Abstract
IN the course of experiments on divergent-beam X-ray photography, we applied extinction theory to find how the contrast σ of deficiency lines against the background depends on η, the standard deviation of mosaic-block disorientation in the crystal. We obtained a curve (reproduced herewith) with a maximum for a value of η comparable with the angular subtense of the X-ray source at the film. For smaller values of η, σ depends chiefly on the integrated intensity of reflexion from the lattice planes concerned rather than on the ideal line profile; in consequence, as η decreases, the contrast must eventually suffer secondary extinction in the same way as single-crystal spots on rotation photographs. Instead of reaching the ideal maximum of ½ (‘Laue’ case) or 1 (‘Bragg’ case), the curve near the origin, provided η is not too near the diffraction width, is given by the expressions: where Q′ is volume reflecting power and t′ is effective crystal thickness.
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References
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PRINGLE, G., PEACE, A. X-Ray Extinction in Type II Diamonds and Topaz. Nature 169, 36 (1952). https://doi.org/10.1038/169036a0
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DOI: https://doi.org/10.1038/169036a0
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