grids is shown in Fig. 2. Figure 3 shows how the disc was accommodated in the envelope. The fine metal grids with variable pitch were produced electrolytically from a glass matrix. Their geometrical configuration is illustrated in

Fro. 2. Photograph of metal disc with grids.

IMAGING PROPERTIES O F ELECTRON-OPTICAL SYSTEMS

525

FIU.3. Metal disc assembled in demountable model.

Fig. 4. The grids are secured in the plane of the apertures above the tungsten cathodes by mean8 of an outer disc made of aluminium foil. The focusing electrode of the intensifier consists of an aluminium foil on the wall of the glass envelope. The anodes, in common with the output screen, are secured in the upper, narrowed section of the glass

526

v. JAREB

AND B. NOVOTN+

bulb; their positions may be adjusted. A diagrammatic cross-section of the demountable model of the X-ray image intensifier with variable magnification is shown in Fig. 5(a). The electrode configuration is illustrated in Fig. 5(b). This configuration, obtained after a series of measurements on the demountable model, was chosen for its very good imaging properties. The resolution of the experimental model of the intensifier was investigated for various electrode arrangements by viewing the demagnified electron images of the metal grids through a micro-

FIG.6. (a) Cross-section of the demountable model of X-ray image intensifier with variable magnification showing microscope M, luminescent screen S, anode A, correctoranode A,, focusing electrode F, and aluminium disc K with thermionic cathodes. (b) Cross-section of the electrode configuration showing luminescent screen S, anode A, corrector-anode A,, and focusing electrode F.

scope. The imaging characteristics are shown in Fig. 6. The resolving power of the electron-optical system of the intensifier can be determined both in the centre and on the periphery of the cathode from the number of distinguishable grid meshes. The image distortion is apparent from the geometry of the electron images of the grids. The curvature of the image plane can be estimated from the ratio of the voltages required to focue the images in the centre and at the periphery of the cathode. I n a similar way the influence upon the sharpness and geometry of the image of both the anode curvature and the size of the anode aperture can be analysed.

IMAQING PROPERTIES OF ELECTRON-OPTIUAL SYSTEMS

I 200 -

-9

527

Un=20 kV

>

100

-

0

I

I

I

5

10

15

Up(kV)

FIG.6. The variation of focusing potential U,, (upper curve) and dernagnification 1/M (lower curve) corrector-anode potential U p .

The Computational Method

To determine the imaging properties of the electron-optical systems by means of a computer, a programme was prepared for the solution of the problem of both the distribution of the electrostatic field and the form of the electron trajectories in a rotationally symmetrical electrostatic field. The distribution of the electrostatic potential in an electron-optical system with rotational symmetry is given by the solution of Laplace’s equation. The numerical solution of this equation is usually derived from methods based on the approximation of derivatives by finite differences. The difference form of the Laplace’s equation suitable for numerical computation is given by the expression1 2Vl 2v2 hz) hl(h1 hz) + h,(h,

+

+

___ 2vo _ [h, - h2

+

+

+

(2r

(2r h4)V3 + (2r - hdV4 h3(h3 h4) h4(h3 h4)

+

+

+

] =o,

- h3)V0 h3h,r

h4

(1)

where V ois the successively approximated potential at the point which is at a distance r from the axis of symmetry, and V , t o V , are the values of potentials at the adjacent points of the network which are a t distances h, t o h4 from the original point. For some special cases, the general equation Eq. (1)can be simplified. If, for instance, the adjacent points of the network are a t equal distances, h, = ha = h3 = h, = h,

528

v. J A R E ~AND

B.

NOVOTNJ

Also for points on the axis the following simplified relations can be used:

and

Vl

+ Vz + 4V3 - 6Vo

==

0.

