Contents 1. lntroduction............................................................... 3 Early development, photoemitter and secondary-emitter development, applications development, photomultiplier and solid-state detectors compared 2. Photomultiplier Design . , . 10 Photoemission, practical photocathode materials, opaque and semitransparent photocathodes, glass transmission and spectral response, thermionic emission, secondary emission, time tag in photoemission and secondary emission 3. Electron Optics of Photomultipliers , . 26 Electron-optical design considerations, design methods for photomultiplier electron optics, specific photomultiplier electron-optical configurations, anode configurations 4. Photomultiplier Characteristics . , . 36 Photocathode-related characteristics, gain-related characteristics, dark current and noise, time effects, pulse counting, scintillation counting, liquid scintillation counting, environmental effects 5. Photomultiplier Applications . 80 Summary of selection criteria, applied voltage considerations, mechanical considerations, optical considera- tions, specific photomultiplier applications Appendix A. Typical Photomultiplier Applications and Selection Guide . 119 Appendix B.Glossary of Terms Related to Photomultiplier lubes and Their Applica- I . 125 Appendix C.Spectral Response Designation Systems . 132 Appendix D. Photometric Units and Photometric-to-Radiant Conversion............. I 37 Appendix E.Spectral Response and Source-Detector Matching......... .......... I 43 Appendix F.Radiant Energy and Sources......................................148 Appendix G. Statistical Theory of Noise in Photomultiplier Tubes. I 60 Index........................................................................ 177 information furnished by BURLE INDUSTRIES, INC. is believed to be accurate and reliable. However, no responsibility of liability is assumed by BURLE for its use, nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or other rights of BURLE INDUSTRIES, INC. BURLE ® and BURLE INDUSTRIES, INC.® are registered trademarks of BURLE TECHNOLOGIES, INC. Marca(s) Registrada(s). Copyright © 1980 by BURLE TECHNOLOGIES, INC. All rights reserved. No part of this book may be copied or reproduced in any form or by any means -- graphic, electronic, or mechanical or incorporated into any information storage or retrieval system, without the written permission of the copyright owner. Supersedes PMT-62, 8-80 Printed in U.S.A./ 10-89 TP-136 2 1. Introduction The photomultiplier is a very versatile and without need for additional signal amplifica- sensitive detector of radiant energy in the tion. Extremely fast time response with rise ultraviolet, visible, and near infrared regions times as short as a fraction of a nanosecond of the electromagnetic spectrum. A schemat- provides a measurement capability in special ic diagram of a typical photomultiplier tube applications that is unmatched by other is given in Fig. 1. The basic radiation sensor radiation detectors. is the photocathode which is located inside a vacuum envelope. Photoelectrons are emit- ted and directed by an appropriate electric EARLY DEVELOPMENT field to an electrode or dynode within the The development (history) of the photo- envelope. A number of secondary electrons multiplier is rooted in early studies of secon- are emitted at this dynode for each imping- dary emission. In 1902, Austin and Starke1 ing primary photoelectron. These secondary reported that the metal surfaces impacted by electrons in turn are directed to a second cathode rays emitted a larger number of elec- dynode and so on until a final gain of trons than were incident. The use of secon- perhaps 106 is achieved. The electrons from dary emission as a means for signal the last dynode are collected by an anode amplification was proposed as early as which provides the signal current that is read 1919.2 In 1935, Iams and Salzberg 3 of RCA out. reported on a single-stage photomultiplier. The device consisted of a semicylindrical photocathode, a secondary emitter mounted on the axis, and a collector grid surrounding the secondary emitter. The tube had a gain of about eight. Because of its better frequen- cy response the single-stage photomultiplier was intended for replacement of the gas- filled phototube as a sound pickup for movies. But despite its advantages, it saw only a brief developmental sales activity PHOTOELECTRONS before it became obsolete. 