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
FAX (301) 975 5334

From: 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>
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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