Isotope persisted down to the present because their rate of decay is so slow (e.g. uranium-238 and potassium-40). Post-primordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium, carbon-14), or by the decay of a radioactive primordial isotope to a radioactive radiogenic nuclide daughter (e.g. uranium to radium). A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction, such as when neutrons from natural nuclear fission are absorbed by another atom. As discussed above, only 80 elements have any stable isotopes, and 26 of these have only one stable isotope. Thus, about two-thirds
Isotope geochemistry successfully used in forensic science to fingerprint bullets, because each batch of ammunition has its own peculiar Pb/Pb vs Pb/Pb ratio. Samarium–neodymium is an isotope system which can be utilised to provide a date as well as isotopic fingerprints of geological materials, and various other materials including archaeological finds (pots, ceramics). Sm decays to produce Nd with a half life of 1.06x10 years. Dating is achieved usually by trying to produce an isochron of several minerals within a rock specimen. The initial Nd/Nd ratio is determined. This initial ratio is modelled relative to CHUR - the Chondritic Uniform Reservoir -