Absolute Positions of Pendellösung Fringes in X-Ray Cases

Abstract

The positions of Pendellösung fringes with respect to the entrance surface were studied experimentally. In section-type experiments, the following results were obtained. (1) At the entrance surface the intensity of the wave field takes the maximum. (2) The distance between the first and second fringes is larger than the averaged one. (3) The positions of fringes in a deeper region are shifted by a quarter of the averaged fringe distance towards the entrance surface, compared with the fringe positions expected from the conventional plane-wave theory of dynamical crystal diffraction. The traverse-type experiments were also carried out.
These results are predictable by a spherical wave theory (N. Kato: Acta cryst. 14 (1961) 526, 627). The fact (3) is due to the phase jumps of the crystal waves at a kind of caustic region near the entrance surface. The phenomenon is similar to the phase anomaly near the focal point of lenses.