The total photofission cross sections for the actinide nuclei ${}^{232}\mathrm{Th},$ ${}^{233}\mathrm{U},$ ${}^{235}\mathrm{U},$ ${}^{238}\mathrm{U},$ and ${}^{237}\mathrm{Np}$ have been measured from 68 to 264 MeV using tagged photons at the Saskatchewan Accelerator Laboratory. The fission fragments were detected using parallel-plate avalanche detectors. The results show that the fission probability for ${}^{238}\mathrm{U}$ is 20% lower than that for ${}^{237}\mathrm{Np}$ and 40% higher than that for ${}^{232}\mathrm{Th}.$ Less significant differences were also found among the individual uranium isotopes. These results contradict the assumption that the fission probability for ${}^{238}\mathrm{U}$ is approximately equal to unity in this energy range. It has also been observed that the fission probability as a function of energy for all these isotopes is constant, with the exception of that for ${}^{232}\mathrm{Th},$ which increases with energy, although it seems to be reaching a saturation value. Comparison between the total photofission cross section for ${}^{237}\mathrm{Np}$ and the photoabsorption cross sections for lighter nuclei shows a behavior consistent with a broadening of the $\Delta$ resonance with increasing atomic mass.

• Received 28 May 1999

DOI:https://doi.org/10.1103/PhysRevC.61.034604