Hi, I would only add one thing to Reinhard's articulate statement: Another advantage to the variable slits is complete freedom the set them to a fixed value that would maximize intensities (from the sample) for a given starting 2-theta angle.
Regards, Jim James P. Cline Ceramics Division National Institute of Standards and Technology 100 Bureau Dr. stop 8520 [ B113 / Bldg 217 ] Gaithersburg, MD 20899-8523 USA jcl...@nist.gov (301) 975 5793 FAX (301) 975 5334 -----Original Message----- From: Reinhard Kleeberg [mailto:kleeb...@mineral.tu-freiberg.de] Sent: Tuesday, December 08, 2009 2:13 AM To: David Lee Cc: Russ Field; rietveld_l@ill.fr Subject: Re: Automated slits for Panalytical MPD The clear advantage of variable slit is to have more intensity at medium/higher diffraction angles, compared to fixed slit systems adapted to low starting angles. This is nice for quantitative Rietveld analysis of samples having important intensities at low as well as at medium/high angles, e.g. clay minerals (00l and 060 reflections). I personally love variable slit measurements with our old (conventional detector) instruments for routine quantitative Rietveld analysis, showing good peak-background ratio and pure alpha1/alpha2 doublet (because of graphite monochromator, traditional point detectors), combined with "acceptable high" intensity from the variable slit. Of course, any "data treatment" like internal conversion from variable to fixed-slit data (as recommended/necessary in some old-fashioned software) before Rietveld analysis should be avoided. As David wrote, it is no big deal to model the instrumental function for variable slits (inclusive the intensities) in a fundamental parameter approach, allowing even a reasonable structure refinement, see http://www.bgmn.de/vardiv.html However, practical problems may arise at very high angles resp. very high opening of the variable slit, if the slit divergence reaches the magnitude of the tube take-off angle or the breadth of the diffracted-beam monochromator. This cannot be correctly modeled by the Monte-Carlo simulation, simply because the intensity distribution within the broad tube beam bundle and the precise position and dimension of the curved monochromator are unknown/uncertain. So extreme slit opening should be avoided even with a fundamental parameter modeling. Reinhard Kleeberg David Lee schrieb: > The only advantage I see with a variable slit is that you keep a > constant area on the surface. This gives a constant sampling volume > for samples that are thin compared to the x-ray penetration depth. > In contrast, the sampling volume is fixed with a thick sample and > fixed slit for typical powder samples. > > I agree that the variation in resolution with angle makes Reitveld > analysis much harder. A full fundamental parameter analysis software > that includes the variable slit might work. > > David Lee, Ph.D. > DTLee Scientific, llc > http://www.dtlee.com > 614-562-6230 > > On Dec 7, 2009, at 10:03 PM, Russ Field wrote: > >> HI All >> >> I am seeking opinion on the installation of automated slits on a >> Panalytical MPD >> >> Pros and cons. >> >> Comments from a previous post are shown below by an esteemed >> colleague >> >> I see little use for varable divergence slits for all sorts of >> reasons (firstly as there is no rock solid conversion from automatic >> to fixed intensities, secondly the resolution changes with angle with >> variable slits that can't be modelled unless a more sofisticated >> model than that of Highcore Plus is used). >> >> >> Regards >> >> Russell >> >> Russell Field BSc (Hons), >> Scientific Officer, >> Dept of Physical Geography, >> Macquarie University, >> NSW 2019 >> ============================== >> Ph: 02 9850 8341 >> Fax: 02 9850 8420 >> Mobile No. 0417 681 959 >> Email: rfi...@els.mq.edu.au >> >> >> > >
