Diffractometer alignment and polarization determination of X-ray beams by means of the Borrmann effect

The Borrmann effect, that is, the anomalous transmission of X-ray beams through sufficiently thick perfect crystals in Laue geometry, is a sensitive diffraction tool which has many fundamental and practical applications: the accurate alignment of any full four-circle instrument (equipped with a cradle in asymmetric design) with respect to the incident beam, measurement of the divergence of any X-ray beam, and the precise determination of the beam polarization, incident or diffracted. For these applications, it is shown that no modifications to the diffractometer are required. The measurements simply consist of measuring φ and χ rocking curves of the anomalous transmitted beam where the ω angle is used as a parameter. The analyses require the findings of the peak centroid positions, the full widths at half maximum and the rocking-curve integrated intensities. Some results are presented as illustrations of the method. These Borrmann-effect applications promise to be more helpful for diffractometers installed at synchrotrons than for in-house laboratories.