Abstract
Large-scale structures in the fission cross sections of the lower-charge actinides are commonly interpreted as pure vibrational (transmission) resonances associated with a well in the fission barrier. This well is often hypothesised to be a tertiary well, the secondary well of a double-humped barrier being too deep (according to most theoretical models) to allow undamped vibrational modes at the excitation energies involved. The author considers the possible width and strength ratios of low-lying states associated with secondary and tertiary wells, investigating the deviation, due to coupling of single-particle and vibrational motions, of these properties from the simple expectations of a simple transmission model this deviation is found to be considerable. Comparison of the calculated resonance properties with experimental data is made; this slightly favours the tertiary well hypothesis. Overall, the results indicate that more detailed fission cross section and angular distribution measurements in conjunction with more realistic model calculations of the type outlined in this paper could help bring a definitive answer to the problem of the structure of the light actinide fission barriers.