Accurate simulation of thermal neutron filter effects in the design of research reactor beam applications

Thermal neutron filters are routinely used for spectral shaping in neutron beam applications. In many situations, the neutron source is the core of a research reactor that faces several beamports. Consequently, the neutrons leaking into the beamport will have an energy spectrum that extends over several decades from the low energy (thermal) range to the high energy (fast) range that is typical of prompt fission neutrons. At the PULSTAR reactor of North Carolina State University, sapphire and bismuth filters are used within the beamports to reduce the epi-thermal and fast neutron components and gamma-rays. The main objective is to produce thermal neutron beams with a high neutron-to-gamma ratio. The thermal beams are used to drive facilities for neutron imaging and neutron diffraction. To assure that these facilities are accurately designed, thermal neutron scattering cross section libraries are generated for sapphire and bismuth at 300 K. The libraries do not account for coherent elastic (i.e., "Bragg") scattering, which is equivalent to assuming the use of a single crystal filter with a preferential orientation relative to the neutron beam. MCNP Monte Carlo simulations show that, using the created libraries, the phenomenon of neutron filtration can be captured and diagnosed to ensure accurate beam and facility designs.