Techniques for Nuclear and Particle Physics Experiments

Transcription

1 Techniques for Nuclear and Particle Physics Experiments

2 William R. Leo Techniques for Nuclear and Particle Physics Experiments A How-to Approach Second Revised Edition With 256 Figures, 40 Tables and Numerous Worked Examples Springer-Verlag Berlin Heidelberg GmbH

3 Dr. William R. Leo Route de St. Maurice 34. CH-1814 La Tour de Peilz Switzerland ISBN ISBN (ebook) DOI / Library of Congress Cataloging-in-Publication Data. Leo. William R Techniques for nuciear and particle physics experiments: a how-to approach 1 William R. Leo. - 2nd rev. ed. p. cm. Includes bibliographical references and index. ISBN I. Particles (Nuclear physics)- Technique. 2. Particles (Nuclear physics)- Experiments. 3. Nuclear physics-technique. 4. Nuclear physics-experiments. 5. Nuclear counters. I. Title. QC L '2'078-dc This work is subject to copyright. All rights are reserved. whether the whole or part of the material is concerned. specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way. and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September in its current version. and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. Springer-Verlag Berlin Heidelberg 1987, 1994 Originally published by Springer-Verlag Berlin Heidelberg New York in 1994 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: K+V Fotosatz GmbH, Beerfelden SPIN: / Printed on acid-free paper

4 To my wife Elisabeth for her love and encouragement

5 Preface to the Second Edition Not quite six years have passed since the appearance of the first edition of this book. This is not a long period. Yet the rapid pace of scientific and technological development today is such that any book on experimental technique must be wary of becoming obsolete in some way or another even in such a short span of time. Thus, when the publisher Springer-Verlag informed me of the need for a new printing of this book, I decided it was an opportune moment to update some of the chapters as well as to include some new material. The result is this second edition. The most notable changes have been in Chapters 2 and 3. In the latter, which concerns radiation protection, most of the sections have been rewritten to take into account the new recommendations from the International Commission on Radiation Protection, the most important of which are the new dose limits for exposure to ionizing radiation. In addition, emphasis has now been put on the use of SI units in dosimetry, i.e., the Gray and Sievert, which have now become standard. In Chapter 2, new material has been added in addition to updated information. In particular, Cherenkov radiation and electron-photon shower production are now treated more thoroughly. These are not phenomena normally encountered in a student laboratory, but they are presented here so as to provide a foundation for understanding detectors based on these effects. Hopefully this will increase the usefulness of this book especially for those entering high-energy physics. The section on multiple scattering in the Gaussian approximation has also been updated with a new and more accurate empirical formula. Throughout these chapters and indeed the entire book, an updating of the references has also been made. During this period, of course, many new papers and books on various experimental techniques have appeared, most of a very specific nature. I have had to be selective therefore and have included only those which bear directly on the more general aspects of a technique or method, or provide new data. However, I can in no way claim to have included all possible new references and I apologize for those that I have missed. Finally, I have included a number of new examples in the text which I hope will enhance understanding of the material. Like the rest of the examples, these are all based on real problems which have been encountered either by myself or by students that I have taught. That I am writing this preface to the second edition at all is a pleasant surprise for me as it attests to the success of the first edition. For this I am infinitely grateful to the people who have helped with its realization, not the least of which are the readers who have written to me with their comments, suggestions and corrections to the first edition. Where possible, I have tried to incorporate these in one way or another in this edition. Hopefully, I have not disappointed them. Last but not least, my deepest gratitude is due to Prof. Catherine Leluc for her invaluable aid and advice once again, and to my wife and children for their infinite patience. La Tour de Peilz, November 1993 William R. Leo

