Radionuclide Concentrations in Foor and the Environment - Chapter 2 docx

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 Ack

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 14C, 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]

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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 (40K), 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 108 years have since decayed to undetectable levels Furthermore, the primordial radionuclides with half-lives greater than

1010 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 232Th, one of the most abundant of the primordial radionuclides, has a half-life of 1.4 × 1010 years and is found at concentrations of 1.5 to 20 ppm in most crustal rocks [1] 238U, another abundant primordial radionuclide, is typically found at concentrations in the low parts per million in minerals and rocks Both 232Th and 238U 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

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 234U, 235U, and 238U 238U, by far the most abundant of the three, has a half-life of 4.47 × 109 years Thus about half of its original primordial level

at Earth’s formation remains In comparison, 235U 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 234U, 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, 234U is a good example of a secondary radionuclide, as it is produced in small quantities by the radioactive decay of the parent 238U (see Figure 2.1) As we discuss later, 235U 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)

87 Rb 4.8 × 10 10 3–9

187 Re 4.0 × 10 10 3–5 × 10 4

232 Th 1.4 × 10 10 1–20

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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

238U and 232Th, respectively Figure 2.3 shows the decay processes for 235U 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 40K, which has a half-life of 1.28 × 109 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 40K atoms 40K can decay by γ emission (11% of the decay pathway) to give 40Ar, and this is the basis for the potassium/argon methods used

to age date very old rocks, meteorites, etc 40K can also emit a β particle and lead

to the formation of 40Ca (89% of the decay processes) Because of its ubiquity and biological uptake, 40K 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 Isotopes and updates [5–7].

4.7 × 10 9 y 234 Th 24 d 4.2 m

2.4

× 10 5 y 3.8 d 222 Rn 1.6 × 10 3 y 226Ra 7.7 × 10 4 y

214 Pb

3 m

214 Bi 20 m 214 Po 6.4 × 10 −5s 210Pb

210 Bi

22.3 y

5 d

210 Po

138 d

27 m

206 Pb

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Radionuclide Sources 27

A very important and widespread secondary radionuclide is 222Rn This noble gas, with a half-life of 3.8 days, is produced from the longer-lived 226Ra (half-life 1,600 years) formed by the decay of 238U (see Figure 2.1) As a gas, 222Rn 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 210Pb, which has a half-life of 22.3 years The fine aerosol

210Pb and its daughters 210Bi (half-life 5 days) and 210Po (half-life 138 days) have been used to estimate the residence times of submicron aerosols in the environ-ment [8–10] 222Ra 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 222Rn 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]

210Pb is another very ubiquitous secondary radionuclide that is formed from

238U decay via 222Rn (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 228 Ac 6.1 h

1.9 y

56 s

212 Pb 216 Po 220 Rn 224 Ra

11 h

208 Tl

61 m (64%)

3.1 m

61 m

(36%)

3.1

× 10 −7s

208 Pb

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28 Radionuclide Concentrations in Food and the Environment

in the lower to mid troposphere once it is produced from the gaseous 222Rn, 210Pb

can spread over significant distances Indeed, a significant amount of 210Pb is

usually present in the upper sections of soil cores because of the atmospheric

deposition of 210Pb (half-life 22.3 years) Concentrations of 210Pb 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 210Pb in subsurface soils is due to 222Rn decay as the gas diffuses from

soil and rock matrices This background level of 210Pb is considered to be

sup-ported by the local soil environment The “excess” 210Pb found in the soil closer

to the surface is due to 222Rn gas that is dispersed through the lower atmosphere

and decays to produce 210Pb, which becomes attached to fine aerosol particles

and is deposited on soil surfaces by wet and dry deposition The presence of this

“excess” 210Pb 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 14C, 238U, and other radionuclides

are found in the oceans as well These accumulations are due to the equilibration

of 14CO2 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 231Th 25 h

235 U 231 Pa 227 Ac

22 y (99%)

4 s 219 Rn 11 d 223 Ra 19 d

211 Pb

8 ×

10 −4s

211 Bi 207 Tl 207 Pb

223 Fr

22 y (1%)

22 m 3.3 × 10 4 y

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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

3H, 7Be, 10Be, 14C, and 22Na 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., 10Be,

26Al, and 21Ne) 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

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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 14C and 7Be 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” (14C 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

14C is one of the more important natural radionuclides, being produced in the atmosphere by cosmic particle bombardment of nitrogen atoms Once formed, the atomic 14C is rapidly oxidized to carbon dioxide in the upper atmosphere The 14C-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 14C is carried throughout the food chain, ultimately labeling all living things on the surface of the planet With a half-life of 5.73 × 103 years, the abundance of 14C 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 14C content [19] 14C 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 10Be, 26Al, and 36Cl 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 14C 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 26Al is meteoric material that strikes the atmosphere or Earth’s surface Cross sections for atmospheric production of this radioisotope are small, because 26Al

is largely produced from argon, which composes only 1% of the atmosphere by volume In contrast, the production of 26Al 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

Radionuclide Sources 31

difference has led to the use of 26Al 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 18F, 22Na, 24Na, 31Si, 32Si, 32P,

33P, 35S, 37Ar, 38Cl, 38Mg, 38S, 39Ar, 39Cl, and 80Kr, as well as stable radionuclides like 3He [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 141Ba, which was correctly explained by Lise Meitner and Otto Frisch [25] as a fission product of 235U 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 235U 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

32 Radionuclide Concentrations in Food and the Environment

atoms, leading to a chain reaction 235U 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 235U (0.72% natural abundance) from the predominantly 238U 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 235U required for atomic weapons is a difficult and expensive task

In a nuclear reactor, the chain reaction with 235U releases energy and neutrons and produces a number of side products, including isotopes of plutonium from neutron capture by 238U in the fuel rods 239Pu produced through exposure of 238U

to neutrons is also fissionable Both 235U and 239Pu 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 235U and 239Pu, respectively Other fissionable materials, including 233U and 232Th, could conceiv-ably be used in nuclear fuel cycles, although currently 235U and 239Pu are the main fuels used 239Pu is produced from 238U by neutron irradiation, usually in 235U/238U

“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 235U, 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 235U, the major fission products are 137Cs and 90Sr 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 238U with neutrons was considered the only source

of 238Pu, 239Pu, 240Pu, and 241Pu 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 239Pu, which is fissionable and has a long half-life (2.4 ×

104 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 239Pu 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 239Pu 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 239Pu Some other anthropogenic radionuclides include 131I, which has a half-life of DK594X_book.fm Page 32 Tuesday, June 6, 2006 9:53 AM