Three-body forces and persistence of spin-orbit shell gaps in medium-mass nuclei: Toward the doubly magic ${}^{\mathbf{78}}$Ni

Rapid Communication

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

K. Sieja and F. Nowacki

Phys. Rev. C 85, 051301(R) – Published 7 May 2012

Abstract

We present state-of-the-art shell-model calculations in a large model space ( $p f$ for protons, $f p g d$ 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 $^{78}$ Ni, 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 $^{78}$ Ni. 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.