Environmental radioactivity
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2008-06-22T17:47:18Z
Lathrop1885
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/* Sea and river silt */
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The '''environmental [[radioactivity]]''' page is devoted to the subject of radioactive materials in the [[human]] [[Ecosystem|environment]]. While some [[isotopes]] are only found on [[Earth]] as a result of human activity (e.g. <sup>90</sup>Sr and <sup>99</sup>Tc), and some isotopes are only present due to natural processes (e.g. <sup>40</sup>K), a few isotopes are present as a result of both natural processes and human activities (e.g. [[tritium]]). The concentration and location of some natural isotopes (such as <sup>238</sup>U) can be affected by human activity. For more details of [[uranium]] and [[radium]] please see [[Uranium in the environment]] and [[radium in the environment]].
== Background level in soils ==
Radioactivity is present everywhere (and has been since the formation of the earth). According to the [[IAEA]], one kilogram of soil typically contains the following amounts of the following three natural radioisotopes 370 Bq <sup>40</sup>K (typical range 100-700 Bq), 25 Bq <sup>226</sup>Ra (typical range 10-50 Bq), 25 Bq <sup>238</sup>U (typical range 10-50 Bq) and 25 Bq <sup>232</sup>Th (typical range 7-50 Bq).<ref>Generic Procedures for Assessment and Response during a Radiological Emergency, IAEA TECDOC Series number 1162, published in 2000 [http://www-pub.iaea.org/MTCD/publications/PubDetails.asp?pubId=5926]</ref> These values are average values and some soils may varry greatly from these norms.
=== Sea and river silt ===
A recent report on the [[Sava]] river in [[Serbia]] suggests that many of the river silts contain about 100 Bq kg<sup>-1</sup> of natural radioisotopes (<sup>226</sup>Ra, <sup>232</sup>Th and <sup>238</sup>U).<ref>Z. Vukovic, V. Sipka, D. Todorovic and S. Stankovic, ''Journal of Radioanalytical and Nuclear Chemistry'', 2006, '''268''', 129-131.</ref> Also according to the [[United Nations]] the normal concentration of uranium in soil is 300 μg kg<sup>-1</sup> to 11.7 mg kg<sup>-1</sup>.<ref>United Nations Scientific Committee on the Effects of Atomic Radiation, 1993, Report to the General Assembly, with scientific annexes, New York</ref>. It is well known that some plants are able to absorb and concentrate metals within their tissues (see [[Phytoremediation, Hyperaccumulators|hyperaccumulators]] for further detail) and it is known that [[iodine]] was first isolated from [[seaweed]] in [[France]] which suggests that seaweed is an iodine hyperaccumulator.
For instance Busby quotes ''Garland et al. 1989'' who reported the plutonium activity in Welsh inter tidal sediments which suggests that the closer a site is to Sellafield the higher the concentration of plutonium in the silt is. Some relationship can be seen but the scatter of points is large (R² = 0.3683) if the data is fitted to an exponential line.
== Man-made ==
{{Pollution}}
The additional radioactivity in the biosphere caused by human activity due to the releases of man-made radioactivity and of Naturally Occurring Radioactive Materials (NORM} can be divided into several classes.
# Normal licensed releases which occur during the regular operation of a plant or process handling man-made radioactive materials.
#* For instance the release of <sup>99</sup>Tc from a [[nuclear medicine]] department of a hospital which occurs when a person given a Tc imaging agent expels the agent.
# Releases of man-made radioactive materials which occur during an industrial or research accident.
#* For instance the [[Chernobyl accident]].
# Releases which occur as a result of military activity.
#* For example a nuclear weapons test.
# Releases which occur as a result of a [[crime]].
#* For example the [[Goiânia accident]] where thieves, unaware of its radioactive content, stole some medical equipment and as a result a number of people were exposed to radiation.
# Releases of Naturally Occurring Radioactive Materials (NORM) as a result of mining etc.
#* For example the release of the trace quantities of Uranium and Thorium in coal, when it is burned in power stations.
