A trace radioisotope is a radioisotope that occurs naturally in trace amounts (i.e. extremely small). This natural formation can be from the decay of heavier nuclei such as uranium-235 decaying into thorium-231. Natural occurrence of radioisotopes can also be driven by cosmic rays. This is the method that creates hydrogen-3 and carbon-14. Isotopes with half-lives greater than about 80 million years also remain in trace amounts from the formation of the Earth. Potassium-40 and vanadium-50 fit into this category. Plutonium (pu-244) such as the others appears naturally in small amounts, but since it is the only natural non-synthetic isotope of plutonium, it is considered 100 percent of the natural abundance.
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