Radiation Detection And Measurement 3rd Ed - Glenn F

Radiation Detection and Measurement\nTHIRD EDITION\nGLENN F. KNOLL Radiation Detection and Measurement\nThird Edition\nGlenn F. Knoll\nProfessor of Nuclear Engineering and Radiological Sciences\nUniversity of Michigan\nAnn Arbor, Michigan\n\nJohn Wiley & Sons, Inc.\nNew York/Chichester/Weinheim/Brisbane/Toronto/Singapore Contents\nChapter 1 Radiation Sources 1\nI. Units and Definitions 2\nII. Fast Electron Sources 3\nIII. Heavy Charged Particle Sources 6\nIV. Sources of Electromagnetic Radiation 11\nV. Neutron Sources 19\n\nChapter 2 Radiation Interactions 29\nI. Interaction of Heavy Charged Particles 30\nII. Interaction of Fast Electrons 43\nIII. Interaction of Gamma Rays 48\nIV. Interaction of Neutrons 55\nV. Radiation Exposure and Dose 57\n\nChapter 3 Counting Statistics and Error Prediction 65\nI. Characterization of Data 66\nII. Statistical Models 70\nIII. Application of Statistical Models 79\nIV. Error Propagation 86\nV. Optimization of Counting Experiments 92\nVI. Limits of Detectability 94\nVII. Distribution of Time Intervals 97\n\nChapter 4 General Properties of Radiation Detectors 103\nI. Simplified Detector Model 103\nII. Modes of Detector Operation 104\nIII. Pulse Height Spectra 110\nIV. Counting Curves and Plateaus 111\nV. Energy Resolution 113\nVI. Detection Efficiency 116\nVII. Dead Time 119\n\nChapter 5 Ionization Chambers 129\nI. The Ionization Process in Gases 129\nII. Charge Migration and Collection 133\nIII. Design and Operation of DC Ion Chambers 136\nIV. Radiation Dose Measurement with Ion Chambers 140\nV. Applications of DC Ion Chambers 145\nVI. Pulse Mode Operation 148 Chapter 6\nProportional Counters\n\nI. Gas Multiplication\nII. Design Features of Proportional Counters\nIII. Proportional Counter Performance\nIV. Detection Efficiency and Counting Curves\nV. Variants of the Proportional Counter Design\n\nChapter 7\nGeiger-Mueller Counters\n\nI. The Geiger Discharge\nII. Fill Gases\nIII. Quenching\nIV. Time Behavior\nV. The Geiger Counting Plateau\nVI. Design Features\nVII. Counting Efficiency\nVIII. Time-to-First-Count Method\nIX. G-M Survey Meters\n\nChapter 8\nScintillation Detector Principles\n\nI. Organic Scintillators\nII. Inorganic Scintillators\nIII. Light Collection and Scintillator Mounting\n\nChapter 9\nPhotomultiplier Tubes and Photodiodes\n\nI. Introduction\nII. The Photocathode\nIII. Electron Multiplication\nIV. Photomultiplier Tube Characteristics\nV. Ancillary Equipment Required with Photomultiplier Tubes\nVI. Photodiodes as Substitutes for Photomultiplier Tubes\nVII. Scintillation Pulse Shape Analysis\nVIII. Hybrid Photomultiplier Tubes\nIX. Position-Sensing Photomultiplier Tubes\nX. Photoionization Detectors Chapter 10\nRadiation Spectroscopy with Scintillators\n\nI. General Consideration in Gamma-Ray Spectroscopy\nII. Gamma-ray Interactions\nIII. Predicted Response Functions\nIV. Properties of Scintillation Gamma-Ray Spectrometers\nV. Response of Scintillation Detectors to Neutrons\nVI. Electron Spectroscopy with Scintillators\nVII. Specialized Detector Configurations Based on Scintillation\n\nChapter 11\nSemiconductor Diode Detectors\n\nI. Semiconductor Properties\nII. The Action of Ionizing Radiation in Semiconductors\nIII. Semiconductors as Radiation Detectors\nIV. Semiconductor Detector Configurations\nV. Operational Characteristics\n\nVI. Applications of Silicon Diode Detectors\n\nChapter 12\nGermanium Gamma-Ray Detectors\n\nI. General Considerations\nII. Configurations of Germanium Detectors\nIII. Germanium Detector Operational Characteristics\nIV. Gamma-Ray Spectroscopy with Germanium Detectors Chapter 13\nOther Solid-State Detectors\n\nI. Lithium-Drifted Silicon Detectors\nII. Semiconductor Materials Other than Silicon or Germanium\nIII. Avalanche Detectors\nIV. Photoconductive Detectors\nV. Position-Sensitive Semiconductor Detectors\n\nChapter 14\nSlow Neutron Detection Methods\n\nI. Nuclear Reactions of Interest in Neutron Detection\nII. Detectors Based on the Boron Reaction\nIII. Detectors Based on Other Conversion Reactions\nIV. Reactor Instrumentation\n\nChapter 15\nFast Neutron Detection and Spectroscopy\n\nI. Counters Based on Neutron Moderation\nII. Detectors Based on Fast Neutron-Induced Reactions\nIII. Detectors that Utilize Fast Neutron Scattering\n\nChapter 16\nPulse Processing and Shaping\n\nI. Device Impedances\nII. Coaxial Cables\nIII. Pulse Shaping\n\nChapter 17\nLinear and Logic Pulse Functions\n\nI. Linear and Logic Pulses\nII. Instrument Standards\nIII. Application Specific Integrated Circuits (ASICs)\nIV. Summary of Pulse-Processing Units\nV. Components Common to Many Applications\nVI. Pulse Counting Systems\nVII. Pulse Height Analysis Systems\nVIII. Digital Pulse Processing\nIX. Systems Involving Pulse Timing\nX. Pulse Shape Discrimination\n\nChapter 18\nMultichannel Pulse Analysis\n\nI. Single-Channel Methods\nII. General Multichanneled Characteristics\nIII. The Multichannel Analyzer\nIV. Spectrum Stabilization