The Omega-scan

The specimen is turned by 360° around a certain axis, e.g., the surface normal. The X-ray source and the detector have to be adjusted depending on the crystal type to get a sufficient number of reflections per turn. The angular positions of these reflections are used to evaluate the orientation of the crystal lattice (completely described by three angles) with relation to the rotation axis.

In order to relate the lattice orientation exactly to the surface of a crystal, the direction of the surface is checked by a laser beam. Also other relevant crystal reference faces or directions can be measured by optical tools in a similar way. This measuring technique enables to determine the orientation of arbitrary single crystals in any orientation range with high precision. Usually, a measuring time of some seconds (during one or a few turns of the specimen) is sufficient to get a reproducibility in the range of a few arc seconds. A special application of the Omega-Scan Method is the precision lattice-parameter determination, especially of cubic crystals.