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

Excited states in the $T_z=\frac{1}{2}$ nucleus ${}^{75}\mathrm{Rb}$ were observed for the first time using the ${}^{40}\mathrm{Ca}{(}^{40}\mathrm{Ca},\alpha p$) reaction at 128 MeV. Identification was achieved using events detected by the Daresbury recoil separator in coincidence with $\gamma$ 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 $I^{\pi }=\left({\frac{45}{2}}^{+}\right)$ and $I^{\pi }=\left({\frac{33}{2}}^{-}\right).\mathrm{}$ 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 $ħ\omega \approx 0.75$ 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 $T=1\mathrm{}$ 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 $T=1\mathrm{}$ pairing phases in these $T_z=\frac{1}{2}$ nuclei.