Neutron interferometry: antiphasing effects caused by geometrical aberrations

Intensity profiles of exit beams of a triple-Laue-case neutron interferometer have been measured photographically and compared with profiles calculated from the dynamical theory of perfect-crystal neutron diffraction. In order to avoid smear by an extended source a narrow incident beam only 0.2 mm wide was used. The spreading in the Borrmann triangles of the component crystals S, M, A of the interferometer was taken into account by a spherical wave calculation. The particular aim was to trace the influence of geometrical aberrations of the wafer thicknesses and deviations from so-called ideal geometries on the profile shapes and the phase homogeneity of the interferometer. It could be shown that previously observed `plait'- or `chessboard'-like patterns occurring in exit beams of neutron interferometers can be fully explained by the action of spherical wave interference disturbed by imperfections of the geometry. The agreement found between theory and experiment is very good. From the results, experimentally confirmed geometrical tolerances for the manufacture of neutron interferometers can be deduced.