Measurement of the Subthreshold Neutron-Induced Fission Cross Section of ²⁴⁰Pu Relative to ²³⁵U from 5 to 300 keV

The fission cross-section ratio ²⁴⁰Pu:²³⁵U has been measured as a function of neutron energy from 5 to 300 keV using ionization fission chambers, the time-of-flight technique, and the threshold cross-section method at the Lawrence Livermore National Laboratory's 100-MeV electron linear accelerator. Distinct structure is observed in the cross-section ratio over this subthreshold energy range.