Atomic masses of the neutron-rich isotopes ${}^{121–128}\mathrm{Cd}$, ${}^{129,131}\mathrm{In}$, ${}^{130–135}\mathrm{Sn}$, ${}^{131–136}\mathrm{Sb}$, and ${}^{132–140}\mathrm{Te}$ have been measured with high precision (10 ppb) using the Penning-trap mass spectrometer JYFLTRAP. Among these, the masses of four $r$-process nuclei ${}^{135}\mathrm{Sn}$, ${}^{136}\mathrm{Sb}$, and ${}^{139,140}\mathrm{Te}$ were measured for the first time. An empirical neutron pairing gap expressed as the odd-even staggering of isotopic masses shows a strong quenching across $N=82$ for Sn, with a $Z$ dependence that is unexplainable by the current theoretical models.

• Received 5 March 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.032501