High-spin states in neutron-rich nuclei ${}^{103,105}\mathrm{Mo},$${}^{103}\mathrm{Nb}$ have been studied using the ${}^{238}\mathrm{U}(\alpha ,f)$ fusion-fission reaction. The deexcitation $\gamma$ rays were detected by Gammasphere in coincidence with the detection of both fission fragments by the Rochester $4\pi$ heavy-ion detector array, CHICO. The measured fission kinematics were used to deduce the masses and velocity vectors for both fission fragments. This allowed Doppler-shift corrections to be applied to the observed $\gamma$ rays on an event-by-event basis and the origin of $\gamma$ rays from either fission fragment to be established. With such advantages, the yrast sequences for these nuclei have been extended to the band crossing region. This band crossing is ascribed to the alignment of a pair of $h_{11/2}$ neutrons, which is supported by the observed blocking effect for the $\nu h_{11/2}$ band in ${}^{105}\mathrm{Mo}$ while there is no evidence for blocking in the alignment measured for either the $\nu d_{5/2}$ band in ${}^{103}\mathrm{Mo}$ or the $\pi g_{9/2}$ band in ${}^{103}\mathrm{Nb}.$ The observed upbend, rather than the sharp backbend seen in the Ru-Pd region, indicates a strong interaction between the ground-state and the aligned $h_{11/2}$ bands.

• Received 4 March 2002

DOI:https://doi.org/10.1103/PhysRevC.65.064325