Our world is radioactive and has been since it was created. Over 60 radionuclides can be found in nature, and they can be placed in three general categories:
•Primordial – been around since the creation of the Earth
•Cosmogenic – formed as a result of cosmic ray interactions
•Human produced – enhanced or formed due to human actions
Primordial radionuclides are left over from when the world and the universe was 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. These included Uranium U-235 (7.04 x 108 yr), Uranium U-238 (4.47 x 109 yr), Thorium Th-232 (1.41 x 1010 yr), Radium Ra-226 (1.60 x 103 yr), Radon Rn-222 (3.82 days), Potassium K-40 (1.28 x 109 yr)
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 radiation’s, and produces radioactive nuclides. They can have long half-lives, but the majority have shorter half-lives than the primordial nuclides. Here are some common cosmogenic nuclides: Carbon C-14 (5730 yr), Tritium T-3 (12.3 yr), Beryllium Be-7 (53.28 days).
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, we have seen a marked decrease since the halting of above ground testing of nuclear weapons. Here are a few nuclides: Tritium H-3 (12.3 yr), Iodine I-131 (8.04 days), Iodine I-129 (1.57 x 107 yr), Cesium Cs-137 (30.17 yr), Strontium Sr-90 (28.78 yr), Technetium Tc99 (2.11 x 105 yr), Plutonium Pu-239 (2.41 x 104 yr).