Dear Tony,
Am 03/05/2016 um 12:58 schrieb Tony Raftery:
What the world needs is better control of antiscatter. Mis-set
anti-scatter or absent anti-scatter slits make life a misery if one
has any intention of modelling a diffraction pattern.
exactly. This holds not only for mysterious "bumps" coming from plastic
sample holders what should not be hit by the primary bundle, but also
for strange "edges" at low angles, even if theta-compensating divergence
slits are in use.
I have great hopes for automatic knife edges but I can see the great
scope for it all going dreadfully wrong. I have yet to see any model
for how such a thing should work (as in how far the slit should be
above the sample as a function of diffractometer radius or divergence
slit width). Manual knife edges are great (properly aligned at about
10um tolerance in height, and about 0.1 deg in chi [that’s hard to
do]) – but then only useful for a restricted (to very restricted)
2theta range otherwise “asymmetrically limiting the
incident/diffracted beam bundle“.
A simple drawing of the beam path shows the origin of the "scattered"
radiation what can be reduced by the "knife edge": The "background edge"
in our patterns was at approximately 7 deg 2theta, even if an
theta-compensating divergence slit and a 0d-detector was used!
Connecting the 7 deg detector position and the "knife edge" position
(what really shifts this edge toward lower angles when going down)
clearly hits a position at the tube housing approximately 3-4 mm below
the beam center, i.e. the lower side of the borehole in the shutter
opening. This is the source of fluorescence radiation (Cu, Zn if brass
is the material), passing through the divergence slit above the sample,
and hitting i) at lower angles directly the detector or ii) at higher
angles the right side of the sample holder. This can be effectively
removed by a second "anti-scatter" slit immediately in front of the tube
window: This slit must just be significantly smaller than the opening in
the tube housing. In fact, all our old instruments have such a second
slit position, no problem. A newer machine (no brand names) has there at
least a filter frame. We glued parallel strips (distance 2.4 mm) of
tantalum (by tape) on the filter frame, and the problem was solved,
clean background down to 3 deg 2theta, the knife edge could be removed
completely.
The take home message: A clean primary beam bundle just hitting the
sample helps a lot, and in the 1970ies this fact was still known to the
manufacturers ;-)
But of course programmable knife edge works as well and is a better
business for the company ;-)
Reinhard
I have great hopes for automatic variable divergence slits also – just
do or don’t think of the anti-scatter issues (as in requires a
programmable a-scatter on both the incident and diffracted beam side
[diffracted side is the problem as it depends on detector parameters
for a 1D detector]) .
regards,
Tony Raftery
*Tony Raftery* | Projects Manager – X-ray & Particles Laboratory
Central Analytical Research Facility (CARF) | Queensland University
of Technology (QUT)
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*From:*rietveld_l-requ...@ill.fr [mailto:rietveld_l-requ...@ill.fr]
*On Behalf Of *Reinhard Kleeberg
*Sent:* Tuesday, 3 May 2016 7:18 PM
*To:* Matthew Rowles <rowle...@gmail.com>
*Cc:* rietveld_l@ill.fr
*Subject:* Re: Parafocussing definition?
Am 03/05/2016 um 03:00 schrieb Matthew Rowles:
K&A (and also Brentano, upon rereading) say that in order to be
focussing, the sample also needs to be toroidally curved; curved
along ACB as well as be rotated about the axis AB; so it is a
little more than the traditional flat-sample effect
Yes. And as ACB is not constant, a "focusing sample" should change
their curvature (bent radius) following the focusing circle versus
2theta, getting more bent with higher diffraction angle, what is hard
to do... That's why Bragg-Brentano geometry (flat sample) is the
working compromise.
