The Omega Scan

A measuring technique for the orientation determination of crystals

In the EFG apparatuses for the orientation determination of single crystals, a special X-ray diffraction method is used. This method, developed by Prof. Bradaczek, has been called “Omega-Scan“.

The specimen is rotating at fixed measuring arrangement.

During one turn an X-ray diffraction diagram (= Omega-Scan diagram) is measured.

From the Omega-Scan diagram are calculated (e.g.): the inclination angle between one crystallographic axis (here: the a axis) and the reference plane as well as the direction of the inclination in this plane.

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.

Advantages of the Omega-Scan Method compared with other X-ray diffraction techniques are:

  • Stable and relative simple arrangement
    (X-ray tube and detector in fixed positions, only one measuring circle, no monochromator).

  • All the data necessary for the complete orientation determination are measured by one turn.

  • High precision at low measuring time.

From these reasons, the method is suited especially for serial measurements and industrial applications.