DISLOCATION IMAGES IN X-RAY SECTION TOPOGRAPHS OF CURVED CRYSTALS

The contrast of dislocation images in X-ray section topographs of curved crystals has been investigated both experimentally and by computer simulation. Good agreement is found between experiment and simulation in both symmetric and asymmetric Laue geometries. Very little change is observed in the symmetric Laue images for radii of curvature as small as 18 m. In the asymmetric geometry, the background intensity rises and the Kato fringe visibility decreases as the curvature increases. The dynamical image becomes more localized and dominates the image as the direct image visibility falls. Our results explain the reversal of dislocation contrast in Lang projection topographs of curved crystals taken in asymmetric geometry.