Radionuclide Concentrations in Foor and the Environment - Chapter 9 pot

269 9 Radiation Detection Methods Ashraf Khater CONTENTS 9.1 Introduction 270 9.2 Radiation Interaction with Matter 271 9.2.1 Heavy Charged Particles 272 9.2.2 Beta Particles 274 9.2.3 Gamma and X-rays 275 9.2.3.1 Photoelectric Absorption 276 9.2.3.2 Compton Scattering 277 9.2.3.3 Pair Production 277 9.3 Radiation Detectors 279 9.3.1 Gas-Filled Detectors 280 9.3.1.1 Ionization Chambers 282 9.3.1.2 Proportional Counters 283 9.3.1.3 Geiger-Muller Counters 284 9.3.2 Scintillation Detectors 285 9.3.2.1 Inorganic Scintillators 287 9.3.2.2 Organic Scintillators 288 9.3.3 Semiconductor Detectors 290 9.3.3.1 Germanium Detectors 293 9.3.3.2 Silicon Detectors 296 9.3.4 Other Types of Radiation Detectors 298 9.4 Basic Radiation Detection System 298 9.4.1 Preamplifier 299 9.4.2 Amplifier 299 9.4.3 Pulse Height Analysis and Counting Techniques 299 9.4.4 Shielding 299 9.5 Radioactivity Analysis 302 9.5.1 2 π α / β Counting with a Gas Flow Counter 303 9.5.2 Liquid Scintillation Spectrometer 305 9.5.3 γ -ray Spectrometry 308 9.5.4 β Particle Spectrometry 315 9.5.5 α Particle Spectrometry 316 DK594X_book.fm Page 269 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 270 Radionuclide Concentrations in Food and the Environment 9.5.6Radiochemical Analysis 318 9.5.6.1Determination of Uranium Isotopes 319 9.5.6.2Determination of Plutonium Isotopes 325 Acknowledgment 331 References 331 9.1 INTRODUCTION Sources of ionizing radiation are inside and surrounding us all the time and everywhere. This radiation comes from radionuclides which occur naturally as trace elements in rocks and soils of the earth as a consequence of radioactive decay. Radionuclides also exist in the atmosphere, lithosphere, hydrosphere, and biosphere. Since the middle of the last century, and the discovery of nuclear radiation, much attention has been focused on the different sources of ionizing radiation and their useful applications and harmful effects on the human body and its environment. In addition to naturally occurring radioactive materials (NORMs), technologically enhanced naturally occurring radioactive materials (TENORMs) and man-made (artificially produced) radionuclides have been intro- duced into the environment from the proliferation of different nuclear applica- tions. All of these sources have contributed to the increase in the levels of environmental radioactivity and radiation doses. Radioecology is concerned with the behavior of radionuclides in the envi- ronment. It deals with the understanding of where radioactive materials originate and how they migrate, react chemically, and affect the ecosphere after their release into the environment. All these aspects are very dynamic processes where the environment greatly affects and is affected by the fate of radioactive substances. So the main goals of studying radioactivity in the environment and food are to provide a scientific basis for the effective utilization of radioactivity, such as geochronology, and to predict the impacts to man and his environment due to different radionuclides. Radiation detection and radioactivity analysis are the main topic of this chapter. The different types of radiation sources (NORMs, TENORMs, and man- made) are summarized in detail in Chapter 1 and Chapter 2 of this book. This chapter deals with three main themes: interactions of radiation with matter, radiation detectors, and radioactivity analysis of environmental and food samples. Heat and light are radiations that you can feel or see directly, but there are other kinds of radiation, such as γ , X-ray, and neutrons, that humans cannot recognize or feel directly. Radiation can be classified into two categories: non- ionizing, such as visible light, and ionizing, such as γ rays and X-rays. Ionizing radiation has the ability to ionize the atoms and molecules of the media it passes through. Ionizing radiation can be classified into two categories: directly ionizing and indirectly ionizing. Based on their electrical properties, ionizing radiation can be classified into charged radiations, such as α and β particles, and uncharged radiations, such as γ rays and neutrons. Also, according to their penetration power, radiation can be classified as soft or hard radiation. DK594X_book.fm Page 270 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radiation Detection Methods 271 Radiations are mainly classified into four groups: • Heavy charged particles, including all particles with a mass greater than or equal to one atomic mass unit (amu), such as α particles, protons, and fission products. • Charged particles, including