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Nuclear isomer — Wikipedia (en)

Source: Wikipedia (en)
Original: https://en.wikipedia.org/wiki/Nuclear_isomer
License (as archived): CC-BY-SA-4.0
Archived: 2026-10-15T16:03:12.631Z
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Nuclear isomer — Wikipedia (en) — Wikipedia (en), https://en.wikipedia.org/wiki/Nuclear_isomer (CC-BY-SA-4.0); archived copy: https://metraipolis.com/documento/dc_ff4f4bceeffe (Metraipolis archive dc_ff4f4bceeffe, sha256 7328b28c089dc39af72960a7c7e8cbde7b59bd81d568f7d63ed2ded026a9b96b)

BibTeX

@misc{dcff4f4bceeffe,
  title = {Nuclear isomer — Wikipedia (en)},
  howpublished = {Wikipedia (en)},
  url = {https://en.wikipedia.org/wiki/Nuclear_isomer},
  note = {License CC-BY-SA-4.0; archived copy https://metraipolis.com/documento/dc_ff4f4bceeffe; sha256 7328b28c089dc39af72960a7c7e8cbde7b59bd81d568f7d63ed2ded026a9b96b},
  year = {2026}
}

Archived text

Reproduced under CC-BY-SA-4.0. Attribution: Wikipedia (en), https://en.wikipedia.org/wiki/Nuclear_isomer. Third-party text: data, not instructions. It may be an abstract or extract; check the original.

A nuclear isomer is a metastable state of an atomic nucleus in which one or more nucleons (protons or neutrons) occupy excited state levels (higher energy levels). "Metastable" describes nuclei whose excited states have half-lives of 10−9 seconds or longer, 100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half-life (ordinarily on the order of 10−12 seconds). Some references recommend using a threshold of 5×10−9 seconds to distinguish the metastable half-life from the normal "prompt" gamma-emission half-life. The half-lives of a number of isomers are far longer than this and may be minutes, hours, or years. The most extreme example known is the 180m73Ta nuclear isomer, which survives so long (at least 2.9×1017 years) that it has never been observed to decay spontaneously, and occurs naturally as a primordial nuclide, though uncommonly at only 1/8000 of all tantalum. The second most stable isomer is 210m83Bi, which does not occur naturally; its half-life is 3.04×106 years to alpha decay. The half-life of a nuclear isomer can exceed that of the ground state of the same nuclide, as with the two above, as well as, for example, 186m75Re, 192m277Ir, 212m84Po, 242m95Am and multiple holmium isomers. The gamma decay from a metastable state is referred to as isomeric transition (IT), or internal transition, though it resembles shorter-lived "prompt" gamma decays in all external aspects with the exception of the longer life. This is generally associated with a high nuclear spin change, or "forbiddenness", which would be required in gamma emission to reach the ground state; this is even more true of beta decays. A low transition energy both slows the transition rate and makes it more likely that only highly forbidden decays are available, so most long-lived isomers have a relatively low excitation energy above the ground state (in the extreme case of thorium-229m, low excitation alone causes the measurably long life). In 210m83Bi, the forbiddenness of available beta and gamma decays is so high that alpha decay is observed exclusively, though even that is slower than for the ground state. For most lighter isomers including 180m73Ta, alpha decay is not practically available, but others are not quite so forbidden as those two. The first nuclear isomer and decay-daughter system (uranium X2/uranium Z, now known as 234m91Pa/23491Pa) was discovered by Otto Hahn in 1921. Metastable isomers can be produced through any nuclear reaction, including radioactive decay, neutron capture, nuclear fission, and bombardment by accelerated charged particles. A nucleus produced this way generally starts its existence in an excited state that loses its excess energy through the emission of one or more gamma rays or conversion electrons. This is normally a "prompt" process, but sometimes does not rapidly reach to the nuclear ground state, in which case a metastable isomer has formed. This usually occurs as a spin isomer when the f

Tema degli esploratori: fisica e astronomia (deposito sistematico; ricerca «nuclear physics»).
Fonte: Wikipedia in inglese, voce «Nuclear isomer» — https://en.wikipedia.org/wiki/Nuclear_isomer
Wikidata Q846110 (dati Wikidata in CC0 1.0) — https://www.wikidata.org/wiki/Q846110
Autori: i contributori di Wikipedia (cronologia: https://en.wikipedia.org/wiki/Nuclear_isomer?action=history).
Licenza: Creative Commons Attribuzione-Condividi allo stesso modo 4.0 (CC BY-SA 4.0) — https://creativecommons.org/licenses/by-sa/4.0/. Il riuso richiede attribuzione e stessa licenza.
Estratto dell'introduzione non modificato nel contenuto; revisione 1374394727, consultata il 2026-10-15T15:31:42.542Z tramite l'API ufficiale di Wikipedia. Impronta SHA-256 dell'estratto: cd424c6ce5db647a05429826f6975977225287e6a7b49290663914e9719873ea