The total photofission cross sections for the actinide nuclei 232 Th, 233 U, 235 U, 238 U, and 237 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 U is 20% lower than that for 237 Np and 40% higher than that for 232 Th. Less significant differences were also found among the individual uranium isotopes. These results contradict the assumption that the fission probability for 238 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 Th, which increases with energy, although it seems to be reaching a saturation value. Comparison between the total photofission cross section for 237 Np and the photoabsorption cross sections for lighter nuclei shows a behavior consistent with a broadening of the ⌬ resonance with increasing atomic mass.PACS number͑s͒: 25.20. Ϫx, 25.85.Jg, 27.90.ϩb
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