, New Zealand and Austria. The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. Atmospheric nuclear weapon tests almost doubled the concentration of 14C in the Northern Hemisphere.|300x300px doses in the continental United States resulting from all exposure routes from all atmospheric
nuclear tests conducted at the
Nevada Test Site from 1951 to 1962. 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 99Tc 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, which have caused a
global fallout, peaking in 1963 (the
Bomb pulse), and up to 2.4 million deaths by 2020. • 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.
Cows Jiří Hála claims in his textbook "Radioactivity, Ionizing Radiation and Nuclear Energy" that
cattle only pass a minority of the
strontium,
caesium,
plutonium and
americium they ingest to the humans who consume
milk and
meat. Using milk as an example, if the cow has a daily intake of 1000 Bq of the preceding isotopes then the milk will have the following activities. • 90Sr, 2 Bq/L • 137Cs, 5 Bq/L • 239Pu, 0.001 Bq/L • 241Am, 0.001 Bq/L
Soil Jiří Hála's
textbook states that soils vary greatly in their ability to bind radioisotopes, the
clay particles and
humic acids can alter the distribution of the isotopes between the soil water and the soil. The distribution coefficient Kd is the ratio of the soil's radioactivity (Bq g−1) to that of the soil water (Bq ml−1). 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 Kd = 1000 •
Pu-239 Kd = 10000 to 100000 •
Sr-90 Kd = 80 to 150 •
I-131 Kd = 0.007 to 50
The Trinity test One dramatic source of man-made radioactivity is a
nuclear weapons test. The glassy
trinitite created by the first atom bomb contains
radioisotopes formed by
neutron activation and
nuclear fission. In addition some natural radioisotopes are present. A recent paper 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 in the graph) was taken from between 40 and 65 meters of ground zero while the other sample was taken from further away from the
ground zero point. The 152
Eu (half life 13.54 year) and 154Eu (half life 8.59 year) were 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
60Co (half life 5.27 year) 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 60Co from the tower would have been scattered over the site reducing the difference in the soil levels. The
133Ba (half life 10.5 year) and
241Am (half life 432.6 year) 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 137Cs level is higher in the sample that was further away from the ground zero point – this is thought to be because the precursors to the 137Cs (137I and 137Xe) and, to a lesser degree, the caesium itself are volatile. The natural radioisotopes in the glass are about the same in both locations. levels within about 100 miles (160 km).
Activation products The action of
neutrons on stable
isotopes 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. Nuclear bomb tests have increased the
specific activity of carbon, whereas the use of fossil fuels has decreased it. See the article on
radiocarbon dating for further details.
Fission products Discharges from nuclear plants within the
nuclear fuel cycle introduce fission products to the environment. The releases from
nuclear reprocessing plants tend to be medium to long-lived 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 short-lived radioisotopes (when the amounts are expressed in activity
Bq)).
Short lived d graph of the external gamma dose for a person in the open near the Chernobyl site. An example of a short-lived fission product is
iodine-131, this can also be formed as an activation product by the
neutron activation of
tellurium. In both bomb fallout and a release from a power reactor accident, the short-lived 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.
Medium lived An example of a medium lived is 137Cs, which has a half-life of 30 years. Caesium is released in bomb fallout and from the nuclear fuel cycle. A paper has been written on the radioactivity in
oysters found in the
Irish Sea, these were found by
gamma spectroscopy to contain 141Ce, 144Ce, 103Ru, 106Ru, 137Cs, 95Zr and 95Nb. In addition, a
zinc activation product (65Zn) was found, this is thought to be due to the
corrosion of
magnox fuel cladding in cooling ponds. The concentration of all these isotopes in the Irish Sea attributable to nuclear facilities such as Sellafield has significantly decreased in recent decades. 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. The caesium isotopes in the fallout have had an effect on farming. A large amount 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. Soudek
et al. reported in 2006 details of the uptake of 90Sr and 137Cs into
sunflowers 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
mushrooms can carry a considerable amount of 137Cs which can be transferred to humans through the
food chain. One of the best countermeasures in dairy farming against 137Cs is to mix up the soil by deeply
ploughing the soil. This has the effect of putting the 137Cs out of reach of the shallow
roots 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 humans due to 137Cs as the gamma
photons 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 137Cs 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 (not to be confused with the
nuclear half-life) of the caesium). The physical or nuclear half-life of 137Cs is about 30 years, which is a constant and can not be changed; however, the biological half-life 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
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
paints have not been successful.
Long lived Examples of long-lived isotopes include iodine-129 and Tc-99, which have nuclear half-lives of 15 million and 200,000 years, respectively.
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
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 largest 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 the actinides into the rock, 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. ==Natural== File:Radiation levels by altitude.jpg|thumb|Radiation levels by altitude on Earth.
Activation products from cosmic rays Cosmogenic isotopes (or
cosmogenic nuclides) are rare
isotopes created when a high-energy
cosmic ray interacts with the
nucleus of an
in situ atom. These isotopes are produced within earth materials such as
rocks or soil, in
Earth's atmosphere, and in extraterrestrial items such as
meteorites. By measuring cosmogenic isotopes,
scientists are able to gain insight into a range of
geological and
astronomical processes. There are both
radioactive and
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 atmosphere; 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.
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.
Applications in geology listed by isotope Applications of dating Because cosmogenic isotopes have long half-lives (anywhere from thousands to millions of years), scientists find them useful for geologic
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
geomorphic and
sedimentary events and processes. Specific applications of cosmogenic isotopes include: • exposure dating of earth surfaces, including
glacially scoured
bedrock,
fault scarps,
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
lithium sodium tungstate (LST). 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
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.
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. The radon (222Rn) released into the air decays to 210Pb and other radioisotopes, and the levels of
210Pb can be measured. The rate of deposition of this radioisotope is dependent on the weather. Below is a graph of the deposition rate observed in
Japan.
Uranium–lead dating Uranium–
lead dating is usually performed on the mineral
zircon (ZrSiO4), though other materials can be used. Zircon incorporates uranium
atoms 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 the advantages of this method 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. ==See also==