Fission Barriers Deduced from the Analysis of Fission Isomer Results

Abstract

Available experimental data on fission-isomer excitation functions have been reanalyzed using an improved statistical model to determine values for the energies relative to the ground state of the secondary minimum and second maximum in the fission barrier for a series of plutonium, americium, and curium isotopes. The statistical model incorporates realistic decay-width calculations for neutron emission, fission, and $\gamma$-ray emission which are based on nuclear level densities derived from appropriate single-particle-level spectra. Values for the curvature $\hslash {\omega }_B$ of the second barrier are also estimated from the observed fission-isomer half-lives. The fission-barrier shapes determined from the analysis of experimental results are compared to theoretical calculations of barriers performed by several groups. The experimental barrier parameters agree with a variety of theoretical calculations to an accuracy of about 1 MeV. Systematic deviations between the experimental and theoretical results suggest that the surface asymmetry constant $\kappa$ in the liquid-drop mass formula should be substantially larger than the value of 1.7826 used in the second mass formula of Myers and Swiatecki.