Alexandra, For your PANalytical MPD, at 5 degrees 2theta, a 1/2 deg divergence slit gives you a beam length (on the sample) of about 48mm. Probably this is much larger than the actual size of your sample!
At 20 degrees 2theta, the length is down to about 12mm for the same divergence (1/2 deg). So if you see that the background is fine above 20 deg, but not below that, your problem may be due to unwanted background scatter from the sample holder. Also consider that for a 12mm long sample, above 20 degrees 2theta, all the beam is illuminating the sample, whereas below this angle, some of the beam misses the sample, and by 5 degrees, only a quarter of the beam hits the sample. So your peak intensities are all compromised at low angles. One solution is to reduce the divergence so that the beam does not overflow the sample. A variable slit does this for you, but (1) it changes the resolution as a function of angle as compared with fixed slits, (2) you need to be careful that at high angles you don’t have too large a divergence (you don’t want the divergence to become greater than half the anti-scatter slit, otherwise the anti-scatter slit will cut off the beam). A smaller fixed slit will also do the trick, but will of course compromise your intensities (and potentially the counting statistics, unless you use a sample spinner) at higher angles. If your sample is 12mm in length, then to avoid beam overflow at 5 degrees 2theta a divergence slit of 1/8 degree must be used. I hope this is helpful, Andrew -- E. Andrew Payzant Senior R&D Staff Member High Temperature Materials Laboratory Oak Ridge National Laboratory 1 Bethel Valley Road PO Box 2008, MS 6064 Building 4515, Room 113 Oak Ridge, TN, 37831-6064 ph: (865) 574-6538 FAX: (865) 574-3940 web: <http://www.ms.ornl.gov/DTP/payzant.shtml> On 12/1/09 9:41 AM, "Leonid Solovyov" <l_solov...@yahoo.com> wrote: Dear Alexandra, The 1/4 divergence and 1/2 anti-scatter slits are narrow enough to give a reasonably low background after 5 degrees 2Theta. First I would check that the background is not related to the sample itself. It may be done by measuring an empty sample holder. The background may also be due to scattering from sample holder edges if the holder diameter is small and the primary beam mask is wide or/and the Soller slit divergence is big. The automated sample changer can not produce background, but I am not sure if the beam knife can be installed together with the changer. The beam knife cuts the air scattering which is essential in the low-angle region. The standard beam knife supplied by PANal is rather thick and if it is installed too close to the sample surface it may attenuate intensity at high angles. With fixed 1/4 divergence slit the knife should be put about 3-4 mm under the sample surface which must be safe for measurements up to 100 degrees 2Theta. To check if the knife attenuates high angle reflections it is necessary to measure the same sample with and without knife and then compare the intensities. On my X’Pert I simply replaced the standard knife by a thin (ca. 1 mm) stainless steel screen, and installed it 3 mm above the sample surface which allows preserving intensity up to 150 degrees 2Theta even with the 1/2 divergence primary slit and have a reasonable low-angle background. Of course, the knife should also be properly centered on the diffractometer axis and aligned vertically. Best regards, Leonid ******************************************************* Leonid A. Solovyov Institute of Chemistry and Chemical Technology 660049, K. Marx 42, Krasnoyarsk, Russia www.icct.ru/eng/content/persons/Sol_LA sites.google.com/site/solovyovleonid ******************************************************* --- On Mon, 11/30/09, Alexandra Seclaman <seclaman_alexan...@yahoo.com> wrote: > From: Alexandra Seclaman <seclaman_alexan...@yahoo.com> > Subject: RE: Configuration of PANalytical X'pert Pro > To: rietveld_l@ill.fr > Date: Monday, November 30, 2009, 2:05 PM > Thank > you for all of your answers, it was more than I hoped for. > > I did a Rietveld refinement on stibnite (Sb2S3). And I > mesured it from 5 2theta to 90. But? from 5 to 20 2Θ i > had a serious background noise and general trend similar to > that of glass. This trend and the background noise are there > no matter what I measure. > I can't get to the lab until Wednesday, but from what I > recall, from the X ray source to the X'celerator > detector: there are 1/4 +1/2 fixed divergence,the widthest > mask and the slit (i'm not shure if that is the proper > name) in front of the X'celerator is also the widthest. > > The automated system of "handling" the sample is > installed, should i changed it? > And I forgot to mention, the system is MPD. > > I can give you more details on Wednesday.? > > I'm sorry for my not technical terms, but is very hard > for me > to speak about diffraction in English. > > > Alexandra Seclaman > > seclaman_alexan...@yahoo.com > > a.c.secla...@gmail.com > > --- On Sun, 11/29/09, Tony Raftery > <a.raft...@qut.edu.au> wrote: > > From: Tony Raftery <a.raft...@qut.edu.au> > Subject: RE: Configuration of PANalytical X'pert Pro > To: "Alexandra Seclaman" > <seclaman_alexan...@yahoo.com> > Date: Sunday, November 29, 2009, 11:57 PM > > Alexandra, > > while there is no such thing as a standard configuration, > choices can be made to help in the modelling. Another > corresponant (to the Rietveld list)? made some > suggestions that I only partly agree with. A lot depends on > you goals ans available instrumentation - for instance if > you have an X'cellerator detector and a monochromator. > If you are interested in the low angle region (as I think > you are) or not. If you were looking at some phases or > phases where the peaks began after > 25 deg 2theta, you could afford to go to larger divergence > than if the peaks started at 5 deg 2theta. > > I see little use for beam knifes except over restricted low > angle ranges (they always interfere at high angle, where > they interfere depends on how they are set). It 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). > > What I agree with is that you need to use and properly > adjust the anti-scatter slits. > > Fro memory (I am away from the lab for a few weeks) my > conditions are for scans 3.5 deg to 140 deg 2theata - a > general scan, well crystalled phases > fixed divergence 0.5 deg, a-scatter (incident) 1 deg, > sollers (incident) 0.04 rad > 3.4mm (diffracted) a-scatter, mono fot > X'cellerator, X'cellerator set full )about 2 deg) > step 0.02 deg (or what X'cellerator allows, in my case > 0.0167 deg), step-time to give about 10,000 counts full > scale > ________________________________________ > From: Alexandra Seclaman [seclaman_alexan...@yahoo.com] > Sent: Saturday, 28 November 2009 3:09 AM > To: rietveld_l@ill.fr > Subject: Configuration of PANalytical X'pert Pro > > Hello, > > I have done a Rietveld refinement using a PANalytical > X'pert Pro but the standard configuration is giving me a > lot of troubles. This configuration is somewhat > "blind" to the small 2 theta angles and it > introduces (because of the poorly focused X ray beam) a > heavy background. I've manage to remove that background > by refinement but I'm sure that the quality of the > acquisition can be improved. > I am no engineer, so the inner workings of that machine are > somewhat a mystery to me. I am asking if someone else has > used a PANalytical for Rietveld and if you can give me a > better configuration of slots, masks etc. I have tried > different configurations with no success. > > Please take into consideration, while writing your reply, > that I'm still a student and I have yet a lot to learn. > > Thank you! > > Alexandra Seclaman > seclaman_alexan...@yahoo.com > a.c.secla...@gmail.com > > > > > > >
