nuclear isomer

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 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 seconds). Some references recommend using a threshold of 5×10 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. An extreme example is the 73Ta nuclear isomer, which survives so long (at least 2.9×10 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 83Bi, which does not occur naturally; its half-life is 3.04×10 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, 75Re, 77Ir, 84Po, 95Am and multiple holmium isomers.