Extinct radionuclide

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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[edit]

A partial list of radionuclides not found on Earth, but for which decay products are present:

Isotope Halflife (Myr) Daughter
Plutonium-244 80.8 Thorium-232, fission products (especially xenon)
Samarium-146 68.7 Neodymium-142 (stable)
Niobium-92 34.7 Zirconium-92
Iodine-129 15.7 Xenon-129 (stable)
Curium-247 15.6 Uranium-235
Lead-205 15.3 Thallium-205
Hafnium-182 8.91 Tungsten-182
Palladium-107 6.53 Silver-107
Technetium-98 4.2 or 6.6 Ruthenium-98
Dysprosium-154 3.01 Samarium-150
Iron-60 2.62 Nickel-60
Technetium-97 2.6 or 4.2 Molybdenum-97
Caesium-135 2.33 Barium-135
Gadolinium-150 1.798 Neodymium-142 (stable)
Zirconium-93 1.53 Niobium-93
Aluminium-26 0.717 Magnesium-26 (stable)

Notable isotopes with shorter lives still being produced on Earth include:

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).

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