An isomeric transition is a nuclear process in which a nucleus with excess energy following the emission of an alpha particle or a beta particle emits energy without changing its number of protons or neutrons. Isomeric transitions can occur through the emission of a gamma ray or through the process known as internal conversion.

In certain cases, the excited nuclear state following the emission of a beta particle may be nearly stable and the nucleus may be able to stay in this nearly stable state for minutes, hours, or even days before emitting a gamma ray. The isomer, as it is called, behaves as a separate radioactive material decaying exponentially with the emission of a gamma ray only. An example is 99m-Tc, which is produced when 99-Mo emits a beta particle. 99m-Tc has a half-life of 6 hours and emits a 140 keV gamma ray.