| Radioactivity in Nature | |
Our world is radioactive and has been since it was created. Over 60 radionuclides (radioactive elements) can be found in nature, and they can be placed in three general categories:
Radionuclides are found naturally in air, water and soil. They are even found in us, being that we are products of our environment. Every day, we ingest and inhale radionuclides in our air and food and the water. Natural radioactivity is common in the rocks and soil that makes up our planet, in water and oceans, and in our building materials and homes. There is nowhere on Earth that you can not find Natural Radioactivity. Radioactive elements are often called radioactive isotopes or radionuclides or just nuclides. There are over 1,500 different radioactive nuclides. Often, radionuclides are symbolized based on the element and on the atomic weight, as in the case of radioactive hydrogen or tritium with an atomic weight of 3 is shown as H-3 or 3H. As another example, Uranium with the atomic weight of 235 would be shortened to U-235 or 235U. Much of the information and many of tables found on this page are adapted from information found in Environmental Radioactivity from Natural, Industrial and Military Sources by Merrill Eisenbud and Tom Gesell, Academic Press, Inc. 4th Edition. Some tables are adapted from the National Council on Radiation Protection reports 94 and 95. References are listed at the bottom of this page. Several of the tables below were produced by calculations based on available data. Note: Many of the units used in science are broken down into smaller units or expressed as multiples, using standard metric prefixes. As examples, a kilobecquerel (kBq) in 1000 becquerels, a millirad (mrad) is 10-3 rad, a microrem (µrem) is 10-6 rem, a nanogram is 10-9 grams, and a picocurie is a 10-12 curies. These are examples of units used frequently throughout this short paper. To find definitions of terms or units you’re not familiar with, look on our glossary page. Common abbreviations used on this page are: m – meter, m3 – cubic meter, g – gram, kg – kilogram, Bq – becquerel, Sv – sievert, Gy – gray, Ci – curie, ppm – parts per million, yr – year, hr – hour, L – liter In the United States, the annual estimated average effective dose equivalent from radiation is 360 mrem per adult. This is broken down as:
Shown in the table above, 82% of the total average annual effective dose is from natural sources of radiation, and of that, most is from radon. Of the other 18%, the majority is from medical diagnosis and treatments, with <1% from nuclear power and fallout. This can perhaps be more easily seen with a graph (6K) You can also calculate your own background radiation from this EPA website. See Radiation and Us for more info on average U.S. doses of radiation. United States Geological Survey map of estimated total gamma exposure for the U.S. (78 k) Primordial radionuclidesPrimordial radionuclides are left over from when the world and the universe were created. They are typically long lived, with half-lives often on the order of hundreds of millions of years. Radionuclides that exist for more than 30 half-lives are not measurable. The progeny or decay products of the long lived radionuclides are also in this heading. Here is some basic information on some common primordial radionuclides:
Some nuclides like 232Th have several members of its decay chain. You can roughly follow the chain starting with 232Th 232Th –> 228Ra –> 228Ac –> 228Th –> 224Ra –> You can see how the decay process works with this interesting Decay In Action website Some other primordial radionuclides are 50V, 87Rb, 113Cd, 115In, 123Te, 138La, 142Ce, 144Nd, 147Sm, 152Gd, 174Hf, 176Lu, 187Re, 190Pt, 192Pt, 209Bi. United States Geological Survey Digital maps of estimated potassium, equivalent uranium-238, equivalent thorium-232 concentrations for the conterminous U.S. CosmogenicCosmic radiation permeates all of space, the source being primarily outside of our solar system. The radiation is in many forms, from high speed heavy particles to high energy photons and muons. The upper atmosphere interacts with many of the cosmic radiations, and produces radioactive nuclides. They can have long half-lives, but the majority have shorter half-lives than the primordial nuclides. Here is a table with some common cosmogenic nuclides:
Some other cosmogenic radionuclides are 10Be, 26Al, 36Cl, 80Kr, 14C, 32Si, 39Ar, 22Na, 35S, 37Ar, 33P, 32P, 38Mg, 24Na, 38S, 31Si, 18F, 39Cl, 38Cl, 34mCl. Human ProducedHumans have used radioactivity for one hundred years, and through its use, added to the natural inventories. The amounts are small compared to the natural amounts discussed above, and due to the shorter half-lives of many of the nuclides, have seen a marked decrease since the halting of above ground testing of nuclear weapons. Here are a few human produced or enhanced nuclides:
Other Interesting Aspects of Natural RadioactivityNatural Radioactivity in soilHow much natural radioactivity is found in a volume of soil that is 1 square mile, by 1 foot deep? The following table is calculated for this volume (total volume is 7.894 x 105 m3) and the listed activities. It should be noted that activity levels vary greatly depending on soil type, mineral make-up and density (~1.58 g/cm3 used in this calculation). This table represents calculations using typical numbers.
