Borrmann effect for alignment of a four-circle diffractometer

A very simple method is described to align accurately any full four-circle diffractometer with respect to the incident beam. This method relies on anomalous transmission measurements. It has been found that the centroids of the Ge 220 anomalous transmission intensity distributions measured at the four main positions of the χ circle, that is 0, 90, 180, and 270°, not only provide an accurate determination of the reference angles for ω and φ, but also provide an accurate method to align the χ circle with respect to the direction of the incident beam. This procedure promises to be even more helpful for diffractometers installed at synchrotron radiation laboratories than for in-house installations.