Theta compensating slits (WAS: Re: Diffraction patterns etc. (fwd)

Lachlan Cranswick ( Lachlan.Cranswick@minerals.csiro.au )
Thu, 16 Nov 95 23:43:00 EST

> From: "Stu Miller"
> I wanted to reply to a question by Scott Meilicke on theta compensating
> slits.
>
> ** snip **
> As an aside,
> Does anybody know if Rietan94 or any other code can take into account
> theta compensating slits? I can't disable them on our Philips
> machine...
>
> ** snip **
>
> We also have theta compensating slits (ADS) on our Philips and before
> using the data for any refinement (I use SIROQUANT) I use an Excel
> spreadsheet to convert the ADS data to fixed divergence slits FDS using
> a simple formula supplied in the Philips documentation for the ADS.
> SIROQUANT also has a bundled calibration for converting (or
> calibrating) the ADS to FDS. I have not done a rigorous comparison but
> I think they are similar.
>
> There is a caveat here I believe that I'm sure others are more
> knowledgeable about than myself so I'll only stir the pot. This is the
> problem of penetration depth. Although you can convert ADS intensities
> to FDS intensities based on iluminated _area_, you cannot acurately
> compensate for the increased _volume_ of illumination when using ADS vs
> FDS at low angles. There was a paper in Powder Diffraction some time
> ago by Matulus and others that discussed this exact problem (sorry I
> can't find the reference right now). Any other opinions on this?
>

This is all from memory so you should check with your Philips
rep before considering this :-

One possibility is to fix the ADS so that it behaves as a fixed
slit system. This would negate having to treat (fudge?)
the data?

Assuming the Philips mechanical ADS subtends a 20mm path
length on a 17.5cm goniometer radius.
(Check with KBr powder or other appropriate sample)

Go into "direct control" and go to 40 deg 2-theta.
Fix the ADS with the grub screw, then disengage the
clutch system. This should now be emulating a 1 degree
fixed slit system. Be careful and clear headed when this
is done as it is possible to damage the ADS if you
don't disengage the clutch correctly.

--------------------------

We found the Sollers in our mechanical Philips ADS were not as
good as the fixed slit Sollers in collimating the X-ray beam.
Thus we obtained better peak shapes with less low angle peak
asymmetry using the fixed slit system.

I hope this makes some sense as it has been around 3
years since I had to do this given we now use fixed slits.

Cheers,

Lachlan.

-- Lachlan M.D. Cranswick          _--_|\  lachlan@dmp.CSIRO.AUCSIRO - Division of Minerals   /CSIRO \ tel +61 3 9647 0367339 Williamstown Rd,           \_.--.*/ fax +61 3 9646 3223Port Melbourne, Australia, 3207      v   (http://www.dmp.csiro.au/tour/lachlan.htm - Still under construction)