(4)

Equations (1) to (4)are the basic relations which were used for the solution of the axially symmetrical field. The program was written in the symbolical language ALGOL for computation on the NationalElliott 503 computer. The procedure, used for computing the field, can be described as follows. A fine network of squares is drawn in the r-z plane of the system of electrodes to be investigated. The lower edge of the network is identical with the axis of symmetry, while the remaining edges overlap the outer contours of the electrodes and form rectangles. By using this structure the tables of fixed potentials can be prepared, serving, in common with the control parameters, as the input values for the calculation on the automatic computer. A programme, which is recorded in the memory of the computer, controls, in common with the input values, the successive approximation of the potential in individual regions of the network. The Young-Frankel super-relaxation method was used to accelerate the convergence of the iteration procedure. The equations defining the motion of electrons in the axially symmetrical electrostatic field were solved by the use of the predictor-corrector method.2 A flow-diagram showing these procedures for the solution of fields and trajectories is given in Fig. 7. Rectangular framing of the instructions means the instruction: “put into the computer”; round framing expresses the condition for a conditional transfer instruction and, finally, oblique framing denotes the text displayed by the highspeed printer or the content of the printed data. The solution of the problem begins with the computation of the electrostatic field. By repeating the whole cycle the difference between successive values of the potential at an arbitrary point is diminished to the pre-selected value. As soon as this value has been attained at all points in the field, the calculation is terminated. The machine then reads the input values of the trajectory from the data tape, prints the heading and the input values of the trajectory, and carries out the computation of the first four points by means of the Runge-Kutta formulae. Then follows the printing of the values of all four points and the computation of further co-ordinates by the predictor-corrector method. As soon as the calculation reaches the plane of the phosphor screen the printer prints the word “END”. The value of the focusing voltage is determined for the two trajec-

IMAQINQ PROPERTIES OF ELECTRON-OPTICAL SYSTEMS

529

w

Ilntroducing tape of initial values .]

*

lReoding of initial values for eleC.1 (Field calculation I

I

(Counting d 20 cycles of network rec.1

0

~~

FIG.7. Flow-diagram for computation of the imaging properties of the device.

tories with particular input values. The computer follows the effect of 1 % changes of the given focusing voltage on the positions a t which the two trajectories strike the phosphor screen. When the distance between these points of intersection is less than 0.1 mm the focusing voltage is determined and the calculation of the next trajectory is begun. The calculation is finished by printing out the value of the focusing voltage, the values of the potentials a t all points, and by

FIG.8. The computed equipotential lines and the trajectories of electrons.

531

IMAGING PROPERTIES OF ELECTRON-OPTICAL SYSTEMS

drawing the equipotential lines. The magnitudes of the potential along the equipotential lines are given by the values of the potential at points on the axis of the system. The pre-set programme enables an automatic repetition of the complete calculation for other values of the variable parameters. TABLEI Input Values of Trajectories Trajectory

1 2 3 4 5 6 7

ro(mm) 65 131 65 131 65 131 0

zdmm)

dro(mm/sec)

dzo(mm/sec)

7, 11 30 7, 11 30 7, 11 30 0

-1.286 X 10' -2.59 X lo8 0 0 -2.59 X 10' -5.336 x 108 1.285 x lo8

6.79 X 10' 6.336 x 10' 5.53 x 10' 5.93 x lo8 5.336 x 108 2.59 x lo8 5.79 x 108

The programme described above has been used to compute the distribution of the electrostatic field and the shape of electron trajectories in the electron-optical system of the image intensifier shown in Fig. 5(a). The computed equipotential lines and the trajectories of electrons with initial velocity of 5.93 x lo5 mlsec are plotted in Fig. 8. The potential of the corrector anode is Up = 19 kV,and the

20

15

12 0 3 3 -

B

1"

7

6

FIG.9. The parts of the trajectories near the output screen (output radius 65 mm from the axis).

