23 2 Radionuclide Sources Jeffrey S. Gaffney and Nancy A. Marley CONTENTS 2.1 Introduction 23 2.2 Primordial and Secondary Radionuclides 24 2.3 Cosmogenic Radionuclides 29 2.4 Anthropogenic Sources 31 2.5 Concluding Remarks 33 Acknowledgments 34 References 35 2.1 INTRODUCTION We live on a planet that was created by the initial forces of the “big bang” and continues to be affected by both natural events and human activities. The global environment that surrounds us contains small amounts of radioactive (unstable) elements or radionuclides (radioisotopes) that are derived from primordial, sec- ondary, cosmogenic, and anthropogenic sources. Radionuclides in the air, soil, water, and rocks that make up the Earth’s geosphere and atmosphere can be transferred into the biosphere by many organisms and bioaccumulated in the food chain. Indeed, the well-known uptake by living organisms of measurable amounts of naturally produced radionuclides, such as 14 C, is used as a means of differen- tiating living from “fossil” carbon. Most of the radioactivity to which we are exposed daily comes from background natural sources commonly occurring in our surrounding environment and the buildings in which we live. Chapter 1 defines radionuclides and discusses the most common types of ionizing radiation, namely α particles (energetic helium nuclei), β particles (ener- getic electrons), and γ radiation (high-frequency, highly energetic electromagnetic radiation). This chapter deals with the natural and anthropogenic sources of radionuclides found in the environment. Addressing all of the more than 1,500 known radionuclides is beyond the scope of this chapter. We will focus on isotopic species that are important contributors to overall radionuclide abundances in various media, whose distributions in air, water, and soil are the topic of later chapters. More detailed information can be found in more extensive books on the sources of radionuclides, both natural and man-made [1]. DK594X_book.fm Page 23 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 24 Radionuclide Concentrations in Food and the Environment Traditionally radionuclides have been separated into three categories or types: (1) primordial and secondary, (2) cosmogenic, and (3) anthropogenic. Primordial radionuclides, such as uranium, thorium, and certain isotopes of potassium, have very long lifetimes and were produced at or before the creation of planet Earth. Secondary radionuclides are derived through radioactive decay of the long-lived primordial parent nuclides. These decay products are commonly referred to as daughters. Along with the parent sources, the daughters constitute radiogenic decay families or “chains” that are an important source of natural radioactivity. Cosmogenic radionuclides are formed by the interaction of cosmic rays with Earth’s atmosphere or lithosphere, while anthropogenic radionuclides are formed from human activities that create artificial radionuclides or enhance the levels of certain radionuclides already present on Earth. In this chapter we discuss the three types of radionuclide sources separately and highlight some of the more important examples. 2.2 PRIMORDIAL AND SECONDARY RADIONUCLIDES The primordial radionuclides have radioactive decay half-lives that are approxi- mately Earth’s age or older (i.e., about 4 to 5 billion years). Primordial radio- nuclides (and the radioactive decay products they produce) are an important source of Earth’s radioactivity. These radionuclides play an important role in the Earth’s processes. Indeed, primordial radionuclides, in particular a potassium isotope of mass 40 ( 40 K), have been suggested as a key source of long-term heat in the Earth’s core over the past 4.5 billion years [2]. The human population is exposed to radiation from primordial radionuclides directly, as a result of external exposure, or through incorporation of these radionuclides into the body through inhalation or ingestion. The primordial radionuclides present when the Earth was formed that have half-lives less than 10 8 years have since decayed to undetectable levels. Furthermore, the primordial radionuclides with half-lives greater than 10 10 years do not make significant contributions directly to background radiation because their half-lives are long and their specific radioactivity levels are low. However, they do contribute significantly to natural background levels of radio- activity through their radioactive progeny or daughters, which often have much shorter half-lives and lead to a chain of radioactive isotope production. The primordial radionuclides compose a significant portion of the natural radionuclides present on Earth because they are significantly long-lived and have half-lives long enough to have been present at the beginning of the Earth’s formation. Table 2.1 lists some of the more important primordial radionuclides and their half-lives. Included are uranium and thorium isotopes having half-lives on the order of 1 to 10 billion years. 