Experimental spectrum averaged cross sections (SACS) in 252Cf(sf) neutron field and its impact on the evaluation of neutron standards

Abstract

A new evaluation of spectrum averaged cross sections (SACS) of ²³⁵U, ²³⁸U, and ²³⁹Pu measured in the ²⁵²Cf(sf) reference neutron field is presented and found to be consistent with the original Mannhart SACS evaluation in IRDF-2002. The comprehensive vetted experimental database that includes SACS ratio and absolute SACS measurements of major actinides is being used to update the SACS database employed as input of the new GMApy code to derive the Neutron Standards. An update of the current neutron standards based on Time Projection Chamber (TPC) shape data, a new comprehensive uncertainty quantification, and revised SACS experimental database is proposed which result in a 0.7% increase of the evaluated ²³⁹Pu(n,f)/²³⁵U(n,f) crosssection ratio in the 1–5 MeV energy region. The increase is due to a 0.3% reduction of the standard ²³⁵U(n,f) cross section and a 0.4% increase of the ²³⁹Pu(n,f) reference cross section in the 1–5 MeV energy region. Those changes are well within estimated USU fission cross-section uncertainties of 1.2%, but are relevant for the evaluated mean values.