6 Preface to the First Edition This book is an outgrowth of an advanced laboratory course in experimental nuclear and particle physics the author gave to physics majors at the University of Geneva during the years The course was offered to third and fourth year students, the latter of which had, at this point in their studies, chosen to specialize in experimental nuclear or particle physics. This implied that they would go on to do a "diplom" thesis with one of the high- or intermediate-energy research groups in the physics department. The format of the course was such that the students were required to concentrate on only one experiment during the trimester, rather than perform a series of experiments as is more typical of a traditional course of this type. Their tasks thus included planning the experiment, learning the relevant techniques, setting up and troubleshooting the measuring apparatus, calibration, data-taking and analysis, as well as responsibility for maintaining their equipment, i.e., tasks resembling those in a real experiment. This more intensive involvement provided the students with a better understanding of the experimental problems encountered in a professional experiment and helped instill a certain independence and confidence which would prepare them for entry into a research group in the department. Teaching assistants were present to help the students during the trimester and a series of weekly lectures was also given on various topics in experimental nuclear and particle physics. This included general information on detectors, nuclear electronics, statistics, the interaction of radiation in matter, etc., and a good deal of practical information for actually doing experiments. Many of the chapters in this book are essentially based on notes which were prepared for these lectures. The information contained in this book, therefore, will hopefully provide the reader with a practical "guide" to some of the techniques, the equipment, the technical jargon, etc., which make up the world of current experimental nuclear and particle physics but which never seem to appear in the literature. As those in the field already know, the art of experimental physics is learned through a type of "apprenticeship" with a more experienced physicist or physicists, not unlike medieval artisans. It is to these "apprentices" that I address these chapters. The book is laid out in three parts. The first four chapters treat some of the fundamental background knowledge required of experimental nuclear or particle physicists, such as the passage of radiation through matter, statistics, and radiation protection. Since detailed descriptions of the theory can be found elsewhere, these chapters only summarize the basic ideas and present only the more useful formulae. However, references are provided for the reader desiring more information. In this form, then, these chapters may serve as a reference. A basic understanding of quantum mechanics and fundamental nuclear physics is assumed throughout. Chapters 5-10 are primarily concerned with the functioning and operation of the principal types of detectors used in nuclear and particle physics experiments. In addition to the basic principles, sections dealing with modern detectors such as the time-

7 x Preface to the First Edition projection chamber or silicon microstrip detectors have also been included. It might be argued, of course, that some of these detectors are too specialized or still too novel to be included in a textbook of this level. However, for the student going on to more advanced work or the experienced researcher, it is these types of detectors he will most likely encounter. Moreover, it gives the student an idea of the state of the art and the incredible advances that have been made. Hopefully, it will provide food for thought on the advances that can still be made! The final chapters, 11-18, are concerned with "nuclear electronics" and the logic which is used in setting up electronics systems for experiments. This has always been a difficult point for many students as most approaches have been from a circuit design point of view requiring analysis of analog circuits, which, of course, is a subject unto itself. With the establishment of standardized systems such as NIM and CAMAC and the availability of commercial modules, however, the experimental physicist can function very well with only a knowledge of electronic logic. These chapters thus treat the characteristics of the pulse signals from detectors and the various operations which can be performed on these signals by commercially available modules. Chapter 18 also presents an introduction to the CAMAC system, which, up to a few years ago, was used only in high-energy physics but which now, with the advent of microcomputers, may also be found on smaller experiments undertaken by students. Although this book is based on a specific laboratory course, the treatment of the topics outlined above is general and was made without specific reference to any particular experiment, except, perhaps, as an example. As such I hope the book will also be of use to researchers and students in other domains who are called upon to work with detectors and radiation. I would like to thank the many people who have at some point or another helped realize this book. In particular, my very special thanks are due to Dr. Rene Hausammann, Dr. Catherine Lechanoine-Leluc, Dr. Jacques Ligou, and Dr. Trivan Pal for having read some of the chapters and for their helpful comments and suggestions. I am also grateful to J.-C. Bostedeche and J. Covillot who helped construct, establish and maintain the experiments in the laboratory, to C. Jacquat for the many hours spent on the drawings for this book and to the many authors who have kindly allowed me to use figures from their articles or books. Finally, I would like to thank Elisabeth, who, although not a physicist, was the first to have the idea for this book. Lausanne, March 1987 William R. Leo

8 Contents 1. Basic Nuclear Processes in Radioactive Sources Nuclear Level Diagrams Alpha Decay Beta Decay Electron Capture (EC) Gamma Emission Isomeric States Annihilation Radiation Internal Conversion Auger Electrons Neutron Sources Spontaneous Fission Nuclear Reactions Source Activity Units The Radioactive Decay Law Fluctuations in Radioactive Decay Radioactive Decay Chains Radioisotope Production by Irradiation Passage of Radiation Through Matter Preliminary Notions and Definitions The Cross Section Interaction Probability in a Distance x. Mean Free Path Surface Density Units Energy Loss of Heavy Charged Particles by Atomic Collisions Bohr's Calculation - The Classical Case The Bethe-Bloch Formula Energy Dependence Scaling Laws for de/dx Mass Stopping Power de/dx for Mixtures and Compounds Limitations of the Bethe-Bloch Formula and Other Effects Channeling Range Cherenkov Radiation Energy Loss of Electrons and Positrons Collision Loss Energy Loss by Radiation: Bremsstrahlung Electron-Electron Bremsstrahlung Critical Energy... 40

