Evidence for the pair-breaking process in {sup 116,117}Sn

The nuclear level densities of {sup 116,117}Sn below the neutron separation energy have been determined experimentally from the ({sup 3}He,{alpha}{gamma}) and ({sup 3}He,{sup 3}He{sup '}{gamma}) reactions, respectively. The level densities show a characteristic exponential increase and a difference in magnitude due to the odd-even effect of the nuclear systems. In addition, the level densities display pronounced step-like structures that are interpreted as signatures of subsequent breaking of nucleon pairs.