Collectivity and instability of the N=Z=28 shell gap and strongly deformed bands with g9/2 configuration in Ni 56

Abstract

The positive-parity excited states of Ni56 and instability of the N=Z=28 shell gap are discussed on the basis of the antisymmetrized molecular dynamics calculation using the Gogny D1S effective interaction. It is found that 56Ni is quite soft against oblate deformation and the N=Z=28 closed-shell configuration amounts to only 62% in the ground state. It is also found that the shell gap easily disappears by prolate deformation, which leads to the coexistence of the almost spherical ground band, excited β and γ bands, and a prolate superdeformed band with (f7/2)-m(p3/2)m configurations within the small excitation energies. Furthermore, in the highly excited region, it is predicted that a couple of strongly deformed bands with (sd)-m(pf)-n(g9/2)m+n configurations are built on the 0+ states around 20 MeV. © 2014 American Physical Society.