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 $\pi g_{9/2}$ and $\nu h_{11/2}$ 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.

• Received 9 December 2014
• Revised 31 January 2015

DOI:https://doi.org/10.1103/PhysRevC.91.021303