What is Radioactivity ?

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:
1. Primordial – from before the creation of the Earth

2. Cosmogenic – formed as a result of cosmic ray interactions

3. Human produced – enhanced or formed due to human actions (minor amounts compared to natural)
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.

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) is 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.
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.
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In the United States, the annual estimated average effective dose equivalent from natural radiation is 310 per adult. This is broken down as:

Note – the above are average dose to the US. This is calculated by taking the total collective dose in the US and dividing by the US population. Shown in the table above, in 1987, there was 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. In 2006, the medical exposures dominate the average. The increase was due mostly to the higher utilization of computed tomography (CT) and nuclear medicine. Therefore, unless you were undergoing CT scans or nuclear medicine, your annual background dose did not change significantly.

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Primordial radionuclides

Primordial 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 –>
220Rn –> 216Po –> 212Pb –> 212Bi –> 212Po –> 208Pb (stable)

You can see how the decay process works with this interesting Decay Science Trek from University of Colorado.

Some other primordial radionuclides are 50V, 87Rb, 113Cd, 115In, 123Te, 138La, 142Ce, 144Nd, 147Sm, 152Gd, 174Hf, 176Lu, 187Re, 190Pt, 192Pt, 209Bi.

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Cosmogenic

Cosmic 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.

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Human Produced

Humans 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: