Extinct radionuclide

An extinct radionuclide is a radionuclide that was formed by nucleosynthesis before the formation of the Solar System, about 4.6 billion years ago, but has since decayed to undetectability. Extinct radionuclides were present in the early Solar System either from stellar or from cosmogenic nucleosynthesis, and became part of the composition of meteorites and protoplanets. All extinct radionuclides have half-lives shorter than 100 million years; not all possible radionuclides have been identified.

Some extinct radionuclides may also be still found in nature because they are continuously generated or replenished by natural processes, such as cosmic rays (cosmogenic nuclides), background radiation, or the decay chain or spontaneous fission of other radionuclides, but their primordial fraction is still extinct.

Examples of extinct radionuclides include iodine-129 (the first to be noted in 1960, inferred from excess xenon-129 concentrations in meteorites, in the xenon-iodine dating system), aluminium-26 (inferred from extra magnesium-26 found in meteorites), and iron-60. The method of detecting the former existence of such isotopes is in general from detection of anomalous concentrations of their decay products.

The Solar System and Earth are formed from primordial nuclides and extinct nuclides. Extinct nuclides have decayed away, but primordial nuclides still exist in their original state (undecayed). There are 251 stable primordial nuclides, and 35 primordial radionuclides of which some fraction remains to the present.

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