232 Th, one of the most abundant of the primordial radionuclides, has a half-life of 1.4 × 10 10 years and is found at concentrations of 1.5 to 20 ppm in most crustal rocks [1]. 238 U, another abundant primordial radionuclide, is typically found at concentrations in the low parts per million in minerals and rocks. Both 232 Th and 238 U are concentrated in coals and peats, indicating that the bioaccumulation of these species has occurred over long DK594X_book.fm Page 24 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radionuclide Sources 25 periods of time. The humin and humic materials that are known to produce coals and peat are strong chelating agents for these and other radionuclides [3]. Indeed, the first discoveries of radioactivity and the isolation of important radionuclides by the Curies and other pioneers in this area came in work with pitchblende and peat known to be enriched in radionuclides through the interaction of organic materials with rocks and minerals containing radioactive isotopes and elements. Uranium was identified as an element by the German chemist Martin Klaproth, who isolated it from samples of pitchblende in 1789. It was not until 1841 that uranium was isolated in metallic form by the French chemist Eugene-Melchior Peligot. Most of the early interest in this element grew from its ability to add color to ceramics and paints. In 1896 the applied physicist Antoine Henri Becquerel reported that all uranium salts are radioactive. This work led to his sharing the 1906 Nobel Prize in physics with Pierre and Marie Curie for the discovery of spontaneous radioactivity [4]. The three naturally occurring isotopes of uranium are 234 U, 235 U, and 238 U. 238 U, by far the most abundant of the three, has a half-life of 4.47 × 10 9 years. Thus about half of its original primordial level at Earth’s formation remains. In comparison, 235 U is fairly depleted from its original levels, having passed through more than six half-lives since Earth’s origin. These two isotopes are both primordial, but 234 U, having a much shorter half-life, would have essentially disappeared from the planet after more than 18,000 half- life periods since its formation. However, 234 U is a good example of a secondary radionuclide, as it is produced in small quantities by the radioactive decay of the parent 238 U (see Figure 2.1). As we discuss later, 235 U and other isotopes that are fissionable by neutrons have played an important role in anthropogenic radio- nuclide production. The uranium isotopes are all radioactive, and their decay produces a number of secondary radioactive elements that continue to decay until they reach stable nuclei. This decay chain of radionuclides is commonly referred to as the uranium decay series. Similarly thorium, another primordial isotope with a long half-life, also has a decay series that leads to the formation of numerous naturally occurring TABLE 2.1 Some Important Primordial Radionuclides Radionuclide Half-life (Years) Estimated Abundance in Crust (ppm) 40 K 1.38 × 10 9 2–3 87 Rb 4.8 × 10 10 3–9 138 La 1.1 × 10 11 1 × 10 2 to 2 × 10 2 147 Sm 1.1 × 10 11 0.5–1 187 Re 4.0 × 10 10 3–5 × 10 4 232 Th 1.4 × 10 10 1–20 235 U 7.0 × 10 8 0.3 × 10 2 to 3 × 10 2 238 U 4.5 × 10 9 0.5–5 DK594X_book.fm Page 25 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 26 Radionuclide Concentrations in Food and the Environment secondary radionuclides. Thus the key primordial radionuclides of uranium and thorium decay to many other radioactive isotopes that occur in the environment at different levels of abundance, depending on their own decay rates and those of their parents. Figure 2.1 and Figure 2.2 show the decay schemes for primordial 238 U and 232 Th, respectively. Figure 2.3 shows the decay processes for 235 U. Only the major pathways are shown in these figures, with the significant γ emitters highlighted in bold type. More detailed information on the isotopic decay processes, including minor pathways, can be obtained from the Table of Isotopes [5–7]. Other primordial isotopic species on the Earth’s surface include 40 K, which has a half-life of 1.28 × 10 9 years. Potassium is quite an abundant element, composing more than 2% of the Earth’s crustal mass. Of that amount, about 1.0 × 10 –4 (0.01%) is 40 K atoms. 