532

v. J A R E ~AND B. NOVOTNP

image is assumed to be demagnified by a factor of 7. The input values of individual trajectories are given in Table I. Some equipotential lines for a corrector-anode potential of U p = 7 kV (for which a 12 x image demagnification is assumed) are plotted for the zone in front of the output screen. Figure 9 shows those parts of the trajectories near the output screen (radius 65 mm) for the two values of the voltage U p . A similar graph for an output radius of 131 mm is illustrated in Fig. 10. UD = 7kV

-2

up/

\U0=20kV

\ \ \

FIG.10. The parks of the trajectories near the output screen (output radius 131 mm from the axis).

The computed results correspond closely with the measurements on the experimental model. The method using the model yields data which describe how electrons strike the output screen, while the computational method enables the trajectories of electrons during their passage through the electron-optical system to be followed. Both methods are an effective aid for the design of new concepts for imaging systems using wide electron beams.

REFERENCES 1. Weber, C., Philips Tech. Rev. 24, 130 (1962). 2. Carr6, B. A. and Wreathall, W. M., Radio Electronic Engr 27, 446 (1964).

DISCUSSION P. FELENBOK:

are you using?

How much computer time do you need and how many memories

IMAGING PROPERTIES OF ELECTRON-OPTICAL SYSTEMS

533

v. J A R E ~ :Calculating the potentida at one thousand points with the desired accuracy of better than 0.1 V, takes on average 6 min for about 100 approximations; the calculation of one trajectory takes about 16 sec. For the calculation of the fields and trajectories only the inner memory of the computer is used. D . R. CHARLES: 1. Are the meshes to simulate the initial velocities of the electrons? 2. Is your programme able to take account of the initial velocities of the electrons and of space charge? V. J A R E ~ :1. The metal meshes with variable pitch do not simulate the velocities of the electrons. The ah adow images of these meshes modulate the electrons from the individual tungsten cathodes so that, after imaging by the system under investigation on to the output screen of the intensifier, the imaging properties of the system may be measured. 2. The programme takes account of the initial velocities of the electrons; the influence of space charge is not considered.

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ABSTRACT

Computation of Imaging Properties of Image Tubes from an Analytic Potential Representation? F. SCHAFFt and W. HARTH Institut fur Technische Elektronik der Technischen Hochschule Munchen, West Germany

Numerical methods of investigating the imaging properties of image tubes by computation of electron trajectories have so far used field tables t o represent the imaging potential distribution. These tables have t o be computed from a given electrode geometry and to be stored in the computer memory. If the curvature of the photocathode is large enough, however, as it is in some commonly used image diodes, it has been shown that it is also possible to represent the potential distribution analytically, i.e. by a linear combination of potential functions. For that purpose the so-called flat-ring co-ordinate system is used both t o establish the potential eigenfunctions and to replace the electrode device by more simple geometrical and electrical boundary conditions. For one image diode, imaging distance, magnification, distortion, tangential and sagittal image surfaces were evaluated and compared with similar results obtained for the same tube by another author who had used the field table method. An effort was then made to calculate back from the potential representation to the electrode geometry which had been used before to determine the boundary conditions. This gave an idea of the influence which various parts of the electrode system exert on the imaging field. Variations in the potential representation were then introduced by changing the boundary conditions, and the effects on the imaging properties studied. As an example, it was noticed that both image distance and magnification are closely connected t o the field strength on the cathode surface and depend hardly a t all on the field distribution elsewhere. Finally, from a potential representation that yielded a higher resolution of the image, the corresponding shapes of the focusing electrodes were computed. This work has shown how the image tube system considered initially could be modified to produce some improvement in the imaging properties, 1 For full paper see 2. Angew. Phy.9. 23, 64 (1967). $ Present address: CERN, Geneva, Switzerland. 535

This Page Intentionally Left Blank

The Design of Electrostatic Zoom Image Intensifiers J. VINE Westinghouse Research Laboratories, Pittsburgh, Pennsylvania, U.S.A.