=== Farming and the transfer to humans of deposited radioactivity ===
Just because a radioisotope lands on the surface of the soil, does not mean it will enter the [[human]] food chain. After release into the environment radioactive materials can reach humans in a range of different routes, and the chemistry of the element usually dictates the most likely route.
[[Image:Atmospheric radiation to human.jpg|thumb|center|600px|Airborne radioactive material can have an effect on humans via a range of routes.]]
==== Cows ====
Jiří Hála's textbook (Radioactivity, Ionizing Radiation and Nuclear Energy, ISBN 807302053X explains how [[cattle]] only pass a minority of the [[strontium]], [[caesium]], [[plutonium]] and [[americium]] they ingest to the humans who consume [[milk]] and [[meat]]. For instance, for milk if the cow has a daily intake of 1000 Bq of the following isotopes then the milk will have the following activities.
* <sup>90</sup>Sr, 2 Bq dm<sup>-3</sup>
* <sup>137</sup>Cs, 5 Bq dm<sup>-3</sup>
* <sup>239</sup>Pu, 0.001 Bq dm<sup>-3</sup>
* <sup>241</sup>Am, 0.001 Bq dm<sup>-3</sup>
==== Soil ====
Jiří Hála's [[textbook]] states that soils vary greatly in their ability to bind radioisotopes, the [[clay]] particles and [[humic acid]]s can alter the distribution of the isotopes between the soil water and the soil. The distribution coefficient K<sub>d</sub> is the ratio of the soil's radioactivity (Bq g<sup>-1</sup>) to that of the soil water (Bq ml<sup>-1</sup>). If the radioactivity is tightly bonded to by the minerals in the soil then less radioactivity can be absorbed by crops and [[grass]] growing in the soil.
* [[Cs-137]] K<sub>d</sub> = 1000
* [[plutonium|Pu-239]] K<sub>d</sub> = 10000 to 100000
* [[strontium|Sr-90]] K<sub>d</sub> = 80 to 150
* [[Iodine|I-131]] K<sub>d</sub> = 0.007 to 50
=== The trinity test ===
One dramatic source of man-made radioactivity is a [[nuclear weapons]] test. The glassy [[trinitite]] formed by the first atom bomb contains [[radioisotope]]s formed by neutron activation and nuclear fission. In addition some natural radioisotopes are present. A recent paper (P.P. Parekh, T.M. Semkow, M.A. Torres, D.K. Haines, J.M. Cooper, P.M. Rosenberg and M.E. Kitto, ''Jorunal of Environmental Radioactivity'', 2006, '''85''', 103-120) reports the levels of long lived radioisotopes in the trinitite. The trinitite was formed from [[feldspar]] and [[quartz]] which were melted by the heat. Two samples of trinitite were used, the first (left hand side bars) was taken from between 40 and 65 [[meter]]s of ground zero while the other sample was taken from further away from the [[ground zero]] point.
[[Image:Trinityglassactivity.png|center|450px|thumb|Levels of radioactivity in the trinity glass from two different samples as measured by gamma spectroscopy on lumps of the glass]]
The <sup>152</sup>Eu and <sup>154</sup>Eu was mainly formed by the neutron activation of the [[europium]] in the soil, it is clear that the level of radioactivity for these isotopes is highest where the neutron dose to the [[soil]] was larger. Some of the <sup>60</sup>Co is generated by activation of the [[cobalt]] in the soil, but some was also generated by the activation of the cobalt in the [[steel]] (100 foot) tower. This <sup>60</sup>Co from the tower would have been scattered over the site reducing the difference in the soil levels.
The <sup>133</sup>Ba and <sup>241</sup>Am are due to the neutron activation of barium and plutonium inside the bomb. The [[barium]] was present in the form of the nitrate in the chemical explosives used while the plutonium was the [[fissile]] fuel used.
