Neutron Fission Cross Sections of ²³⁹Pu and ²⁴⁰Pu Relative to ²³⁵U

The ratios of the neutron fission cross sections σ_f(²⁴⁰Pu)/σ_f(²³⁹Pu), σ_f(²⁴⁰Pu)/σ_f(²³⁵U), and σ_f(²³⁹Pu)/σ_f(²³⁵U) have been measured simultaneously with a multiplate ionization fission chamber using the Oak Ridge Electron Linear Accelerator as a neutron source over the neutron energy range from 5 keV to 20 MeV. The ²⁴⁰Pu ratio data are in overall agreement with ENDF/B-V with exceptions in relatively narrow neutron energy regions. Below 150 keV and from 10 to 20 MeV, the present ²³⁹Pu/²³⁵U fission ratios indicate significant discrepancies when compared to ENDF/B-V. These ratios are important for thermal and fast reactor applications.