Brian: The comments are quite helpful. Thanks. I ran surfaces with high relief regularly for some special purposes. We can consider the radiated area consisting many tiny sub-areas with different degrees of displacements. With a Bragg-Brentano configuration, it is foreseeable that the high relief would cause more peak broadening at low 2theta angle side and less on the high angle side. In extreme cases, double peaklets can be seen. Is there a way GSAS II handles the high relief? J Sent from Yahoo Mail on Android On Thu, Feb 2, 2017 at 10:44 PM, Toby, Brian H.<[email protected]> wrote: Shay, I have to disagree with much of the previous discussion. Yes, there are aberrations in two-theta with sample displacement in a pseudo-parallel beam instrument and they are different for displacement along the beam direction and for displacement perpendicular to the beam (but in the detection plane). Typically you can only see the first correction when data are collected over a very wide angular range (>150 degrees) so this is almost never noticed with x-rays. The functional form for the corrections is coded into GSAS-II, so if you want read through the source code you can see what they are or you can do some simulations to see the effect. The functional form for these corrections are completely different from those for Bragg-Brentato geometry, which is extremely sensitive to the sample height and in some cases sample transparency. The Debye-Sherrer geometry corrections are typically very minor. When only the second term is needed, the correction can be replaced with small shifts in the two-theta zero correction and lattice constants. This is why in practice with synchrotrons and CW neutrons one does not see the problems with sample placement that leads to large peak shifts in a Bragg-Brentano geometry, making it much easier to index patterns from synchrotron data, but it is not true that there are no corrections. Brian On Feb 2, 2017, at 9:14 AM, Shay Tirosh <[email protected]> wrote: Dear Rietvelders Does the 2-teta shift due to sample displacement in parallel beam is similar to Bragg Brentano mode?If not then what is the mathematical expression in parallel beam? Thanks Shay -- _________________________________________________ Dr. Shay TiroshInstitute for Nanotechnology & Advanced MaterialsBar Ilan University Ramat Gan, 52900Israel Phone: +972-(0)30-531-7320Mobile: +972-(0)54-8834533Email: [email protected]_________________________________________________ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Please do NOT attach files to the whole list <[email protected]> Send commands to <[email protected]> eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/[email protected]/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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