Mass Distribution of Fission Products Following Photofission of Uranium-238

The mass-yield distribution of fission products following photofission of ²³⁸U using bremsstrahlung energies of 22, 24, and 26 MeV were measured by radio-chemically isolating the fission products belonging to 24 mass chains. The absolute activities of these nuclides were determined by β‾ and γ counting techniques, and the cumulative fission yields were calculated relative to ¹⁴⁰Ba. The peak-to-valley ratio was found to be effectively constant; thus, the average photon-energy-inducing fission is essentially equal for all three energies studied in this work. The fission yields in the mass region from 130 to 140 were examined for evidence of fine structure around A = 132 or A = 133. Although there were some deviations in this region from a “smooth curve,” it did not yield conclusive evidence for fine structure. Finally, to set the slope of the heavy wing of the mass yield curves, cumulative fission yields of mass chains in the rare-earth region (in addition to others) were measured.