9 XII Contents Radiation Length Range of Electrons The Absorption of fj Electrons Multiple Coulomb Scattering Multiple Scattering in the Gaussian Approximation Backscattering of Low-Energy Electrons Energy Straggling: The Energy Loss Distribution Thick Absorbers: The Gaussian Limit Very Thick Absorbers Thin Absorbers: The Landau and Vavilov Theories The Interaction of Photons Photoelectric Effect Compton Scattering Pair Production Electron-Photon Showers The Total Absorption Coefficient and Photon Attenuation., The Interaction of Neutrons Slowing Down of Neutrons. Moderation Radiation Protection. Biological Effects of Radiation Dosimetric Units The Roentgen Absorbed Dose Relative Biological Effectiveness (RBE) Equivalent Dose Effective Dose Typical Doses from Sources in the Environment Biological Effects High Doses Received in a Short Time Low-Level Doses Dose Limits Shielding Radiation Safety in the Nuclear Physics Laboratory Statistics and the Treatment of Experimental Data Characteristics of Probability Distributions Cumulative Distributions Expectation Values Distribution Moments. The Mean and Variance The Covariance Some Common Probability Distributions The Binomial Distribution The Poisson Distribution The Gaussian or Normal Distribution The Chi-Square Distribution Measurement Errors and the Measurement Process Systematic Errors Random Errors Sampling and Parameter Estimation. The Maximum Likelihood Method 91

10 Contents XIII Sample Moments The Maximum Likelihood Method Estimator for the Poisson Distribution Estimators for the Gaussian Distribution The Weighted Mean Examples of Applications Mean and Error from a Series of Measurements Combining Data with Different Errors Determination of Count Rates and Their Errors Null Experiments. Setting Confidence Limits When No Counts Are Observed Distribution of Time Intervals Between Counts Propagation of Errors Examples Curve Fitting The Least Squares Method Linear Fits. The Straight Line Linear Fits When Both Variables Have Errors Nonlinear Fits Some General Rules for Rounding-off Numbers for Final Presentation General Characteristics of Detectors Sensitivity Detector Response Energy Resolution. The Fano Factor The Response Function Response Time Detector Efficiency Dead Time Measuring Dead Time Ionization Detectors Gaseous Ionization Detectors Ionization and Transport Phenomena in Gases Ionization Mechanisms Mean Number of Electron-Ion Pairs Created..., Recombination and Electron Attachment Transport of Electrons and Ions in Gases Diffusion Drift and Mobility Avalanche Multiplication The Cylindrical Proportional Counter Pulse Formation and Shape Choice of Fill Gas The Multiwire Proportional Chamber (MWPC) Basic Operating Principle Construction Chamber Gas

11 XIV Contents Timing Resolution Readout Methods Track C l u ~... e r s MWPC Efficiency The Drift Chamber Drift Gases Spatial Resolution Operation in Magnetic Fields The Time Projection Chamber (TPC) Liquid Ionization Detectors (LID) Scintillation Detectors General Characteristics Organic Scintillators Organic Crystals Organic Liquids Plastics Inorganic Crystals Gaseous Scintillators Glasses Light Output Response Linearity Temperature Dependence Pulse Shape Discrimination (PSD) Intrinsic Detection Efficiency for Various Radiations Heavy Ions Electrons Gamma Rays Neutrons Photomultipliers Basic Construction and Operation The Photocathode The Electron-Optical Input System The Electron-Multiplier Section Dynode Configurations Multiplier Response: The Single-Electron Spectrum Operating Parameters Gain and Voltage Supply Voltage Dividers Electrode Current. Linearity Pulse Shape Time Response and Resolution Noise Dark Current and Afterpulsing Statistical Noise Environmental Factors Exposure to Ambient Light Magnetic Fields

12 Contents XV Temperature Effects...,..., Gain Stability, Count Rate Shift Scintillation Detector Mounting and Operation Light Collection Reflection Coupling to the PM Multiple Photomultipliers Light Guides Fluorescent Radiation Converters Mounting a Scintillation Detector: An Example Scintillation Counter Operation Testing the Counter Adjusting the PM Voltage The Scintillation Counter Plateau Maintaining PM Gain Semiconductor Detectors Basic Semiconductor Properties Energy Band Structure Charge Carriers in Semiconductors Intrinsic Charge Carrier Concentration Mobility Recombination and Trapping Doped Semiconductors Compensation The np Semiconductor Junction. Depletion Depth The Depletion Depth Junction Capacitance Reversed Bias Junctions Detector Characteristics of Semiconductors Average Energy per Electron-Hole Pair Linearity The Fano Factor and Intrinsic Energy Resolution Leakage Current Sensitivity and Intrinsic Efficiency Pulse Shape. Rise Time... " Silicon Diode Detectors Diffused Junction Diodes Surface Barrier Detectors (SSB) Ion-Implanted Diodes Lithium-Drifted Silicon Diodes - Si(Li) Position-Sensitive Detectors Continuous and Discrete Detectors Micro-Strip Detectors Novel Position-Sensing Detectors Germanium Detectors Lithium-Drifted Germanium - Ge(Li) Intrinsic Germanium

