The medium-spin structure of the nuclei ${}^{93}\mathrm{Rb}$ and ${}^{95}\mathrm{Rb}$ is studied following the neutron-induced fission of ${}^{235}\mathrm{U}$ at the PF1B neutron guide, using the FIFI spectrometer, and at the Lohengrin mass spectrometer of the Institut Laue-Langevin Grenoble. These nuclei, plus ${}^{91}\mathrm{Rb}$, are also studied following the spontaneous fission of ${}^{248}\mathrm{Cm}$ and ${}^{252}\mathrm{Cf}$ sources, using the EUROGAM-II and Gammasphere detector arrays, respectively. A high-spin isomeric state, with a half-life of 111(11) ns, is found in ${}^{93}\mathrm{Rb}$ at an excitation energy of 4422.4 keV, which most likely corresponds to the fully aligned $[\pi (g_{9/2})\otimes \nu (g_{7/2}{h}_{11/2})]{}_{{27/2}^{-}}$ configuration. An analogous configuration is proposed for the 5297.9-keV level observed in ${}^{91}\mathrm{Rb}$. A new $E3$ decay branch of the 1133.9-keV isomer in ${}^{91}\mathrm{Rb}$ is found, for which the rather low transition rate of $B(E3)=3.8(10)$ W.u. is determined. The energy of the isomeric state of ${}^{95}\mathrm{Rb}$ is now proposed to be at 810.6 keV, with a spin of ($9/2^{+}$), and its half-life determined to be $T_{1/2}=94(7)$ ns. A cascade of prompt transitions is observed on top of the 810.6-keV isomer in ${}^{95}\mathrm{Rb}$. The near-yrast structures of ${}^{91}\mathrm{Rb}$, ${}^{93}\mathrm{Rb}$, and ${}^{95}\mathrm{Rb}$ are compared to the results of shell-model calculations, which support the proposed $27/2^{-}$ interpretation of states in ${}^{91}\mathrm{Rb}$ and ${}^{93}\mathrm{Rb}$. An analogous $27/2^{-}$ state is expected to occur in ${}^{95}\mathrm{Rb}$, as a long-lived isomer at 3.24 MeV. No such isomeric decay could be observed in a measurement using the Lohengrin spectrometer, which shows that, if it exists, its population, following the fission of ${}^{235}\mathrm{U}$, is at least four times lower than that of the analogous $27/2^{-}$ isomer in ${}^{97}\mathrm{Y}$.

• Received 2 April 2010

DOI:https://doi.org/10.1103/PhysRevC.82.024302