Level structure of99Nb

Abstract

The $\ensuremath{\beta}$ decay of ${}^{97}\mathrm{Sr}$ to ${}^{97}\mathrm{Y}$ has been investigated using ion-guide on-line mass separation and a 10 Ge-detector array to record $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences to a detection limit well below that of former studies. Similarities are found in the $\ensuremath{\beta}$-decay patterns of ${}^{99}\mathrm{Zr}$ and of its isotone ${}^{97}\mathrm{Sr}$ and also in the $\ensuremath{\gamma}$-ray decay rates and branchings of the corresponding levels in their respective daughters ${}^{99}\mathrm{Nb}$ and ${}^{97}\mathrm{Y}$. This indicates a persisting influence of the ${d}_{5/2}$ neutron shell closure for ${}^{99}\mathrm{Nb}$. The level structure of ${}^{99}\mathrm{Nb}$ and the $\ensuremath{\beta}$-feeding pattern are discussed in the frame of the interacting boson-fermion plus broken pair model and the microscopic quasiparticle phonon model.