Systematic intensity errors caused by spectral truncation : origin and remedy
Faculty of Sciences. Chemistry
Acta crystallographica: section A: foundations of crystallography. - Copenhagen
, p. 629-641
University of Antwerp
The wavelength dispersion of graphite(002)-monochromated X-ray beams has been determined for a Cu, a Mo and an Rh tube. The observed values for Delta lambda/lambda were 0.03, 0.14 and 0.16, respectively. The severe reduction in monochromaticity as a function of wavelength is determined by the absorption coefficient mu of the monochromator. mu (monochromator) varies with lambda (3). For an Si monochromator with its much larger absorption coefficient, Delta lambda/lambda values of 0.03 were found, regardless of the X-ray tube. This value matches a beam divergence defined by the size of the focus and of the crystal. This holds as long as the monochromator acts as a mirror, i.e. mu (monochromator) is large. In addition to monochromaticity, homogeneity of the X-ray beam is also an important factor. For this aspect the mosaicity of the monochromator is vital. In cases like Si, in which mosaicity is practically absent, the reflected X-ray beam shows an intensity distribution equal to the mass projection of the filament on the anode. Smearing by mosaicity generates a homogeneous beam. This makes a graphite monochromator attractive in spite of its poor performance as a monochromator for lambda < 1 <Angstrom>. This choice means that scan-angle-induced spectral truncation errors are here to stay. These systematic intensity errors can be taken into account after measurement by a software correction based on the real beam spectrum and the applied measuring mode. A spectral modeling routine is proposed, which is applied on the graphite-monochromated Mo K alpha beam. Both elements in that spectrum, i.e. characteristic alpha (1) and (2) emission lines and the Bremsstrahlung, were analyzed using the 6,3,18 reflection of Al2O3 (s = 1.2 Angstrom (-1)). The spectral information obtained was used to calculate the truncation errors for intensities measured in an omega /2 theta scan mode. The results underline the correctness of previous work on the structure of NiSO4 . 6H(2)O [Rousseau, Maes & Lenstra (2000). Acta Cryst. A56, 300-307].