The <sup>137</sup>Cs level is higher in the sample which was further away from the ground zero point– this is thought to be because the precursors to the <sup>137</sup>Cs (<sup>137</sup>I and <sup>137</sup>Xe) and the caesium to a lesser degree are volatile. The natural radioisotopes in the glass are about the same in both locations.
[[Image:Trinity fallout.png|300px|right|thumb|Fallout around the Trinity site. The radioactive cloud moved towards northeast with high [[röntgen]] levels within about 100 miles (62 km).]]
=== Activation products ===
The action of [[neutrons]] on stable [[isotope]]s can form [[radioisotopes]], for instance the neutron bombardment ([[neutron activation]]) of [[nitrogen]]-14 forms [[carbon]]-14. This radioisotope can be released from the [[nuclear fuel cycle]], this is the radioisotope responsible for the majority of the dose experienced by the population as a result of the activities of the [[nuclear power]] industry. For a discussion of the physics of neutron activation see [[Neutron activation analysis]].
Nuclear bomb tests have increased the [[radioactivity|specific activity]] of carbon, whereas the use of fossil fuels has decreased it. See the page on [[Radiocarbon dating]] for further details.
=== Fission products ===
''See [[fission product]]s for more detail'', two sources of these [[radioisotope]]s is [[nuclear fallout]] from [[atomic bomb]]s and nuclear accidents such as [[chernobyl]]. For a good paper about the isotropic signature of the local bomb fallout from a ground burst see T. Imanaka, S. Fukutani, M. Yamamoto, A. Sakaguchi and M. Hoshi, ''J. Radiation Research'', 2006, '''47''', Suppl A121-A127.
Discharges from plants within the [[nuclear fuel cycle]] introduce fission products to the environment, the releases from [[nuclear reprocessing]] plants tend to be medium to longlived radioisotopes, this is because the [[nuclear fuel]] is allowed to cool for several years before being dissolved in the [[nitric acid]]. The releases from [[nuclear reactor]] accidents and bomb detonations will contain a greater amount of the shortlived radioisotopes (when the amounts are expressed in activity {[[Bq]]}).
==== Short lived ====
An example of a shortlived fission product is [[Iodine-131]], this can also be formed as an activation product by the [[neutron]] activation of [[tellurium]].
[[Image:US fallout exposure.png|center|300px|thumb|Per capita [[thyroid]] doses in the continental United States resulting from all exposure routes from all atmospheric [[nuclear testing|nuclear tests]] conducted at the [[Nevada Test Site]] from 1951-1962.]]
In both bomb fallout and a release from a power reactor accident, the shortlived isotopes cause the dose rate on day one to be much higher than that which will be experienced at the same site many days later. This holds true even if no attempts at decontamination are made. In the graphs below, the total gamma dose rate and the share of the dose due to each main isotope released by the Chernobyl accident are shown.
[[Image:Totalexternaldoseratecher.png|thumb|center|300px|The external gamma dose for a person in the open near the Chernobyl site.]]
[[Image:Airdosechernobyl2.jpg|thumb|center|300px|The contributions made by the different isotopes to the dose (in air) caused in the contaminated area in the time shortly after the accident. This image was drawn using data from the OECD report, the Korean table of the isotopes and the second edition of 'The radiochemical manual'.]]
==== Medium lived ====
The classic example is <sup>137</sup>Cs, the [[caesium]] is released in bomb fallout and from the [[nuclear fuel cycle]]. A paper has been written on the radioactivity found in [[oyster]]s found in the [[Irish Sea]], these were found by [[gamma spectroscopy]] to contain <sup>141</sup>Ce, <sup>144</sup>Ce, <sup>103</sup>Ru, <sup>106</sup>Ru, <sup>137</sup>Cs, <sup>95</sup>Zr and <sup>95</sup>Nb. In addition a [[zinc]] activation product (<sup>65</sup>Zn) was found, this is thought to be due to the [[corrosion]] of [[magnox]] fuel cladding in cooling ponds.<ref>A. Preston, J.W.R. Dutton and B.R. Harvey, ''Nature'', 1968, '''218''', 689-690.</ref> It is likely that the modern releases of all these isotopes from [[Windscale]] is smaller.