INTRODUCTION Electrostatic image intensifier design is an excellent subject for the application of large computers, as has been illustrated by several published papers.la2 The main reasons for this are firstly, that the mathematical model of the tube is good, so that the electron-optical problem can be accurately presented t o the computer and the computed results readily interpreted, and secondly, that experimental design study is impeded by many problems that do not arise from the electron optics. The most important single development in electrostatic image tube design was the introduction of the spherical cathode surface by Morton and Ramberg,3 an innovation which reduced geometrical aberrations to a considerably lower level than had previously been attainable with a flat cathode. The three important geometrical aberrations are image curvature, astigmatism, and distortion, and the main problem of electron-optical design is to minimize these while keeping the tube dimensions within reasonable limits. The remaining important aberration is chromatic aberration, due to the spread in the emission energies of the electrons, which sets the ultimate resolution limit of the tube. It is well known that this resolution limit is directly proportional to the electric field strength a t the cathode surface. DESIGNTRENDSIN DIODES Figure 1 shows three important design parameters in a schematic representation of a basic image diode. They are the radius of curvature of the photocathode Rc, the cathode-anode separation x, and the diameter of the anode aperture d. Varying these parameters one a t a time produces the following effects on the focal length f and magnification M : (i) increased R, increases f end M ; (ii) increased d increases f 537

538

J. VINE

and M ; (iii) increased x decreases f and M . It is also important to consider the variation of two parameters simultaneously to maintain constant magnification, in which case a reduction of either d or R, results in a reduction o f f . Both these trends produce higher field strength a t the cathode, providing higher ultimate resolution capability.

FIG.1. Basic design parameters.

However, it is usually found that the shorter the focal length for a given magnification, the worse are the geometric aberrations, particularly distortion. Thus, a shorter tube will generally have higher center resolution but poorer image uniformity. The introduction of a control electrode between cathode and anode

produces a triode capable of being focused. Figure 2 shows a configuration in which this electrode has been made to coincide approximately with an equipotential surface of the basic diode. Some computed principal rays are shown, and the image surfaces for three values of focus voltage V,. The typical properties illustrated are that f and M

ELECTROSTATIC ZOOM IMAGE INTENSIBIERS

539

increase with V,; the image surfaces tend to scale directly with M , with little change in shape and the principal rays do not change significantly.

ZOOMTUBEDESIGN

A zoom tube can be produced by introducing into the triode a fourth electrode either in the anode space or in the cathode space. The distinction is not necessarily clear cut, but the terminology is convenient and its implications will be made clearer in what follows. Anode Space Zooming This is most simply achieved by separating the screen from the anode, as shown schematically in Fig. 3. The essential feature of such a tube is that it comprises two independent parts, the cathode lens and the zoom lens, separated by a field-free space within the anode. This

Screen

I

FIG.3. Variable magnification tube employing a screen lens.

separation simplifies the design problem considerably, since the basic properties of the two parts can be computed separately, and the results of combining them in any manner are then calculable by the simple formulae of Gaussian optics. The required properties of the image triode are described by two curves, namely the magnification M and focal length f as functions of focus voltage V,. The simple zoom lens shown in Fig. 3 can be termed a “screen” lens,? since the output screen forms an essential element of the lens. Figure 4 shows the action of the lens schematically by means of ray diagrams utilizing the cardinal points. The lens is either (a) reducing, or (b) magnifying, according as the screen voltage V , is greater or less than the anode voltage V,. The cardinal points of the lens have been computed for a range of values of the ratio VJV,. These data are not presented here. Since

t This terminology should not be confused with screen lenses formed by parallel wire screens.

J. VINE

540

Screen

Screen

Imoge

--_

--F 2 F :

5

2.0

u--

v--

(b)

FIQ.4. Imaging action of the screen lens. (a) V , < V,,, demagnifying; (b) Vs> V,, magnifying.

the image is required in practice to lie at the screen, only one conjugate pair is of interest for each value of VJV,. Therefore the cardinal point data are reduced to the data of practical interest represented by the two curves shown in Fig. 5. These show the object position Zoblwith respect to the screen and the magnification M , as functions of the voltage ratio. It can be seen that the voltage variations are large, and that there is a range of M over which the object position varies very

Anode

E

.c c .-

'c

ul c

r"

1.2 -

1.0 -

080.6

-

04 -

0.02

0.05

0.1

0.2

0.5

1'0

2.0

5.0

10

20

V,/G FIQ.5. Properties of the zoom lens (computed).

