Interference fringes from a Si plane-parallel crystal have been observed in the topography of the transmitted beam in the Bragg mode when the crystal is very weakly bent. The period of the interference fringes becomes large as the distance from the incident point of the X-ray to the emitted point of the beam increases. The interference fringes are caused by interference between the transmitted beams in the Bragg mode with twice internal reflections from the bottom and top surfaces and those without any internal reflection. When these interference fringes are observed, another interference fringes are observed in the emitted beams from the lateral surface even when the distance from the incident point to the crystal edge is 13.5 mm. This indicates that X-rays propagate over a long distance, i.e., over a centimeter distance in the crystal, and the very weakly bent crystal acts as an X-ray waveguide.
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