Low-lying levels in Cr59: Inadequacy of the fp model space and onset of deformation

Abstract

The low-lying levels in $^{59}\mathrm{Cr}$ have been studied with the $^{13}\mathrm{C}(^{48}\mathrm{Ca},2p)$ reaction at a beam energy of $130\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Data on $^{59}\mathrm{Mn}$, produced via the $pn$-evaporation channel, have set limits on the spin of the ground state in $^{59}\mathrm{Cr}$ and enabled some spin-parity assignments of excited states to be made. The structure obtained in this study is clearly inconsistent with results of shell-model calculations within the full $fp$ shell and requires inclusion of the $\ensuremath{\nu}{g}_{9∕2}$ orbital. The sequence of states is understood within Nilsson model calculations assuming a moderate oblate ground-state deformation.