Isomeric transition

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
₈₃Bi, 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
₇₃Ta, alpha decay is not practically available, but others are not quite so forbidden as those two.