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An extinct radionuclide is a radionuclide that was formed by nucleosynthesis before the formation of the Solar System, about 4.6 billion years ago, and incorporated into it, but has since decayed to virtually zero abundance, due to having a half-life shorter than about 100 million years.
Short-lived radioisotopes that are found in nature are continuously generated or replenished by natural processes, such as cosmic rays (cosmogenic nuclides), background radiation, or the decay chain or spontaneous fission of very long-lived isotopes such as uranium or thorium.
Short-lived isotopes that are not generated or replenished by natural processes are not found in nature, so they are known as extinct radionuclides. Their former existence is inferred from a superabundance of their stable decay products.
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 Solar System and Earth formed from primordial nuclides and extinct nuclides. Extinct nuclides have decayed away, but primordial nuclides still exist in their original state (undecayed). There are 253 stable primordial nuclides, and remainders of 33 primordial radionuclides that have very long half-lives.
List of extinct radionuclides
A partial list of radionuclides not found on Earth, but for which decay products are present:
|Plutonium-244||80.8||Thorium-232, fission products (especially xenon)|
|Technetium-98||4.2 or 6.6||Ruthenium-98|
|Technetium-97||2.6 or 4.2||Molybdenum-97|
Notable isotopes with shorter lives still being produced on Earth include:
- Manganese-53 and beryllium-10 are produced by cosmic ray spallation on dust in the upper atmosphere.
- Uranium-236 is produced in uranium ores by neutrons from other radioisotopes.
- Iodine-129 is produced from tellurium-130 by cosmic-ray muons and from cosmic ray spallation of stable xenon isotopes in the atmosphere.
Radioisotopes with half-lives shorter than one million years are also produced: for example, carbon-14 by cosmic ray production in the atmosphere (half-life 5730 years).
- Presolar grains
- Radiometric dating
- List of nuclides which includes a list of radionuclides in order by half-life