Structure of neutron-rich nuclei in A∼100 region observed in fusion-fission reactions

Neutron-rich nuclei around Asimilar to100 were populated as fission fragments produced by the U-238(alpha,f) fusion-fission reaction. The deexcitation gamma rays were detected by Gammasphere in coincidence with the detection of both fission fragments by the Rochester 4pi heavy-ion detector array, CHICO. This technique allows Doppler-shift corrections to be applied for the observed gamma rays on an event-by-event basis thus establishing the origin of gamma rays from either fission fragment. In addition, it allows observation of gamma-ray transitions from states with short lifetimes and offers the opportunity to study nuclear species beyond the reach of the spontaneous fission process. With these advantages, one can extend the spectroscopic study to higher spins than those derived using the thick-target technique, and to more neutron-rich nuclei than those derived from spontaneous fissions. Among the new and interesting phenomena identified in this rapid shape-changing region, the most distinct result is the evidence for a prolate-to-oblate shape transition occurring at Pd-116, which may have important implications to our understanding of the shell structure for neutron-rich nuclei.