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fast communications
Newly observed diffraction phenomena that appear in the transmitted divergent-beam (pseudo-Kossel) patterns given by perfect or nearly perfect crystals are reported. They are seen within the areas of overlapping intensity distributions where two or more Kossel lines cross. The pseudo-Kossel patterns were generated using a scanning electron microscope to produce a small X-ray source (diameter < 10 μm) within a 2 μm-thick copper film coating single surfaces of polished, plate-shaped, nearly perfect diamond specimens. Experimental conditions determined that the observed width of Kossel lines was dominated by Cu Kα1 wavelength spread. Imposed upon the overall area of crossing of these dispersion-broadened lines, there appear fine streaks of enhanced transmitted intensity that delineate loci along which, as the wavelength changes, coherent multiple diffraction occurs, accompanied by enhanced Borrmann transmission. These fine-scale features, only a few arcseconds in angular width, add markers to the broad-line Kossel pattern that should be exploitable in lattice-parameter measurements.