Three-body forces and persistence of spin-orbit shell gaps in medium-mass nuclei: Toward the doubly magic 78Ni

We present state-of-the-art shell-model calculations in a large model space (pf for protons, fpgd for neutrons), which allows us to study simultaneously excitations across the Z = 28 and N = 50 shell gaps. We explore the region in the vicinity of Ni-78, which is a subject of intense experimental investigations. Our calculations correctly account for the known low-lying excited states in this region, including those which may correspond to cross-shell excitations. We observe the minimum of the N = 50 mass gap at Z = 32 consistent with experimental data and its further increase toward Z = 28, indicating a robustness of the N = 50 gap in Ni-78. The evolution of the N = 50 gap along the nickel chain is shown to bear similarities to what is known in oxygen and calcium chains, providing a new opportunity for the studies of three-body monopole effects in medium-mass nuclei.