X-ray diffracted intensity for double reflection channel cut Ge monochromators at extremely asymmetric diffraction conditions

The width and the integrated intensity of the 220 x-ray double diffraction profile and the shift of the Bragg condition due to refraction have been measured in a channel cut Ge crystal in an angular range near the critical angle of total external reflection. The Bragg angle and incidence condition were varied by changing the x-ray energy. In agreement with the extended dynamical theory of x-ray diffraction, the integrated intensity of the double diffraction remained almost constant even for grazing incidence condition very close to the critical angle C for total external reflection. A broadening of the diffraction profile not predicted by the extended theory of x-ray diffraction was observed when the Bragg condition was at angles of incidence lower than 0.6?. Plane wave topographs revealed a contrast that could be explained by a slight residual crystal surface undulation of 0.3 degrees due to the etching to remove the cutting damage and the increasing effect of refraction at glancing angles close to the critical angle. These findings confirm that highly asymmetric channel cut Ge crystals can work as efficient monochromators or image magnifiers also at glancing angles close to the critical angle and that the main limitation is the crystal surface preparation