Soil and Environmental Analysis: Modern Instrumental Techniques - Chapter 8 ppsx

8 Measurement of Radioisotopes and Ionizing Radiation Olivia J. Marsden and Francis R. Livens The University of Manchester, Manchester, England I. INTRODUCTION Approximately 1700 different isotopes are known, of which around 275 are stable. The remainder are radioactive; that is, their nuclear configurations are unstable and can change to more stable forms by nuclear transformations that are collectively known as radioactive decay. These radioactive decay processes are accompanied by the emission of particles and/or photons from the nucleus. Isotopes (or nuclides) are distinguished by the number of protons and neutrons (collectively known as nucleons) they contain and are commonly designated using mass number (A: number of protons þ neutrons) and atomic number (Z: number of protons). For example, 14 6 Cis an isotope of carbon in which the nucleus contains 14 nucleons, of which six are protons. The proton number defines the chemical identity of the atom, since the proton charge must be balanced by the appropriate number of electrons, but it also duplicates the information provided by the chemical symbol and, in practice, is often omitted, hence 14 C. Differences in the neutron number may control the stability, or otherwise, of a nucleus but have only subtle effects on chemistry, although these can be exploited in studies of stable isotope fractionation in natural systems, for example 2 H/ 1 H, 13 C/ 12 C, 15 N/ 14 N, 17 O/ 16 O, 34 S/ 32 S (see Chap. 9). Only a minority of the unstable isotopes are formed in nature. Most are man-made, and the majority of these are available only in such small amounts, or are so short-lived or both, that they are unlikely to be TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. encountered in the environment or to be of any use as radiotracers. The naturally occurring radioisotopes fall into three groups: 1. Primordial isotopes that have existed since the formation of the Earth about 4.5 Â10 9 years ago. These have half-lives (see later) comparable to the age of the Earth, that is, in the range 10 8 to 10 12 yr. Examples include 235 U(t ½ 7.5 Â10 8 yr) and 138 La (t ½ 1.35 Â10 11 yr). 2. Short-lived isotopes formed by the decay of long-lived parents. These have widely varying half-lives, ranging from 3 Â10 À7 s( 212 Po) to 2.4 Â10 5 yr ( 234 Pa) and are constantly being formed by decay of the parent isotope and removed by their own decay. In many cases, these isotopes form part of long decay series, in which multiple transforma- tions occur before a stable nucleus is reached. An example is the decay of 238 U through a sequence of 14 a -andb-decay steps to the stable isotope 206 Pb (Fig. 1). 3. Short-lived isotopes formed constantly by nuclear reactions in the atmosphere and transported to the Earth’s surface by atmospheric mixing and wet and dry deposition. Examples include 3 H(t ½ 12.3 yr), 7 Be (t ½ 53.4 d), and 14 C(t ½ 5736 yr). The exploitation of nuclear reactions in nuclear weapons and nuclear power, and the use of radioisotopes in industrial and medical applications, has led to the global dispersion of radioisotopes. Some of these are also formed to a significant extent in nature (for example, 3 H and 14 C) but many are not (examples include 239 Pu, 237 Np, 137 Cs, 125 I, 129 I, 35 S). Figure 1 Isotopes produced by the decay of 238 U. 346 Marsden and Livens TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. II. THEORY OF RADIOACTIVITY A. Nuclear Stability The phenomenon of radioactivity originates in the instability of some nuclear configurations. Very simply, for a nucleus to remain intact, the Coulomb repulsion arising from the interaction of the protons must be less than the ‘‘strong’’ attractive force between all nucleons. Thus the forces arising from the presence of protons in the nucleus have both attractive and repulsive components, whereas those arising from the presence of neutrons are predominantly attractive. The balance between attractive and repulsive forces controls nuclear stability and is dependent on the neutron/proton ratio in the nucleus. In the lightest nuclei, up to about A ¼40, a neutron/ proton ratio of about 1.0 is adequate for stability, but beyond this point, the Coulomb repulsion for each extra proton rises faster than the attractive force for each extra neutron, and so the neutron/proton ratio needed for stability increases progressively up to a value of about 1.5 in 209 Bi, the heaviest stable nucleus. Nuclei with an unstable nuclear configuration (i.e., with an imbalance of protons or neutrons) can move toward stability by changing the neutron/proton ratio. The most common route toward stability is the transformation of a neutron into a proton (for a neutron-rich nucleus) or of a proton into a neutron (for a proton-rich nucleus). There are three ways in which this can occur and these are collectively known as beta decay processes. In the neutron-rich nuclei, the reaction: 1 0 n ! 