Low-energy states of 31 81 Ga 50 : Proton structure of the nuclei close to Ni 78

The \ensuremath{\beta} decay of the neutron-rich ${}_{30}^{81}{\mathrm{Zn}}_{51}$ has been investigated at the PARRNe mass separator at the IPN Orsay. The sources of $^{81}\mathrm{Zn}$ were produced using the ISOL (Isotopic Separation On Line) technique by the fission of ${}^{\mathrm{nat}}$U exposed to the neutron flux produced by the 26-MeV deuteron beam delivered by the MP-Tandem. With \ensuremath{\gamma} and $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence measurements, excited levels were attributed to ${}_{31}^{81}{\mathrm{Ga}}_{50}$ for the first time. A partial decay scheme for $^{81}\mathrm{Zn}$ is proposed. The proposed level scheme is well reproduced by shell model calculations using the most recent effective empirical interaction. We show that the structure of this nucleus is consistent with that of the heavier odd-proton $N=50$ isotones within the assumption of strong proton $Z=28$ and neutron $N=50$ effective shell effects. The observed states can be associated to rather simple and clean configurations of three protons placed in the $1{f}_{5/2}$ and $2{p}_{3/2}$ orbits.