Methodology for Generating Covariance Data of Thermal Neutron Scattering Cross Sections

International audienceThis paper details and implements a framework for evaluating thermal neutron scattering crosssections that provide Sðα; βÞ data and covariance data for hydrogen in light water. This methodology involvesperturbing model parameters of molecular dynamics potentials and fitting the simulation results to experi mental data. The framework is general and can be applied to any material or simulation method. The fit ismade using the Unified Monte Carlo method to experimentally measure double-differential scattering crosssections of light water at the Spallation Neutron Source at Oak Ridge National Laboratory. Mean values andcovariance data were generated for model parameters, phonon density of states, double-differential crosssections, and total scattering cross sections. These posterior parameter values were very similar to their priorvalues with a maximum relative error of 0.54%. This falls within in the Unified Monte Carlo–calculateduncertainties on the order of 2.7%. Additionally, posterior double-differential cross sections agree favorablywith ENDF/B-VIII.0 cross sections. The new thermal scattering law was tested by comparing it againstbenchmarks from the International Criticality Safety Benchmark Evaluation Project Handbook, which showeda slight improvement over the ENDF/B-VIII.0 library. Additionally, the covariance matrix of the phonondensity of states was validated to confirm that the spread of keff from the density of states used to generate thecovariance matrix was similar to the spread of keff from the density of states of the sampled covariance matrix