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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