13 XVI Contents Gamma Spectroscopy with Germanium Detectors Other Semiconductor Materials Operation of Semiconductor Detectors Bias Voltage Signal Amplification Temperature Effects Radiation Damage Plasma Effects Pulse Signals in Nuclear Electronics Pulse Signal Terminology Analog and Digital Signals Fast and Slow Signals The Frequency Domain. Bandwidth The NIM Standard Modules Power Bins NIM Logic Signals TTL and ECL Logic Signals Analog Signals Signal Transmission 13.1 Coaxial Cables Line Constituents The General Wave Equation for a Coaxial Line The Ideal Lossless Cable Characteristic Impedance Reflections Cable Termination. Impedance Matching Losses in Coaxial Cables. Pulse Distortion Cable Response. Pulse Distortion Electronics for Pulse Signal Processing Preamplifiers Resistive vs Optical Feedback Main Amplifiers Pulse Shaping Networks in Amplifiers CR-RC Pulse Shaping Pole-Zero Cancellation and Baseline Restoration Double Differentiation or CR-RC-CR Shaping Semi-Gaussian Shaping Delay Line Shaping Biased Amplifiers Pulse Stretchers Linear Transmission Gate Fan-out and Fan-in Delay Lines Discriminators

14 Contents XVII Shapers Single-Channel Analyzer (Differential Discriminator) Analog-to-Digital Converters (ADC or AID) ADC Linearity Multichannel Analyzers Digital-to-Analog Converters (DAC or DI A) Time to Amplitude Converters (TAC or TPHC) Scalers Ratemeter Coincidence Units Majority Logic Units Flip-Flops Registers (Latches) Gate and Delay Generators Some Simple and Handy Circuits for Pulse Manipulation Attenuators Pulse Splitting Pulse Inversion Filtering and Shaping Pulse Clipping..., High-Pass Filter or CR Differentiating Circuit RC Low-Pass Filter or Integrating Circuit Pulse Height Selection and Coincidence Technique A Simple Counting System Pulse Height Selection SCA Calibration and Energy Spectrum Measurement..., A Note on Calibration Sources Pulse Height Spectroscopy with Multichannel Analyzers Basic Coincidence Technique Adj usting the Delays. The Coincidence Curve Adjusting Delays with the Oscilloscope Accidental Coincidences Combining Pulse Height Selection and Coincidence Determination. The Fast-Slow Circuit Pulse Shape Discrimination Electronic Logic for Experiments Basic Logic Gates: Symbols Boolean Laws and Identities The Inhibit or Busy Triggers One-Body Scattering Two-Body Scattering Measurement of the Muon Lifetime Timing Methods and Systems Walk and Jitter Time-Pickoff Methods

15 XVIII Contents Leading Edge Triggering (LE) Fast Zero-Crossing Triggering Constant Fraction Triggering (CFT) Amplitude and Risetime Compensated Triggering (ARC) Analog Timing Methods The START-STOP Time-to-Amplitude Converter Time Overlap TAC's Digital Timing Methods The Time-to-Digital Converter (TDC) The Vernier TDC Calibrating the Timing System Computer Controlled Electronics: CAMAC CAMAC Systems The CAMAC Standard Mechanical Standards Electrical Standards: Digital Signals The CAMAC Dataway Common Control Signals (Z,C,I) Status Signals Timing Signals Data Signals Address Signals Command Signals Pin Allocations Dataway Operations Dataway Timing Block Transfers Multi-Crate Systems - The Branch Highway CAMAC Software Appendix A. A Review of Oscilloscope Functions A.l Basic Structure A.l.1 Bandwidth and Risetime A.2 Controls and Operating Modes A.2.1 Input Coupling A.2.2 Vertical and Horizontal Sensitivity A.2.3 Triggering (Synchronization) A.2.4 Display Modes A.3 Applications and Examples A.3.1 Signal Viewing A.3.2 Comparison of Signals B. Physical and Numerical Constants C. Resistor Color Code References Subject Index