The masses of 40 neutron-rich nuclides from $Z=51$ to 64 were measured at an average precision of $\delta m/m={10}^{-7}$ using the Canadian Penning Trap mass spectrometer at Argonne National Laboratory. The measurements, of fission fragments from a ${}^{252}$Cf spontaneous fission source in a helium gas catcher, approach the predicted path of the astrophysical $r$ process. Where overlap exists, this data set is largely consistent with previous measurements from Penning traps, storage rings, and reaction energetics, but large systematic deviations are apparent in $\beta$-endpoint measurements. Differences in mass excess from the 2003 Atomic Mass Evaluation of up to 400 keV are seen, as well as systematic disagreement with various mass models.

• Received 1 July 2011

DOI:https://doi.org/10.1103/PhysRevC.85.045805