Cross-shell excitations from the fp shell: Lifetime measurements in Zn61

Lifetimes of excited states in the neutron-deficient nucleus $^{61}\mathrm{Zn}$ were measured employing the recoil-distance Doppler-shift (RDDS) and the electronic fast-timing methods at the University of Cologne. The nucleus of interest was populated as an evaporation residue in $^{40}\mathrm{Ca}(^{24}\mathrm{Mg},n2p)^{61}\mathrm{Zn}$ and $^{58}\mathrm{Ni}(\ensuremath{\alpha},n)^{61}\mathrm{Zn}$ reactions at 67 and 19 MeV, respectively. Five lifetimes were measured for the first time, including the lifetime of the $5/{2}_{1}^{\ensuremath{-}}$ isomer at 124 keV. Short lifetimes from the RDDS analysis are corrected for Doppler-shift attenuation (DSA) in the target and stopper foils. Ambiguous observations in previous measurements were resolved. The obtained lifetimes are compared to predictions from different sets of shell-model calculations in the $fp$, ${f}_{5/2}p{g}_{9/2}$, and multishell $fp\text{\ensuremath{-}}{g}_{9/2}{d}_{5/2}$ model spaces. The band built on the $9/{2}_{1}^{+}$ state exhibits a prolate deformation with $\ensuremath{\beta}\ensuremath{\approx}0.24$. Especially, the inclusion of cross-shell excitation into the $1{d}_{5/2}$ orbital is found to be decisive for the description of collectivity in the first excited positive-parity band.