1 1 p þ þ 0 À1 e À occurs, with the electron (negatron) being ejected from the nucleus. Note that, in this reaction, A remains the same, while Z increases by 1, so that, in general, in this type of decay, A Z M ! A Zþ1 N þ 0 À1 e À A specific example is that of 90 Sr: 90 38 Sr ! 90 39 Y þ 0 À1 e À In proton-rich nuclei, the reverse reaction may occur: 1 1 p þ ! 1 0 n þ 0 1 e þ Measurement of Radioisotopes and Ionizing Radiation 347 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. The positively charged particle (positron) emitted in this case is identical to an electron in all respects except its charge, which is equal in magnitude to that of the electron but positive in sign. In general, in positron emission, A Z M ! A ZÀ1 N þ 0 1 e þ An example of this type of decay is that of 11 C: 11 6 C ! 11 5 B þ 0 1 e þ The negatron and positron are sometimes denoted  À and  þ respectively. An alternative way of converting a proton into a neutron is for orbital electron capture to occur: 1 1 p þ þ 0 À1 e À ! 1 0 n No particles are emitted in this reaction. Since the captured electron originates in one of the inner orbitals, usually the K shell, electron capture is often referred to as K capture. An example of this decay type is 182 77 Ir þ 0 À1 e À ! 182 76 Os In the heaviest nuclei, alternative decay modes may be observed. Alpha decay is characteristic of neutron-rich nuclei with A > 209 and of a small number of lanthanide isotopes. In this decay mode, a helium nucleus (or a-particle) is ejected from the parent nucleus: A Z M ! AÀ4 ZÀ2 N þ 4 2 He 2þ An example is the decay of 239 Pu: 239 94 Pu ! 235 92 U þ 4 2 He 2þ Spontaneous fission is an alternative decay mode found in some of the heaviest known isotopes. Many of these decay by both a-emission and spontaneous fission to varying degrees (e.g., 252 Cf: 96.9% by a, 3.1% by spontaneous fission). 348 Marsden and Livens TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. B. Gamma Ray Emission Both a- and b-decay processes may proceed via one or more excited states of the product nucleus, although transitions directly to the product ground state do occur and indeed are the dominant transitions in a number of important isotopes (e.g., 3 H, 14 C, 32 P, 35 S, 90 Sr, 241 Pu). Where the first decay product is an excited state, this can de-excite to the ground state by a number of mechanisms. The most important of these is the emission of one or more photons of electromagnetic radiation, which are known as g-rays. These photons are characterized by high energies (typically 40 to 2000 keV), and the energy with which they are emitted is that separating the nuclear states between which the g-transition has occurred. In most cases, the transition from excited to ground state occurs unmeasurably fast and is effectively coincident with the accompanying a-orb-decay event. However, in some cases, the excited states have measurable lifetimes, in which case they are known as ‘‘nuclear isomers’’ or ‘‘metastable states.’’ One of the best known of these is 110m Ag (t ½ 249.9 d), which was released in significant quantities during the Chernobyl reactor accident. It is important to recognize that, because there may be a variety of routes from parent ground state to product ground state, the decay of a particular isotope may be accompanied by the emission of photons with a number of different energies and also that not all decay events lead to the emission of a g-photon of a particular energy (or indeed any g-photons at all). The proportion of decays that lead to a particular g-transition is known as the ‘‘abundance’’ and is usually expressed as a percentage. Thus the a- decay of 241 Am proceeds directly to the 237 Np ground state in 65% of cases and via an excited state at 59.5 keV in the remaining 35%. De-excitation takes place in one step, so 241 Am is described as an a-emitter giving a 59.5 keV g-ray with 35% abundance. C. Decay Energies Since all radioactive decay events arise as an unstable nucleus moves toward a more stable (i.e., lower energy) state, the energy difference between the initial and final states has to be dissipated during the decay process. Both initial and final nuclear states are of well defined energies, and it is easy to calculate the total expected decay energy. Where the product nucleus is formed in a nuclear excited state, which subsequently de-excites to the ground state (see above), the decay energy corresponds to the gap between initial and excited product states. In a-decay processes, the energy associated with the transition from parent to product states is almost all dissipated as kinetic energy associated Measurement of Radioisotopes and Ionizing Radiation 349 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. with the a-particle. The product nucleus has some recoil energy, but conservation of momentum and the usually much larger mass of the product nucleus leave the a-particle with the majority of the kinetic energy. For example, 238 92 U ! 