Trace radioisotope

A trace radioisotope is a radioactive isotope of an element which exists naturally in tiny amounts (i.e. traces). Generally speaking, trace radioisotopes have half-lives much shorter than the age of the Earth.
Trace radioisotopes occur in the atmosphere and the crust of the Earth. They are constantly produced by natural processes such as cosmic ray fission: high-energy cosmic ray striking atoms and causing them to split into smaller nuclei). 
Another mechanism is neutron capture in uranium ores causing the production of nuclei such as uranium-236 and plutonium-239, which are both radioactive isotopes. Spontaneous fission and alpha decay of heavy nuclei such as thorium and uranium also produce radioisotopes in traces as their intermediate decay products.