Photofission cross sections of 233U and 239Pu near threshold induced by gamma rays from thermal neutron capture

The photofission cross sections of 233U and 239Pu have been studied by using monochromatic and discrete photons, in the energy interval from 5.43 to 9.72 MeV, produced by thermal neutron capture. The photofission data are presented and compared with those of other authors. A possible structure was observed in the 233U cross section near 7.23 MeV. According to the liquid drop model the heights of the simple fission barriers were determined: (5.6±0.2) MeV and (5.7±0.2) MeV for 233U and 239Pu respectively. The relative fissionability of the samples was also determined at each excitation energy and shown to be energy independent: (2.12±0.25) for 233U and (3.32±0.41) for 239Pu. (orig.)