Systematic intensity errors caused by spectral truncation: origin and remedy
Abstract
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 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 of the monochromator. (monochromator) varies with 3. For an Si monochromator with its much larger absorption coefficient, 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. (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 < 1 A. 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 beam. Both elements in that spectrum, i.e. characteristic 1 and 2 emission lines and the Bremsstrahlung, were analyzed using the 6318 reflection of Al2O3 (s = 1.2 A-1). The spectral information obtained was used to calculate the truncation errors for intensities measured in an 2 scan mode. The results underline the correctness of previous work on the structure of NiSO4.6H2O Rousseau, Maes & Lenstra (2000). Acta Cryst. A56, 300-307.