Photofission Cross Section of Plutonium-238 and Plutonium-239

The photofission yields of ²³⁸Pu and ²³⁹Pu were measured up to 11.5 MeV with the University of Cincinnati betatron. Corona discharge spark detectors were used to detect the fission fragments from isotopically pure foils in the presence of the very high alpha-particle activity of the plutonium. The photofission cross sections were deduced from the yields by both the Penfold-Leiss procedure and Cook's least-structure analysis. They show the onset of the giant resonance. The cross section for ²³⁸Pu at 7.5 MeV is 28 ± 5 mb and 21 ± 4 mb for ²³⁹Pu. At 11.0 MeV, the corresponding values were 303 ± 94 mb and 221 ± 69 mb. Between 7.5 and 11 MeV, the ²³⁸Pu photofission cross section is ∼35% larger than that for ²³⁹Pu.