40 K can decay by γ emission (11% of the decay pathway) to give 40 Ar, and this is the basis for the potassium/argon methods used to age date very old rocks, meteorites, etc. 40 K can also emit a β particle and lead to the formation of 40 Ca (89% of the decay processes). Because of its ubiquity and biological uptake, 40 K is the most significant natural source of radioactivity ingested by humans. FIGURE 2.1 Uranium 238 decay, showing the main paths for the production of various radionuclides. Clear arrows indicate β decay and gray arrows are α processes. Half-lives for the decay processes are indicated inside the arrows. The major γ emitters are in bold letters. For complete radioactive decay processes, refer to Table of Isotope s and updates [5–7]. 4.7 × 10 9 y 234 24 d 4.2 m 238 U 234 Pa 234 U 2.4 × 10 5 y 3.8 d 1.6 × 10 3 y 7.7 × 10 4 y 222 Rn 226 Ra 218 Po 230 214 Pb 3 m 214 Bi 214 Po 210 Pb 20 m 6.4 × 10 −5 s 210 Bi 22.3 y 5 d 210 Po 138 d 27 m 206 Pb DK594X_book.fm Page 26 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radionuclide Sources 27 A very important and widespread secondary radionuclide is 222 Rn. This noble gas, with a half-life of 3.8 days, is produced from the longer-lived 226 Ra (half- life 1,600 years) formed by the decay of 238 U (see Figure 2.1). As a gas, 222 Rn can diffuse through the crustal material into the atmosphere, where it can be transported over continental regions. Its decay products attach themselves to fine atmospheric aerosols in the respirable size range. The dominant secondary radi- onuclide in this chain is 210 Pb, which has a half-life of 22.3 years. The fine aerosol 210 Pb and its daughters 210 Bi (half-life 5 days) and 210 Po (half-life 138 days) have been used to estimate the residence times of submicron aerosols in the environ- ment [8–10]. 222 Ra and its progeny have been of particular concern as environ- mental hazards, particularly in homes and buildings where air infiltration rates can be low. Significant 222 Rn from ground-source uranium parents (see Figure 2.1) can concentrate in the lower levels of buildings (cellars, basements, etc.) and lead to potential inhalation risks in indoor environments [11]. 210 Pb is another very ubiquitous secondary radionuclide that is formed from 238 U decay via 222 Rn (see Figure 2.1). Because it attaches itself to fine aerosols FIGURE 2.2 Thorium 232 decay, showing the main paths for the production of various radionuclides. Clear arrows indicate β decay and gray arrows are α processes. Half-lives for the decay processes are indicated inside the arrows. The major γ emitters are in bold letters. For complete radioactive decay processes, refer to Table of Isotopes and updates [5–7]. 1.4 × 10 10 y 228 Ra 5.8 y 6.1 h 228 Ac 232 228 1.9 y 56 s 0.15 s 3.7 d 212 Pb 216 Po 220 Rn 224 Ra 11 h 212 Bi 212 Po 208 Tl 61 m (64%) 3.1 m 61 m (36%) 3.1 × 10 −7 s 208 Pb DK594X_book.fm Page 27 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 28 Radionuclide Concentrations in Food and the Environment in the lower to mid troposphere once it is produced from the gaseous 222 Rn, 210 Pb can spread over significant distances. Indeed, a significant amount of 210 Pb is usually present in the upper sections of soil cores because of the atmospheric deposition of 210 Pb (half-life 22.3 years). Concentrations of 210 Pb usually decrease as a function of distance downward in soil cores, gradually diminishing from the surface to a fairly constant level within about 1 m of the surface. The background level of 210 Pb in subsurface soils is due to 222 Rn decay as the gas diffuses from soil and rock matrices. This background level of 210 Pb is considered to be sup- ported by the local soil environment. The “excess” 210 Pb found in the soil closer to the surface is due to 222 Rn gas that is dispersed through the lower atmosphere and decays to produce 210 Pb, which becomes attached to fine aerosol particles and is deposited on soil surfaces by wet and dry deposition. The presence of this “excess” 210 Pb in surface soils due to atmospheric deposition has been useful for estimating soil sedimentation rates and erosion rates in many environments [12–17]. While many of the primordial and cosmogenic radionuclides are concentrated in Earth’s lithosphere, significant amounts of 14 C, 238 U, and other radionuclides are found in the oceans as well. These accumulations are due to the equilibration of 14 CO 2 with ocean waters and the dissolution of minerals into fresh and ocean waters from rocks and soil erosion. Many primordial and secondary radionuclides are also found in fresh surface waters and groundwaters at low concentrations. FIGURE 2.3 Uranium 235 decay, showing the main paths for the production of various radionuclides. Clear arrows indicate β decay and gray arrows are α processes. Half-lives for the decay processes are indicated inside the arrows. The major γ emitters are in bold letters. For complete radioactive decay processes, refer to Table of Isotopes and updates [5–7]. 