RE: advice on new powder diffractometer
As I have said in reply to Leonid, I don't know why we have such a large difference in our experimental setup. It may be that our 0.02 slits are misaligned -- I will have to check this. The question of what is meant by signal to noise ratio in connection with powder diffraction is one which I have been trying to find out about. You see this term used quite often in papers and in general discussion, but I have still to discover how one works it out in a properly defined way with regards to powder diffraction. It is well defined in radio frequency signals. One could for example take, as you have said the peak height minus the background and divide by the background as one measure, a value which would tend to zero as the peak becomes smaller. This definition would also imply that increasing measurement time has no effect on signal to noise ratio, since both peak and background would simply scale up with time, in which case we might as well measure for negligible amount of time. Or else one could take the peak intensity divided by the square root of the background: this at least would improve with measurement time. For instance suppose we have a peak above background of 1 counts and a background of 1000 counts, this would give a signal to noise ratio of roughly 322. If we measure ten times longer, the peak intensity becomes 10 and the background becomes 1, giving a signal to noise ratio of 1000, an improvement! So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? Mike ** -Original Message- From: Van der Lee [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 07:39 To: rietveld_l@ill.fr Subject: Re: advice on new powder diffractometer After having purchased the set of 0.02 soller slits in Dec. 03 I have done some comparison tests on the (111) silicon peak under identical conditions except for the change of the primary and secondary sollers: background Bpeak height PH 0.0295 8580 0.04 32018900 negliable change on the FWHM so not an intensity drop with a factor of 25, neither 4, but only slightly larger than 2. How do I define signal/background ratio? There is not much scientific in it, it serves only to compare qualitatively these kind of 'optical' elements; it should be something like (PH-B)/B or more generally Bragg scattering intensity to non-Bragg-scattering intensity. Arie Dear Mike, Normally, changing sollers must not influence the signal/background ratio. Wider sollers, however, make the primary beam wider and if the sample diameter is small then a parasitic scattering from the sample holder edges may appear. I am really surprised that moving from 0.04 Soller slits to 0.02 you got 25 times intensity reduction. When I change the primary soller from 0.04 to 0.02 the intensity drops ~2 times, so if you changed both the primary and the secondary sollers the intensity should decrease ~4 times, but not 25 times. Leonid Solovyov How do you define signal to noise in powder diffraction? I have seen this term used several times, but I have not found a definition so far with regard to powder diffraction per se. I have just done two runs on a Panalytical one with 0.04 soller slits and one with 0.02 (both with a CuKa1 premonochromator) both for about 9 hours. The strongest peak for the 0.02 case is 2700 counts, half width 0.08 degrees and a background of 25 counts. The same peak with the 0.04 slits has 7 counts, half width 0.11 degrees and a background of 700 counts. Mike Glazer __ __ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ -- *** A. van der Lee Institut Européen des Membranes CNRS - UMR 5635 Université de Montpellier II - Case Courrier 047 Place E. Bataillon 34095 MONTPELLIER Cedex 5 - FRANCE Tel : 33 (0) 4 67 14 91 35 Fax : 33 (0) 4 67 14 91 19 Website X-ray scattering facility ICG/IEM: http://www.iemm.univ-montp2.fr/xrayweb/main_uk.html
RE: advice on new powder diffractometer
Leonid I don't know why we have such a large difference, but for some reason we do. I will send you privately the two diagrams. Mike -Original Message- From: Leonid Solovyov [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 03:56 To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Dear Mike, Normally, changing sollers must not influence the signal/background ratio. Wider sollers, however, make the primary beam wider and if the sample diameter is small then a parasitic scattering from the sample holder edges may appear. I am really surprised that moving from 0.04 Soller slits to 0.02 you got 25 times intensity reduction. When I change the primary soller from 0.04 to 0.02 the intensity drops ~2 times, so if you changed both the primary and the secondary sollers the intensity should decrease ~4 times, but not 25 times. Leonid Solovyov
Re: advice on new powder diffractometer
