First Observation of Neutron Deflections by Single Crystal Prisms

For a monochromatic neutron beam incident on a single crystal prism, we derived analytic expressions for variations of the deflection δcr and transmitted fraction IO of forward diffracted neutrons, with the incidence angle θ. In the vicinity of a Bragg reflection, δcr deviates sharply from the deflection δam for an equivalent amorphous prism, reaching opposite extrema at either end of the total reflectivity domain and exhibits a 3 orders of magnitude greater sensitivity to θ than δam. We have experimentally observed the variation of δcr and IO across a Bragg reflection for single crystal silicon prisms of various apex angles in several asymmetric Bragg configurations. The observed δcr deviates from δam by upto 27% with sensitivities |dδcr/dθ| upto 0.43 arcsec/arcsec. The results are in harmony with theory. This observation has led to the design of a super-collimator Bragg prism producing neutron beams with sub-arcsecond angular widths.