Triaxiality and the alignedh11∕2neutron orbitals in neutron-rich Zr and Mo isotopes

Abstract

Neutron-rich Zr and Mo isotopes were populated as fission fragments produced by the $^{238}\mathrm{U}(\ensuremath{\alpha},\mathit{f})$ fusion-fission reaction. The level schemes of these nuclei have been extended beyond the first band crossing region, which can be ascribed to the ${h}_{11∕2}$ neutron pair alignment. The spin alignment and signature splitting for the $\ensuremath{\nu}{h}_{11∕2}$ orbitals in term of triaxiality is addressed. The crossing frequency of the aligned bands in even Zr and Mo isotopes can be reproduced well by calculations using the cranked shell model. Compared to the Zr isotopes, band crossings in Mo isotopes shift to a lower rotational frequency due to the emerging importance of the $\ensuremath{\gamma}$ degree of freedom. Within the framework of particle-rotor model, the difference in the signature splitting observed for the $\ensuremath{\nu}{h}_{11∕2}$ bands between the odd Zr and Mo isotopes can be attributed to the triaxial degree of freedom in the Mo isotopes. The surprising shift of the band crossing to a higher rotational frequency in $^{106}\mathrm{Mo}$ is interpreted as a manifestation of the deformed subshell closure at $N=64$.