Natural Radioactivity in the OceanHow much natural radioactivity is found in the world’s oceans? All water on the Earth, including seawater, has some radionuclides in it. In the following table, the oceans’ volumes were calculated from the 1990 World Almanac:
The activities used in the table below are from 1971 Radioactivity in the Marine Environment, National Academy of Sciences:
FoodEvery food has some small amount of radioactivity in it. The common radionuclides in food are potassium 40 (40K), radium 226 (226Ra) and uranium 238 (238U) and the associated progeny. Here is a table of some of the common foods and their levels of 40K and 226Ra.
Ref: Handbook of Radiation Measurement and Protection, Brodsky, A. CRC Press 1978 and Environmental Radioactivity from Natural, Industrial and Military Sources, Eisenbud, M and Gesell T. Academic Press, Inc. 1997.Human bodyYou are made up of chemicals, and it should be of no surprise that some of them are radionuclides, many of which you ingest daily in your water and food. Here are the estimated concentrations of radionuclides calculated for a 70,000 gram adult based ICRP 30 data:
It would be reasonable to assume that all of the radionuclides found in your environment would be in you in some small amounts. The average annual dose equivalent from the internal deposited radionuclides is given in the table at the top of this page. Natural Radioactivity in Building MaterialsAs mentioned before, building materials have some radioactivity in them. Listed below are a few of common building materials and their estimated levels of uranium, thorium and potassium.
Oklo Natural ReactorAdapted from August 1976 Scientific American article on Oklo by Cowan. In 1972, natural nuclear reactor was found in a Western Africa in the Republic of Gabon, at Oklo. While the reactor was critical, approximately 1.7 billion years ago, it released 15,000 megawatt-years of energy by consuming six tons of uranium. It operated over several hundred thousand years at low power. It was discovered by a French mining geologist while assaying samples for the Oklo Uranium mine. Today, the fissionable Uranium 235 has an natural abundance of 0.7202%, but the scientist noticed some samples from Oklo to be 0.7171%. While this difference was small, it started the scientists to ponder and take a look closer at the Oklo site. Later, samples were found more depleted, down to 0.44%. This difference could only be explained if some of the fuel, the 235U, had been used up in a fission reaction. Upon further investigation, abnormally high amounts of fission products were found in six separate reactor zones. Like present day power reactors, a natural reactor would require several special condition, namely fuel, a moderator, a reflector, lack of neutron absorbing poisons and some way to remove the heat generated. At Oklo, the area was naturally loaded with uranium by water transport and deposition. The concentration of Uranium 235 is artificially enriched for most modern reactors, but at the time of the Oklo reactor it was naturally enriched with an abundance of approximately 3%. This is because when the world was formed, there was a certain amount of 235U, and it has been decaying ever since. So, because 235U has a shorter half-life than 238U, so one billion years ago ,235U made up a larger percentage of the natural uranium. The 3% 235U was enough for a sustained nuclear reaction. Oklo site was saturated with groundwater, which served as a moderator, reflector and cooling for the fission reaction. There was a lack of poisons before the reaction began, and fission products like xenon and neodymium serve as neutron absorbing poisons, absorbing neutrons, acting to limit the power. To confirm that there was a natural fission reactor, the scientists started looking for other evidence. First they wanted to look for some element that might have been produced in a reactor, but would have little natural occurrence elsewhere. They looked at several, but neodymium gave strong indications of the reactor had indeed operated. Neodymium has seven stable isotopes, but only six are fission products. The abundance of the neodymium at Oklo sites was compared to other areas and to the neodymium found in modern reactors. Once the samples were compared, the abundance of neodymium was found to be almost exactly that found in present day reactors. All in all, the fission products studied matched what would have been the result of a sustained nuclear reaction. There is even evidence that the reactor bread its own fuel, bombarding the 238U with neutrons, making the easily fissionable 239Pu. Some other interesting information has come out of this, over half of the thirty fission products found there were confined to the reactor zones, with all plutonium immobilized. The strontium was mainly confined to the local zones, with some release to environment estimated from krypton 85 and cesium 137 One of the greatest works of the 20th century was the building of the first atomic pile (nuclear reactor) in Chicago in 1941 by Enrico Fermi. It took some of the brightest minds in modern physics and great engineering efforts to duplicate what nature did two billion years earlier. Editors note: Despite some wild baseless claims, there is no evidence or even credible theory that the Oklo natural reactor was anything but a natural phenomenon. The 6 reactor zones are spread over a huge area that was a uranium mine during the time it was first discovered. The reactor zones were the result of natural physical processes, active for thousands of years. For more information on the Oklo Reactor, try:
