Shape Coexistence, Shape Transitions, and Triaxial Deformation in Neutron-Rich A~100 Nuclei Populated by Fusion-Fission Reactions

Neutron-rich nuclei with mass~100 were populated by the ²³⁸U(α,f) fusion-fission reaction. The deexcitation γ rays were detected by Gammasphere in coincidence with the detection of both fission fragments by the Rochester 4π heavy-ion detector array, CHICO. This technique allows Doppler-shift corrections to be applied for the observed γ rays thus establishing the origin of γ rays from either fission fragment. In addition, it allows observation of γ-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 fission. Among the new and interesting phenomena identified in this rapid shape-changing region, the most distinct result is evidence for a prolate-to-oblate shape transition occurring at both Ru and Pd isotopes. This represents the first observation of a transition from prolate to oblate shape in a given isotope chain.