β particles (negative electrons), positrons (positive electrons), internal conversion electrons, and auger electrons. • Electromagnetic radiations, including γ -rays (following β particles decay or nuclear reactions), characteristic x-rays, annihilation radiation and bremsstrahlung. • Neutrons, including fast neutrons, intermediate neutrons, epithermal neutrons, thermal neutrons, and cold neutrons. Neutrons can be gen- erated from spontaneous fission, radioisotope (alpha-neutron) sources, photo-neutron sources, or reactions from accelerated charged particles. The backbone of studying environmental radioactivity and radioecology is radiation detection and radioactivity analysis. The radiation detectors are one of the main components of radiation detection and measurement systems, which include the detector, the signal processing unit, and the output display device, such as a counter or spectrometer. Radiation detectors basically depend on the interaction of incident radiation with the detector material, which produces a detectable output signal. For each type of radiation, there is one or more suitable type of detector or detection system; each has advantages and disadvantages. 9.2 RADIATION INTERACTION WITH MATTER Knowledge of the mechanisms by which ionizing radiation interacts with matter is fundamental to an understanding of specific radiation topics such as instru- mentation, dosimetry, and shielding. Recall that the basic building block of matter is the atom, which consists of a nucleus, a positively charged central core con- taining protons and (with one exception) neutrons, surrounded by orbiting elec- trons. In a neutral atom, each electron supplies a negative charge to counter the positive charges found within the nucleus. Ionizing radiations, those radiations that possess sufficient energy to eject electrons from neutral atoms, include α particles, β particles, γ -rays, and x-rays. These radiations transfer energy to matter via interactions with the atom’s constituent parts. Radiation detection is based on the different mechanisms of radiation’s inter- action with matter. These mechanisms depend on both the physical properties of the radiation and the physical and structural properties of the detector materials. The interaction of radiation with matter will be explained here on two levels: the microscopic level, to understand the mechanisms of losing radiation energy inside the matter, and the macroscopic level, to understand the effect of different absorber materials on the intensity of radiation during and after passing through an absorber. DK594X_book.fm Page 271 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 272 Radionuclide Concentrations in Food and the Environment The following expressions are related to the interaction of radiation with matter and should be defined first: • Radiation stopping power (specific energy loss): the average energy loss per unit path length, usually expressed in megaelectron volts per centimeter (MeV/cm). • Radiation range: the linear distance behind which no particle passes through the absorber material. It depends on the type and energy of the particle and on the material through which the particle passes. • Radiation range straggling: the variation in the path length for individ- ual particles that have the same initial energy. • Radiation path length: the total distance traveled by the particle in the absorber material, where it is linear for heavy charged particles and nonlinear for charged particles. • Mean free path: the average length of the path the radiation travels without interaction with the absorber material. • Specific ionization: the average number of ion pairs (electron and positive ion pairs) formed per centimeter in the radiation track. • Mean ionization energy: the average energy required to form one ion pair in the matter. It is nearly independent of the energy of the radiation, its charge, and its mass. 9.2.1 H EAVY C HARGED P ARTICLES On the microscopic level, when charged particles travel through the absorber material, they undergo elastic and inelastic collisions with the orbital electrons of the absorbing material. Heavy charged particles interact with the matter under the effect of the Coulomb force (electrostatic force) between the positively charged particles, such as α particles and protons, and the negative orbital elec- trons of the constituent atoms of the absorber material. Rutherford scattering (i.e., interactions with nuclei of the matter atoms) are possible, but they are rare and are not normally significant in the response of radiation detectors. Under the effect of the Coulomb