A high-precision direct Penning trap mass measurement has revealed a 0.5-MeV deviation of the binding energy of ${}^{134}\mathrm{Sn}$ from the currently accepted value. The corrected mass assignment of this neutron-rich nuclide restores the neutron-shell gap at $N=82$, previously considered to be a case of “shell quenching.” In fact, the new shell gap value for the short-lived ${}^{132}\mathrm{Sn}$ is larger than that of the doubly magic ${}^{48}\mathrm{Ca}$ which is stable. The $N=82$ shell gap has considerable impact on fission recycling during the $r$ process. More generally, the new finding has important consequences for microscopic mean-field theories which systematically deviate from the measured binding energies of closed-shell nuclides.

• Received 11 November 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.072501