Identification of excited states in theTz=+12nucleus75Rb:The quest for experimental signatures of collective neutron-proton correlations

Abstract

Excited states in the ${T}_{z}=\frac{1}{2}$ nucleus ${}^{75}\mathrm{Rb}$ were observed for the first time using the ${}^{40}\mathrm{Ca}{(}^{40}\mathrm{Ca},\ensuremath{\alpha}p$) reaction at 128 MeV. Identification was achieved using events detected by the Daresbury recoil separator in coincidence with $\ensuremath{\gamma}$ rays detected in the 45 element EUROGAM I Ge-detector array. Threefold events were used to build a decay scheme which consists of two rotational bands observed to ${I}^{\ensuremath{\pi}}=({\frac{45}{2}}^{+})$ and ${I}^{\ensuremath{\pi}}=({\frac{33}{2}}^{\ensuremath{-}}).$ The positive parity band in ${}^{75}\mathrm{Rb}$ behaves similarly to a negative-parity band in ${}^{74}\mathrm{Kr}$ and contains a region of alignment at $\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approx}0.75$ MeV. These data, and those of ${}^{77}\mathrm{Sr},$ can be interpreted by treating protons and neutrons separately in a cranked shell model approach despite a recent suggestion for the presence of $T=1$ neutron-proton pairing correlations in the neighboring self-conjugate, odd-odd ${}^{74}\mathrm{Rb}$ ground state band. Our study suggests that some experimental observables such as the energy levels and moments of inertia, may not be able to differentiate between different $T=1$ pairing phases in these ${T}_{z}=\frac{1}{2}$ nuclei.