Quenching of the neutronN=82shell gap near120Srwith monopole-driving core excitations

Abstract

Properties of the experimentally inaccessible $N=82$ isotones below ${}^{132}\mathrm{Sn}$ have been a major open question for nuclear structure and nuclear astrophysics. Evolution of the neutron $N=82$ shell gap along this isotonic chain with even proton numbers $36--48$ is investigated by large-scale shell model calculations, which allow core excitations across both the $N=82$ neutron and $Z=50$ proton shell gaps. It is found that when moving away from ${}^{132}\mathrm{Sn},$ the $N=82$ shell gap, measured by the excited ${2}^{+}$ states with the neutron core-excited configurations, decreases gradually due to the monopole interaction acting dynamically between the $\ensuremath{\pi}{g}_{9/2}$ and $\ensuremath{\nu}{h}_{11/2}$ orbits. At ${}^{120}\mathrm{Sr},$ the neutron core-excited configuration is sufficiently low and becomes the dominant component in the first excited ${2}^{+}$ state, which results in a quenching of the $Z=40$ subshell. Measurement of $E2$ transition probabilities in ${}^{120}\mathrm{Sr}$ is proposed to confirm this novel shell-quenching mechanism.