Photon scattering on U238 and the interpretation of near-threshold photofission resonances

Cross sections for photon elastic scattering have been measured for U238 between 4.8 and 6.4 MeV with an energy resolution of about 50 keV. These data have been used along with existing photofission data to infer the total photoabsorption cross section T and the fission transmission coefficient Tf below neutron threshold. We find that the inferred T varies smoothly with energy and generally follows the extrapolated tail of the giant dipole resonance, and that Tf shows a broad bump between 5.6 and 5.9 MeV. These results strongly support the contention that resonances seen in near-threshold photofission cross sections are due to the fission and not the photon channel.