Lawrence Berkeley National Laboratory

Background: Neutron-rich nuclei in the A≈100 mass region are interesting due to a rapid shape transition, especially pronounced in the Zr isotopes, and more recently observed in Nb isotopes. Nb98, with only one proton and one neutron outside the subshell closure nucleus of Zr96, is amenable to shell model calculations. Purpose: To further examine the rapid shape transition, the yrast structure of Nb98 was established in this work. This was the only yrast structure missing from all immediate neighbors to Zr96. Method: The yrast structure of Nb98 was studied in the fission of the compound systems formed in three heavy-ion induced reactions, Mg24 (134.5 MeV) +Yb173, Na23 (129 MeV) +Yb176, and O18 (91 MeV) +Pb208. Prompt γ-ray spectroscopy was accomplished using the Gammasphere array. Results: Excitation energies up to 3 MeV were observed for the first time in Nb98. The yrast structure above the previously known (5)+ isomer was established. In the process of studying Nb98 the yrast structure of positive-parity states in Mo99 was extended to 3.7 MeV excitation energy, the previously known Nb99 level scheme was enriched, and two new levels were added in the level scheme of Zr97. Conclusions: The coupling of the odd proton occupying the g9/2 orbital to the yrast states in the core nucleus of Zr97 can account for all observed states in Nb98. The yrast structure for the positive-parity states of Mo99 is compared to the deformed ground-state bands of the Ru101 isotone and of Mo98,100.