Neutron-rich ${}^{96,97}\mathrm{Sr}$ and ${}^{98,99}\mathrm{Zr}$ nuclei were populated as fission fragments produced by the ${}^{238}\mathrm{U}(\alpha ,\mathrm{f})$ fusion-fission reaction. The yrast states of these nuclei have been extended up to $\approx 20\hslash$, which is about $6\hslash$ on average beyond the previously known spin, by studying the prompt $\mathrm{\gamma }$ rays in coincidence with the detection of both fission fragments. This extension allows the observation of yrast states with spins beyond $\approx 4^{+}$ in ${}^{96}\mathrm{Sr}$ and ${}^{98}\mathrm{Zr}$ evolving from vibrationlike states to rotationlike states. With an additional neutron, the yrast states with excitation energies above $\approx 600\mathrm{keV}$ in ${}^{97}\mathrm{Sr}$ and ${}^{99}\mathrm{Zr}$ have the characteristics of members of rotationlike bands for both positive- and negative-parity states. However, their underlying single-particle configurations cannot be determined uniquely by the measured intensity ratios of $\mathrm{\Delta }\mathrm{I}=1$ to $\mathrm{\Delta }\mathrm{I}=2$ transitions because of possible configuration mixing. The sharp variations in the yrast structure of these nuclei are discussed in terms of the gradualness of the onset of quadrupole deformation.

• Received 7 September 2004

DOI:https://doi.org/10.1103/PhysRevC.70.064312