50

ELECTROSTATIC ZOOM IMAUE INTENSIFIERS

d

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J. VINE

542

little, although it should be remarked that Zobfis proportional to the lens diameter D. Figure 6 shows a design example developed from the triode of Fig. 2. Principal rays and image surfaces are shown for: (a), M = 0.91 and (b),M = 0.45. It is noted that there is a 10 : 1 voltage variation for a 2 : 1 change in M . The effect of the screen lens on the image surfaces is slight, but would be greater if the lens were of smaller diameter. Since Zob,is proportional to D, a smaller diameter screen lens would necessitate less adjustment of the focus. If the required voltage-ratio variation is achieved by changing V a ) then the resolution capability at the photocathode varies proportionately, On the other hand, if the screen voltage V , is varied then the tube gain varies in a way that accentuates the output brightness change that occurs due to the magnification change itself. Limited variations of both voltages might be employed in practice to optimize the resolution and gain changes. A practical tube similar to this type has been described by Woodhead, Taylor and S ~ h a g e n . ~

Cathode Space Zooming This type of zoom image intensifier is illustrated by an actual design example in Fig. 7. The design problem is more complicated because the system may not be considered as two independent parts. Study of the four-electrode system places greater demands on the computational techniques employed.

i

Cathod

M.051

M= 1.0

tangential focus. FIQ.7. Cathode space zoom tube showing image surfaces. 0, 0, sagittal focus.

The mode of use of a zoom tube is such that the output diameter remains fixed, while the input diameter varies inversely with M . This is illustrated by the two principal rays shown in the figure. Thus, the

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most exacting requirements on the lens occur a t low M , since it is in this condition that the beam diameter within the lens field is largest. Therefore, the initial diode design should be satisfactory a t low magnification; then as M increases, the beam diameter reduces, helping to minimize the effects of field distortions. If in addition the tube is made long, the geometrical aberrations a t high magnification will be further reduced, as was mentioned above. Thus the device shown in Fig. 7 is basically a low magnification diode of large focal length, the latter being achieved by the use of a large anode aperture. The limitation on focal length is set by resolution requirements, because the greater length is accompanied by reduced field a t the cathode, with corresponding increase in chromatic aberration. To form the zoom tube, two additional electrodes are introduced between cathode and anode. Their positions correspond approximately with equipotential surfaces, so that when operating a t low M the tube essentially reproduces the performance of the basic diode. The performance a t high M is dependent upon the choice of equipotentials. I n this condition the potential of G2 equals the anode potential so that GZ becomes essentially the anode of the tube, the final electrode being situated in field-free space. Thus, the diode formed by the cathode and GZ must achieve high magnification in a relatively short focal length, necessitating the use of a small aperture for G2. The field distortion necessary to achieve a focus by adjusting the potential of G1 is then minimized. These principles provide guide lines that help to optimize the tube performance overall, but much still depends on the location and detailed shaping of the electrodes. A trial and error study of these effects can be economically conducted by computation. The principal rays and image surfaces shown in Fig. 7 are typical results of such a process. I n this configuration the field a t the cathode is largely determined by V,,, so that a variation in limiting resolution approaching 4 : l might be expected over the magnification range shown. Referred to the output this variation would be only about 2 :1. On the other hand, curvature of the image varies quite markedly with M , with the result that edge resolution tends to fall as the center resolution rises. REFERENCES 1. Vine, J., IEEE Trans. Nucl. Sci. ED-13, 544 (1966). 2. Wreathall, W. M., I n “Advances in Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan and E. Kahan, Vol. 22A, p. 583. Academic Press, London (1966). 3. Morton, G. A. and Ramberg, E. G., Physics 7, 461 (1936). 4. Woodhead, A. W., Taylor, D. G. and Schagen, P., Philipe Tech. Rev. 25, 88 (1963).

