High-energy neutron emission in thermal neutron-induced fission of 235 U

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 252 Cf(sf) and 235 U( n th , f ) fission neutrons. The employed SACS in the 235 U( n th , 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 252 Cf(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 252 Cf(sf) and 235 U( n th , f ) PFNS can be used to probe the high-energy tail of the 235 U( n th , f ) fission neutron spectrum as the 252 Cf(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 235 U( n th , f ) ENDF / B-VII.1, ENDF / B-VIII.0, or JEFF -3.3 PFNS evaluations. The ENDF / B-VIII.0 evaluation of the 235 U( n th , f ) PFNS agrees well with derived SACS ratio data within quoted uncertainties. Other libraries predict a significantly lower fraction of 235 U( n th , f ) fission neutrons above 11 MeV of the outgoing neutron energy.