An experimental perspective on triggered gamma emission from nuclear isomers

The study of triggered depopulation of nuclear isomers, with accompanying gamma emission, is a rapidly-changing field that is only now attaining some degree of maturity. Because isomer decays via electromagnetic transitions are strongly inhibited, the interaction of these levels with externally-produced photons provides an important probe of nuclear structure. Also, since some isomers may store large amounts of energy for long times, a number of applications have been proposed, including the creation of a gamma-ray laser. Early experiments conclusively demonstrated triggering of the 1015-year isomer of 180Ta, but even a partial correlation of the effect with known levels could only be obtained recently. Investigations of triggering for the 31-year isomer of 178Hf were initially guided just by systematics and experiments are characterized by considerable controversy. Against this background, the field of triggered gamma emission is entering a new phase in which improved level data allow targeting of specific potentially-useful transitions. This paper summarizes the current state-of-the-art and discusses the changing nature of the field.