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Electron Optics of a Photoconductive Image Converter M. E. BARNETT, C. W. BATES, Jr.t and L. ENGLAND Department of Applied Physics, Imperial College, University of London, England

INTRODUCTION The feasibility of direct view image tubes, using electron mirror read-out from infra-red sensitive photoconductive layers, is well known.la2 Figure 1 shows the simplest version of such a tube, reported some twenty years ago.2 Light falling on a photoconductive layer creates a potential relief on the surface facing the beam. The potential of the layer is held near gun-cathode potential in such a way that the layer acts as an electron mirror, the majority of the beam being reflected without striking the layer. The reflected beam is modulated by the potential relief, and an image is formed on the output phosphor screen. The conversion of the light signal into a potential relief has been quite well discussed in the original references, but the electron optics of the device has never been treated quantitatively and hence rational design and device assessment has not been possible.

GEOMETRICAL OPTICS Figure 2 shows equivalent electron-optical representations of an electron mirror tube. Approximate expressions for the important design parameters can be derived as follows. The mirror anode can be treated as a thin aperture lens, and the retarding field approximated by a uniform field in which the trajectories are parabolic. The focal length f of the aperture lens, treated as thin, is given from Hoeft’s modification of the small aperture formula3 by

t Academic Visitor at Imperial College, London. Associates, Pa10 Alto, California, U.S.A. 645

Permanent address: Varian

546

M. E. BARNETT, C. W. BATES, JR. AND L. ENGLAND

&

Glass objective

I

Fluorescent screen

Magnifying glass lens Light -optical mirror

FIG.1. Sohaffernicht’seleotron mirror tube.

where V A is the anode potential and V o the potential of the point at the centre of the anode hole, R is the radius of the anode hole and d the distance between the mirror and the mirror anode. V o is given by4

this equation being a good approximation for the range 0

< -Rd < 2-3‘

The anode aperture can thus be replaced (Pig. 2(b)) by a diverging lens of focal length 1--. (3) ( 3 Owing to the parabolic trajectories in the retarding field, the effective

f=4d

plane of reversal (virtual mirror plane) in the equivalent optical

ELECTRON OPTICS O F A PHOTOCONDUCTIVE IMAGE CONVERTER

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representation of Fig. 2(b) is a t a distance 2d from the anode plane. The combination of diverging lens and plane mirror can be replaced (Fig. 2(c)) by a convex mirror of focal length fm, whose principal plane lies behind the photoconductive mirror plane. It is quite easy to show geometrically that, within the range of Rld quoted above, the focal length of the mirror is well represented by

">

- 0.42-d ,

f,,,= :(l and that

(4)

h = f-.m 4

1;'

fl

1---

h

y-*----I

\

I I

I

I I I

-I-

I

I

I

(C)

Fm. 2. Equivalent representations of an electrostatic electron mirror.

Equation (4) becomes exact when Rld -+ 0, but for larger values of Rid is only accurate to about 10%. Using the equivalent representation of Fig. 2(b), the magnification y/x (Fig. 2(a)) can be shown to be 4d 4d 4d 4d I+-+m=Y=[$)[ 2 + - + z + 4z d f ) + ' ] _____ f L 2d X 2d l+-+l, I+-+-

i;"

f

L

f being given by Eq. (3). I n the tube of Pig. 1 the approximate values of the ratios were R -_ 5 _

