Measurement of spectrum averaged cross section of 23Na(n,2n) in VR-1 reactor

Abstract Cross-section data are fundamental quantities that affect the accuracy of all calculations in nuclear applications. The 23Na(n,2n)22Na spectrum-averaged cross section has an irreplaceable role due to its sensitivity in the very high energy region of prompt fission neutron spectrum, but its measurement is complicated. Despite its importance, there are only a few contradictory measurements. In the current experiment, this spectrum-averaged cross section was measured in the VR-1 reactor with high water to uranium ratio, where the actual neutron spectrum is harder than actual fission spectrum. The value corrected to 235U(nth,f) PFNS is 3.903 ± 0.121 µb, which is in good agreement with the previous measurement in the benchmark reference neutron field in the LR-0 reactor. It is also in good agreement with the IRDFF-II tabulated value. Considering the good knowledge of pointwise cross-section data in IRDFF-II evaluation, this result serves as excellent independent validation of ENDF/B-VIII.0 235U(nth,f) PFNS above 10 MeV.