Modelling of wavelength cut-off filters and polarising mirrors in a neutron guide

This paper details the modelling and parametrisation of mirror-in-guide neutron deflector devices used in transmission as wavelength cut-off filters and beam polarisers for neutron scattering instrumentation. The construction of so-called 'phase-space' or 'acceptance diagrams' allows a rapid and quantitative evaluation of parameters such as the critical angles of the mirror and guide coatings, mirror angle, and transmission properties of the mirror-in-guide system. Despite its apparent simplicity, mirror-in-guide devices result in surprisingly complex properties for their neutron transmission as a function of wavelength, divergence and neutron spin due to the possibility of multiple reflections from the surrounding guide walls and multiple strikes on the mirror. Depending on the choice of parameters such devices can be configured as low-pass wavelength filters, beam polarisers or periodic notch and band-pass filters for neutron scattering instrumentation. Geometrically constructed phase-space diagrams are compared with ray-traced neutron Monte-Carlo simulations to evaluate more detailed effects such as the weighting of the transmission probability which cannot be easily evaluated using phase-space diagrams. The results of phase-space modelling and simulations are used to define the specifications of the wavelength cut-off filters and beam polarisers for the new small-angle neutron scattering (SANS) instrument, D33, currently under construction at the Institut Laue Langevin.