Michael Glazer wrote the following on 19/02/2008 09:42: Or else one could take the peak intensity divided by the square root of the background: this at least would improve with measurement time. For instance suppose we have a peak above background of 1 counts and a background of 1000 counts, this would give a signal to noise ratio of roughly 322. If we measure ten times longer, the peak intensity becomes 10 and the background becomes 1, giving a signal to noise ratio of 1000, an improvement! So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? This at least coincides with data collection practice in single crystal diffraction: there it is said that in order to improve the 'signal to noise' ratio with a factor of two you have to count 4 times longer, or, alternatively, double the generator power. Arie
RE: advice on new powder diffractometer
So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? Why does it matter? A higher Bragg/Background ratio does not necessarily mean better data if counting statistics are poor. Exaggerating slightly :-) consider a single peak with a ratio of 100/1 compared to a peak with a ratio 1/1000. The second measurement will give the lowest error, not the first which has a much higher signal/noise. And you measure lots of points on a slowly varying background, so you have a much better estimate of background than the normal error of a single point. Please don't encourage people to simply maximise signal/noise. Similarly, low profile R-factor's can be obtained with low resolution data and high background. That does not mean that low resolution data produces smaller errors in structural parameters. I worry about people treating measurement and refinement as black boxes with simplified measures of quality such as R-factors, signal-to-noise etc. You have to look at the physical reality of the model and the estimated errors in its parameters, while not cheating by removing data that doesn't fit for unknown reasons, adding too much a priori information such as constraints, or throwing in extra garbage parameters to improve the R-factors. __ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE [EMAIL PROTECTED] +33.476.98.41.68 http://www.NeutronOptics.com/ __
RE: advice on new powder diffractometer
It seems to me that if there is no internationally accepted definition of signal to noise ratio in powder diffraction, then let me suggest that this forum might be a way to define it once and for all. How about this? SNR = (P-B)/SQRT(B). Of course this would only give a value for a single peak. I suppose one could also define a global SNR as a sum over all Global SNR = sum over peaks (P-B)/SQRT(B). Does this make sense? Mike A more overall value would be a normalized sum of the ratios (Ip-Ib)/sqrt(Ib), where Ip and Ib are the profile intensities of the pattern and the background for each datapoint. Leonid Looking for last minute shopping deals? Find them fast with Yahoo! Search. http://tools.search.yahoo.com/newsearch/category.php?category=shopping
RE: advice on new powder diffractometer
I think that care has to be taken when defining B=noise. In many cases, B contains some amorphous material scattering, air scattering, TDS, etc which one might account for using a proper model. So they will scale with time while the error part of noise will not. miguel On 19 Feb 2008 at 9:21, Michael Glazer wrote: It seems to me that if there is no internationally accepted definition of signal to noise ratio in powder diffraction, then let me suggest that this forum might be a way to define it once and for all. How about this? SNR = (P-B)/SQRT(B). Of course this would only give a value for a single peak. I suppose one could also define a global SNR as a sum over all Global SNR = sum over peaks (P-B)/SQRT(B). Does this make sense? Mike -Original Message- From: Van der Lee [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 09:09 To: rietveld_l@ill.fr Subject: Re: advice on new powder diffractometer Michael Glazer wrote the following on 19/02/2008 09:42: Or else one could take the peak intensity divided by the square root of the background: this at least would improve with measurement time. For instance suppose we have a peak above background of 1 counts and a background of 1000 counts, this would give a signal to noise ratio of roughly 322. If we measure ten times longer, the peak intensity becomes 10 and the background becomes 1, giving a signal to noise ratio of 1000, an improvement! So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? This at least coincides with data collection practice in single crystal diffraction: there it is said that in order to improve the 'signal to noise' ratio with a factor of two you have to count 4 times longer, or, alternatively, double the generator power. Arie -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
RE: advice on new powder diffractometer
