Open AccessOn predicting scan profiles: the nature of the `aberration function'
Author(s) -
Destro R.,
Marsh R. E.
Publication year - 1993
Publication title -
acta crystallographica section a
Language(s) - English
Resource type - Journals
eISSN - 1600-5724
pISSN - 0108-7673
DOI - 10.1107/s0108767392007645
Subject(s) - monochromator , spectral power distribution , optics , function (biology) , truncation (statistics) , crystal (programming language) , distribution function , intensity (physics) , distribution (mathematics) , radiation , physics , computational physics , mathematics , mathematical analysis , computer science , wavelength , statistics , quantum mechanics , evolutionary biology , biology , programming language
In an earlier treatment [Destro & Marsh (1987 . cta Cryst. A43, 711-718], an attempt was made to predict the shapes of high-angle theta-2theta scan profiles by convoluting a low-angle profile with the presumedly known spectral distribution function for the incident (crystal-monochromatized Mo Kalpha) radiation but it was found necessary to introduce a third component, an 'aberration function', that varied with the Bragg angle theta. It is shown here that the primary purpose of the aberration function is to correct for defects in the spectral-distribution function. In particular, the effective intensity ratio between the Kalpha2 and Kalpha1 spectral lines can apparently deviate greatly (by more than 10%) from the theoretical value of 0.499, depending upon the alignment of the monochromator crystal, and an appreciable amount of white radiation may also be present. By a suitable modification of the spectral-distribution function, high-angle scan profiles can be predicted from accurate measurements of a low-angle profile; as a result, scan-truncation losses can, for the most part, be removed. However, modeling the spectral distribution function appropriate to a particular experiment remains a difficult empirical procedure.