Evidence for the pair-breaking process in Sn116,117

The nuclear level densities of Sn-116,Sn-117 below the neutron separation energy have been determined experimentally from the (He-3,alpha gamma) and (He-3, He-3'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.