Extinct radionuclide

An extinct radionuclide is one that scientists believe was formed by primordial processes, such as stellar nucleogenesis in the supernova(s) that contributed radioisotopes to the early solar system, about 4.6 billion years ago. Generally, radioisotopes with a decay half-life shorter than about 100 million years are not found in nature, except for those generated continuously by a natural process, such as cosmic rays, or a decay chain of much longer lived isotopes, such as uranium or thorium. Other short-lived isotopes are thus seen only as extinct radionuclides, whose former existence is inferred now from a superabundance of their stable decay products.