Performance of neutron guide systems for low energy accelerator-driven neutron facilities

Low Energy accelerator-driven Neutron Facilities have the potential to become competitive to research reactors and spallation sources to generate neutron beams for scattering experiments. A low energy accelerator-driven neutron facility is developed at the Jülich Centre for Neutron Science. This source is expected to provide thermal and cold neutrons with high brilliance and is therefore called "High Brilliance neutron Source" (HBS). In this work, we study the performance of neutron guide systems at HBS by using neutron ray-tracing simulations. Elliptical and ballistic guides with elliptic diverging/converging section have been used in simulations for various moderator-to-guide distances and guide entrance cross-sections. Results show that the beam properties have a strong dependence on the distance between guide entry and moderator. We demonstrate that the ballistic guide system can achieve a comparable neutron flux and brilliance transfer as the true elliptical guide for thermal neutrons if a proper distance between guide entrance and moderator is chosen. For low-divergence cold neutrons, the selected ballistic guide is showing even better performance than the elliptical one.