Observation of neutron forward diffraction by single crystal prisms

Thermal neutrons traversing an amorphous prism undergo a deflection in the arcsecond regime. On changing the incidence angle by a few degrees, the deflection changes in the arcsecond range. For a single crystal prism operating in a Bragg configuration, however, dynamical diffraction effects bring about strong deviations from this scenario. We derived analytic expressions for the deflection as well as transmitted fraction of forward diffracted neutrons from the single crystal prism. In the vicinity of a Bragg reflection, the neutron deflection deviates sharply from that for an amorphous prism reaching opposite extrema at either end of the total reflectivity domain and exhibits a 3 orders of magnitude greater sensitivity to the incidence angle variation. Using a 2 arcsec wide 5.24 Å neutron beam from a 7-bounce Bonse-Hart monochromator, we observed the variations in the deflection and transmission of the neutron beam across a Bragg reflection, for several single crystal silicon prisms. The observed single crystal prism deflections deviate from amorphous prism deflections by up to 27% with deflection sensitivities up to 0.43 arcsec per arcsec variation in the incidence angle. The results agree well with theory and provide a smooth control over the neutron deflection. These observations have led to the design of a super-collimator Bragg prism, which produced a neutron beam with a sub-arcsecond angular width.