Nuclear Isomer

A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons (protons or neutrons). "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10−12 seconds). As a result, the term "metastable" is usually restricted to refer to isomers with half-lives of 10−9 seconds or longer. Some sources recommend 5 × 10−9 s to distinguish the metastable half life from the normal "prompt" gamma emission half life.

Occasionally the half-lives are far longer than this, and can last minutes, hours, years, or in one notable case 180m
73Ta, so long that it has never been observed to decay (at least 1015 years). Sometimes, the gamma decay from a metastable state is given the special name of an isomeric transition, but save for the long-lived nature of the meta-stable parent nuclear isomer, this process resembles shorter-lived gamma decays in all external aspects. The longer lives of nuclear isomers (metastable states) are often due to the larger degree of nuclear spin change which must be involved in their gamma emission to reach the ground state. This high spin change causes these decays to be relatively forbidden, and thus delayed. However, other reasons for delay in emission, such as low or high available decay energy, also have effects on decay half life.

The first nuclear isomer and decay-daughter system (uranium X₂/uranium Z, now known as 234m
91Pa/234
91Pa) was discovered by Otto Hahn in 1921.