I would like to add to Reinhard Neder's clear explanation that modern analysis software such as TOPAS, GSAS-II, and FULLPROF are able to model the axial divergence asymmetry at low angles efficiently. Earlier implementations and older software, using approximations like Pearson VII, not so much, and so the sacrifice in intensity that Davide Levy describes is often accepted as a tradeoff for easier analysis. Even if an asymmetric peak shape is not visible to the eye, axial divergence shifts the observed maximum from the nominal position given by Bragg's law, which can be troublesome.
Best, Peter ************************** Peter W. Stephens SUNY Distinguished Professor Emeritus Department of Physics and Astronomy Stony Brook University On Thu, Nov 14, 2024 at 3:56 AM reinhard.neder <reinhard.ne...@fau.de> wrote: > Dear Davide, > > I assume the diffractometer type you are talking about is a Bragg-Brentano > diffractometer with flat sample. > > Such a diffractometer commonly uses a ~ 1cm long footprint of the electron > beam onto the target (Cu, Co, Mo etc). Correspondingly the detector is > about a good cm wide as well. The purpose of the Soller-collimator is to > cut the primary beam into a group of parallel slices along this 1 cm > footprint on the source. Especially at lower 2Theta (5 to 20° 2Theta) this > enhances the instrumental resolution / reduces the asymmetry of the > observed reflection profile. > > Two effects cause this asymmetric broadening at lower 2Theta: > > 1) Any x-ray path that travels along the intended directions along the > soller collimator will still cause a Debye-Scherrer cone of diffracted > intensity. The broad detector will integrate the curved signal into a > single intensity at the nominal 2Theta position. As the parts of the > Debye-Scherrer cone to the side of the central position are geometrically > at a lower position, the detector will report this intensity at a lower > 2Theta. This effect is more prominent at lower 2Theta. Imagine a flat area > detector, where you would see the complete powder rings on the detector. > > 2) Without the Soller collimator, you will allow xray beams that will > travel "diagonally" across your sample. Lets assume the sample is > horizontal, and source and detector travel along a vertical arc. If we now > observe this from the top down, a beam that originates from the back part > of the electron footprint on the source diagonally onto the middle of the > sample and then on to the front section of the detector will cause a > differently oriented Debye-Scherrer cone than a beam path that travels > strictly in the plane defined by the Soller collimator. The effect causes > several Scherrer cones to appear on the detector the so called "umbrella" > effect. > > Try a sample with much larger lattice parameter with Reflections at the 5 > to 20° range and you should see a more drastic increase in the asymmetry of > the reflection profile at the low 2Theta side of your reflections. In the > data you presented you can actually see the effect as well. The red line is > (slightly) more asymmetric at the low 2Theta side. at this "high" 2Theta > the effect is mus less than at say 10°. > > If all your samples have small lattice parameters, you will get away > without a Soller collimator and you will be able to enjoy the higher > intensity. For samples with higher lattice parameters, lower symmetry you > will have more reflection overlap even at low 2Theta. As always it is a > compromise between the desire for high intensities and small angular > resolution. > > Hope this helps, yes I should have made / referenced a few schematic > drawings... > > Best > > Reinhard Neder > > > Am 14.11.24 um 08:52 schrieb davide.lev...@gmail.com: > > Dear All, > I would like to know how the primary Soller Slits influence the quality of > the diffraction pattern, as from some test I did, I don't see major > differences in the peak shape. The use of these slits, on the other hand, > strongly cut the peak intensities. > > Do you have any strong argument to use the primary Soller Slit for > qualitative and qualitative analysis? > > Thank for your answer > > Davide > > > > > > Dr. Davide Levy, Ph.D. > > > > X-ray diffraction lab. responsible , *TAU.nano* > > > > *Jan Koum Center for Nanoscience and Nanotechnology* > > > > *🌐** https://nano.tau.ac.il/ <https://nano.tau.ac.il/>* > > > > 📞 +972-3- 6407815 > > > > 📱0528698231 > > > > 📧 dav...@tau.ac.il > > > > > > > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com> > <alan.he...@neutronoptics.com> > Send commands to <lists...@ill.fr> <lists...@ill.fr> eg: HELP as the subject > with no body text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > -- > Prof. Dr. Reinhard Neder > Kristallographie und Strukturphysik > Friedrich-Alexander-Universität Erlangen-Nürnberg > phone +49 9193 85 25 191 > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com > > > Send commands to <lists...@ill.fr> eg: HELP as the subject with no body > text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > >
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