Abstract

X-ray topographs of a new type have revealed fine fringes in the diffraction images of wedge-shaped parts of perfect and nearly perfect crystals. The fringes are analogous to those seen in electron microscope images of wedge-shaped parts of magnesium oxide crystals and can be interpreted according to the theory applicable to the electron case. Fringe spacing depends upon X-ray wavelength, wedge-angle, inclination of reflecting plane to the wedge surfaces, and the structure amplitude of the reflection. Discovery of these fringes shows that (a) parts of real crystals behave as ideally perfect from the X-ray diffraction standpoint (b) the dynamical theory of diffraction may be applied quantitatively under practical experimental conditions, and (c) structure amplitudes of low-order reflections may be determined by fringe-spacing measurements, without any need for measuring reflection intensities. Tests of the theory on prepared wedges of silicon and quartz indicate a slight systematic discrepancy of 4 to 5 % between calculated and observed values of structure amplitude, and suggest also that Wei's (1935) F-values for quartz 10T1 and 1T22 are too low by 9% and 4% respectivcly. Experiments suggest that to explain fully the observations some modification of the dynamical theory is required in the direction of allowing for a spherical wavefront of the incident beam.

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