An important part of the [[Chernobyl]] release was the caesium-137, this isotope is responsible for much of the long term (at least one year after the fire) external exposure which has occurred at the site. Also the caesium isotopes in the fallout have had an effect on farming.
<!-- Image with unknown copyright status removed: [[Image:Radioactive_fallout_caesium137_after_Chernobyl.jpg|thumb|400px|right|Map of radioactive fallout caesium-137 after Chernobyl catastrophe. In kilobecquerels per square meter (kBq/m²). Copyright J.Smith and N.A. Beresford, "Chernobyl: Catastrophe and Consequences" (Praxis, Chichester, 2005). See also [http://www.irsn.fr/vf/05_inf/05_inf_1dossiers/05_inf_17_tchernobyl/film_nuage_web.html an animated map] of radioactive fallout caesium-137, produced by the French ''[[Institut de radioprotection et de sûreté nucléaire]]''{{rfu-c|[[2006-12-14]]}}{{replacethisimage}}]] -->
A good source of data on the subject of [[caesium]] in [[Chernobyl]] fallout exists at [http://www.uiar.org.ua/Eng/index.htm], this is the ''Ukrainian Research Institute for Agricultural Radiology''.
A great deal of caesium was released during the [[Goiânia accident]] where a radioactive source (made for medical use) was stolen and then smashed open during an attempt to convert it into scrap metal. The accident could have been stopped at several stages; first, the last legal owners of the source failed to make arrangements for the source to be stored in a safe and secure place; and second, the scrap metal workers who took it did not recognise the markings which indicated that it was a radioactive object.
P. Soudek, Š. Valenová, Z. Vavříková and T. Vaněk, ''Journal of Environmental Radioactivity'', 2006, '''88''', 236-250 report details of the uptake of <sup>90</sup>Sr and <sup>137</sup>Cs into [[sunflower]]s grown under [[hydroponic]] conditions. The caesium was found in the leaf veins, in the stem and in the [[apical]] leaves. It was found that 12% of the caesium entered the plant, and 20% of the strontium. This paper also reports details of the effect of [[potassium]], [[ammonium]] and [[calcium]] ions on the uptake of the radioisotopes.
Caesium binds tightly to [[clay]] minerals such as [[illite]] and [[montmorillonite]]; hence it remains in the upper layers of soil where it can be accessed by plants with shallow roots (such as grass). Hence [[grass]] and [[mushroom]]s can carry a considerable amount of <sup>137</sup>Cs which can be transferred to humans through the [[food chain]]. One of the best countermeasures in dairy farming against <sup>137</sup>Cs is to mix up the soil by deeply [[ploughing]] the soil. This has the effect of putting the <sup>137</sup>Cs out of reach of the shallow [[root]]s of the grass, hence the level of radioactivity in the grass will be lowered. Also after a nuclear war or serious accident the removal of top few cm of [[soil]] and its burial in a shallow trench will reduce the long term gamma dose to [[human]]s due to <sup>137</sup>Cs as the gamma [[photon]]s will be attenuated by their passage through the [[soil]]. The more remote the trench is from humans and the deeper the trench is the better the degree of protection which will be afforded to the human population.
In [[livestock]] farming an important countermeasure against <sup>137</sup>Cs is to feed to animals a little [[prussian blue]]. This [[iron]] [[potassium]] [[cyanide]] compound acts as an [[ion-exchanger]]. The cyanide is so tightly bonded to the iron that it is safe for a human to eat several grams of prussian blue per day. The prussian blue reduces the [[biological half life]] (different from the [[half-life|nuclear half life]]) of the caesium. The physical or nuclear half life of <sup>137</sup>Cs is about 30 years. This is a constant which can not be changed but the biological half life is not a constant. It will change according to the nature and habits of the organism for which it is expressed. [[Caesium]] in humans normally has a biological half life of between one and four months. An added advantage of the prussian blue is that the caesium which is stripped from the animal in the [[feces|droppings]] is in a form which is not available to plants. Hence it prevents the caesium from being recycled. The form of prussian blue required for the treatment of humans or animals is a special grade. Attempts to use the [[pigment]] grade used in [[paint]]s have not been successful.