92cs-32288 Multistage Devices Fig. 1 - Schematic representation of a photo- multiplier tube and its operation In 1936, Zworykin, Morton, and Malter, all of RCA4 reported on a multistage photomultiplier. Again, the principal con- For a large number of applications, the templated application was sound-on-film photomultiplier is the most practical or sen- pickup. Their tube used a combination of sitive detector available. The basic reason electrostatic and magnetic fields to direct for the superiority of the photomultiplier is electrons from stage to stage. A photograph the secondary-emission amplification that of a developmental sample is given in Fig. 2. makes it possible for the tube to approach Although the magnetic-type photomultiplier “ideal” device performance limited only by provided high gain, it had several dif- the statistics of photoemission. Amplifica- ficulties. The adjustment of the magnetic tions ranging from 103 to as much as 108 field was very critical, and to change the gain provide output signal levels that are com- by reducing the applied voltage, the patible with auxiliary electronic equipment magnetic field also had to be adjusted. 3 Photomultiplier Handbook Another problem was that its rather wide Snyder 8 and by Janes and Gloverg, all of open structure resulted in high dark current RCA. The basic electron-optics of the cir- because of feedback from ions and light cular cage was thus well determined by 1941 developed near the output end of the device. and has not changed to the present time For these reasons, and because of the although improvements have been made in development of electrostatically focused processing, construction, and performance photomultipliers, commercialization did not of the 931A product. follow. The success of the 931 type also resulted from the development of a much improved 10 photocathode, Cs3Sb, reported by Gorlich in 1936. The first experimental photo- multipliers had used a Ag-O-Cs photocath- ode having a typical peak quantum efficien- cy of 0.4% at 800 nm. (The Ag-O-Cs layer Fig. 2 - Magnetic-type multistage photomul- was also used for the dynodes.) The new tiplier reported by Zworykin, Morton, and Cs3Sb photocathode had a quantum effi- Malter in 1936. ciency of 12% (higher today) at 400 nm. It was used in the first 931’s, both as a photocathode and as a secondary-emitting material for the dynodes. The design of multistage electrostatically focused photomultipliers required an analysis of the equipotential surfaces be- tween electrodes and of the electron trajec- PHOTOEMITTER AND SECONDARY- tories. Before the days of high-speed com- EMITTER DEVELOPMENT puters, this problem was solved by a Photocathode Materials mechanical analogue: a stretched rubber membrane. By placing mechanical models of Much of the development work on the electrodes under the membrane, the photomultiplier tubes has been concerned height of the membrane was controlled and with their physical configuration and the corresponded to the electrical potential of related electron optics. But a very important the electrode. Small balls were then allowed part of the development of photomultiplier to roll from one electrode to the next. The tubes was related to the photocathode and trajectories of the balls were shown to cor- secondary-emission surfaces and their pro- respond to those of the electrons in the cor- cessing. RCA was very fortunate during the responding electrostatic fields. Working 1950’s and 60’s in having on its staff, prob- with the rubber-dam analogue, both J.R. ably the world’s foremost photocathode ex- Pierce5 of Bell Laboratories and J.A. pert, Dr. A.H. Sommer. His treatise on Rajchman6 of RCA devices linear arrays of Photoemissive Materials1 1 continues to pro- electrodes that provided good focusing prop- vide a wealth of information to all erties. Although commerical designs did not photocathode process engineers. result immediately from the linear dynode Sommer explored the properties of array, The Rajchmann design with some numerous photocathode materials-par- modifications eventually was, and still is, ticularly alkali-antimonides. Perhaps his used in photomultipliers-particularly for most noteworthy contribution was the high-gain wide-bandwidth requirements. multialkali photocathode (S-20 spectral response). This photocathode, Na2KSb:Cs, First Commercial Devices is important because of its high sensitivity in The first commercially successful the red and near infrared; the earlier Cs3Sb photomultiplier was the type 931. This tube photocathode spectral response barely ex- had a compact circular array of nine tends through the visible, although it is very dynodes using electrostatic focusing. The sensitive in the blue where most scintillators first such arrangement was described by emit. Zworykin and Rajchman.7 Modifications Bialkali photocathodes were also de- were later reported by Rajchmann and veloped by Sommer and have proven to be 4 Introduction better in some applications than the Cs3Sb