Photofission of actinide nuclei in the quasideuteron and lower part of theDelta energy region - art. no. 034604

The total photofission cross sections for the actinide nuclei Th-232, U-233 , U-235, U-238, and Np-237 have been measured from 68 to 264 MeV using tagg ed photons at the Saskatchewan Accelerator Laboratory. The fission fragment s were detected using parallel-plate avalanche detectors. The results show that the fission probability for U-238 is 20% lower than that for Np-237 an d 40% higher than that for Th-232. Less significant differences were also f ound among the individual uranium isotopes. These results contradict the as sumption that the fission probability for U-238 is approximately equal to u nity in this energy range. It has also been observed that the fission proba bility as a function of energy for all these isotopes is constant, with the exception of that for Th-232, which increases with energy, although it see ms to be reaching a saturation value. Comparison between the total photofis sion cross section for Np-237 and the photoabsorption cross sections for li ghter nuclei shows a behavior consistent with a broadening of the Delta res onance with increasing atomic mass.