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

The structure of neutron-rich nuclei beyond Ni-78 was studied using postaccelerated radioactive beams of Ga-83,Ga-84,Ga-85 utilizing beta gamma and beta-n gamma 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 nu 3s(1/2) and nu 2d(5/2) 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 Ge-84 are interpreted as an indication of a possible weakening of the doubly magic Ni-78 core for nuclei beyond N = 50. The experimental evidence is supported by spherical HFB calculations using the SkO(T) functional involving the tensor term in the nucleon-nucleon interaction.