Practical Gamma-ray Spectrometry

Transcription

1 Practical Gamma-ray Spectrometry 2nd Edition Gordon R. Gilmore Nuclear Training Services Ltd Warrington, UK John Wiley & Sons, Ltd

2 Contents Preface to the Second Edition xv A source of generic information Preface to the First Edition xvii Internet Resources within the Book xix 23 1 Radioactive Decay and the Origin of 2 Interactions of Gamma Radiation with Gamma and X-Radiation 1 Matter Introduction Introduction Beta Decay Mechanisms of Interaction [3- or negatron decay Photoelectric absorption ß+ or positron decay Compton scattering Electron capture (EC) Pair production Multiple stable isotopes Total Attenuation Coefficients Alpha Decay Interactions within the Detector Spontaneous Fission (SF) The very large detector Minor Decay Modes The very small detector Gamma Emission The 'real' detector The electromagnetic spectrum Summary Some properties of nuclear 2.5 Interactions within the Shielding 33 transitions Photoelectric interactions Lifetimes of nuclear energy Compton scattering 34 levels Pair production Width of nuclear energy levels Bremsstrahlung Internal conversion Attenuation of Gamma Radiation Abundance, yield and 2.8 The Design of Detector Shielding 36 emission probability Ambiguity in assignment of 38 nuclide identity Other Sources of Photons 12 3 Semiconductor Detectors for Gamma-Ray Annihilation radiation 12 Spectrometry Bremsstrahlung Introduction Prompt gammas Semiconductors and Gamma-Ray X-rays 13 Detection The Mathematics of Decay and The band structure of solids 40 Growth of Radioactivity Mobility of holes The decay equation Creation of charge carriers by Growth of activity in reactors 16 gamma radiation Growth of activity from decay Suitable semiconductors for The Chart of the Nuclides Newer semiconductor materials A source of nuclear data The Nature of Semiconductors 43 of a parent 17 gamma-ray detectors

3 viii Contents 3.4 The Manufacture of Germanium Detectors Introduction The manufacturing process Lithium-drifted detectors The detector configurations available Absorption in detector caps and dead layers Detectors for low-energy measurements Well detectors 3.5 Detector Capacitance Microphonic noise 3.6 Charge Collection in Detectors Charge collection time Shape of the detector pulse Timing signals from germanium detectors Electric field variations across the detector Removing weak field regions from detectors Trapping of charge carriers Radiation damage 3.7 Packaging of Detectors Construction of the detector mounting Exotic detectors Loss of coolant Demountable detectors Customer repairable detectors Electrical cooling of detectors 4 Electronics for Gamma -Ray Spectrometry 4.1 The General Electronic System Introduction Electronic noise and its implications for spectrum resolution Pulse shapes in gamma spectrometry systems Impedance inputs and outputs The impedance of cabling Impedance matching 4.2 Detector Bias Supplies 4.3 Preamplifiers Resistive Feedback preamplifiers Reset preamplifiers The noise contribution of 45 preamplifiers The rise time of 45 preamplifiers Amplifiers and Pulse Processors The functions of the 47 amplifier Pulse shaping The Optimum pulse shape The Optimum pulse shaping 49 time constant The gated integrator 49 amplifier Pole-zero cancellation Baseline shift Pile-up rejection Amplifier gain and overview 4.5 Resolution Enhancement New semiconductor materials Multichannel Analysers and their Analogue-to-Digital Converters Introduction Pulse range selection The ADC input gate The ADC MCA conversion time and 55 dead time Choosing an ADC Linearity in MCAs Optimum spectrum size MCA terms and definitions Arrangement of the MCA 59 function Simple MCA analysis functions Live Time Correction and Loss-Free 61 Counting Live time clock correction The Gedcke Haie method Use of a pulser Loss-free counting (LFC) MCA throughput Spectrum Stabilization Analogue stabilization Digital stabilization Coincidence and Anticoincidence 66 Gating Multiplexing and Multiscaling 4.11 Digital Pulse Processing Systems

