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

Medium spin excited levels in Br-88 populated in fission of U-235 induced by neutrons have been observed for the first time. The measurement of gamma radiation following fission has been performed using the EXILL array of Ge detectors at the cold-neutron beam facility PFIB of the Institut Laue-Langevin (ILL), Grenoble. The ground state of Br-88 is proposed to be 1(-), changing the adopted (2(-)) value. The low-energy, newly observed levels are members of the pi p(3/2)nu(d(5/2))(3) and pi f(5/2)(-1)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 Br-88. The position of the g(9/2) proton intruder in the Ni-78 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 Br-88.