7.0 × 10 8 y 231 25 h 235 U 231 Pa 227 Ac 22 y (99%) 4 s 11 d 19 d 219 Rn 223 Ra 215 Po 227 211 Pb 8 × 10 −4 s 211 Bi 207 Tl 207 Pb 36 m 2.1 m 4.8 m 223 Fr 22 y (1%) 22 m 3.3 × 10 4 y DK594X_book.fm Page 28 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radionuclide Sources 29 These are typically chemically bound or chelated by dissolved organic substances, principally humic and fulvic acids, that can limit the bioavailability of these materials in the natural environment [3]. 2.3 COSMOGENIC RADIONUCLIDES Cosmogenic radionuclides are formed by interactions of highly energetic cosmic particles with Earth’s atmosphere and surface that lead to the formation of radio- active isotopes [18]. Some important cosmogenically produced radionuclides and their lifetimes are shown in Table 2.2. Galactic cosmic rays that are capable of nuclear interactions lead to the direct formation of radionuclides and also generate secondary particles, particularly neutrons, that can result in the production of important radionuclides including 3 H, 7 Be, 10 Be, 14 C, and 22 Na. Most of these interactions occur in Earth’s atmo- sphere, particularly in the stratosphere and upper troposphere. However, some minor production of radioisotopes also occurs at the Earth’s surface (e.g., 10 Be, 26 Al, and 21 Ne) and their presence is an important indicator of cosmic ray activity. The differences in production rates in the atmosphere and surface are primarily due to the strong attenuation of many cosmic particles and secondary particles by Earth’s atmosphere, with the result that more high-energy particle interactions occur in the upper atmosphere than at the surface, where the flux is smaller. Production rates of many cosmogenic radionuclides depend on incoming cosmic particle intensities, which can be affected by Earth’s magnetic field or can vary due to solar activity (e.g., sunspots or solar flares). The variations in the cosmic radiation fluxes lead to some temporal and spatial variability in the pro- duction of these radionuclides. Thus relatively short-term variations (on the order of days) and seasonal variations can result from solar events and changes in cosmic ray intensities. In addition, variability in latitudinal production arises because Earth’s magnetic field can focus incoming cosmic rays, leading to more significant production at higher latitudes. The northern lights (the aurora borealis) TABLE 2.2 Some Important Cosmogenically Produced Radionuclides and Their Half-Lives Radionuclide Half-Life Major Source 3 H 12.3 years Atmospheric N, O 7 Be 53.3 days Atmospheric N, O 10 Be 1.5 × 10 6 years Atmosphere N, O, surface O 14 C 5.73 × 10 3 years Atmospheric N 26 Al 7.1 × 10 5 years Surface Si, meteorites 36 Cl 3.1 × 10 5 years Surface Ca, K DK594X_book.fm Page 29 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 30 Radionuclide Concentrations in Food and the Environment are evidence of the increased production of radionuclides and the atmospheric effects of cosmic rays on the atmosphere at higher latitudes. Cosmic radiation consists primarily of highly energetic particles, including α particles and neutrons. Their effects in the upper atmosphere lead to nuclear interactions with nitrogen and oxygen atoms and molecules resulting in the production of 14 C and 7 Be and other radionuclides, directly or via secondary neutron interactions. These same types of nuclear reactions can occur during aboveground nuclear tests that release energetic particles into the atmosphere. These reactions produce secondary particles (neutrons, protons, etc.) that can generate the same radionuclides as normal cosmic radiation exposures. Thus, during the 1950s, significant amounts of “bomb carbon” ( 14 C produced from aboveground nuclear tests) were produced, along with other radionuclides that will be discussed later in this chapter, by the same processes that occur naturally. 