High Background Radiation AreasBackground radiation levels are from a combination of terrestrial (from the 40K, 232Th, 226Ra, etc.) and cosmic radiation (photons, muons, etc.). The level is fairly constant over the world, being 8-15 µrad/hr. The US EPA has an on-line calculator to let you calculate your own annual background dose. Around the world though, there are some areas with sizable populations that have high background radiation levels. The highest are found primarily in Brazil, India and China. The higher radiation levels are due to high concentrations of radioactive minerals in soil. One such mineral, Monazite, is a highly insoluble rare earth mineral that occurs in beach sand together with the mineral ilmenite, which gives the sands a characteristic color. The principal radionuclides in monazite are from the 232Th series, but there is also some uranium its progeny, 226Ra. In Brazil, the monazite sand deposits are found along certain beaches. The external radiation levels on these black sands range up to 5 mrad/hr (50 µGy/hr), which is almost 400 times normal background in the US. Some of the major streets of the surrounding cites have radiation levels as high as 0.13 mrad/hr (1.3 µGy/hr), which is more than 10 times the normal background. Another high background area in Brazil is the result of large rare earth ore deposits that form a hill that rises about 250 meters above the surrounding area. An ore body near the top of the hill is very near the surface, and contains an estimated 30,000 tons of thorium and 100,000 tons of rare earth elements. The radiation levels near the top of the hill are 1 to 2 mrad/hr (0.01 to 0.02 mGy/hr) over an area of about 30,000 m2. The plants found there have absorbed so much 228Ra, that they can will produce a self “x-ray” if placed on a sheet of photographic paper (an autoradiographed). On the Southwest coast of India, the monazite deposits are larger than those in Brazil. The dose from external radiation is, on average, similar to the doses reported in Brazil, 500-600 mrad/yr (5 – 6 mGy/yr), but individual doses up to 3260 mrad/yr (32.6 mGy/yr) have been reported. An area in China, has does rates that is about 300-400 mrad/yr (3-4 mGy/yr). This is also from monazite that contains thorium, uranium and radium. From BEIR V, National Research Council report on Health Effects of Low Levels of Ionizing Radiation:
Cosmic RadiationCosmic radiation as discussed above, upon interaction with our atmosphere produces cosmogenic radionuclides. It also is responsible for a whole body doses. Cosmic radiation is really divided into two types, primary and secondary. Primary cosmic radiation is made up of extremely high energy particles (up to 1018 eV), and are mostly protons, with some larger particles. A large percentage of it comes from outside of our solar system and is found throughout space. Some of the primary cosmic radiation is from our sun, produced during solar flares. Little of the primary cosmic radiation penetrates to the Earth’s surface, the vast majority of it interacts with the atmosphere. When it does interact, it produces the secondary cosmic radiation, or what we actually see here on Earth. These reactions produce other lower energy radiations in the form of photons, electrons, neutrons and muons that make it to the surface. The atmosphere and the Earth’s magnetic fields also act as shields against cosmic radiation, reducing the amount that reaches the Earth’s surface. With that in mind, it is easy to see that the annual dose you get from cosmic radiation depends on what altitude you are at. From cosmic radiation the U.S., the average person will receive a dose of 27 mrem per year and this roughly doubles every 6,000 foot increase in elevation. Typical Cosmic Radiation Dose rates:
There is only about a 10% decrease at sea level in cosmic radiation rates when going from pole to the equator, but at 55,000 feet the decrease is 75%. This is on account of the effect of the earth’s and the Sun’s geomagnetic fields on the primary cosmic radiations. Flying can add a few extra mrem to your annual dose, depending on how often you fly, how high the plane flies, and how long you are in the air.
Other sites with good information on cosmic radiation is The Exposure Of New Zealand Aircrew To Cosmic Radiation, Australian Aircrew To Cosmic Radiation and SEC Radiation Hazard page Astronauts are exposed to cosmic radiation, but they are also exposed to radiation as they pass through the Van Allen radiation belts that circle the Earth.
References and Additional Information Sources
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