A New International Evaluation of the Neutron Cross Section Standards

Abstract

The measurements of most neutron cross sections are made relative to neutron cross section standards. The last complete evaluation of the standards, which was for ENDF/B-VI, took place almost 15 years ago. The cross section standards produced for the ENDF/B-VI evaluation were for the H(n, n), 3He(n, p), 6Li(n, t), 10B(n, α), 10B(n, α1γ), C(n, n), Au(n, γ) and 235U(n, f) reactions. Some additional important reactions including the 238U(n, f) cross section, which is used as a standard cross section in the NEANDC/INDC Nuclear Standards File, were evaluated. These standards were accepted internationally to ensure that all evaluation projects were using the same set of standards. Significant experimental activity on the standards has occurred since that evaluation was completed which will cause changes in a new evaluation of the standards. International efforts are now underway to update the previous work by including standards measurements made since that evaluation was completed and to improve the evaluation process. Due to the need for high energy standards, the energy region will be extended to about 200 MeV for some of the cross section standards. Work on the standards evaluation has begun with an investigation of the completed experiments to determine uncertainties and correlations within and among data sets. Further work could lead to an improved evaluation procedure compared with that used in ENDF/B-VI. The status of the work on this evaluation will be discussed.