14 C is one of the more important natural radionuclides, being produced in the atmosphere by cosmic particle bombardment of nitrogen atoms. Once formed, the atomic 14 C is rapidly oxidized to carbon dioxide in the upper atmosphere. The 14 C-labeled carbon dioxide, quite a stable molecule, is mixed from the upper atmosphere down into the troposphere, where it is taken up by plants during photosynthesis. As herbivores and omnivores ingest plants for food, the 14 C is carried throughout the food chain, ultimately labeling all living things on the surface of the planet. With a half-life of 5.73 × 10 3 years, the abundance of 14 C has been used to differentiate recent carbon present in samples from “fossil” carbon derived from petroleum that is hundreds of millions of years old and is quite “dead” with regard to 14 C content [19]. 14 C is also the basis for carbon dating of organic artifacts in archeology. Cosmic particle-driven neutron spallation reactions near the Earth’s surface can lead to the formation of some important radionuclides that have been used for geochronology, such as 10 Be, 26 Al, and 36 Cl. Estimation of the production rates of cosmogenic nuclides requires an understanding of the cross sections for the nuclear reactions, along with estimates of cosmic ray fluxes that vary with geomagnetic latitude and altitude. Modeling that incorporates experimentally derived cross sections for gases and minerals has been used to estimate radionuclide production rates [20,21]. These production rates are then compared with direct measurements to evaluate the estimated results and also to probe past cosmic ray activity by examining the variance and concentrations of surface radionuclides of various lifetimes. Since the first measurements of 14 C by Willard Libby and coworkers [22], these cosmogenic isotopes have been used for geochronology, becoming important tools for the “dating” of events in geochemistry and geomorphology. An extraterrestrial source for some of the heavier cosmogenic nuclides such as 26 Al is meteoric material that strikes the atmosphere or Earth’s surface. Cross sections for atmospheric production of this radioisotope are small, because 26 Al is largely produced from argon, which composes only 1% of the atmosphere by volume. In contrast, the production of 26 Al can be quite high on the mineral surfaces of meteors because of the higher cosmic ray exposures in space. This DK594X_book.fm Page 30 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC Radionuclide Sources 31 difference has led to the use of 26 Al to evaluate meteoritic material deposition on the Earth’s surface and to measure ice surface ages in the Antarctic [23,24]. There are indeed many trace-level cosmogenically produced radionuclides besides the ones we have discussed here, including 18 F, 22 Na, 24 Na, 31 Si, 32 Si, 32 P, 33 P, 35 S, 37 Ar, 38 Cl, 38 Mg, 38 S, 39 Ar, 39 Cl, and 80 Kr, as well as stable radionuclides like 3 He [1]. Many of the cosmogenic radioisotopes with longer half-lives are difficult to measure with conventional radiochemical counting methods. Because they have low radioactivity levels, they are measured directly by using accelerator mass spectrometry methods that have enhanced sensitivity and speed. Cos- mogenic radioisotopes have been used to estimate surface ages of the Earth because their general production rates have remained fairly constant over time. Examination of the surface concentrations of the longer-lived radionuclides clearly indicates that Earth’s surface has been exposed to cosmic radiation for millions of years, at a minimum. These data have been used as an effective argument against the concept of a much shorter time for Earth’s creation that has been put forth by some creationist philosophies. 2.4 ANTHROPOGENIC SOURCES Most of the radionuclides present on Earth are from primordial or cosmogenic sources, as noted above. During the early 1930s, a series of events that would change history and the world we live in began in the physics and chemistry communities. Following Enrico Fermi’s lead in exploring the interactions of heavy nuclei with neutrons, Otto Hahn and Fritz Strassman attempted to make heavier elements (transuranics) by bombarding uranium with neutrons. They were able to identify the production of 141 Ba, which was correctly explained by Lise Meitner and Otto Frisch [25] as a fission product of 235 U. Soon, Niels Bohr and others recognized that the release of very large amounts of energy from nuclear fission might be useful for both peaceful and military applications. Letters from Bohr to Einstein and from Einstein to President Franklin Roosevelt ultimately led to the initiation of the Manhattan Project in the U.S. in June 1942 [26]. As part of the Manhattan Project, a group led by Enrico Fermi began to build a uranium-based reactor that they hoped would demonstrate the potential for a controlled chain