High-energy neutron emission in thermal neutron-induced fission of U235

Spectrum-averaged cross-section (SACS) ratio measurement showing reduced uncertainties of measured SACS in two independent fission neutron fields is presented. The used prompt fission neutron fields correspond to the ²⁵²Cf(sf) and ²³⁵U(nₜₕ,f) fission neutrons. The employed SACS in the ²³⁵U(nₜₕ,f) prompt fission neutron spectrum (PFNS) were measured using three different light water reactors: LR-0 and VR-1 zero power reactors and the LVR-15 10 MWt reactor. The employed SACS in the ²⁵²Cf(sf) prompt fission neutron field were measured using a certified high-flux neutron source. Existing correlations among measured SACS in the two different neutron fields are estimated and used to reduce the uncertainty of measured SACS ratio for IRDFF dosimetry reactions. The derived set of measured SACS ratios with reduced uncertainty extends previous works into the higher-energy fission neutron range up to 20--30 MeV. The SACS ratio in ²⁵²Cf(sf) and ²³⁵U(nₜₕ,f) PFNS can be used to probe the high-energy tail of the ²³⁵U(nₜₕ,f) fission neutron spectrum as the ²⁵²Cf(sf) reference neutron spectrum is relatively well known. Derived experimental SACS ratio data. featuring low uncertainty are compared to the calculated dosimetry SACS ratio using the IRDFF-II dosimetry cross sections and the ²³⁵U(nₜₕ,f) ENDF/B-VII.1, ENDF/B-VIII.0, or JEFF-3.3 PFNS evaluations. The ENDF/B-VIII.0 evaluation of the ²³⁵U(nₜₕ,f) PFNS agrees well with derived SACS ratio data within quoted uncertainties. Other libraries predict a significantly lower fraction of ²³⁵U(nₜₕ,f) fission neutrons above 11 MeV of the outgoing neutron energy.