Multifaceted yrast structure and the onset of deformation inSr96,97andZr98,99

Abstract

Neutron-rich $^{96,97}\mathrm{Sr}$ and $^{98,99}\mathrm{Zr}$ nuclei were populated as fission fragments produced by the $^{238}\mathrm{U}(\ensuremath{\alpha},\mathrm{f})$ fusion-fission reaction. The yrast states of these nuclei have been extended up to $\ensuremath{\approx}20\ensuremath{\hbar}$, which is about $6\ensuremath{\hbar}$ on average beyond the previously known spin, by studying the prompt $\mathrm{\ensuremath{\gamma}}$ rays in coincidence with the detection of both fission fragments. This extension allows the observation of yrast states with spins beyond $\ensuremath{\approx}{4}^{+}$ 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 $\ensuremath{\approx}600\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ 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 $\mathrm{\ensuremath{\Delta}}\mathrm{I}=1$ to $\mathrm{\ensuremath{\Delta}}\mathrm{I}=2$ 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.