Stability of the N=50 shell gap in the neutron-rich Rb, Br, Se, and Ge isotones

The low- and medium-spin states of the N = 50 neutron-rich, ₃₇ ⁸⁷Rb, ₃₅ ⁸⁵Br, ₃₄ ⁸⁴Se and ₃₂ ⁸²Ge isotones have been populated in deep-inelastic processes produced by the interaction of 460 MeV 82Se ions with a 192Os target. The subsequent gamma decay has been investigated at the Laboratori Nazionali di Legnaro using the GASP gamma-ray detector array. The comparison of the experimentally observed excited states with shell-model calculations performed with and without neutron degrees of freedom has allowed the investigation of the role of the neutron-core breaking excitations and therefore of the N = 50 shell gap. The inclusion of neutron configurations in the shell-model calculations results in an improved agreement between the experimental and calculated level energies at medium and high spin. These results highlight the considerable contribution of neutron-core particle-hole excitations across the N = 50 shell gap to the level configurations. The overall agreement of the measured excited states with the shell-model predictions is indicative of the persistence of the N = 50 shell gap down to Z = 32.