Measurement of the U 236 (n,f) cross section for neutron energies from 0.4 MeV to 40 MeV from the back-streaming white neutron beam at the China Spallation Neutron Source

A new measurement of the neutron-induced fission cross section of U236 relative to the standard U235(n,f) cross section from 0.4-40 MeV was carried out by the time-of-flight method and a multicell fast fission ionization chamber at the China Spallation Neutron Source-Back-n white neutron source. The measured resonance peaks and the distribution of fission event-energy spectrum from the U235(n,f) reaction are in good agreement with that of the ENDF/B-VIII.0 evaluation, which confirms the reliability of the experimental setup. The measured U236/235U fission cross section ratios in 0.4-40 MeV neutron energy region are compared to previous measurements. The U236 fission cross section with the relative experimental uncertainty of 11.2%-2.9% in 0.4-1 MeV and 2.9%-2.8% in 1-40 MeV is determined and compared to calculation, previous measurements, and main evaluations. The present theoretical result with calculation program UNF reproduces the experimental data and the LANSCE data better than ENDF/B-VIII.0 evaluation does in the secondary fission region. The result confirms current evaluations from 0.4-40 MeV in most energy points and provides the experimental data for evaluating the discrepancies in the relevant energy region.