and Relocation\nV. Spectrum Analysis Chapter 1\nRadiation Sources\n\nThe radiations of primary concern in this text originate in atomic or nuclear processes. They are conveniently categorized into four general types as follows:\n\nCharged particulate radiation\n{ Fast electrons\n Heavy charged particles\n Neutrons\n}\n\nUncharged radiation\n{ Electromagnetic radiation\n}\n\nFast electrons include beta particles (positive or negative) emitted in nuclear decay, as well as energetic electrons produced by any other process. Heavy charged particles denote a category that encompasses all energetic ions with mass of one atomic mass unit or greater, such as alpha particles, protons, fission products, or the products of many nuclear reactions. The electromagnetic radiation of interest includes X-rays emitted in the rearrangement of electron shells of atoms, and gamma rays that originate from transitions within the nucleus itself. Neutrons generated in various nuclear processes constitute the final major category, which is often further divided into slow neutrons and fast neutron subcategories (see Chapter 14).\n\nThe energy range of interest spans over six decades, ranging from about 10 eV to 20 MeV. (Slow neutrons are technically an exception but are included because of their technological importance.) The lower energy bound is set by the minimum energy required to produce ionization in typical materials by the radiation or the secondary products of its interaction. Radiations with energy greater than this minimum are classified as ionizing radiations. The upper bound is chosen to limit the topics in this coverage to those of primary concern in nuclear science and technology.\n\nThe main emphasis in this chapter will be the laboratory-scale sources of these radiations, which are likely to be of interest either in the calibration and testing of radiation detectors described in the following chapters, or as objects of the measurements themselves. Natural background radiation is an important additional source and is discussed separately in Chapter 20.\n\nThe radiations of interest differ in their \"hardness\" or ability to penetrate thicknesses of material. Although this property is discussed in greater detail in Chapter 2, it is also of considerable concern in determining the physical form of radiation sources. Soft radiations, such as alpha particles or low-energy X-rays, penetrate only small thicknesses of material. Radioisotope sources must therefore be deposited in very thin layers if a large fraction of these radiations is to escape from the source itself. Sources that are physically thicker are subject to \"self-absorption,\" which is likely to affect both the number and the energy spectrum of the radiations that emerge from its surface. Typical thicknesses for such sources are therefore measured in micrometers. Beta particles are generally more penetrating, and sources up to a few tenths of a millimeter in thickness can usually be tolerated. Harder\n\n1 2 Chapter 1 Radiation Sources\n\nI. UNITS AND DEFINITIONS\n\nA. Radioactivity\n\nThe activity of a radioisotope source is defined as its rate of decay and is given by the fundamental law of radioactive decay\n\ndN\ndt = -λN\n\nwhere N is the number of radioactive nuclei and λ is defined as the decay constant. The historical unit of activity has been the curie (Ci), defined as exactly 3.7 × 10^10 disintegrations/second, which owes its definition to its origin as the best available estimate of the activity of 1 gram of pure 226Ra. Its submultiples, the millicurie (mCi) or microcurie (µCi), generally are more suitable units for laboratory-scale radioisotope sources.\n\nAlthough still widely used in the literature, the curie is destined to be replaced gradually by its SI equivalent, the becquerel (Bq). At its 1975 meeting, the General Conference of Weights and Measures (CGPM) adopted a resolution declaring that becquerel, defined as one disintegration per second, has become the standard unit of activity. Thus\n\n1 Bq = 1 s^-1 = 2.703 × 10^-11 Ci\n\nRadioactive sources of convenient size in the laboratory are most reasonably measured in kilobecquerels (kBq) or megabecquerels (MBq).\n\nIt should be emphasized that activity measures the source disintegration rate, which is not synonymous with the emission rate of radiation produced in its decay. Frequently, given radiation will be emitted in only a fraction of all the decays, so a knowledge of the decay scheme of the particular isotope may be necessary to infer a radiation emission rate from its activity. Also, the decay of a given radioisotope may lead to a radioactive product whose activity also contributes to the radiation yield from the source. A complete listing of radioisotope decay schemes is tabulated in Ref. 1.