Research Highlights1Part2

X-ray diffraction topography provides a simple, fast, and direct technique for imaging crystal disorder. In conventional single-crystal diffraction (e.g., as performed by macromolecular crystallographers), the sample is illuminated using a beam with a finite angular divergence and the diffraction pattern is recorded using a detector placed ~0.11.0 m from the sample. Under these conditions, the finite beam divergence and large sample-to-detector distance produce largely featureless diffraction spots. But if the incident beam has a very small angular divergence and a high spatial resolution detector is placed very close to the sample, the diffracted beams from different parts of the crystal are spatially resolved and the diffraction spots provide images of the crystal. Contrast in these images arises due to variations in lattice orientation (mosaicity) and lattice spacing (strain) that cause variations in diffracted intensity from point to point in the crystal. Contrary to what its name suggests, X-ray topography is a bulk imaging technique: the “topography” is of the diffracting planes within the crystal. The images that it produces can be crudely viewed as the “dark-field” counterpart to the “bright-field” images produced by the direct transmitted beam in conventional radiography.