in general, I made the observation that intensity may drop more than expected from theory due to alignment problems. Alignment becomes increasingly critical for smaller divergences as we normally have to sintonize 2 slits, one in the incident and one in the diffracted beam. With low divergences, a small twist of the soller axis around theta or the X-ray beam may cut an important portion of intensity. BTW, in our old Philips with 2 Soller 0.04 rad slits and sec beam graphite monochromator, using 1º div slits and a 0.2 mm receiving slit gives 12550 cps and FWHM=0.119º (including alpha2 which is visible as a shoulder) for the 2th=26.6º quartz peak and ~15 cps for the background (zero bk Si sample holder) below this peak. miguel On 19 Feb 2008 at 8:29, Michael Glazer wrote: Leonid I don't know why we have such a large difference, but for some reason we do. I will send you privately the two diagrams. Mike -Original Message- From: Leonid Solovyov [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 03:56 To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Dear Mike, Normally, changing sollers must not influence the signal/background ratio. Wider sollers, however, make the primary beam wider and if the sample diameter is small then a parasitic scattering from the sample holder edges may appear. I am really surprised that moving from 0.04 Soller slits to 0.02 you got 25 times intensity reduction. When I change the primary soller from 0.04 to 0.02 the intensity drops ~2 times, so if you changed both the primary and the secondary sollers the intensity should decrease ~4 times, but not 25 times. Leonid Solovyov -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Antw: RE: advice on new powder diffractometer
From my understanding 'noise' is the random signal superimposed to the background and the peaks. The signal to noise ratio would be peak above background / random signal, and this should improve with longer measurement time. Nic Michael Glazer [EMAIL PROTECTED] 02/19/08 9:42 am As I have said in reply to Leonid, I don't know why we have such a large difference in our experimental setup. It may be that our 0.02 slits are misaligned -- I will have to check this. The question of what is meant by signal to noise ratio in connection with powder diffraction is one which I have been trying to find out about. You see this term used quite often in papers and in general discussion, but I have still to discover how one works it out in a properly defined way with regards to powder diffraction. It is well defined in radio frequency signals. One could for example take, as you have said the peak height minus the background and divide by the background as one measure, a value which would tend to zero as the peak becomes smaller. This definition would also imply that increasing measurement time has no effect on signal to noise ratio, since both peak and background would simply scale up with time, in which case we might as well measure for negligible amount of time. Or else one could take the peak intensity divided by the square root of the background: this at least would improve with measurement time. For instance suppose we have a peak above background of 1 counts and a background of 1000 counts, this would give a signal to noise ratio of roughly 322. If we measure ten times longer, the peak intensity becomes 10 and the background becomes 1, giving a signal to noise ratio of 1000, an improvement! So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? Mike ** -Original Message- From: Van der Lee [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 07:39 To: rietveld_l@ill.fr Subject: Re: advice on new powder diffractometer After having purchased the set of 0.02 soller slits in Dec. 03 I have done some comparison tests on the (111) silicon peak under identical conditions except for the change of the primary and secondary sollers: background Bpeak height PH 0.0295 8580 0.04 32018900 negliable change on the FWHM so not an intensity drop with a factor of 25, neither 4, but only slightly larger than 2. How do I define signal/background ratio? There is not much scientific in it, it serves only to compare qualitatively these kind of 'optical' elements; it should be something like (PH-B)/B or more generally Bragg scattering intensity to non-Bragg-scattering intensity. Arie Dear Mike, Normally, changing sollers must not influence the signal/background ratio. Wider sollers, however, make the primary beam wider and if the sample diameter is small then a parasitic scattering from the sample holder edges may appear. I am really surprised that moving from 0.04 Soller slits to 0.02 you got 25 times intensity reduction. When I change the primary soller from 0.04 to 0.02 the intensity drops ~2 times, so if you changed both the primary and the secondary sollers the intensity should decrease ~4 times, but not 25 times. Leonid Solovyov How do you define signal to noise in powder diffraction? I have seen this term used several times, but I have not found a definition so far with regard to powder diffraction per se. I have just done two runs on a Panalytical one with 0.04 soller slits and one with 0.02 (both with a CuKa1 premonochromator) both for about 9 hours. The strongest peak for the 0.02 case is 2700 counts, half width 0.08 degrees and a background of 25 counts. The same peak with the 0.04 slits has 7 counts, half width 0.11 degrees and a background of 700 counts. Mike Glazer __ __ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ -- *** A. van der Lee Institut Européen des Membranes CNRS - UMR 5635 Université de Montpellier II - Case Courrier 047 Place E. Bataillon 34095 MONTPELLIER Cedex 5 - FRANCE Tel : 33 (0) 4 67 14 91 35 Fax : 33 (0) 4 67 14 91 19 Website X-ray scattering facility ICG/IEM: http://www.iemm.univ-montp2.fr/xrayweb/main_uk.html
