Level scheme study of Mo91 : Weak-coupling approximation in the N=50 region

The level scheme of $^{91}\mathrm{Mo}$ has been studied using the GALILEO $\ensuremath{\gamma}$-ray spectrometer at Laboratori Nazionali di Legnaro, INFN. Five transitions together with four levels are identified for the first time by the $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence measurement, considerably enriching the negative band in $^{91}\mathrm{Mo}$. The discrepancy in the previous parity assignments for the states higher than 3807.5 keV has been clarified with the help of new experimental results. A shell-model calculation employing the JUN45 interaction is performed in order to get a further understanding of the level structure of $^{91}\mathrm{Mo}$. Being close to $N=50$ shell closure, the weak-coupling approximation between the valence ${g}_{9/2}$ neutron-hole with the even-even core $(^{92}\mathrm{Mo})$ is discussed in $^{91}\mathrm{Mo}$.