From:   SMTP%"oomotoso@nrcan.gc.ca" 30-DEC-2002 23:39:32.58
To:     "'cockcroft@img.cryst.bbk.ac.uk'" <cockcroft@img.cryst.bbk.ac.uk>
CC:     
Subj:   Diffractometer selection

Hello Jeremy,

I hope you had a great holiday!

The following questions are of personal interest so I didn't post them on
the discussion list.  My lab is trying to get a new diffractometer and I am
hoping you could help answer a few questions.  

1 - Is small band pass incident beam monochromator only usable with a
focussing geometry?  I can't figure out how we can combine a commercially
available mirror to produce parallel beam after the incident beam
monochromator.

2 - In terms of angular resolution, intensities and peak shapes, what are
the differences between collimating polycapillary tubes and multilayered
mirrors for parallel beam production given similar post sample accessories?
Looking through the literature,  I read that conventional multilayered
mirror can reduce the equatorial divergence to ~ 0.05 deg but not the axial
divergence (except with toroidal mirrors which appear uncommon for
laboratory sources).  I also read that polycapillary tubes tend to reduce
both axial and equatorial divergence.  Given that the axial divergence is
important in profile fitting, which of the two is a better compromise
between resolution and intensities when looking at multi-mineral samples
with severe overlaps and wide concentration ranges.

3-  One polycapillary manufacturer claims parallel beam divergence of 0.22
deg.  Is this really parallel? Also, if I use a divergent slit of 0.17 deg
in a Bragg-Bretano machine, is the beam considered parallel?

All the best in the New Year.

Dipo Omotoso

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