RE: advice on new powder diffractometer
Alan I absolutely agree with you regarding Rietveld refinement. The same argument is true for adjusting peak shapes to get the best fit, playing around with different peak formulae. Often such tweaking, while it makes the observed and calculated fit look nicer, has little effect on the atomic positions, which in the end is what one is trying to derive. One should not lose sight of the model. But my question about defining SNR is more to do with the use of powder data in general, and especially as used in industry or forensics. For example distinguishing in a legal sense between two materials may depend on which of two powder patterns has the higher SNR. I keep seeing this term used in papers and books but never defined. Mike -Original Message- From: Alan Hewat [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 10:10 To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? Why does it matter? A higher Bragg/Background ratio does not necessarily mean better data if counting statistics are poor. Exaggerating slightly :-) consider a single peak with a ratio of 100/1 compared to a peak with a ratio 1/1000. The second measurement will give the lowest error, not the first which has a much higher signal/noise. And you measure lots of points on a slowly varying background, so you have a much better estimate of background than the normal error of a single point. Please don't encourage people to simply maximise signal/noise. Similarly, low profile R-factor's can be obtained with low resolution data and high background. That does not mean that low resolution data produces smaller errors in structural parameters. I worry about people treating measurement and refinement as black boxes with simplified measures of quality such as R-factors, signal-to-noise etc. You have to look at the physical reality of the model and the estimated errors in its parameters, while not cheating by removing data that doesn't fit for unknown reasons, adding too much a priori information such as constraints, or throwing in extra garbage parameters to improve the R-factors. __ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE [EMAIL PROTECTED] +33.476.98.41.68 http://www.NeutronOptics.com/ __
RE: advice on new powder diffractometer
Gentlemen, I really do not want to interfere too much, but you should (probably) start with the definition of noise and signal. In the theory of signals both terms have their well-defined meaning. So you can either accept the definition used by the signal community and identify what is what in your powder patterns or develop your own definition and put it on a good statistical base. My apologies once more, Lubo On Tue, 19 Feb 2008, Michael Glazer wrote: Alan I absolutely agree with you regarding Rietveld refinement. The same argument is true for adjusting peak shapes to get the best fit, playing around with different peak formulae. Often such tweaking, while it makes the observed and calculated fit look nicer, has little effect on the atomic positions, which in the end is what one is trying to derive. One should not lose sight of the model. But my question about defining SNR is more to do with the use of powder data in general, and especially as used in industry or forensics. For example distinguishing in a legal sense between two materials may depend on which of two powder patterns has the higher SNR. I keep seeing this term used in papers and books but never defined. Mike -Original Message- From: Alan Hewat [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 10:10 To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? Why does it matter? A higher Bragg/Background ratio does not necessarily mean better data if counting statistics are poor. Exaggerating slightly :-) consider a single peak with a ratio of 100/1 compared to a peak with a ratio 1/1000. The second measurement will give the lowest error, not the first which has a much higher signal/noise. And you measure lots of points on a slowly varying background, so you have a much better estimate of background than the normal error of a single point. Please don't encourage people to simply maximise signal/noise. Similarly, low profile R-factor's can be obtained with low resolution data and high background. That does not mean that low resolution data produces smaller errors in structural parameters. I worry about people treating measurement and refinement as black boxes with simplified measures of quality such as R-factors, signal-to-noise etc. You have to look at the physical reality of the model and the estimated errors in its parameters, while not cheating by removing data that doesn't fit for unknown reasons, adding too much a priori information such as constraints, or throwing in extra garbage parameters to improve the R-factors. __ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE [EMAIL PROTECTED] +33.476.98.41.68 http://www.NeutronOptics.com/ __
