I thought I would weigh in on the discussion concerning VCT data collection:
The VCT data collection methods result in the best data per unit time. But: I
have two Siemens machines which are equipped with Mbraun, quartz wire,
scanning, position sensitive detectors. The apertures (in the plane of
diffraction) are set at approximately 4.5 degrees, which means that at a scan
rate of .6 degrees a minute (a 4 hour scan of 150 degrees two-theta) and a
step width of .01 degrees, the count time is 7.8 minutes per step. While this
analysis may exaggerate the benefit somewhat; the count rate is roughly an
order of magnitude higher than conventional detectors. The additional counts
are collected from particles which are not "in focus" in the Bragg-Brentano
geometry, leading to both a drawback and a benefit: 1) Peak shapes are broader
at the base and more asymmetric than those from conventional detectors; FWHM
values are nominally the same. 2) The collection of data from additional
particles dramatically reduces effects of particle counting statistics.
While I have not performed a study as have Madsen & Hill, I have collected
data used for certification of SRMs on one of the Siemens machines (a D500
also equipped with an incident beam monochromator). In experiments designed
to check for homogeneity of an SRM, 20 sets of data were collected on mixtures
of alpha and beta silicon nitride and alumina. Rietveld analysis of these
scans resulted in quantitative data with standard deviations of less than 0.5
percent of the measured mass fraction. Crystallographic parameters were
stable, consistent and reasonable.
The ultimate situation would be a PSD with a VCT data collection method. This
would be very difficult to implement with the present generation of PSDs.
However, new, CCD type, technologies should make such a route possible in the
(hopefully) near future.
Jim
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James P. Cline CLINE@CREDIT.NIST.GOV
Ceramics Division VOICE (301) 975 5793
A256/223 FAX (301) 990 8729
National Institute of Standards and Technology
Gaithersburg, MD. 20899 USA
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