Lifetime study of particle-hole excitations in the semimagic nucleus94Ru

Abstract

The recoil-distance Doppler-shift technique was employed to determine lifetimes of high-spin states in the semimagic nucleus ${}^{94}\mathrm{Ru}$. The nuclei were populated using the reaction ${}^{58}\mathrm{Ni}{(}^{40}\mathrm{Ca},4p)$ at a beam energy of 145 MeV, and the $\ensuremath{\gamma}$ radiation from their decay was detected in six EUROBALL cluster detectors. A total of 23 reduced transition probabilities and limits for fifteen further transitions were extracted and compared to large-scale shell model calculations, considering different configuration spaces and residual interactions. The information deduced on transition strengths turned out to be essential for the correct assignment of the calculated to the experimental excited states. The results indicate that the ${13}_{2}^{\ensuremath{-}}$ (6919 keV), ${14}_{1}^{\ensuremath{-}}$ (7970 keV), and the ${15}_{1}^{\ensuremath{-}}$ (8133 keV) levels have pure proton $\ensuremath{\pi}{(f}_{5/2}{)}^{\ensuremath{-}1}$ $\ensuremath{\pi}{(g}_{9/2}{)}^{5}$ configurations, whereas all other excited states above 6.3 MeV are built from a neutron ${g}_{9/2}\ensuremath{\rightarrow}{d}_{5/2}$ excitation across the $N=50$ shell closure, coupled to up to six valence protons. Strong $M1$ transitions were found in a stretched dipole cascade within the sequence of neutron core-excited states at positive parity, while the strengths of the transitions between core-excited and pure proton states were proven to be small, similar as in ${}^{95}\mathrm{Rh}$.