New subshell closure at N=58 emerging in neutron-rich nuclei beyond Ni78

The structure of neutron-rich nuclei beyond Ni78 was studied using postaccelerated radioactive beams of Ga83,84,85 utilizing βγ and β-nγ 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 ν3s1/2 and ν2d5/2 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 21+ and proposed 41+ states observed in N=52 Ge84 are interpreted as an indication of a possible weakening of the doubly magic Ni78 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.