Atomic Masses in the Intermediate Region

The double-focusing mass spectrometer previously described has been used to measure 28 atomic masses in the region between gallium and niobium. The new information combined with previous determinations permits one to draw a rather complete packing fraction curve for the region from sulfur through xenon. Analysis shows that masses for stable and unstable isotopes of an element can be fitted by parabolas, one for odd $A$ and one for even $A$. The two parabolas have the same shape (except just above a proton shell) but are displaced in $A$ as well as mass. The displacements are such that, for an isobar, the minimum mass does not occur at the same $Z$ for the four nuclear families (even even, etc.), in contradiction to usual assumptions.