The Neutron-Production Cross Section of ²³⁸Pu in a Fast Spectrum

The production cross sections where α = σ_c/σf for ²³⁸Pu and ²³⁹Pu, have been compared in the fast neutron flux at the center of a bare spherical critical assembly of ²³⁹Pu. These quantities averaged by the fast-neutron spectrum indicate nearly the same properties for the even-even ²³⁸Pu nucleus as for the odd-even ²³⁹Pu nucleus. The ratio measured in a neutron flux peaked at 0.25 MeV with an average neutron energy = 1.67 MeV is (²³⁸Pu)/(²³⁹Pu) = 1.01 ± 0.06, and (²³⁸Pu) = 3.76 ± 0.23 b. The results of this study indicate that ²³⁸Pu metal probably has a critical mass of the same order of magnitude as ²³⁹Pu metal.