Angular distribution in alpha-induced fission of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$

The angular distribution of fission products has been measured as a function of mass asymmetry in the fissioning systems ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$(${\mathrm{\alpha }}_{39.1\mathrm{M}\mathrm{e}\mathrm{V}}$, f), ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$(${\mathrm{\alpha }}_{29.0\mathrm{M}\mathrm{e}\mathrm{V}}$, f), and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$(${\mathrm{\alpha }}_{39.1\mathrm{M}\mathrm{e}\mathrm{V}}$, f) using the recoil-catcher technique and off-line gamma spectrometry. The present data have been analyzed along with similar data reported earlier by us for ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$(${\mathrm{\alpha }}_{29\mathrm{M}\mathrm{e}\mathrm{V}}$, f) and ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$(${\mathrm{\alpha }}_{29\mathrm{M}\mathrm{e}\mathrm{V}}$, f) and the literature data on mass-averaged products in the same fissioning systems in the alpha energy region 16–40 MeV. The angular anisotropies of the average symmetric and asymmetric products in these fissioning systems have been evaluated based on the transition state model assuming two fission modes with characteristic shapes and barrier heights and considering the multichance fission effect at various alpha energies. Agreements between the calculated and experimental anisotropies indicate that the angular distributions for the symmetric and asymmetric modes are determined at and beyond the corresponding second saddle points in the deformation energy surface. The effects of the multichance fission are also seen to play a significant role depending on the properties of the nuclides concerned.