A trace radioisotope is a radioisotope that occurs naturally in trace amounts (i.e., extremely small). Generally speaking, trace radioisotopes have half-lives that are short in comparison to the age of the earth, since primordial nuclides tend to occur in larger than trace amounts. Trace radioisotopes are therefore present only because they are continually produced on Earth by natural processes. One exception to this rule is Plutonium-244 which, with a half-life of about 80 million years, is just barely stable enough that some of it has remained from the Earth's formation. Natural processes which produce trace radioisotopes include cosmic ray bombardment of stable nuclides, ordinary alpha and beta decay of the long-lived heavy nuclides, Thorium-232, Uranium-238, and Uranium-235, spontaneous fission of natural Uranium-238, and nuclear transmutation reactions induced by natural radioactivity, such as the production of Plutonium-239 and Uranium-236 from neutron capture by natural uranium.
The elements that are created only from decay are listed below.