4 Contents ix 5 Statistics of Counting Resolution: Origins and Control Introduction 101 Introduction Statistical statements Counting Distributions The binomial distribution The Poisson and Gaussian distributions Sampling Statistics Confidence limits Combining the results from different measurements Propagation of uncertainty Peak Area Measurement Simple peak integration Peaked-background correction Optimizing Counting Conditions Optimum background width Optimum spectrum size Optimum counting time Counting Decision Limits Critical limit (L c) Spectrometer Calibration Upper limit (Lv ) 116 Introduction Confidence limits Detection limit (4) Determination limit (L Q) Other calculation options Minimum detectable activity (MDA) Uncertainty of the (Lv) and MDA An example by way of summary Special Counting Situations Non-Poisson counting Low numbers of counts Non-Poisson statistics due to pile-up rejection and loss-free counting Uncertainty Budgets Introduction Accuracy and precision Types of uncertainty Types of distribution Uncertainty on sample preparation Counting uncertainties Calibration uncertainties An example of an uncertainty budget Charge Production to p Germanium versus silicon Germanium versus sodium iodide Temperature dependence of resolution 6.3 Charge Collection cou Mathematical form of cou 6.4 Electronic Noise Parallel noise Series noise Flicker noise Total electronic noise and shaping time 6.5 Resolving the Peak Width Calibration References 7.2 Reference Data for Calibration 7.3 Sources for Calibration 7.4 Energy Calibration Errors in peak energy determination 7.5 Peak Width Calibration Factors affecting peak width Algorithms for peak width estimation Estimation of the peak height Anomalous peak widths 7.6 Efficiency Calibration Which efficiency? Full-energy peak efficiency Are efficiency calibration curves necessary? The effect of source-to-detector distance Calibration errors due to difference in sample geometry An empirical correction for sample height Effect of source density on efficiency Efficiency loss due to random summing (pile-up) True coincidence summing Corrections for radioactive decay

5 x Contents Electronic timing problems 7.7 Mathematical Efficiency Calibration ISOCS LabSOCS Other programs 8 True Coincidence Summing 8.1 Introduction 8.2 The Origin of Summing 8.3 Summing and Solid Angle 8.4 Spectral Evidence of Summing 8.5 Validity of Close Geometry Calibrations Efficiency calibration using QCYK mixed nuclide sources 8.6 Summary 8.7 Summing in Environmental Measurements 8.8 Achieving Valid Close Geometry Efficiency Calibrations 8.9 TCS, Geometry and Composition 8.10 Achieving `Summing-free' measurements Using the `interpolative fit' to correct for TCS Comparative activity measurements Using correction factors derived from efficiency calibration curves Correction of results using `bodged' nuclear data 8.11 Mathematical Summing Corrections 8.12 Software for Correction of TCS GESPECOR Calibrations using summing nuclides TCS correction in spectrum analysis programs 9 Computer Analysis of Gamma-Ray Spectra 9.1 Introduction 9.2 Methods of Locating Peaks in the Spectrum Using regions-of-interest Locating peaks using 160 channel differences Derivative peak searches Peak searches using 162 correlation methods Checking the acceptability 163 of peaks Library Directed Peak Searches Energy Calibration Estimation of the Peak Centroid Peak Width Calibration Determination of the Peak Limits Using the width calibration Individual peak width 168 estimation Limits determined by a moving average minimum Measurements of Peak Area Full Energy Peak Efficiency Calibration Multiplet Peak Resolution by Deconvolution Peak Stripping as a Means 172 of Avoiding Deconvolution The Analysis of the Sample Spectrum Peak location and 175 measurement Corrections to the peak 175 area for peaked background Upper limits and minimum detectable activity Comparative activity estimations Activity estimations using 176 efficiency curves Corrections independent 179 of the spectrometer Nuclide Identification Simple use of look-up tables Taking into account other 180 peaks The Final Report Setting Up Nuclide and Gamma-Ray Libraries Buying Spectrum Analysis Software The Spectrum Analysis Programs Referred to in the Text

6 Contents xi 10 Scintillation Spectrometry 10.1 Introduction The Scintillation Process 205 Introduction 10.3 Scintillation Activators 10.4 Life time of Excited States Temperature Variation of the Scintillator Response 10.6 Scintillator Detector Materials Sodium iodide NaI(T1) Bismuth germanate BGO Caesium iodide CsI(T1) and CsI(Na) Undoped caesium iodide CsI Barium fluoride BaF Caesium fluoride CsF Lanthanum halides LaC1 3(Ce) and LaBr3 (Ce) Other new scintillators 10.7 Photomultiplier Tubes 10.8 The Photocathode The Dynode Electron Multiplier Chain Photodiode Scintillation Detectors Construction of the Complete Detector Detector shapes Optical coupling of the scintillator to the photomultiplier The Resolution of Scintillation Systems Statistical uncertainties in the detection process Factors associated with the scintillator crystal The variation of resolution with gamma-ray energy Electronics for Scintillation Systems High-voltage supply Preamplifiers Troubleshooting Amplifiers 217 Fault-Finding Multi-channel analysers and spectrum analysis Comparison of Sodium Iodide and Germanium Detectors Choosing and Setting up a Detector, and Checking its Specifications 11.2 Setting up a Germanium Detector System Installation the detector environment Liquid nitrogen supply Shielding Cabling Installing the detector Preparation for powering-up Powering-up and initial checks Switching off the system 11.3 Optimizing the Electronic System General considerations DC level adjustment and baseline noise Setting the conversion gain and energy range Pole-zero (PZ) cancellation Incorporating a pulse generator Baseline restoration (BLR) Optimum time constant 11.4 Checking the Manufacturer's Specification The Manufacturer's Specification Sheet Detector resolution and peak shape Detector efficiency Peak-to-Compton (P/C) ratio Window thickness index Physical parameters Equipment required Fault-Einding guide 12.2 Preamplifier Test Point and Leakage Current Resistive feedback (RF) preamplifiers

