Determination of beam polarization from integrated Borrmann intensities

The beam polarization ratio of a dual graphite monochromator system was studied with the Borrmann polarimeter technique in the wavelength range between 0.7093 and 1.6782 Å. This ratio depends on the polarization of the beam incident on the first `crystal', on the properties of the `crystals' and on geometrical effects, especially those arising from the collimation conditions. All these dependences were studied. Furthermore, the experimental difficulties in using a Borrmann polarimeter were examined with some care and are outlined for the benefit of others who realise the importance of measuring their own beam polarization ratio. In this connection, a number of pictures of scans showing the horizontal and vertical divergences, the background levels arising from Borrmann crystals of inadequate thickness, the effects of crystal settings on χ scans etc. are displayed. In accordance with custom, the results are reported in terms of the exponent n in the expression (beam polarization ratio) K′ = cos ²ⁿ2θ_m, where the 2 in the exponent arises from using a dual monochromator. As is well known, secondary extinction leads to values of n from near zero to two, even if the incident beam is unpolarized, n values ranging from 0.35 to 1.67 were found and these are discussed in terms of polarization of the incident beam and the collimation conditions.