New subshell closure at N = 58 emerging in neutron-rich nuclei beyond 78Ni

The structure of neutron-rich nuclei beyond 78Ni was studied using postaccelerated radioactive beams of 83,84,85Ga utilizing and -n spectroscopy. Our data, when combined with energy level systematics, suggests a possible new spherical subshell closure at N = 58 is created by the nearly degenerated 3s1/2 and 2d5/2 orbitals being well separated from other orbitals above N = 50. The near degeneracy of these states could be evidenced by isomerism in this region. The energies of the 2+1 and proposed 4+1 states observed in N = 52 84Ge are interpreted as an indication of a possible weakening of the doubly magic 78Ni core for nuclei beyond N = 50. The experimental evidence is supported by spherical HFB calculations using the SkOT functional involving the tensor term in the nucleon-nucleon interaction.