RE: advice on new powder diffractometer
Hi Bob, The band of delt/sig is a little wider than that, but I think the real problem is that they sent me the data in cps. I am awaiting the data in counts now. Thanks to you and others for pointing this out. However, in my view, having the data in cps should simply go into the histogram scale factor and should not affect the fit. What is wrong with my logic? Thanks, - Kurt -Original Message- From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED] Sent: Friday, February 15, 2008 6:37 AM To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Hi Kurt, If you plot the results in POWPLOT do the statistical analysis what does the delt/sig vs 2-theta plot look like? If weights are right then the band of delt/sig ought to be roughly 2-3 sig wide. Is your data in cpm? Some instruments make that by default. That will mess up the R-factors. Bob
RE: advice on new powder diffractometer
Kurt (and others), Intensity in cps isn't a problem if the conversion is also applied to the esd and both intensity and esd is output to be read by GSAS. The problem is that the instrument software doesn't give you the esds at all but just a table (in the old 10 column format) of the intensities as cps. GSAS then assumes these are counts and that esd = sqrt(counts) according to the usual Gaussian statistics. So chi**2 will be off by the reciprocal of the count time per step, i.e. too small. BTW this does not affect the resulting parameters (or esds) one bit - it only makes the chi**2 wrong. BTW**2: Poisson statistics only comes into play when the count is less than ~20 so that zero is within (say) 4-5 sigma of the count. Above that Gaussian statistics ~ Poisson statistics. So if all points in the scan have 20 counts (including background) then the assumptions about Gaussian statistics for the counts are reasonable. BTW**3: If the background was say 20 counts with a sig ~4-5 then the R-! value for that background will be ~20%. So if your pattern has very few peaks (say only 10% of the points) then R-values can easily be much larger than 10%. Following this logic a bit further, a pattern with a higher background will give lower R-values so IMHO comparing R-values from one experiment to another is a particularly pointless exercise. Ditto for asking how good does my R-factor have to be? because it just depends. Bob -Original Message- From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: Tue 2/19/2008 10:37 AM To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Hi Bob, The band of delt/sig is a little wider than that, but I think the real problem is that they sent me the data in cps. I am awaiting the data in counts now. Thanks to you and others for pointing this out. However, in my view, having the data in cps should simply go into the histogram scale factor and should not affect the fit. What is wrong with my logic? Thanks, - Kurt -Original Message- From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED] Sent: Friday, February 15, 2008 6:37 AM To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Hi Kurt, If you plot the results in POWPLOT do the statistical analysis what does the delt/sig vs 2-theta plot look like? If weights are right then the band of delt/sig ought to be roughly 2-3 sig wide. Is your data in cpm? Some instruments make that by default. That will mess up the R-factors. Bob
RE: advice on new powder diffractometer
Hi all, I never thought of background as noise - to me the noise should be defined as the esd of the background points. You could get that by averaging 10 or 20 background points and getting the standard deviation. I'm told that for typical counting statistics, that should be close to the square root of the background, so Mike's idea of using that square root seems reasonable. - Kurt -Original Message- From: Alan Hewat [mailto:[EMAIL PROTECTED] Sent: Tuesday, February 19, 2008 3:10 AM To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? Why does it matter? A higher Bragg/Background ratio does not necessarily mean better data if counting statistics are poor. Exaggerating slightly :-) consider a single peak with a ratio of 100/1 compared to a peak with a ratio 1/1000. The second measurement will give the lowest error, not the first which has a much higher signal/noise. And you measure lots of points on a slowly varying background, so you have a much