==== Long lived ====
A pair of good examples would be [[iodine]]-129 and Tc-99.
=== Plutonium and the other actinides ===
In popular culture plutonium is credited with being the ultimate threat to ''life and limb'' which is wrong; while ingesting plutonium is not likely to be good for one's health, other radioisotopes such as [[radium]] are more toxic to humans. Regardless, the introduction of the ''transuranium'' elements such as [[plutonium]] into the [[Natural environment|environment]] should be avoided wherever possible. Currently the activities of the [[nuclear reprocessing]] industry have been subject to great debate as one of the fears of those opposed to the industry is that large amounts of plutonium will be either mismanaged or released into the environment. In the past one of the biggest releases of plutonium into the environment has been [[nuclear bomb]] testing.
*Those tests in the air scattered some plutonium over the entire globe; this great dilution of the plutonium has resulted in the threat to each exposed person being very small as each person is only exposed to a very small amount.
*The underground tests tend to form molten rock which rapidly cools and seals in the actinides so rendering them unable to move, again the threat to humans is small unless the site of the test is dug up.
*The safety trials where bombs were subject to simulated accidents pose the greatest threat to people; some areas of land used for such experiments (conducted in the open air) have not been fully released for general use despite in one case an extensive decontamination.
''For further details see [[actinides in the environment]].''
== Natural ==<!-- This section is linked from [[Cosmic ray]] -->
=== Activation products from cosmic rays ===
'''Cosmogenic isotopes''' (or ''cosmogenic nuclides'') are rare [[isotope]]s created when a high-energy [[cosmic ray]] interacts with the [[atomic nucleus|nucleus]] of an ''[[in situ]]'' [[atom]]. These isotopes are produced within earth materials such as [[rock (geology)|rock]]s or [[soil]], in [[Earth|Earth's]] [[Earth's atmosphere|atmosphere]], and in extraterrestrial items such as [[meteorite]]s. By measuring cosmogenic isotopes, [[scientist]]s are able to gain insight into a range of [[geology|geological]] and [[astronomy|astronomical]] processes. There are both [[radioactive isotope|radioactive]] and [[stable isotope|stable]] cosmogenic isotopes. Some of these radioisotopes are [[tritium]], [[carbon]]-14 and [[phosphorus]]-32.
==== Production modes ====
Here is a list of radioisotopes formed by the action of [[cosmic rays]] on the atomosphere, the list also contains the production mode of the isotope. ''These data were obtained from the SCOPE50 report, see table 1.9 of chapter 1''.