\n\nThe specific activity of a radioactive source is defined as the activity per unit mass of the radioisotope sample. If a pure or \"carrier-free\" sample is obtained that is unmixed with any other nuclear species, its specific activity can be calculated from\n\nactivity = λN/A\n\nspecific activity = ------------\n mass NM/A\n\n(1.2)\nwhere M = molecular weight of sample\n A = Avogadro's number (= 6.02 × 10^23 nuclei/mole)\n λ = radioisotope decay constant (= ln 2/half-life)\n\nOne should be aware that Eq.(1.1) represents the decay rate only, and the net value of dN/dt may be altered by other production or disappearance mechanisms. As one example, the radioisotope may be produced as the daughter product of the decay of a parent species also present in the sample. A production term is present for the daughter that is given by the decay of the parent multiplied by the fraction of such sources that leads to the daughter species. If the half-life of the parent is very long, the number of daughter nuclei increases until the daughter activity reaches an equilibrium value (after many daughter halves have passed) when the production and decay rates are equal, and dN/dt = 0 for the number of daughter model. Chapter 1 Fast Electron Sources\n\nII. FAST ELECTRON SOURCES\n\nA. Beta Decay\n\nThe most common source of fast electrons in radiation measurements is a radioisotope that decays by beta-minus emission. The process is written schematically\n\n2X → 2Y + β^- + ν\n\nwhere X and Y are the initial and final nuclear species, and ν is the antineutrino. Because neutrinos and antineutrinos have an extremely small interaction probability with matter, they are undetectable for all practical purposes. The recoil nucleus Y appears with a very\n\nEnergy\n\nThe traditional unit for measurement of radiation energy is the electron volt or eV, defined as the kinetic energy gained by an electron by its acceleration through a potential difference of 1 volt. The multiples of kiloelectron volt (keV) and megaelectron volt (MeV) are more common in the measurement of energies for ionizing radiation. The electron volt is a convenient unit when dealing with particulate radiation because the energy gained from an electric field can easily be obtained by multiplying the potential difference by the number of electric charges carried by the particle. For example, an alpha particle that carries an electron charge of +2 will gain an energy of 2 keV when accelerated by a potential difference of 1000 volts.\n\nThe SI unit of energy is the joule (J). When dealing with radiation energies, the submultiple [femtojoule (fJ)] is more convenient and is related to the electron volt by the conversion\n\n1 eV = 1.602 × 10^-19 J\n\nor\n\n1 fJ (or 10^-15 J) = 6.241 × 10^15 eV\n\nIt is not clear to what extent the electron volt will be phased out in future usage because its physical basis and universal use in the literature are strong arguments for its continued application to radiation measurements.\n\nThe energy of an X- or gamma-ray photon is related to the radiation frequency by\n\nE = hν\n\nwhere h = Planck's constant (6.626 × 10^-34 J s, or 4.135 × 10^-15 eV·s)\nv = frequency\n\nThe wavelength λ is related to the photon energy by\n\nλ = 1.240 × 10^-6\n\nwhere λ is in meters and E in eV.\n Table 1.1 Some \"Pure\" Beta-Minus Sources\nNuclide Half-Life Endpoint Energy (MeV)\n3H 12.26 y 0.0186\n14C 5730 y 0.156\n32P 14.28 d 1.710\n33P 24.34 d 0.248\n35S 87.9 d 0.167\n36Cl 3.08 x 10^7 y 0.714\n45Ca 165 d 0.252\n60Ni 92 y 0.067\n90Sr/90Y 27.7 y/64 h 0.546/2.27\n94Tc 2.12 x 10^4 y 0.292\n147Pm 2.62 y 0.224\n211Rn 3.81 y 0.766\nData from Lederer and Shirley.\n\nsmall recoil energy, which is ordinarily below the ionization threshold, and therefore it can- not be detected by conventional means. Thus, the only significant ionizing radiation produced by beta decay is the fast electron or beta particle itself.\n\nBecause most radionuclides produced by neutron bombardment of stable materials are beta-active, a large assortment of beta emitters are readily available through production in a reactor flux. Species with many different half-lives can be obtained, ranging from thousands of years down to as short a half-life as is practical in the application. Most beta decays populate an excited state of the product nucleus, so that the subsequent de-excitation gamma rays are emitted together with beta particles in many common sources. Some examples of nuclides that decay directly to the ground state of the product and are therefore \"pure beta emitters\" are shown in Table 1.1.\n\nEach specific beta decay transition is characterized by a fixed decay energy or Q-value. Because the energy of the recoil nucleus is virtually zero, this energy is shared between the beta particle and the \"invisible\" neutrino. The beta particle thus appears with an energy that varies from decay to decay and can range from zero to the \"beta endpoint energy,\" which is numerically equal to the Q-value. A representative beta energy spectrum is illustrated in Fig. 1.1. The Q-value for a given decay is normally quoted assuming that\n\nFigure 1.1 The decay scheme of 36Cl (3.08 x 10^7 y) and the resulting beta particle energy distribution.