New subshell closure atN=58emerging in neutron-rich nuclei beyondNi78

Abstract

The structure of neutron-rich nuclei beyond $^{78}\mathrm{Ni}$ was studied using postaccelerated radioactive beams of $^{83,84,85}\mathrm{Ga}$ utilizing $\ensuremath{\beta} \ensuremath{\gamma}$ and $\ensuremath{\beta}\ensuremath{-}n \ensuremath{\gamma}$ 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 $\ensuremath{\nu}3{s}_{1/2}$ and $\ensuremath{\nu}2{d}_{5/2}$ 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 $N=52$ $^{84}\mathrm{Ge}$ are interpreted as an indication of a possible weakening of the doubly magic $^{78}\mathrm{Ni}$ core for nuclei beyond $N=50$. The experimental evidence is supported by spherical HFB calculations using the ${\mathrm{SkO}}_{T}$ functional involving the tensor term in the nucleon-nucleon interaction.