Efficiency of a thermal neutron scatterer with the shape of a flat disk studied by Monte-Carlo simulation

Small-Angle Neutron Scattering (SANS) instruments are usually installed in such a way that the sample has no direct sight of the reactor core. One possible way to achieve this, is to place the instrument at the exit of a tangential beam tube, and to use a good thermal neutron scatterer to redirect the source neutrons towards the sample. The choice of the scatterer geometry and position in the tube depend both on the local neutron flux conditions and on the neutron energy distribution desired at the sample. Any analytical approach to the choice of the best performing geometry of a scatterer risks to give misleading results due to the simplificative assumptions. A Monte-Carlo simulation by means of the MCNP code was used to study the thermal neutron scattering efficiency of a water scatterer with the shape of a flat disk. The parameters varied in the study were the thickness and the orientation of the disk. Results for different neutron flux distributions are discussed showing that the best performing geometry is extremely dependent on the local neutron flux conditions, particularly on the angular distribution of the neutrons impinging on the scatterer