Axial divergence & asymmetric scanning in GSAS

Apurva ( mehta@SLAC.Stanford.EDU )
Mon, 12 May 1997 15:15:05 +0100

Dear Bob,
We are building a dedicated powder diffractometer on one of the
beamlines here. We have designed the diffractometer with two detector arms.
The first detector is a Si111 analyzer - a high resolution detector. The
high angle detector arm - 70 degrees from the analyzer - is a dovetail that
can accommodate either a 1mrad Soller slit, or a PSD. We are planning to
use GSAS to analyze the data from this diffractometer.
The analyzer data suffers from Axial divergence - beam footprint in
the horizontal direction is large (10mm). We can reduce the axial
divergence but at a cost. It would be better if GSAS can handle asymmetric
peaks arising from axial divergence directly. I have tried using asymmetry
in the Pseudo-voight (fn #2) profile shape to model it, but the profile fit
is less than satisfactory. About a year ago, I had heard rumors that GSAS
would have a new profile function to explicitly deal with axial divergence.
Is there any substance to the rumor? If so, when would the new function be
part of GSAS?
Eventually we are hoping to collect data from both detectors
simultaneously in an asymmetric reflection geometry. (That is, theta fixed
at low incidence angles and 2theta scanned. I believe, people using a large
- INEL like - PSD would have similar diffraction geometry.) Are there any
plans for GSAS to accommodate such scans?

regards

Apurva

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Apurva Mehta
SSRL/SLAC
PO Box 4349, MS 69
Stanford, CA 94309
(415) 926 4791
(415) 926 4100 - FAX

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