Generalization of the parallel position in diffraction using elastically curved crystals

A theoretical and experimental study of double-crystal rocking curves with elastically deformed silicon wafers yields the result that rotating one of the crystals through 180° can change the peak intensity by a large factor, limited only by the sharpness of the intrinsic mosaic spread. The conventional double-crystal rocking curve with equivalent scattering vectors in what is called the parallel position has its counterpart for two crystals with different scattering vectors if each crystal is bent to its proper radius of curvature. Like the parallel-position case, the collimation does not enter into the width of the rocking curve. The rocking curve can be sharper than the usual parallel-position convolution of the two mosaic spreads. The often observed lack of agreement between rocking curves in what seem to be identical geometries is explained as a rotation of the mean of the mosaic spread across a crystal, which is equivalent to a bent crystal.