better estimate of background than the normal error of a single point. Please don't encourage people to simply maximise signal/noise. Similarly, low profile R-factor's can be obtained with low resolution data and high background. That does not mean that low resolution data produces smaller errors in structural parameters. I worry about people treating measurement and refinement as black boxes with simplified measures of quality such as R-factors, signal-to-noise etc. You have to look at the physical reality of the model and the estimated errors in its parameters, while not cheating by removing data that doesn't fit for unknown reasons, adding too much a priori information such as constraints, or throwing in extra garbage parameters to improve the R-factors. __ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE [EMAIL PROTECTED] +33.476.98.41.68 http://www.NeutronOptics.com/ __
RE: advice on new powder diffractometer
I think the problem with creating a new definition of signal to noise is that this is an attempt to make something that corresponds to the quality of the measurement, while the true signal to noise is only a measure of the quality of the signal. This can be used for estimating the time needed for a measurement to obtain the desired statistics, but should not be used as a way to judge the quality of a completed measurement. As far as taking the sum of the squares, it is my understanding that the signal to noise is intended to be applied to a single mode, so the true signal-to-noise could vary widely for different modes. As I mentioned, it can be a very useful predictor of the time needed for the measurement, but probably shouldn't be used in discussing the quality of the measurement. If you are interested in a peak other than the one for the highest mode, you should use that peak when estimating the time needed for a measurement. Another link is http://en.wikipedia.org/wiki/Signal-to-noise_ratio Tom Worlton -Original Message- From: Leonid Solovyov [mailto:[EMAIL PROTECTED] Sent: Tuesday, February 19, 2008 3:51 AM To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer It seems to me that if there is no internationally accepted definition of signal to noise ratio in powder diffraction, then let me suggest that this forum might be a way to define it once and for all. How about this? SNR = (P-B)/SQRT(B). Of course this would only give a value for a single peak. I suppose one could also define a global SNR as a sum over all Global SNR = sum over peaks (P-B)/SQRT(B). Does this make sense? Mike A more overall value would be a normalized sum of the ratios (Ip-Ib)/sqrt(Ib), where Ip and Ib are the profile intensities of the pattern and the background for each datapoint. Leonid Looking for last minute shopping deals? Find them fast with Yahoo! Search. http://tools.search.yahoo.com/newsearch/category.php?category=shopping
RE: advice on new powder diffractometer
I've watched this thread develop over the past few days. Apparently there is little advice forthcoming about the original inquiry. Just one person's observation. Frank May - University of Missouri-St. Louis winmail.dat
Re: advice on new powder diffractometer
Yes it is mainly down to the Soller slits, there was a very large thread on soller slits somewhere in the Rietveld archives about this discussion. I think the down side of changing the soller slits is a move away from the optimum FWHM that can be obtained? Quoting Leonid Solovyov [EMAIL PROTECTED]: Xpert Pros are fantastic machines. Collections times are significantly reduced with their new semi-conductor PSDs. The only snag with the machines is=20 that the asymmetry at low angle is very stark - so much so that in effect you=20 are having to model a multiple diffractometer situation... In fact, the problem of asymmetry is related to the Soller slit choice (and to the diffractometer radius), but not to the detector. By default PANal supplies their machines with Soller slits of 0.04 rad divergence that give highly asymmetric peaks, but one may choose Sollers of 0.02 or 0.01 rad to reduce the asymmetry. Additionally, the secondary monochromator for X=92Celerator or PIXcel detectors makes the peaks even more symmetric as it suppresses the axial divergence. Leonid Solovyov ___= _ Be a better friend, newshound, and=20 know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_y= lt=3DAhu06i62sR8HDtDypao8Wcj9tAcJ=20 William Bisson -- Webmaster and Administrator - CCP14 and BCA http://www.ccp14.ac.uk http://bca.chem.ucl.ac.uk/
Re: advice on new powder diffractometer
Yes it is mainly down to the Soller slits, there was a very large thread on soller slits somewhere in the Rietveld archives about this discussion. I think the down side of changing the soller slits is a move away from the optimum FWHM that can be obtained? Changing sollers from 0.04 rad to 0.02 or 0.01 reduces the asymmetry, the FWHM and the intensity, so the peak shape and the resolution become more optimal but the intensity is sacrificed. Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs
Re: advice on new powder diffractometer
Leonid Solovyov wrote the following on 18/02/2008 16:27: Yes it is mainly down to the Soller slits, there was a very large thread on soller slits somewhere in the Rietveld archives about this discussion. I think the down side of changing the soller slits is a move away from the optimum FWHM that can be obtained? Changing sollers from 0.04 rad to 0.02 or 0.01 reduces the asymmetry, the FWHM and the intensity, so the peak shape and the resolution become more optimal but the intensity is sacrificed. However, the signal to noise ratio becomes better with 0.02/0.01 rad sollers compared to 0.04 rad Arie -- *** A. van der Lee Institut Européen des Membranes CNRS - UMR 5635 Université de Montpellier II - Case Courrier 047 Place E. Bataillon 34095 MONTPELLIER Cedex 5 - FRANCE Tel : 33 (0) 4 67 14 91 35 Fax : 33 (0) 4 67 14 91 19 Website X-ray scattering facility ICG/IEM: http://www.iemm.univ-montp2.fr/xrayweb/main_uk.html
RE: advice on new powder diffractometer
How do you define signal to noise in powder diffraction? I have seen this term used several times, but I have not found a definition so far with regard to powder diffraction per se. I have just done two runs on a Panalytical one with 0.04 soller slits and one with 0.02 (both with a CuKa1 premonochromator) both for about 9 hours. The strongest peak for the 0.02 case is 2700 counts, half width 0.08 degrees and a background of 25 counts. The same peak with the 0.04 slits has 7 counts, half width 0.11 degrees and a background of 700 counts. Mike Glazer -Original Message- From: Van der Lee [mailto:[EMAIL PROTECTED] Sent: 18 February 2008 16:36 To: rietveld_l@ill.fr Subject: Re: advice on new powder diffractometer Leonid Solovyov wrote the following on 18/02/2008 16:27: Yes it is mainly down to the Soller slits, there was a very large thread on soller slits somewhere in the Rietveld archives about this discussion. I think the down side of changing the soller slits is a move away from the optimum FWHM that can be obtained? Changing sollers from 0.04 rad to 0.02 or 0.01 reduces the asymmetry, the FWHM and the intensity, so the peak shape and the resolution become more optimal but the intensity is sacrificed. However, the signal to noise ratio becomes better with 0.02/0.01 rad sollers compared to 0.04 rad Arie -- *** A. van der Lee Institut Européen des Membranes CNRS - UMR 5635 Université de Montpellier II - Case Courrier 047 Place E. Bataillon 34095 MONTPELLIER Cedex 5 - FRANCE Tel : 33 (0) 4 67 14 91 35 Fax : 33 (0) 4 67 14 91 19 Website X-ray scattering facility ICG/IEM: http://www.iemm.univ-montp2.fr/xrayweb/main_uk.html
RE: advice on new powder diffractometer
IMHO, plenty of counter problems have their roots in the past. When the first GM counters were used in powder diffractometry the apparatus (both electronics math) was taken directly from the field of \alpha-particles physics. Because \alpha-particles are rather rare (events), people used binomial or POissonian distributions with mean value=variance. This is hardly true for normal (Gaussian) distribution diffraction. Later when the LS moved to the field of diffraction, weights were automatically set to w=1/var as if the random errors (=noise, or better, white noise) had normal distribution. So far so good, but variance was set to the mean value as if it were Poissonian distribution. Because the LS are derived under the assumption of normal distribution of random errors, the procedure must give biased estimates of the parameters. Now when the solid state detectors are rather fast and efficient it is probably the high time for new data collection strategies, e.g. repeated measurements giving better estimates of the variances. In this way it could be possible to estimate also the noise level. Who knows, BTW :-0 Lubo On Mon, 18 Feb 2008, Michael Glazer wrote: How do you define signal to noise in powder diffraction? I have seen this term used several times, but I have not found a definition so far with regard to powder diffraction per se. I have just done two runs on a Panalytical one with 0.04 soller slits and one with 0.02 (both with a CuKa1 premonochromator) both for about 9 hours. The strongest peak for the 0.02 case is 2700 counts, half width 0.08 degrees and a background of 25 counts. The same peak with the 0.04 slits has 7 counts, half width 0.11 degrees and a background of 700 counts. Mike Glazer -Original Message- From: Van der Lee [mailto:[EMAIL PROTECTED] Sent: 18 February 2008 16:36 To: rietveld_l@ill.fr Subject: Re: advice on new powder diffractometer Leonid Solovyov wrote the following on 18/02/2008 16:27: Yes it is mainly down to the Soller slits, there was a very large thread on soller slits somewhere in the Rietveld archives about this discussion. I think the down side of changing the soller slits is a move away from the optimum FWHM that can be obtained? Changing sollers from 0.04 rad to 0.02 or 0.01 reduces the asymmetry, the FWHM and the intensity, so the peak shape and the resolution become more optimal but the intensity is sacrificed. However, the signal to noise ratio becomes better with 0.02/0.01 rad sollers compared to 0.04 rad Arie -- *** A. van der Lee Institut Européen des Membranes CNRS - UMR 5635 Université de Montpellier II - Case Courrier 047 Place E. Bataillon 34095 MONTPELLIER Cedex 5 - FRANCE Tel : 33 (0) 4 67 14 91 35 Fax : 33 (0) 4 67 14 91 19 Website X-ray scattering facility ICG/IEM: http://www.iemm.univ-montp2.fr/xrayweb/main_uk.html
