U 238 and Th 232 Photofission 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 ${1}^{\ensuremath{-}}$ 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 $^{238}\mathrm{U}$.