force, the heavy charged particle interacts simultaneously with many orbital electrons of the absorbing medium atoms. Because of the large mass differences between the charged particles and the electrons, the energy transfer from the charged particles per collision is very small. The maximum energy transfer in one collision is about 1/500 of the particle energy per nucleon. The charged particles lose their energies after many collisions within the matter. The particle’s energy is decreased with increasing path length and finally stops within the matter after losing its energy. During the energy transfer process, after decreasing the particle’s energy and velocity, the charged particles pick up electrons from the surrounding medium, reduce their charge, and finally become neutral atoms at the end of their track. DK594X_book.fm Page 272 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radiation Detection Methods 273 The heavy charged particles have a linear path and a definite range in a given absorbing material. Depending on the energy transferred to the orbital electrons, either it brings the electrons to a higher orbit with less binding energy (atom excitation) or it remove the electrons, called primary electrons, from the atoms (primary atom ionization). Atomic ionization produces ion pairs where each ion pair is composed of an electron and a positive ion of an absorber atom from which one electron has been removed. The energetic primary electrons, known as δ electrons or δ rays, interact with the absorber atoms and lose their energy via secondary ionization. Secondary ionization is very important for radiation detection and radiation protection, because it indirectly increases the energy transfer to the absorbing medium. The Bethe formula (Equation 10.1) describes the specific energy loss for charged particles: (9.1) (9.2) where ez = charge of the primary charged particle, Z = atomic number of the absorber material, m 0 = electron rest mass, υ = velocity of the primary charged particle, c = speed of light in a vacuum, I = average excitation and ionization energy of the absorber, N = density of the absorber atoms (number of electrons per unit volume). Equation 9.1 is generally valid for the charged particles where the velocity remains larger than that of the orbital electrons in the absorbing atoms. It begins to fail at low particle energies, where the charge exchange between the particles and the absorber atoms becomes significant. The specific energy loss, linear stopping power ( dE / dx ), varies as 1/ υ 2 or inversely with particle energy (1/ E ). The rate of energy transfer is increased with decreasing charged particle velocity because it spends a greater amount of time in the vicinity of any given electron. For different charged particles that have the same velocity, the particle with the greatest charge ( ze ) will have the largest energy loss per track length. For different absorber materials, dE / dx depends on the product NZ , linear stopping power − = dE dx ez m NB 4 42 0 2 . . . π υ BZ m I cc ≡−−       −         .ln. . ln. 2 1 0 22 2 2 2 υυυ DK594X_book.fm Page 273 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 274 Radionuclide Concentrations in Food and the Environment increases with the increasing atomic number of the absorber material (i.e., a higher density material). 9.2.2 B ETA P ARTICLES The interaction of β particles with matter is similar to that of heavy charged particles, where the Coulomb force is the dominant force between the constitutes. β particles interact with the matter and lose their energy through collisions of incident particles with orbital electrons and consequently either excite or ionize the absorber atoms. Because both β particles and electrons have the same mass, the energy loss per collision is larger compared to that for heavy charged particles. Because of the large deviation in the direction of β particles after collision, they follow a much more tortuous path. For fast electrons, the specific energy loss due to collisions has also been derived by Bethe and is written as (9.3) where the symbols have the same meaning as in Equation 9.1. In addition to the energy loss due to atom excitation or ionization, particle energy may be lost by another radiative process, bremsstrahlung “braking” radi- ation. When high-speed charged particles pass close to the intense electric field of the absorber nuclei, the particle suffers strong deceleration and bremsstrahlung radiation are emitted. The energy loss due to bremsstrahlung radiation is minor compared to that from atom excitation and ionization collision processes. It is more significant in absorber materials of high atomic number. The ratio of the contribution of radiative processes and collision processes is given by (9.5) where Z = atomic number of absorber material, E = energy of the incident particle. − = ⋅ ⋅ dE dx e m ZB 2 4 0 2 π υ. B mE I c c = − −−−+ln .