Thermal neutron scattering evaluation framework

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Chapman, Christopher W.
Rahnema, Farzad
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In this work, a thermal neutron scattering data evaluation framework is presented that combines measured scattering data and computer simulations to evaluate the dynamic structure factor (DSF), double differential cross section (DDCS), and their uncertainties. The original parameter set of a given interaction model is randomly sampled according to interaction parameters’ prior probability distribution function. For each set of perturbed parameters, a corresponding DSF and DDCS are computed, and a weight associated with this set of perturbed parameters is obtained using a Unified Monte Carlo (UMC) method from the differences between simulated and measured data. Using these weights, the best estimate of the DSF and its uncertainty is computed as a weighted average of DSF values of all perturbed parameters sets. This is the first time thermal neutron scattering kernel uncertainties have been estimated by sampling the underlying atomic interaction model parameters. This evaluation framework is demonstrated on the TIP4P/2005f light water interaction model combined with DDCS data measured at the Spallation Neutron Source (SNS) Fine Resolution Fermi-Chopper Spectrometer (SEQUOIA) at Oak Ridge National Laboratory (ORNL). Molecular dynamics trajectories computed from randomly sampled TIP4P/2005f parameters by the GROMACS code were processed to yield thermal neutron scattering kernel DSF and DDCS. An ensemble of 60 randomly perturbed TIP4P/2005f interaction parameters yielding satisfactory agreement with experimentally measured characteristics of light water were found. For each of these 60 parameter sets the UMC expressions were used to compute their associated weights based on the quality of agreement between the corresponding DDCS and SNS data. These UMC weights were used to compute a weighted average of the DSF, the corresponding DDCS, and the total scattering cross section, as well as their corresponding uncertainties. The averaged cross sections computed from this DSF were then validated against independent experimental data (including DDCS and total cross section), as well as relevant benchmarks in the International Handbook of Evaluated Criticality Safety Benchmarks (ICSBEP), including the PU-SOL-THERM-033, LEU-COMP-THERM-079, and HEU-COMP-THERM-006 benchmarks. MCNP simulations of these integral benchmark experiments were performed for each DSF in the ensemble to produce a spread of neutron multiplication factors (keff) that represents a measure of uncertainty caused by uncertainty in the DSF for the first time.
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