Photofission of Heavy Nuclei at Energies up to 4 GeV

Total photofission cross sections for ²³⁸U, ²³⁵U, ²³³U, ²³⁷Np, ²³²Th, and ^natPb have been measured simultaneously, using tagged photons in the energy range E_γ = 0.17-3.84 GeV. This was the first experiment performed using the Photon Tagging Facility in Hall B at Jefferson Lab. Our results show that the photofission cross section for ²³⁸U relative to that for ²³⁷Np is about 80% over the entire energy range, implying the presence of important processes which compete with fission. If we assume that for ²³⁷Np the photofission probability is equal to unity, we observe a significant shadowing effect, starting below 1.5 GeV.