( ) (ln ) 0 2 2 2 2 2 2 2 21 221 1 υ υ υυ ccc c 2 2 2 2 2 2 1 1 8 11         + − + −−            () υυ          (9.4) dE dx dE dx EZ radiative collision             ≅ 70 00 DK594X_book.fm Page 274 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radiation Detection Methods 275 Finally, β particles lose their energy inside the absorber and stop at the end of their tracks. Negative β particles act as free electrons in the absorber, while positive β particles interact with free electrons (i.e., matter-antimatter interaction). Annihilation radiation begins with two photons, having an energy of 511 keV for each are generated, which are very penetrable compare to the range of positron. These photons interact with matter and may lead to energy deposition in other locations. The β particle energy spectrum is different from that of α - or γ -rays, where β particles can have values from zero to the maximum (endpoint) energy value. For the majority of β particles, the absorption curves (number of β particles as a function of absorber thickness) have a near exponential shape and are repre- sented by (9.6) where I 0 = counting rate without absorber, I = counting rate with absorber, t = absorber thickness (in g/cm 2 ), n = absorption coefficient. Backscattering is a very important process that can significantly affect the specific energy lost in the matter, and consequently the radiation detection. Some particles undergo large angle deflections along their track that lead to backscat- tering. Backscattered particles on the absorber surface or inside the absorber itself can reemerge from the absorber surface without depositing all their energy in the absorbing medium, which will significantly affect the detection process. Also, backscattering of β particles that reemerge from the surface of some β particle sources due to the thick backing could increase the number of emitted particles from the source surface. 9.2.3 G AMMA AND X-R AYS The electromagnetic radiations, such as γ and x-rays, interact with matter in a completely different way. The concepts of range and specific energy loss are not applicable as for charged particles. Electromagnetic radiations have no electric charge and no mass, and their rest mass is zero. They can pass through an absorber without energy loss (i.e., they have a high penetration power). The relationship between energy ( E ), frequency ( ν ), and wavelength ( λ ) is (9.7) where h is Planck’s constant. I I e nt 0 = − Eh h c ==ν λ DK594X_book.fm Page 275 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 276 Radionuclide Concentrations in Food and the Environment When electromagnetic radiations, γ -rays, x-rays, and bremsstrahlung radia- tion, travel with the velocity of light, they are called photons. γ rays and x-rays have well-defined energies (i.e., monoenergetic) and have different origins. γ -rays originate from the nucleus, while x-rays originate from atoms. Bremsstrahlung radiation is produced by accelerating and decelerating charged particles and has a continuous energy spectrum. There are three main mechanisms of interaction of γ -rays and x-rays with matter that play an important role in radiation detection processes: photoelectric absorption, Compton scattering, and pair production. These interaction mecha- nisms lead to the partial or complete transfer of γ -ray photon energy to electron energy which leads to indirect ionization of the absorber atoms. 9.2.3.1 Photoelectric Absorption This mechanism of interaction is very important for γ - and x-ray measurements. The photon interacts with the absorber atoms and disappears (i.e., photon absorp- tion occurs). Depending on the photon energy, the most bonded orbital electron in the K or L shell will absorb the photon energy to be removed from the atom with a kinetic energy given by , (9.8) where E e = photoelectron kinetic energy, h ν = photon energy, E b = electron binding energy. The photoelectrons are energetic electrons and interact with matter exactly like β particles. These electrons leave the atom and create an electron vacancy in their inner orbit, where either a free electron or an electron from a higher orbit fills this vacancy and generates x-rays. The generated x-rays interact with the absorber and can produce another photoelectron (i.e., photoelectric absorption) with less binding energy (known as an auger