Beam choppers for neutron reflectometers at steady flux reactors

Realizations of the TOF technique for neutron reflectometers at steady flux reactors are compared. Beam choppers for neutron reflectometers divide into choppers of type 1 (Δλ=const) and 2 (Δλ∕λ=const). It follows from Monte-Carlo simulations that choppers of type 1 do not yield to more intricate choppers of type 2, widely used at neutron reflectometers. Because of a very fast drop of neutron reflectivities with the momentum transfer q, non-optimality of measurements with a chopper of type 1 is fully compensated by better statistics at large q, and is not so much essential at small q. To vary the TOF resolution with choppers of type 1, a phasing of two discs and a turning of the system of two discs are suggested. The fluxes of neutrons with wavelengths beyond the working range and the efficiencies of their elimination by means of a bandwidth limiting prechopper are evaluated.