234 90 Th þ 4 2 He 2þ Since momentum before decay is approximately 0, momentum after decay is approximately 0 as well. Thus M d v d ¼ M a v a ð1Þ where d and a denote daughter and a, respectively and v a v d ¼ M d M a ¼ 234 4 ¼ 58:5 ð2Þ The ratio of the kinetic energies E d /E a is thus 234 Âð1Þ 2 4 Âð58:5Þ 2 ¼ 0:017 In other words, the a-part icle carries away 98% of the decay energy as its kinetic energy. When they were first discovered, b-decay processes were difficult to interpret in this way since the particles are emitted with a range of energies from effectively zero up to a maximum value characteristic of a particular isotope (E max ). It was difficult to reconcile the emission of particles with a spectrum of energies with their supposed origin in transitions between well-defined, quantized energy levels. When the existence of the neutrino was proposed in 1927, this paradox was resolved, although the neutrino was not detected experimentally until 1953. The neutrino is a particle with zero rest mass, which is also formed during a positron or negatron emission and which carries away a variable proportion of the total decay energy. Thus the decay energy is the sum of neutrino and b-particle energies. Where the b-particle energy is zero, all the decay energy is removed by the neutrino, and conversely when the b-particle energy is equal to E max the neutrino energy is zero. 350 Marsden and Livens TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. D. Quantitative Treatment of Radioactive Phenomena For any given isotope, the rate of radioactive decay (the activity, denoted A) is proportional only to the number of atoms present in the source, denoted N. Thus if one compares the a-count rate from a 1 ng source of 241 Am with that from a 2 ng source under identical conditions, the first count rate will be half the second. The activity is related to the number of atoms present by the decay constant, l, which has units of time À1 i.e. A ¼ N ð3Þ The SI unit of activity is the Becquerel (Bq), which is equal to 1 disinte- gration per second. Thus a 1 ng source of 241 Am, which has a decay constant of 1.600 Â10 À3 yr À1 (¼5.07 Â10 À11 s À1 ), will have an activity of 10 À9 241 Â 6:022 Â10 23 Â 5:07 Â10 À11 ¼ 126 Bq Since the activity depends only on the number of atoms present and the decay constant characteristic of that isotope, repeated measurement of a radioactive source’s activity will show a decrease in count rate with time, since the decay process is removing atoms, and the fewer the atoms present, the lower the activity. The activity follows first-order kinetics and can be described by a simple exponential function: A t ¼ A 0 e Àt ð4Þ where A t ¼activity at time t, A 0 ¼initial activity, and t ¼time elapsed, or more usefully, ln A t ¼ ln A 0 À t ð5Þ Clearly, since A is proportional to N, N t ¼N 0 e Àlt . After 1000 years, the activity of our 1 ng of 241 Am will therefore have fallen from its initial value of 126 Bq to 25 Bq. We can also characterize a radioactive isotope by the time required for half of the atoms originally present to decay (or for a source’s initial activity to fall to 50% of the initial value). This time is known as the half-life and is denoted t 1/2 . It is straightforward to demonstrate the relationship between Measurement of Radioisotopes and Ionizing Radiation 351 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. l and t 1/2 . After one half-life has elapsed, a source’s activity will have fallen from A 0 to A 0 /2, thus A 0 2 ¼ A 0 e Àt 1=2 ð6Þ so t 1=2 ¼ ln 2  ¼ 0:693  ð7Þ and for our 241 Am example, t 1/2 ¼433 years. E. Radioactive Decay Series The simplest example of a radioactive decay is a one-step transformation of a radioactive parent into a stable product nucleus, for example 14 C: 14 6 C ! 14 7 N þ b À However, in many cases. e.g., 238 U, 235 U, 232 Th, there are multiple decays before a stable product nucleus is reached. The decay scheme of 238 U has been illustrated in Fig. 1 as an example. In this, and in the corresponding schemes of 235 U and 232 Th, the decay of the long-lived parent isotope generates shorter-lived isotopes. These short-lived isotopes may be radiologically significant (e.g., 220 Rn and 222 Rn), or they may be exploitable as tracers (e.g., 210 Pb, 234 U, 234 Pa) in natural systems. The number of atoms of any of these short-lived isotopes which is formed depends on the balance between the production rate (i.e., the decay of the parent) and the removal rate (i.e., the decay of the product). The mathematical relationships between the activities of different members of the natural decay series were first derived by