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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