After a radioactive nucleus undergoes an isobaric transition (beta emission, positron emission, or electron capture), it usually contains too much energy to be in its final stable or daughter state. Nuclei in these intermediate and final states are isomers, since they have the same atomic and mass numbers. Nuclei in the intermediate state will undergo an isomeric transition by emitting energy and dropping to the ground state.
In contrast to normal gamma emission that occurs by dipole radiation, isomeric transitions must occur by higher order multipole transitions that occur on a longer time-scale. If the lifetime for gamma emission exceeds about one nanosecond, the excited nucleus is defined to be in a metastable or isomeric state. The decay process from this excited state is known as an isomeric transition (IT). An example is the decay of the isomer or metastable state of protactinium i.e.
234m91Pa ® 23491Pa + g
with a half-life t1/2 = 1.17 min. and a branching ratio of 0.0013. The letter m after the mass number denotes the metastable state.
J. Magill and J. Galy, Radioactivity Radionuclides Radiation Springer Verlag, 2005