electron) than the original photo- electron. The photoelectric coefficient ( τ ), the probability of photoelectric absorption per unit length, depends on the photon energy ( E ) and the absorber atomic number ( Z ). Photoelectric absorption is the predominant mechanism of interaction for low-energy photons ( E γ ). It is enhanced with increasing absorber atomic number ( Z ). A rough approximation is given by , (9.9) where n and m are constant values that range between 3 and 5. EhE eb = −ν τ γ m E m − () ≅ 1 Constant Z n DK594X_book.fm Page 276 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radiation Detection Methods 277 9.2.3.2 Compton Scattering Compton scattering is an inelastic collision between the incident photon and the weak-bonded electron in the outer shell of the absorber atoms. The incident photon dissipates a part of its energy and deflects with a scattering angle of θ . The recoil electron is removed from the atom with a kinetic energy that depends on the amount of energy transferred from the photon. The energy transfer varies from zero, when θ = 0, to a maximum value, when θ = π . The Compton coefficient decreases with increasing energy and increases linearly with the atomic number Z of the absorber material. The energy of the recoil electron and the scattered photon are given by , (9.10) , (9.11) where E 0 = incident photon energy, E γ = scattered photon energy, E e = recoil electron energy, m 0 = electron rest mass. The Compton scattering coefficient (σ), the probability of occurrence per unit length, is approximated and given by , (9.12) where f(E γ ) is a function of E γ . 9.2.3.3 Pair Production Pair production is the main interaction mechanism for the energetic photon. Practically, it becomes significant for the few megaelectron volt energy photons. Theoretically it is possible for photons with energy (E γ ) of 1.022 MeV, which is equivalent to the energy of two electron rest masses (2 m 0 C 2 ). The photon disappears in the nucleus field of the absorber atoms and one electron-positron pair is generated. The kinetic energy of the electron (E e– ) and the positron (E e+ ) is given by . (9.13) EE Emc γ θ = + () − ()         0 0 2 1 11 cos EEEE Emc Emc e = − = () − () + () − 00 00 2 00 2 1 11 γ θ cos ccosθ ()         σ γ mNZfE − () = () 1 EE EmC mC E ee ee − + − + == − () − () () = −05 05 10 0 2 0 2 γγ 222MeV () DK594X_book.fm Page 277 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 278 Radionuclide Concentrations in Food and the Environment The pair production coefficient (κ), the probability of occurrence per unit length, is a complicated function of Z and E which changes slightly with Z and increases with E: , (9.14) where f(E γ , Z) is a function of E and Z. Both electrons and positrons interact with the absorber as β particles and finally come to rest after losing their kinetic energy. Then the electron acts as a free electron and the positron interacts with the electron (i.e., matter-antimatter inter- action) and generates two inhalation photons, each with an energy of 0.511 MeV. At the macroscopic level, the incident photons interact with the absorber material and their numbers decrease with increasing thickness of the absorber (known as radiation attenuation). Photon attenuation is due to the main interaction mechanisms of photons (photoelectric effect, Compton effect, and pair produc- tions effect), that is, photons are completely absorbing or scattering. There are other mechanisms of photon interaction with matter, but they are insignificant in γ- and x-ray measurement. The linear attenuation coefficient (µ) is the probability per unit length that the photon is interacted with and removed from the beam. The linear attenuation coefficient is the sum of the probabilities of the three main interaction mechanisms (photoelectric, Compton scattering, and pair production) and is given by . (9.15) The mean free path (λ) of a γ-ray photon is related to the linear attenuation coefficient and the half-value thickness (X 1/2 ), and is given by (9.16) The mass attenuation coefficient (µ m ) is much more widely used because of the variation in the absorber density (ρ) and is the same regardless of the physical state of the absorber. It is given by . (9.17) The number of transmitted γ-ray photons (I) through an absorber of thickness t from the incident γ-ray photons (I 0 ) is given by , (9.18) κ γ mNZfEZ − () = () 12 , µ τσκ=++()()()photoelectric compton pair λ µ == 1 14 12 / X µ µ ρ m = IIe t = − 0 µ DK594X_book.fm Page 278 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC [...]