Evaluation of two methods to minimize the anticlastic curvature of cylindrical bent crystals.

A scanning dynamically focusing sagittal X-ray monochromator accepting 3 mrad x 0.1 mrad from a 6 T wiggler has been designed for XAFS measurements. In the energy range 4.1-12.4 keV, the slope error of the second cylindrical crystal caused by anticlastic bending must be less than 1/5 of the Darwin width of the crystal or the photon flux will decrease drastically. Two methods to minimize the anticlastic bending are proposed. Thin crystals with stiffening ribs and thin crystals with an aspect ratio equal to the ;golden value' are evaluated by finite-element analysis and by long-trace-profiler characterization. Both approaches are satisfactory, but the ;golden value' approach is preferred in this case for the second crystal of the new monochromator not only because it is easy to manufacture but also because the surface is smoother than the ribbed crystal.