X-ray dynamical diffraction: the concept of a locally plane wave

The long distance between the source and the experiment and the small source size, now available at third-generation synchrotron sources, leads to new optical characteristics for X-ray diffraction. It is shown that, under certain conditions, the intensity received by a point located on the exit surface of a crystal is described by the diffraction of a locally plane wave. Each point along the surface is influenced by a plane wave with a varying departure from Bragg angle. In such a case, it is possible to visualize the rocking curve of the crystal as a function of the position along the exit surface. This represents a topographic method to obtain the reflectivity curve of the crystal instead of the usual goniometric method. Applications will be described in a forthcoming paper.