Speaker
Description
For accelerator-based neutron sources, a moderator is one of the most important components determining the performance of the neutron beams produced. The neutron scattering cross section data for the moderator materials in the thermal and cold neutron energy region are utilized in the neutronics calculation for the development and improvement of moderator systems. To develop a reliable data set of the neutron scattering cross section for the moderator materials, we have conducted research through both experimental and theoretical analysis. We have measured the total cross section and double differential scattering cross sections (DDX) at the Materials and Life science experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC). As a neutron scattering analysis code, we have used KUNSCA developed in Kyoto University. KUNSCA calculates the scattering cross section without model parameters from molecular dynamics (MD) simulation results, employing the Gaussian approximation, so it is highly versatile.
As an example of experimental results that we have measured, we will show the results of DDX for light water at room temperature. Measurements were conducted at BL14 in J-PARC MLF. We obtained DDX at incident energies of 10.5 meV and 23.7 meV for scattering angle from 30-degree to 110-degree in 10-degree intervals. We used monochromatic neutrons obtained by using disk choppers and measured scattered neutrons with position sensitive He-3 detector bank in the beamline. The absolute DDX of the sample was deduced relative to the vanadium incoherent elastic scattering cross section. We compared the measurement results with the values evaluated by KUNSCA and the values obtained by directly calculating the Van Hove space-time self-correlation function from molecular dynamics trajectory data without the Gaussian approximation. The results showed that direct calculation of the space-time self-correlation function reproduces the experimental data more accurately. From these results, we showed that the scattering cross section in the thermal energy region affects the non-Gaussian motion of light water.
KUNSCA couldn’t calculate the scattering cross section for liquid hydrogen, deuterium and hydrogen deuteride (HD) at cryogenic temperatures. Therefore, we added a new function to calculate the scattering cross section for these materials. We calculated the scattering cross section from trajectory data of ring polymer molecular dynamics simulation, which can compute dynamics for quantum liquid. The calculated results were compared with experimental data for total scattering cross section. As a result, we obtained good agreement for liquid hydrogen and deuterium but overestimated the experimental data for liquid HD. To further improve the accuracy, we plan to measure the total cross section and DDX for these materials in J-PARC MLF.