Gamma emission is not a primary decay process but usually accompanies alpha and beta decay. Typically this type of radiation arises when the daughter product resulting from alpha or beta decay is formed in an excited state. This excited state returns very rapidly (< 10–9 s) to the ground state through the emission of a gamma photon. Instead of having a well-defined range like alpha and beta particles, gamma rays lose characteristically a certain fraction of their energy per unit distance through matter. Gamma rays are highly penetrating and can result in considerable organic damage. Gamma emitting sources require heavy shielding and remote handling.
In contrast to normal gamma emission, isomeric transitions occur on a longer time-scale. If the life-time for gamma emission exceeds about one nanosecond, the excited nucleus is defined to be in a metastable or isomeric state (denoted by m). The decay process from this excited state is known as an isomeric transition (IT).
J. Magill and J. Galy, Radioactivity Radionuclides Radiation Springer Verlag, 2005