Re: INEL 120 PSD & flat plate geometry ...

Ian Madsen ( Ian.Madsen@minerals.csiro.au )
Tue, 29 Apr 1997 10:37:00 +1200

At 02:04 PM 28/4/97 GMT0BST, you wrote:
>
> Fixed Flat Plate Geometry and PSDs
> ----------------------------------
>
> We are currently using an Enraf Nonius FR590 with an
> Inel CPS 120 positiion sensitive detectorand fisxed flat plate
> geometry to carry out X-ray powder diffraction and subsequently
> Rietveld analysis on mixtures of apatites.
> We have been using DBWS and GSAS Rietveld packages with
> Bragg-Brentano geometry as our nearest model to the fixed flat
> plate geometry. This is of course inappropriate for detailed work
> as the absorption correction is very different as are the sample
> displacement calculations etc. We would like to hear from anyone
> who knows of a reliable Rietveld package which treats the fixed
> flat plate geometry correctly.
> We would also like to communicate with anyone using a
> diffractometer with a PSD (particularly with INEL CPS 120
> detectors) so that we could exchange ideas and information.
>
> Yours
> Sincerely
> Rory. M. Wilson
>
> (Dental Biophysics,
> Medical Sciences Building,
> Queen Mary & Westfield College,
> Mile End Road,
> LONDON 1E 4NS)

The following is part of a message I sent directly to Rory Wilson a couple
of weeks ago, but I thought that it might contribute something to the
current discussion on flat plate geometry in the Inel PSD.

**------------------------------------------------**

With regard to the flat plate correction - we also use flat plate geometry
(with fixed incident beam angle) with the INEL. The absorption correction is
different for the incident and diffracted beams and requires an additional
correction to the diffracted intensities.

There is a recent paper by Toraya (J. Appl. Cryst, 26, 774-777, 1993) which
confirms that the correction of James (1967) is OK. That correction is :-

Icorr = I * 2 / [1 + (sin(alpha) / sin(beta))]

where alpha = the angle between the incident beam and the surface of the
sample and
beta = the angle between the diffracted beam and the surface of the sample.
and 2theta = alpha + beta

I have coded this correction into our Rietveld program (a modification of
the LHPM series of Hill and Howard) and tested it for a few phases. Since
the asymmetric sample presentation will affect intensity as a function of
angle, the thermal parameters should be a good measure of whether the
correction works or not. My recollection is that the thermal parameters for
corundum stayed near the reported values for a wide range of incident
(alpha) angles.

You should be able to insert this correction into your DBWS code (our LHPM
code was derived from an early version of DBW) without too many problems -
just remember to apply the correction to the calculated intensities, not to
the observed data.

**------------------------------------------------**

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Ian Madsen
CSIRO Division Of Minerals
P.O. BOX 124
Port Melbourne 3207
Victoria
AUSTRALIA

Phone +61 3 9647 0366 direct
+61 3 9647 0211 switch
FAX +61 3 9646 3223
Email Ian.Madsen@minerals.csiro.au

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