The spatial condensation of the neutron beam by an asymmetric diffraction in thermal-neutron monochromatization

A thermal-neutron monochromator based on an elastically bent perfect silicon crystal in a strongly asymmetric diffraction geometry used for the spatial condensation of the output neutrons is presented and discussed. Its effective `mosaic spread' δ_hθ can be easily controlled by a simple choice of the bending radius and the length of the irradiated part of the crystal plate. A theoretical analysis shows that a gain in the flux density of monochromatic neutrons in the fully asymmetric case in comparison with the symmetric one is equal to the ratio of the width of the incident polychromatic beam and the thickness of the crystal. The experimental integrated reflectivity and focusing obtained are found to be in good agreement with theoretical predictions. The asymmetric arrangement of the bent crystal seems to be suitable for use at a source with a large incident-beam cross section when δ_hθ may achieve a value of about 10⁻² rad and an effective neutron flux at the sample position may achieve the fluxes obtained by the best mosaic monochromator.