reaction starting with 235 U. On December 2, 1942, the first self- sustained chain reaction, using enriched uranium oxide moderated by graphite rods, was achieved at the University of Chicago’s Stagg Field stadium. This initial experiment demonstrating controlled nuclear fission led to the development of atomic weapons and nuclear industries in medicine and energy [26]. It also was the dawn of development of many radionuclides produced by humans for widely ranging uses including nuclear reactors, nuclear medicine, and nuclear weapons. Nuclear fission is the process by which neutrons produce chain reactions in a nuclear reactor. When a fissionable nucleus is hit by a thermal or slow neutron, the nucleus can interact with the neutron and divide (fission) into two smaller nuclei, releasing neutrons and energy that initiate the splitting of more fissionable DK594X_book.fm Page 31 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC 32 Radionuclide Concentrations in Food and the Environment atoms, leading to a chain reaction. 235 U is the most abundant naturally available isotope that can undergo fission. Gaseous diffusion and other methods are used to enrich and separate the small amount of 235 U (0.72% natural abundance) from the predominantly 238 U found in nature. For most nuclear reactors, such as the light-water reactors, the enrichment required for a sustained nuclear reaction is approximately 10-fold. The more significant enrichment of 235 U required for atomic weapons is a difficult and expensive task. In a nuclear reactor, the chain reaction with 235 U releases energy and neutrons and produces a number of side products, including isotopes of plutonium from neutron capture by 238 U in the fuel rods. 239 Pu produced through exposure of 238 U to neutrons is also fissionable. Both 235 U and 239 Pu have been used in nuclear reactors and in atomic weapons. Indeed, the first atomic weapons used in World War II, “Little Boy” and “Fat Man,” were bombs that used 235 U and 239 Pu, respectively. Other fissionable materials, including 233 U and 232 Th, could conceiv- ably be used in nuclear fuel cycles, although currently 235 U and 239 Pu are the main fuels used. 239 Pu is produced from 238 U by neutron irradiation, usually in 235 U/ 238 U “breeder” reactors. Fission reactions lead to the formation of many isotopes (both stable and radioactive) from a wide variety of elements, as many fragment combinations are possible and do occur. For 235 U, the addition of one neutron would lead to two fission nuclei of 118 mass units if the process gave two equally sized nuclei. However, the fission reaction leads mostly to fragments of unequal sizes. For the case of 235 U, the major fission products are 137 Cs and 90 Sr. During aboveground testing of atomic bombs, a significant amount of anthropogenic radionuclides was released into the stratosphere and upper troposphere. This material became attached to particulate matter in the atmosphere and was deposited worldwide as “radioactive fallout.” Early on, bombardment of 238 U with neutrons was considered the only source of 238 Pu, 239 Pu, 240 Pu, and 241 Pu because plutonium was not a known natural radionuclide until its discovery in 1940 by Glenn Seaborg and colleagues. The 15 known isotopes of plutonium are mostly short-lived. The most important of these, as noted above, is 239 Pu, which is fissionable and has a long half-life (2.4 × 10 4 years). Not until the early 1970s did discovery of the remains of a natural fission reactor system in the Oklo district of Gabon, Africa, provide evidence that plutonium production could occur naturally [27–29]. The Oklo area is very high in uranium. Analysis of mines there yielded anomalous isotopic data indicating that neutron chain reaction events might have occurred under natural water medi- ation of the deposits. Furthermore, very low levels of 239 Pu produced more recently by normal neutron capture in uranium cores were measured in samples from the site [27,29]. Although these results demonstrate that natural production of 239 Pu is possible, it is safe to say that most of the plutonium currently present on Earth came from anthropogenic sources. Many of the anthropogenic radionuclides produced from nuclear power or nuclear bomb tests have reasonably short half-lives with the exception of 239 Pu. Some other anthropogenic radionuclides include 131 I, which has a half-life of DK594X_book.fm Page 32 Tuesday, June 6, 2006 9:53 AM © 2007 by Taylor & Francis Group, LLC [...]