Neutron-proton multiplets in the nucleus $^{88}$Br

Medium spin excited levels in $^{88}\mathrm{Br}$ populated in fission of $^{235}\mathrm{U}$ induced by neutrons have been observed for the first time. The measurement of $\ensuremath{\gamma}$ radiation following fission has been performed using the EXILL array of Ge detectors at the cold-neutron beam facility PF1B of the Institut Laue-Langevin (ILL), Grenoble. The ground state of $^{88}\mathrm{Br}$ is proposed to be ${1}^{\ensuremath{-}}$, changing the adopted $({2}^{\ensuremath{-}})$ value. The low-energy, newly observed levels are members of the $\ensuremath{\pi}{p}_{3/2}\ensuremath{\nu}{({d}_{5/2})}^{3}$ and $\ensuremath{\pi}{f}_{5/2}^{\ensuremath{-}1}\ensuremath{\nu}{({d}_{5/2})}^{3}$ multiplets. A triplet of yrast levels observed at around 2 MeV is interpreted as being due to coupling of the ${g}_{9/2}$ proton to the $({d}_{5/2}{)}^{3}$, seniority 3 multiplet, supporting the presence of collective effects in $^{88}\mathrm{Br}$. The position of the ${g}_{9/2}$ proton intruder in the $^{78}\mathrm{Ni}$ core is determined at 5.7 MeV above the ${f}_{5/2}$ proton level. Shell-model calculations predict the same proton-neutron excitations proposed in $^{88}\mathrm{Br}$.