Nuclear isomer

A nuclear isomer is a <a href="/facts/Metastable/H72SMWof">metastable</a> state of an <a href="/facts/Atomic_nucleus/orpIm5Xl">atomic nucleus</a>, in which one or more <a href="/facts/Nucleon/6fguU9Xk">nucleons</a> (protons or neutrons) occupy excited state levels (higher energy levels). "Metastable" describes nuclei whose excited states have <a href="/facts/Half-life/kGVH8BAB">half-lives</a> 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 5×10−9 seconds to distinguish the metastable half life from the normal "prompt" <a href="/facts/Induced_gamma_emission/L6exCbrP">gamma-emission</a> half-life. Occasionally the half-lives are far longer than this and can last minutes, hours, or years. For example, the <a href="/facts/Isotopes_of_tantalum/uPRoIMz1">180m73Ta</a> nuclear isomer survives so long (at least 2.9×1017 years) that it has never been observed to decay spontaneously. The half-life of a nuclear isomer can even exceed that of the ground state of the same nuclide, as shown by 180m73Ta as well as <a href="/facts/Isotopes_of_rhenium/UmIoHfcT">186m75Re</a>, <a href="/facts/Isotopes_of_iridium/F1zVVPRq">192m277Ir</a>, <a href="/facts/Isotopes_of_bismuth/DrbTkEzW">210m83Bi</a>, <a href="/facts/Isotopes_of_polonium/bIdUTjCm">212m84Po</a>, <a href="/facts/Isotopes_of_americium/aqoWt39B">242m95Am</a> and multiple <a href="/facts/Isotopes_of_holmium/zNJ7HJqT">holmium isomers</a>.
Sometimes, the <a href="/facts/Gamma_decay/5Drf913J">gamma decay</a> from a metastable state is referred to as isomeric transition, but this process typically resembles shorter-lived gamma decays in all external aspects with the exception of the long-lived nature of the meta-stable parent nuclear isomer. 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 <a href="/facts/Forbidden_transition/UeungKv0">forbidden transitions</a> and delayed. Delays in emission are caused by low or high available decay energy.
The first nuclear isomer and decay-daughter system (uranium X2/uranium Z, now known as 234m91Pa/<a href="/facts/Protactinium-234/AVGw4A8U">23491Pa</a>) was discovered by <a href="/facts/Otto_Hahn/7alx1Db7">Otto Hahn</a> in 1921.

Nuclear isomer open-in-new

Nuclear isomer