... radioisotopes, including 3H, 11C, 13N, 14C, 15O, 99Tc, 123 I, 125 I, and 131I, are produced by the use of special equipment (nuclear reactors, cyclotrons, etc.) developed in the high-energy physics community and used routinely in nuclear medicine [30] Some of the more important anthropogenic radionuclides are listed in Table 2. 3, along with their half-lives 2. 5 CONCLUDING REMARKS It should be noted that the sources... J.S., Cunningham, M.M., Orlandini, K.A., Paode, R., and Drayton, P.J., Measurement of 21 0Pb, 21 0Po, and 21 0Bi in size fractionated atmospheric aerosols: an estimate of fine aerosol residence times, Aerosol Sci Technol., 32, 569–583, 20 00 11 U.S EPA, A Citizen’s Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon, EPA 4 0 2- K-0 2- 0 06, Indoor Environments Division (6609J), U.S Environmental... Edition, DOE/NE-0046, Office of the Assistant Secretary for Nuclear Energy, U.S Department of Energy, Washington, DC, 19 82 27 Cowan, G.A., A natural fission reactor, Sci Am., 23 5, 36, 1976 28 Myers, W.A and Lindner, M., Precise determination of the natural abundance of 23 7Np and 23 9Pu in Katanga pitchblende, J Inorg Nucl Chem., 33, 323 3– 323 8, 1971 29 Curtis, D., Fabryka-Martin, J., Dixon, P., and Cramer,... ages and bombardment history, Proceedings of the 10th Lunar and Planetary Science Conference, Lunar and Planetary Institute, Houston, 1061–10 72, 1979 24 Evans, J.C and Reeves, J.H., 26 Al survey of Antarctic meteorites, Earth Planet Sci Lett., 82, 22 3 23 0, 1987 25 Meitner, L and Frisch, O.R., Disintegration of uranium by neutrons: a new type of nuclear reaction, Nature, 143, 23 9 24 0, 1939 26 U.S DOE, The. .. June 6, 20 06 9:53 AM 36 Radionuclide Concentrations in Food and the Environment 15 Olley, J.M., Murray, A.S., Mackenzie, D.H., and Edwards, K., Identifying sediment sources in a gullied catchment using natural and anthropogenic radioactivity, Water Resour Res., 29 , 1037–1043, 1993 16 Walling, D.E., Owens, P.N., and Leeks, G.J.L., Fingerprinting suspended sediment sources in the catchment of the River... using beryllium-7, cesium-137, and lead -2 1 0 J Environ Qual., 31, 54–61, 20 02 14 Walling, E.E and Woodward, J.C., Use of radiometric fingerprints to derive information on suspended sediment sources, in Erosion and Sediment Transport Monitoring Programmes in River Basins, publication 21 0, International Association of Hydrological Sciences, Wallingford, UK, 19 92, pp 153–164 © 20 07 by Taylor & Francis Group,... that the short-lived radionuclides produced during fission and neutron release can decay before the rods are reprocessed with the extraction of 23 9Pu and unburned 23 5U Alternatively, the longer-lived radionuclide waste is placed in a geologic repository like the Yucca Mountain site Other sources of radioactivity include medical radioactive sources used for nuclear medicine, typically operated in hospitals... York, 1999 8 Gaffney, J.S., Marley, N.A., and Cunningham, M.M., Natural radionuclides in fine aerosols in the Pittsburgh area Atmos Environ., 38, 3191– 320 0, 20 04 9 Marley, N.A., Gaffney, J.S., Orlandini, K.A., Drayton, P.J., and Cunningham, M.M., An improved method for the separation of 21 0Bi and 21 0Po from 21 0Pb using solid phase extraction disk membranes: environmental applications, Radiochim Acta,... testing of an atomic device aboveground are releases of anthropogenic radionuclides significant on a regional or global scale Other sources of radionuclide release are tied to the nuclear energy industry in the transport of fuel rods and the reprocessing and disposal of spent fuel rods In many instances spent uranium fuel rods are allowed to sit in interim cooling sites (ponds or air-cooled containers)... 5, 1963 France and the Republic of China continued to test nuclear weapons aboveground and in the oceans until 1996, but these tests were few in comparison with the aboveground tests of the U.S and USSR Although it has not been approved by the U.S., a comprehensive test ban treaty drawn up by 37 nations in 1996 is likely to prevent the kind of aboveground nuclear testing that led to the dispersal of . h 22 8 Ac 23 2 22 8 1.9 y 56 s 0.15 s 3.7 d 21 2 Pb 21 6 Po 22 0 Rn 22 4 Ra 11 h 21 2 Bi 21 2 Po 20 8 Tl 61 m (64%) 3.1 m 61 m (36%) 3.1 × 10 −7 s 20 8 Pb DK594X_book.fm Page 27 Tuesday, June 6, 20 06. m 23 8 U 23 4 Pa 23 4 U 2. 4 × 10 5 y 3.8 d 1.6 × 10 3 y 7.7 × 10 4 y 22 2 Rn 22 6 Ra 21 8 Po 23 0 21 4 Pb 3 m 21 4 Bi 21 4 Po 21 0 Pb 20 m 6.4 × 10 −5 s 21 0 Bi 22 .3 y 5 d 21 0 Po 138 d 27 m 20 6 Pb DK594X_book.fm. d 21 9 Rn 22 3 Ra 21 5 Po 22 7 21 1 Pb 8 × 10 −4 s 21 1 Bi 20 7 Tl 20 7 Pb 36 m 2. 1 m 4.8 m 22 3 Fr 22 y (1%) 22 m 3.3 × 10 4 y DK594X_book.fm Page 28 Tuesday, June 6, 20 06 9:53 AM © 20 07 by Taylor