7 xii Contents 12,2.2 Transistor reset and pulsed optical reset preamplifiers 12.3 Thermal Cycling of the Detector The origin of the problem The thermal cycling procedure Frosted detector enclosure 12.4 Ground Loops, Pick-up and Microphonics Ground loops Electromagnetic pick-up Microphonics 13 Low Count Rate Systems 13.1 Introduction 13.2 Counting with High Efficiency MDA: efficiency and resolution MDA: efficiency, background and counting period 13.3 The Effect of Detector Shape Low energy measurements Well detectors Sample quantity and geometry 13.4 Low Background Systems The background spectrum Low background detectors Detector shielding The graded shield Airborne activity The effect of cosmic radiation Underground measurements 13.5 Active Background Reduction Compton suppression systems Veto guard detectors 13.6 Ultra-Low-Level Systems 14 High Count Rate Systems 14.1 Introduction 14.2 Detector Throughput 14.3 Preamplifiers for High Count Rate Energy rate saturation Energy resolution Dead time Amplifiers Time constants and pile-up The gated integrator Pole zero correction Amplifier stability peak shift Amplifier stability 246 resolution Overload recovery Digital Pulse Processing The ADC and MCA Dead Times and Throughput Extendable and 251 non-extendable dead time Gated integrators DSP systems Theory versus practice System Checks Ensuring Quality in Gamma-Ray 257 Spectrometry Introduction Nuclear Data Radionuclide Standards Maintaining Confidence in the 263 Equipment Setting up and 263 maintenance procedures Control charts Setting up a control chart Gaining Confidence in the Spectrum Analysis Test spectra Computer-generated test 270 spectra Test spectra created by 270 counting Assessing 273 spectrum analysis 276 performance Intercomparison exercises Assessment of 279 intercomparison exercises Maintaining Records Accreditation 312

8 Contents xiii Internet Sources of Information 16 Gamma Spectrometry of Naturally Occurring Radioactive Materials (NORM) 16.1 Introduction 16.2 The NORM Decay Series The uranium series 238U The actinium series 235 U The thorium series 232Th Radon loss Natural disturbance of the decay series 16.3 Gamma Spectrometry of the NORM Nuclides Measurement of 7 Be Gamma Spectrometry of Nuclear 313 Industry Wastes Measurement of isotopically modified uranium Measurement of Measurement of 40K 318 Gamma spectrometry of the uranium/thorium series nuclides Allowance for natural background Resolution of the 186 kev peak Other spectral interferences and summing Nuclear Data of the NORM Nuclides Measurement of Chemically Modified NORM Measurement of separated uranium Measurement of separated thorium `Non-natural' thorium Measurement of gypsum a cautionary tale General observations Applications 17.1 Gamma Spectrometry and the CTBT Background The global verification regime Nuclides released in a nuclear explosion Measuring the radionuclides Current status transuranic nuclides Waste drum scanning 17.3 Safeguards Enrichment meters Plutonium spectra Fresh and aged samples Absorption of gamma-rays Hand-held monitors 17.4 PINS Portable Isotopic Neutron Spectrometry Appendix A: Sources of Information 343 A.1 Introduction 343 A.2 Nuclear Data 343 A.2.1 Recent developments in the distribution of nuclear data 344 A.2.2 On line internet sources of gamma-ray emission data 345 A.2.3 Off-line sources of gamma-ray emission data 346 A.2.4 Nuclear data in print 346 A.3 Internet Sources of Other Nuclear Data 347 A.4 Chemical Information 347 A.5 Miscellaneous Information 348 A.6 Other Publications in print 348 Appendix B: Gamma- and X-Ray Standards for Detector Calibration 351 Appendix C: X-Rays Routinely Found in Gamma Spectra 359 Appendix D. Gamma-Ray Energies in the Detector Background and the Environment Appendix E: Chemical Names, Symbols and Relative Atomic Masses of the Elements Glossar), Index