X-ray interference fringes from weakly bent crystal

Topographies of Si 220 reflection in the Bragg mode have been observed from a weakly bent single crystal. When bending is very weak, a novel interference fringe is observed instead of mirage diffraction fringes. The period of this fringe increases with the distance between the incident point of X-rays and the exit point of the diffracted beam. This interference fringe is caused by the interference between a mirage diffraction beam and the reflected beam from the bottom surface. When this interference fringe is observed, the interference fringes are also observed from the lateral surface even when the distance from the incident point to the crystal edge is 12 mm; X-rays propagate over a long distance, i.e., over a centimeter distance in the crystal.