A six-inch double-focusing mass spectrometer has been employed to determine 32 mass differences between neighboring isotopes of even-$Z$ elements in the region $Z=64\mathrm{} \mathrm{to} 82$. Neutron separation energies and neutron pairing energies are easily calculated from the experimental results. These calculated values are sufficiently precise to conclude that lack of uniformity in neutron separation energy and pairing energy systematics appears in regions where other nuclear properties exhibit changes in their systematics. Neutron separation energy data have also been employed for the isotopic assignments of several nuclear reactions.

• Received 3 June 1957

DOI:https://doi.org/10.1103/PhysRev.107.1669