Photofission cross sections of 238U and 235U from 5.0 MeV to 8.0 MeV

Abstract

Cross sections extracted from measured yield curves are reported for photofission of 238U and 235U. Surface barrier detectors were used to measure the fission yields from metal foil targets of 238U (enriched to 99.97%) and 235U (enriched to 97.68%) for maximum photon energies stepped from 5.0 MeV to 8.0 MeV in 0.1 MeV increments. The photons were obtained by Compton scattering γ-rays produced by neutron capture in a nickel plate near the core of the 5 MW Ames Laboratory Research Reactor. An unfolding procedure was used to extract the smoothed cross sections from the measured yield curves. In the 238U(γ, f) cross section, prominent structure is observed at 6.2 MeV and 7.6 MeV with hints of structure at 5.5 MeV and 7.0 MeV. The 235U(γ, f) cross section shows pronounced structure at 5.9 MeV, 6.6 MeV and 7.5 MeV with a hint of structure at 5.4 MeV. These cross sections are compared to previous photofission measurements and are discussed in the framework of Bohr's “fission channel” theory.