Bateman (1910) and are discussed in Choppin et al. (1995). Three cases can be identified, depending on the half-lives of the parent and product isotopes. The most important in the environmental context, and the only one discussed here, is that where a long-lived parent decays to a short-lived product (‘‘secular equilibrium’’). If we isolate an isotopically pure sample of the parent isotope, its decay starts to form the product isotope immediately. Initially, the production rate (¼N parent l parent ) is greater than the removal rate (¼N product l product ), so that the product isotope activity increases with time. Eventually, the situation is reached where production is balanced by 352 Marsden and Livens TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. removal, at which point the product isotope activity is said to be in a ‘‘steady state’’ or ‘‘secular equilibrium’’. An example is provided by the production of 234 Th (t 1/2 24.1 days) from the a-decay of 238 U. The change in the 234 Th activity with time in an initially isotopically pure source of 238 U is illustrated in Fig. 2. III. MEASUREMENT OF RADIOACTIVITY A. Interaction of Radiation with Matter The particles and photons emitted during radioactive decay processes and nuclear reactions are, in general, highly energetic, with energies in the range keV to MeV. These are much greater than the energies typically associated with chemical reactions, where only a few eV are needed to make or break a chemical bond. As a result, the particles and photons are capable of breaking large numbers of chemical bonds or generating a large number of atoms in excited states during their passage through matter, often forming ions and/or free radical species. These processes allow ionizing radiations to be detected very efficiently. The rate of dissipation of energy (linear energy transfer or LET) is an indicator of the efficiency with which a particle or photon loses its kinetic energy during passage through a medium. Different particles and photons have very different LET values; for example, the LET for a-particles in air is of the order of 1 MeV cm À1 , while that for g-photons of moderate energy (about 500 keV) in air is around 1 keV cm À1 . These very different characteristics lead to the use of different types of detectors to measure the different types of radiation even though the vast majority of detectors Figure 2 Ingrowth of 234 Th in 238 U to reach secular equilibrium. Measurement of Radioisotopes and Ionizing Radiation 353 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. measure the ionization or excitation induced by passage of a particle or photon. B. Nucleonics The majority of radiation detectors rely on the interaction of the radiation with the detector medium, which may be a suitable solid, liquid, or gas, and the generation of a measurable electrical signal. This may be generated indirectly, for example through the stimulation of light (visible or UV) emissions, which are converted to electrical impulses, or it may be stimulated directly. Most measurement systems also include amplifiers to boost the often small output from the detectors; analog-to-digital converters (ADCs) to convert the amplifier outputs into digital signals; and data acquisition devices. These may be multichannel analyzers (MCAs, also known as pulse height analyzers, PHAs), where appropriate, which display the output data as a histogram of channel number on the x-axis (proportional to photon or particle energy) against number of counts (proportional to emission intensity), or they may be simpler scalers that just measure the total number of signals. A key parameter in the measurement of detector performance is the energy resolution, that is, the sharpness of the signals generated, which is usually expressed in terms of the full width at half maximum (FWHM), although full width at tenth maximum values are sometimes quoted as well. The other parameter of general importance is the detection efficiency, that is, the probability of a particle or photon being detected. Other parameters can also be defined, for example peak : Compton intensity ratios in g-spectroscopy, or figures of merit in liquid scintillation counting. The use of MCAs allows the accumulation of energy-resolved spectra, which is particularly useful in a- and g-spectroscopies and in some types of b-counting. Accurate timing is, obviously, essential in measuring count rates, and much modern instrumentation is based around personal computers, which are readily adapted to data acquisition and can run the computer programs needed to collect and analyze what can sometimes be complex data. C. Detectors It is probably best to describe each counting technique in turn, since many are adaptable to the measurement of more than one radiation type. Table 1 contains a summary of the techniques that can be used to measure different radiations either in the field or in environmental samples. 