U238andTh232Photofission and Photoneutron Emission near Threshold

Variable-energy photons defined to 100 keV were used to study ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ from 5 to 8 MeV. The inferred fission transmission indicates (a) that the lowest $1^{-}$ fission barrier is about 6.5 MeV in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ and 6.3 MeV in ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, and (b) that there is an energy gap before the rapid opening of additional fission channels above 7 MeV in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$.