Photofission and Photoneutron Emission in Uranium

The yields of the two reactions U²³⁸(γ, n)U²³⁷ and U²³⁸(γ, fission) produced by bremsstrahlung have been measured as a function of the maximum energy of the x-ray spectrum. The ratio of the number of fissions to the number of U²³⁷ atoms produced decreased from 0.390 at 8 Mev to 0.275 at 12 Mev and then increased to 0.465 at 22 Mev. These data are interpreted to show that for maximum x-ray energies in excess of 12 Mev, a fraction of the U²³⁷ atoms formed by the (γ, n) reaction are left with sufficient excitation energy that fission or secondary neutron emission can occur. The cross sections for the reactions as a function of gamma-ray energy have been calculated from the yield data.