Btw, understanding the geometric principles seems to be a bit out of
fashion, as we nowadays can find "strange" instrumental configurations
for common powder diffraction work in many laboratories, for example:
- parallel primary beam optics in combination with narrow point
detectors,
- overmuch long linear detectors combined with a Bragg-Brentano setup,
- small "antiscatter" slits in front of big linear detectors,
- "knife edge" antiscatter slits above the sample, asymmetrically
limiting the incident/diffracted beam bundle,
etc. pp....
Reinhard
So summarising everyone here, and the papers I've been reading,
parafocussing means "sort of focussing given the constraints of
actually making a physical diffractometer". Also, we have a finite
source and detector size, and that with the dimensions of beam
footprint and goniometer radii that we use mean that it all pretty
much works out in the end.
Matthew
On 2 May 2016 at 20:07, Kern, Arnt <arnt.k...@bruker.com
<mailto:arnt.k...@bruker.com>> wrote:
Matthew,
I think Klug & Alexander (1974) give a good explanation about
the origin of the term "parafocusing": 2nd edition, section
"parafocusing methods", page 222 ff.
Cheers,
Arnt
*From:*rietveld_l-requ...@ill.fr
<mailto:rietveld_l-requ...@ill.fr>
[mailto:rietveld_l-requ...@ill.fr
<mailto:rietveld_l-requ...@ill.fr>] *On Behalf Of *Eduard E. Levin
*Sent:* Montag, 2. Mai 2016 13:21
*To:* Cline, James Dr. (Fed); Matthew Rowles
*Cc:* rietveld_l@ill.fr <mailto:rietveld_l@ill.fr>
*Subject:* RE: Parafocussing definition?
Dear James,
For me too, please, if it would not be much of a trouble.
Thank you in advance!
Eduard
*On Mon, 2 May 2016 11:09:31 +0000, Cline, James Dr. (Fed) wrote*
> Bob Cheary and I developed and presented a workshop several
times in the 1990's that included a discussion of this issue.
I can send you the notes for it if you would like them.
>
> Jim
>
>
> James P. Cline
> Materials Measurement Science Division
> National Institute of Standards and Technology
> 100 Bureau Dr. stop 8520 [ B113 / Bldg 217 ]
> Gaithersburg, MD 20899-8523 USA
> jcl...@nist.gov <mailto:jcl...@nist.gov>
> (301) 975 5793 <tel:%28301%29%20975%205793>
> FAX (301) 975 5334 <tel:%28301%29%20975%205334>
>
> *From:* rietveld_l-requ...@ill.fr
<mailto:rietveld_l-requ...@ill.fr>
[mailto:rietveld_l-requ...@ill.fr
<mailto:rietveld_l-requ...@ill.fr>] *On Behalf Of *Matthew Rowles
> *Sent:* Monday, May 02, 2016 1:53 AM
> *To:* RIETVELD_L Distribution List <rietveld_l@ill.fr
<mailto:rietveld_l@ill.fr>>
> *Subject:* Parafocussing definition?
>
>
> Hi all
>
>
>
> I've been trying to find a good explanation of what
parafocussing (wrt Bragg-Brentato geometry) actually is, but
haven't been able to find one.
>
>
>
> Klug and Alexander just reference Brentano's papers.
>
>
>
> "The Basics of Crystallography and Diffraction" 2nd ed say
that B-B geometry is "semi-focussing" because the sample is
flat, and not curved to follow the focussing circle (this
doesn't sound right to me)
>
>
>
> Brentano, J Appl. Phys. 17, 420 (1946) says that a ray
reflecting off the arc defined by ACB where A is the source, C
is the centre of the gonio, and B is the detector (ie the
focussing circle) is automatically parafocussing, because you
only can establish the location of the crystallites, not their
orientation, but then goes on to say that you can actually
find the orientation, as the lattice plane normal bisects the
angle ACB.
>
>
>
> I also haven't been able to find a use of the word
"parafocus" outside of the diffraction literature, so I can't
see how the word is used elsewhere.
>
>
>
> Any ideas?
>
>
>
>
>
> Matthew
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