Systematics of Fission Thresholds

An examination of the experimental data on fission thresholds and ground-state masses of nine nuclei shows that (i) there is experimental evidence for a difference between the masses of even-even, odd-$A$ and odd-odd nuclei in the deformed saddle-point configurations, analogous to the well-known difference for the ground states, (ii) the trend with $\frac{Z^2}{A}$ in the saddle-point masses is consistent with simple theoretical estimates based on a liquid drop type of model. This leads to a semiempirical formula for fission thresholds, which is found to be consistent also with estimates of thresholds made with the aid of the observed spontaneous fission half-lives of 28 nuclei. An analogous semiempirical formula for spontaneous fission half-lives is given.