{| class="wikitable"
|+ Isotopes formed by the action of [[cosmic rays]] on the air
! Isotope !! Mode of formation
|-
! ³H (tritium)
| <sup>14</sup>N (n, <sup>12</sup>C)³H
|-
! <sup>7</sup>Be
| [[Spallation]] (N and O)
|-
! <sup>10</sup>Be
| Spallation (N and O)
|-
! <sup>11</sup>C
| Spallation (N and O)
|-
! <sup>14</sup>C
| <sup>14</sup>N (n, p) <sup>14</sup>C
|-
! <sup>18</sup>F
| <sup>18</sup>O (p, n)<sup>18</sup>F and Spallation (Ar)
|-
! <sup>22</sup>Na
| Spallation (Ar)
|-
! <sup>24</sup>Na
| Spallation (Ar)
|-
! <sup>28</sup>Mg
| Spallation (Ar)
|-
! <sup>31</sup>Si
| Spallation (Ar)
|-
! <sup>32</sup>Si
| Spallation (Ar)
|-
! <sup>32</sup>P
| Spallation (Ar)
|-
! <sup>34m</sup>Cl
| Spallation (Ar)
|-
! <sup>35</sup>S
| Spallation (Ar)
|-
! <sup>36</sup>Cl
| <sup>35</sup>Cl (n, )<sup>36</sup>Cl
|-
! <sup>37</sup>Ar
| <sup>37</sup>Cl (p, n)<sup>37</sup>Ar
|-
! <sup>38</sup>Cl
| Spallation (Ar)
|-
! <sup>38</sup>
| Spallation (Ar)
|-
! <sup>39</sup>Ar
| <sup>38</sup>Ar (n, )<sup>39</sup>Ar
|-
! <sup>39</sup>Cl
| <sup>40</sup>Ar (n, np)<sup>39</sup>Cl & spallation (Ar)
|-
! <sup>41</sup>Ar
| <sup>40</sup>Ar (n, )<sup>41</sup>Ar
|-
! <sup>81</sup>Kr
| <sup>80</sup>Kr (n, ) <sup>81</sup>Kr
|}
==== Transfer to ground ====
The level of [[beryllium]]-7 in the air is related to the [[sun spot]] cycle, as radiation from the sun forms this [[radioisotope]] in the atmosphere. The rate at which it is transferred from the air to the ground is controlled in part by the weather.
[[Image:Be7fromcosmicrays.png|center|thumb|550px|The rate of delivery of Be-7 from the air to the ground in Japan (source M. Yamamoto ''et. al.'', ''Journal of Environmental Radioactivity'', 2006, '''86''', 110-131)]]
==== Applications in geology listed by isotope ====
{| class="wikitable"
|+ Commonly measured long lived cosmogenic isotopes
! [[chemical element|element]] !! [[atomic mass|mass]] !! [[half-life]] (years) !! typical application
|-
| [[helium]] || 3 || - stable - || exposure dating of [[olivine]]-bearing rocks
|-
| [[beryllium]] || 10 || 1.36 million || exposure dating of [[quartz]]-bearing rocks, sediment, dating of ice cores, measurement of erosion rates
|-
| [[carbon]] || 14 || 5,730 || [[Radiocarbon dating|dating]] of organic matter, water
|-
| [[neon]] || 21 || - stable - || dating of very stable, long-exposed surfaces, including [[meteorite]]s
|-
| [[aluminum]] || 26 || 720,000 || exposure dating of rocks, sediment
|-
| [[chlorine]] || 36 || 308,000 || exposure dating of rocks, [[groundwater]] tracer
|-
| [[calcium]] || 41 || 103,000 || exposure dating of [[carbonate rock]]s
|-
| [[iodine]] || 129 || 15.7 million || groundwater tracer
|}
==== Applications of dating ====
Because cosmogenic isotopes have long [[half life|half-lives]] (anywhere from thousands to millions of years), scientists find them useful for geologic [[radiometric dating|dating]]. Cosmogenic isotopes are produced at or near the surface of the Earth, and thus are commonly applied to problems of measuring ages and rates of [[geomorphology|geomorphic]] and [[sediment]]ary events and processes.
Specific applications of cosmogenic isotopes include:
* exposure dating of earth surfaces, including [[glacier|glacially]]-scoured [[bedrock]], [[geologic fault|fault]] [[escarpment|scarp]]s, [[landslide]] debris
* burial dating of sediment, bedrock, ice
* measurement of steady-state [[erosion]] rates
* [[absolute dating]] of organic matter ([[radiocarbon dating]])
* absolute dating of water masses, measurement of groundwater transport rates
* absolute dating of meteorites, lunar surfaces
==== Methods of measurement for the long lived isotopes ====
To measure cosmogenic isotopes produced within solid earth materials, such as rock, samples are generally first put through a process of mechanical separation. The sample is crushed and desirable material, such as a particular mineral ([[quartz]] in the case of Be-10), is separated from non-desirable material by using a density separation in a heavy liquid medium such as LST ([[lithium sodium tungstate]]).