... thicknesses (in mg/cm2) encountered during routine sampling and counting [7] For gross α and gross β measurements, the detector must be calibrated to obtain the counting efficiency (i.e., the ratio of the counting rate to the disintegration rate) α-emitting radionuclides such as 241Am, 230Th, and 238Pu and β-emitting radionuclide such as 90 Sr /90 Y reference standard are prepared in the same sample geometries and. .. made to the P-type oxidized surface by a thin layer of gold and to the N-type surface by a layer of aluminum The depletion region is the sensitive volume of the detector and is formed by to the migration of electrons toward the P-type region and the hole toward the N-type region The width of the depletion region increases with an increase in the bias voltage and can extend to the limit of the breakdown... radiation with the scintillator crystal, the electron can gain enough energy to rise from the valence band to the higher energy level of the conduction band and leave a positive hole in the valence band In the pure crystal, after a certain decay time, an electron returns to the valence band with the emission of a photon This process is inefficient and the librated photon energy is too high to lie in the visible... of incident radiation energy for the different detector types are shown in Figure 9. 9 and Figure 9. 10 Coaxial P-type germanium detectors are used for γ-rays, with an energy range of 100 keV to about 10 MeV, and cannot be used for low-energy - and x-rays because they cannot penetrate the aluminum detector window and high-energy γ-rays might pass through the sensitive volume without interaction For x-ray... cylindrical with a central rod as an anode, and spherical — but they work based on the same principles When the incident radiation travels through the gas (the sensitive volume of the detector) and interacts with the gas atoms and molecules, atom excitation and ionization occur The gas ionization produces electron-ion pairs; their number depends on the energy deposited during the radiation-gas interaction... Francis Group, LLC DK 594 X_book.fm Page 298 Tuesday, June 6, 2006 9: 53 AM 298 Radionuclide Concentrations in Food and the Environment measurements, including α particles and fission fragments spectroscopy They have a P-N structure in which a depletion region is formed by applying reverse bias In the SSB detector, a surface barrier junction is formed by oxidizing the surface of the N-type silicon Electric... positive hole in the valence band, which can also move Sometimes the absorbed energy is not enough to elevate the electron to the conduction band Instead, the electron remains electrostatically bound to the positive hole in the valence band (i.e., excitation) Energy gaps, in which electrons can never be found in the pure crystal, exist between the valence and conduction bands As a result of the interaction... that of the 3 in × 3 in NaI(Tl) scintillation detector © 2007 by Taylor & Francis Group, LLC DK 594 X_book.fm Page 296 Tuesday, June 6, 2006 9: 53 AM 296 Radionuclide Concentrations in Food and the Environment Nal(TI) Detector Counts HPGe Detector 0 500 1000 1500 2000 Energy (keV) FIGURE 9. 11 γ-ray spectra for the same source measured using NaI(Tl) scintillation and HPGe detectors at a detector-to-source... emerging from the lead can be stopped by the cadmium layer and the cadmium K x-rays (22 keV) in turn are stopped in the innermost liner of copper Active shielding is another detection system surrounding the main detection system, where the output pulses of both are going through an electronic circuit (such as anticoincident circuit) to eliminate the background pulses which are counted simultaneously in. .. beryllium In addition to the detector window thickness, x-ray energy will affect the detection efficiency, where the efficiency increases with an increase in the x-ray energy to a maximum level and then decreases with increasing energy because at the high energy the x-rays might pass through the active volume © 2007 by Taylor & Francis Group, LLC DK 594 X_book.fm Page 297 Tuesday, June 6, 2006 9: 53 AM Radiation . 290 9. 3.3.1 Germanium Detectors 293 9. 3.3.2 Silicon Detectors 296 9. 3.4 Other Types of Radiation Detectors 298 9. 4 Basic Radiation Detection System 298 9. 4.1 Preamplifier 299 9. 4.2 Amplifier 299 9. 4.3. 299 9. 4.3 Pulse Height Analysis and Counting Techniques 299 9. 4.4 Shielding 299 9. 5 Radioactivity Analysis 302 9. 5.1 2 π α / β Counting with a Gas Flow Counter 303 9. 5.2 Liquid Scintillation. 277 9. 2.3.2 Compton Scattering Compton scattering is an inelastic collision between the incident photon and the weak-bonded electron in the outer shell of the absorber atoms. The incident photon