Nearly degenerate isomeric states ofCu75

Abstract

The decays of two isomeric states in the neutron-rich nucleus $^{75}\mathrm{Cu}$ have been studied via delayed $\ensuremath{\gamma}$-ray spectroscopy. These states were populated by the fragmentation of a $60.4\phantom{\rule{4pt}{0ex}}A\phantom{\rule{0.16em}{0ex}}\mathrm{MeV} ^{86}\mathrm{Kr}$ primary beam, which impinged on a Be target at the LISE2k spectrometer of GANIL. Isomeric half-lives and branching ratios were measured, and a $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence analysis performed. These have allowed a revised level scheme of $^{75}\mathrm{Cu}$ to be proposed. A comparison with large-scale shell-model calculations using different effective interactions and valence spaces shows the importance of proton excitations across the $Z=28$ shell gap for reproducing the energy spacing between the isomeric states. The coexisting collective and single-particle properties of the $(1/{2}^{\ensuremath{-}})$ and $(3/{2}^{\ensuremath{-}})$ states, respectively, are similar to those of the same states in the neighboring $^{69,71,73}\mathrm{Cu}$. The small energy spacings between the $(1/{2}^{\ensuremath{-}}), (3/{2}^{\ensuremath{-}})$, and $5/{2}^{\ensuremath{-}}$ states are responsible for the isomerism.