The sample is then dissolved, a common isotope carrier added (Be-9 carrier in the case of Be-10), and the aqueous solution is purified down to an oxide or other pure solid.
Finally, the ratio of the rare cosmogenic isotope to the common isotope is measured using [[particle accelerator|accelerator]] [[mass spectrometry]]. The original concentration of cosmogenic isotope in the sample is then calculated using the measured isotopic ratio, the mass of the sample, and the mass of carrier added to the sample.
==== References about cosmogenic isotope dating ====
* [[John Gosse|Gosse, John C.]], and Phillips, Fred M. (2001). "Terrestrial in situ cosmogenic nuclides: Theory and application". ''Quaternary Science Reviews'' '''20''', 1475-1560.
* Granger, Darryl E., Fabel, Derek, and Palmer, Arthur N. (2001). "Pliocene-Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic 26Al and 10Be in Mammoth Cave sediments". ''Geological Society of America Bulletin'' '''113''' (7), 825-836.
==== External links ====
* [http://www.physics.purdue.edu/primelab/introduction/cosmogenic_nuclides.html Purdue University Prime Lab, "Cosmogenic nuclides"]
* [http://www.geocities.com/earthhistory/tcn.htm "Cosmogenic Exposure Dating and the Age of the Earth"]
* [http://depts.washington.edu/cosmolab/index.html Cosmogenic Isotope Laboratory, University of Washington]
* [http://www.esd.mun.ca/~gac/MEDALS/pressHutch.html Geological Association of Canada Awards Distinguished Medal to Young Scientist]
=== Radium and Radon from the decay of long lived actinides ===
[[Radium]] and [[radon]] are in the environment because they are decay products of [[uranium]] and [[thorium]]. ''For further details about thorium see [[actinides in the environment]], for further details of uranium see [[uranium in the environment]] while for further details about radium and radon please see [[radium in the environment]]''.
The radon (<sup>222</sup>Rn) released into the air decays to <sup>210</sup>Pb and other radioisotopes, the levels of [[lead|<sup>210</sup>Pb]] can be measured. The rate of deposition of this radioisotope is dependent on the weather. Here is a graph of the deposition rate observed in [[Japan]].<ref>M. Yamamoto ''et al.'', ''Journal of Environmental Radioactivity'', 2006, '''86''', 110-131)</ref>
[[Image:Lead210inairatjapan.png|500px|center|Lead-210 deposition rate as a function of time as observed in Japan]]
=== Uranium-lead dating ===
The '''uranium-lead radiometric dating''' scheme is one of the oldest available, as well as one of the most highly respected. It has been refined to the point that the error in dates of rocks about three billion years old is no more than two million years.
[[Uranium]]-[[lead]] dating is usually performed on the mineral [[zircon]] (ZrSiO<sub>4</sub>), though it can be used on other materials. Zircon incorporates uranium [[atom]]s into its crystalline structure as substitutes for [[zirconium]], but strongly rejects lead. It has a high blocking temperature, is resistant to mechanical weathering and is chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. These can be dated by a SHRIMP [[ion]] microprobe.
One of its advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a [[half-life]] of about 703 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.
For further details see [[radiometric dating]] and [[exponential decay]].
==References==
{{reflist}}
==Further reading==
*'''Radioactivity, Ionizing Radiation and Nuclear Energy''', by J. Hala and J.D. Navratil
*A review of the subject has been published by [[SCOPE]] in the report [http://www.icsu-scope.org/downloadpubs/scope50/contents.html SCOPE 50 Radioecology after chernobyl].
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[[Category:Geochemistry]]
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[[Category:Radiometric dating]]
[[Category:Environmental isotopes]]
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[[Category:Quaternary]]
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