354 Marsden and Livens TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. [...]... development of ICP-MS, and there are a number of NAA methods for long-lived radionuclides, principally 129I (Muramatsu and Yoshida, 1995; Parry et al., 1995), 232Th and 238U (Edgington, 1967; Fakhi et al., 1 988 ), and 237Np (Ruf and Friedrich, 19 78; Byrne, 1 986 ; May and Pinte, 1 986 ) For the metal ions, NAA has largely been supplanted by ICP-MS, but it remains very attractive for the measurement of 129I (t1/2... mixed with scintillation cocktail, and counted Two separate energy regions are measured, 20 to 500 keV, in which the 90Sr decays are measured, and 500 to 1500 keV, in which 89 Sr decays are measured 2 Separation of 237Np and a-Emitting Isotopes of Pu and Analysis by ICP-MS and a-Spectrometry (Morris and Livens, 1996) The sample is dry-ashed to destroy organic matter and then leached with mineral acids... Kilius, L R and Synal, H A 1997 36Cl and 129I in the Yenisei, Kolyma and Mackenzie rivers Environ Sci Technol 31: 183 4– 183 6 Bondarenko, O A., Salmon, P L., Henshaw, D L., Fews, A P and Ross, A N 1996 Alpha particle spectroscopy with TASTRAK (CR-39 type) plastic and its application to the measurement of hot particles Nucl Inst Meth Phys Res A 369: 582 – 587 Bersina, I G., Brandt, R., Vater, P., Hinke, K and Schutze,... Paper AED CONF 7 3-0 85 020 Eakins, J D and Morrison, R T 19 78 A new procedure for the determination of lead-210 in lake and marine sediments Int J Appl Radiation Isotopes 29:531–536 Edgington, D N 1967 The estimation of Th and U at the sub-microgram level in bone by neutron activation Int J Pure Appl Isotopes 18: 11– 18 Efurd, D W., Drake, J., Roensch, F R., Cappis, J H and Perrin, R E 1 986 Quantification... 54:2552–2556 Mathew, E., Matkar, V M and Pillai, K C 1 981 Determination of plutonium, americium and curium in environmental materials J Radioanal Chem 62:267–2 78 May, S and Pinte, G 1 986 Neutron activation determination of 237NP in irradiated experimental fuels and in waste solutions and distribution studies in sea water and submarine flora and fauna of disposal areas In: Modern Trends in Activation Analysis... M., Handl, J., Popplewell, D S., Woods, M J., Jerome, S., Bates, T H., Holmes, A., Harvey, B R., Odell, K J., Warren, B B and Young, P 1996 Low level radioactivity ocean sediment standard reference material Appl Radiation Isotopes 47:967–970 Jia, G G., Testa, C., Desideri, D., Guerra, F and Roselli, C 19 98 Sequential separation and determination of plutonium, americium-241 and strontium-90 in soils and. .. Taylor, K Hatton, J C., Dighton, J and Howard, D M., 1991 The potential of a root bioassay for determining P-deficiency in high altitude grassland J Appl Ecol 28: 277– 289 Harvey, B R and Lovett, M B., 1 984 The use of yield tracers in the determination of alpha emitting actinides in the marine environment Nucl Inst Meth Phys Res A 223:224–234 Harvey, B R and Young, A K., 1 988 Determination of natural radionuclides... and americium nuclides down to the 10 mBq level Appl Radiation Isotopes 44:957–966 Hindman, F D 1 986 Actinide separations for a-spectrometry using neodymium fluoride coprecipitation Anal Chem 58: 12 38 1241 Hursthouse, A S., Baxter, M S., McKay, K and Livens, F R 1992 Evaluation of methods for the assay of neptunium and other long-lived actinides in environmental materials J Radioanal Nucl Chem 157: 281 –294... has been eliminated, the residual sample is dissolved in 8 M HNO3 and passed through a Sr Spec extraction chromatography column This consists of 4,40 (50 )-bis-t-butylcyclohexano- 1 8- crown-6, a highly selective complexant for Sr2þ, immobilized on an inert support The column is washed with 12 column volumes of 8 M HNO3, which removes Ca and all other metals Strontium is then eluted from the column with... Radioisotopes and Ionizing Radiation 365 determined by thermal ionization mass spectrometry (McCarthy and Nicholls, 1990), and this latter technique has also been used by several other groups to measure long-lived isotopes (Landrum et al., 1969; Efurd et al., 1 986 ; Taylor et al., 19 98; Goodall and Lythgoe, 1999) In terms of ultimate sensitivity, counting techniques are usually better than ICP-MS This is . extensive delocalized aromatic structures, e.g., 2,5-diphenyloxazole (PPO), used as a primary scintillant, and 1,4-bis-[ 2-( 5-phenyloxazolyl)]-benzene (POPOP), used as a secondary scintillant transforma- tions occur before a stable nucleus is reached. An example is the decay of 2 38 U through a sequence of 14 a -andb-decay steps to the stable isotope 206 Pb (Fig. 1). 3. Short-lived isotopes. d and a denote daughter and a, respectively and v a v d ¼ M d M a ¼ 234 4 ¼ 58: 5 ð2Þ The ratio of the kinetic energies E d /E a is thus 234 Âð1Þ 2 4 Âð 58: 5Þ 2 ¼ 0:017 In other words, the a-part