_d - - 3

D

1 D - 12' L - 26' L a 2' implying an electron-optical magnification of about x 4.

_-

548

M. E . BARNETT, C. W. BATES, JR. AND L. ENGLAND

A further matter of interest is the question of what proportion of the output field of view is taken up by the aperture in the output screen which allows passage to the incident beam. This aperture, acting as the field stop for the system, needs t o have a radius ro just large enough to permit the whole output screen (radius yo)to be filled. The important relationship in this case is

which, for the geometry of the tube shown in Fig. 1, comes to about 13, implying that less than 1% of the field of view is lost. The limit on the useful field of view at the mirror is set by the magnitude of the transverse velocity components at the edge of the field. Transverse velocity components increase the distance of closest approach of the reflected electrons with the result that the image in the peripheral region is degraded when compared with that in the axial region. The ratio ofthe transverse energy V,eV to the beam energy VeV is given by

If an input field of view of 1-mm diameter is required in the geometry of Fig. 1, a transverse energy of about 2 eV is found for an electron reflected a t the edge of the field of view (beam energy 5 keV). This is clearly a great obstacle to obtaining a device of reasonable sensitivity over a useful field of view. It seems from Eq. ( 7 ) that a long device working at a low beam voltage is desirable. The situation can be improved somewhat by incorporating a weakly converging lens between the anode and screen, as in the test system described later.

IMAGE FORMATION Unlike conventional photoemissive tubes, the photosurface is not electron-optically conjugated with the output screen S. It is clear from Fig. 2(b) that such conjugation is not possible owing t o the presence of a diverging lens, since for such a lens, object and image space always coincide, whereas the photosurface and screen are on opposite sides of the lens. In fact, for the example under discussion, the screen is conjugated with a plane S' which lies between S and the anode aperture. Thus the electron density distribution a t the output is identical with the virtual electron density distribiition in S', apart from a scale factor due to magnification.

ELECTRON OPTICS O F A PHOTOCONDUCTIVE IMAGE CONVERTER

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The modulation of the reflected beam is caused by the transverse electric fields associated with the signal-induced potential relief on the photosurface. These cause angular deflexions in the electron trajectories which, projected over the distance zo (Fig. 2(b)), are seen as lateral displacements in the virtual object plane S‘. If the displacements of electrons in s’ due to the application of the light signal are small compared with the wavelength of the fundamental spatial harmonic of the light pattern, then a “differentiated” image is obtained, i.e. the electron density distribution in the output is determined by the first derivative of the transverse field at the mirror surface. A true differentiated image can only be obtained however if the image contrast is low, i.e. for small modulation. For large modulation when the displacement in S‘ is large compared with the spatial wavelength, electrons from different parts of the input field become mixed up, with the result that the image bears little apparent relation to the object and the device is effectively useless. Between the limits of large and small modulation the image suffers from a characteristic type of distortion6 in which the unilluminated areas appear larger in the output than they really are, and illuminated areas correspondingly smaller. Further unusual features of the mechanism of image formation are due to the fact that the modulation arises from transverse field components. At a given amplitude of the voltage variation, the magnitude of the transverse field increases with increasing spatial frequency. Thus it might be thought that the sensitivity of the device should actually increase at higher spatial frequencies. However, with increased spatial frequency the range of the field associated with the potential relief decreases so that an increased retarding field E is required to ensure that the slowest electrons approach near enough to the layer to experience the increased transverse field. The angular deflexion due to the potential relief is inversely proportional to E , and furthermore there is a practical upper limit on E . Thus the modulation transfer function (which only has meaning at low image contrast, where the device operates linearly) is moderately complicated. Analysis using a simplified theoretical model5 shows that at a given E , there is a spatial frequency at which the response is a maximum and conversely for a given spatial frequency the response can be maximized by choice of E . Fortunately the most favourable spatial frequency range for operating the device appears to be loa to lo3 lp/mm, which means that restrictions on the field of view are less damaging. It seems that in principle it may be possible at such frequencies to detect potential variations of millivolt amplitudes.6 The most fundamental drawback in the mechanism of image formation, however, lies in the nature of the restriction on the dynamic

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M. E . BARNETT, 0 .

