Correction for the twist and the conical defects of a sagittaly bent crystal

The symmetrical bending of the focusing crystal of a double X-ray monochromator is a difficult problem. Indeed, the slope due to the curvature is usually three orders of magnitude: higher than the accepted slope error (typically, the Darwin width of the crystal). In these conditions, even a low parasitic slope error induced by the bending process may lead to quite a strong intensity decrease. When the bending moment is applied, the main parasitic distortions which may appear are typically the anticlastic curvature, the inhomogeneous sagittal curvature, the conical shape and the twist. On the D2AM beamline at the ESRF, a program called CHKC2 has been developed to correct on-line the latter two distortions: the conical shape and the twist. On this beamline the X-ray beam, which has been collimated by a:grazing angle mirror, is monochromatized first by a flat silicon crystal, and then diffracted by the sagittaly curved crystal. A fluorescent screen gives an image of this diffracted beam. The CHKC2 program records a series of images of the beam diffracted by the sagittaly curved crystal while rotating this crystal with respect to the flat one. An automatic analysis of this topography (plane-wave topography) is then performed in order to evaluate the behavior of the tuned areas and compare it to a model. An accurate correction is proposed to the beamline operator so that optimisation using long multidimensional scanning is no more necessary. (C) 1999 Elsevier Science B.V. All rights reserved.