For the discussion...
I have transcribed a paragraph extracted from a very good review paper written by McCusker, Von Dreele, Cox, Louer, Scardi: J. Appl. Cryst., 32, 36-50 (1999) (good people!) from the Commission on Powder Diffraction, in which they evaluate the use of Automatic Divergence Slits in Rietveld refinements:
"Many laboratory diffractometers are equipped with automatic divergence
slits, which open as a function of 2theta. While there is a substantial
gain in diffracted intensity at high angles with such slits, there is also
a progressive deterioration of the parafocusing condition and therefore of
the resolution as 2theta increases. In Bragg-Brentano geometry, the flat
specimen surface is tangent to the focusing circle and, as 2theta
increases, the radius of this circle diminishes. At the same time, the
illuminated area of the sample is reduced, so that the flat-sample effect
on data resolution is minor, whatever the diffraction angle. However, if
varying slits widths are used, a progressive angular-dependent defocusing
occurs and the quality of the data deteriorates. Furthermore, if the
measured intensities are to be used in a Rietveld refinement, the slits
opening needs to have a precision of at least 1% or, in other words, be
reproducible to a few microns over the entire 2theta range. For these
reasons, the use of such slits for a Rietveld refinement is not
recommended."
Miguel Delgado
Jens Wenzel Andreasen wrote:
Another query which rightfully belongs in the rietveld mailing list, but
bugs me a lot: Why is there so precious little software that models
ADS data and none of them are any of the most well known, widely
used packages such as GSAS and Fullprof?
If you have ADS data and want to use any of these programs, you
have to convert your data to fixed slit data.
I wonder, because I imagine that it would take minimal effort to
incorporate it in the software.
Jens> A query is: how common is it for the data collection
> software to collect ADS data then convert it to
> Fixed Slit data. Would it not be better if the
> data is to be stored and transmitted as "fixed
> divergence slit" - then to generate it as fixed
> divergence slit data?Jens Wenzel Andreasen "If only those geologists would let me
Dept. of Mineralogy alone, I could do very well, but those
Institute of Geology dreadful hammers! I hear the clink of
University of Copenhagen them at the end of every cadece of the
Phone: +45 44 64 74 10 Bible verses." - John Ruskin, 1851