W.

BATES, JR. AND L. ENGLAND

range for linear operation. Using the simplified model referred to above it can be shown that the condition that a pure spatial harmonic in the potential relief should give rise to a pure spatial harmonic in the output current density distribution is

V o being the amplitude and X the spatial wavelength of the potential relief and - V being the mean d.c. potential of the mirror surface. The problem here is that this condition contains the input parameters V , and A, which for arbitary inputs are unknown and distributed according to the Fourier spectrum of the input signal. Equation (8) implies that correct adjustment of the tube requires prior knowledge of the input! This is clearly a most undesirable feature in practice. A PRACTICAL TESTSYSTEM Figure 3 shows a demountable system which has been built in order to investigate the electron optics of the photoconductive image converter. A weak magnetic lens is included which provides a means of magnification control. An important practical consideration is the choice of a suitable photoconductive layer. The layer surface needs to be very smooth, otherwise the effect of surface topography appears in the output image. It should have a thickness of the order of one micron and its resistivity should be not less than l O W cm. These requirements appear to be fulfilled by a mixture of selenium and bismuth in the proportion 95% Se, 5% Bi by atomic eight.^ This combination yields a layer having a peak response at a wavelength of about 1 pm and a long wavelength limit at about 1.6 pm.7 Co-evaporatorr gives a vitreous tion of these elements at a pressure of lod6to layer. Selenium-bismuth layers of this type have been prepared on conducting substrates and have proved to be suitable for use in the test system. The photoconductivity of these layers has not been measured directly but absorption measurements indicate that the bismuth content shifts the long wavelength cut-off for the response of selenium (normally N 7000 8 )into the infra-red as expected. Using the demountable system and evaporated photoconductive layers of the type described it has been possible to obtain image conversion in the infra-red, the filter employed at the input having a narrow pass band centred on 9600 8. Typically, the beam energy is 5 keV and the layer substrate is held at about lOV positive with respect to the cathode, the beam current being adjusted so that the layer surface exposed to the besm stabilizes

ELECTRON OPTICS OF A PHOTOCONDUCTIVE IMAGE CONVERTER

551

+

a t a potential of to 1 V negative with respect t o the cathode. The image obtained using a test grid pattern shows the predicted form of distortion. The build-up of insulating contamination due to the poor vacuum interferes with the proper functioning of the layer, and it seems I Infra-red filter

I

I

Quartz window

A, -

pqq-

Se-Bi photoconductor

Magnetic lens

I

I

I

Viewing window

Phosphor Screen

I

I I

I

FIQ.3. Schematic diagram of demountable infra-red converter.

likely that prolonged tests of the image converter require a sealed-off system. The preceding analysis of the electron optics will enable us t o make a rational choice of geometry for such a system,

REFERENCES 1. Orthuber, R., 2. Angew. Phys. 1, 79 (1948). 2. Schaffernicht, W., “Fiat Review of German Science, Electronics”, Vol. 1, p. 100 (1948). 3. Hoeft, J., 2. Angew. Phys. 11, 380 (1969). 4. Fry, T. C., Amer. Math. iVonthZy 39, 199 (1932). 6. Barnett, M. E. and England, L., Optik 27, 341 (1968). 6. Barnett, M. E., AppZ. Phys. Letter8 12, 229 (1968). 7. Schottiniller, J. C., Bowman, D. L. and Wood, C., J . AppZ. P h p . 39, 1663 (1968).

552

M. E. BARNETT, C. W. BATES, JR. AND L. ENGLAND

DIscussIoN s. JEFFERS: By how much does the addition of bismuth extend the long wavelength cut-off of the layer? L. ENOLAND: Addition of more than 6% of bismuth (by atomic weight) extends the long-wavelength cut-off to approximately 1.6 pm. The wavelength of maximum response does not change significantly from 1 pm for concentrations of bismuth up to 30% by atomic weight.