Simulating convoluted moderators

It is a continuous effort at spallation sources to improve and optimize the directional emission of thermal and sub-thermal neutrons off the moderator faces into beam tubes. Tools have been upgraded to be able to simulate the emission improvements of so-called convoluted moderators, heterogeneous moderators of layered moderator and single crystal plates. The MCNPX code was equipped with a single-crystal neutron scattering model, and neutron reflection/refraction physics. Studies of simple cylindrical convoluted moderator systems of 100 mm diameter and composed of polyethylene and single crystal silicon were performed with the upgraded MCNPX code and reproduced the magnitude of effects seen in experiments compared to homogeneous moderator systems. Applying different material properties for refraction and reflection, and by voiding silicon, we could show that the emission enhancements are primarily caused by the transparency of the silicon/void layers. Parameter studies of the thicknesses of the polyethylene and silicon layers were conducted and arrived at optimal thicknesses of 1.3 mm for polyethylene and 1 mm for silicon.