RE: advice on new powder diffractometer
As Mike implies, for a given counting time, you have much better statistics using the high intensity configuration. This can be true even when the signal to noise ratio is actually lower with the high intensity configuration. So signal/noise is not everything. That said, there is no point in reducing statistical error much below systematic error. I would try hard to physically model the peak asymmetry using a higher intensity configuration, and for a given counting time try to balance statistical error with systematic error, which occurs when the model cannot completely describe the data however you choose the model parameters. Many theoretical arguments about signal/noise forget that the experimentalist has a finite time to collect his data :-) Alan. Michael Glazer said: I have just done two runs on a Panalytical one with 0.04 soller slits and one with 0.02 (both with a CuKa1 premonochromator) both for about 9 hours. The strongest peak for the 0.02 case is 2700 counts, half width 0.08 degrees and a background of 25 counts. The same peak with the 0.04 slits has 7 counts, half width 0.11 degrees and a background of 700 counts. __ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE [EMAIL PROTECTED] +33.476.98.41.68 http://www.NeutronOptics.com/ __
Re: advice on new powder diffractometer
Alan Hewat wrote: Many theoretical arguments about signal/noise forget that the experimentalist has a finite time to collect his data :-) The finite time to analyse and publish can also be rate limiting! In this respect a high background is a fine addition to any instrument; it gives a much lower profile R-factor. This tells us something about the value of the profile R factor, I think ;-) Best, Jon
Re: advice on new powder diffractometer
Xpert Pros are fantastic machines. Collections times are significantly reduced with their new semi-conductor PSDs. The only snag with the machines is that the asymmetry at low angle is very stark - so much so that in effect you are having to model a multiple diffractometer situation... In fact, the problem of asymmetry is related to the Soller slit choice (and to the diffractometer radius), but not to the detector. By default PANal supplies their machines with Soller slits of 0.04 rad divergence that give highly asymmetric peaks, but one may choose Sollers of 0.02 or 0.01 rad to reduce the asymmetry. Additionally, the secondary monochromator for XCelerator or PIXcel detectors makes the peaks even more symmetric as it suppresses the axial divergence. Leonid Solovyov Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ
Re: advice on new powder diffractometer
Dear Kurt, Xpert Pros are fantastic machines. Collections times are significantly reduced with their new semi-conductor PSDs. The only snag with the machines is that the asymmetry at low angle is very stark - so much so that in effect you are having to model a multiple diffractometer situation. If you try and us fundamental parameters to fit the date you run up against a commercial secret as to the separation between each of the semi-conducting strips in the detector. I don't know as yet, any Rietveld program that have profile functions that can adequately model the low angle data from the new X'pert pro machines. If anyone does I'll be glad to know, and from a fundamental parameters approach what the separation and/or width of the semi-conductor strips are? Regards William Bisson Quoting Kurt Leinenweber [EMAIL PROTECTED]: Hi All, I am attempting to assist others to choose a new powder x-ray diffractometer. We are currently looking at a Panalytical X'Pert Pro in alpha-1 configuration, and I would like to know if anyone has any experience with this for Rietveld refinements? For other reasons we would like to keep the sample flat, but then we could not use the alpha-1 configuration, so instead we would consider a hybrid monochromator - I would like to ask if this monochromator makes it impossible to do Rietveld? Also, any other insights and suggestions for systems that are competitive with these would be welcome. We are at an early stage and have time to think about this, will be looking at other possibilities, and I would appreciate any suggestions. Incidentally, I received some data on quartz (supplied by me) from the alpha-1, and refined it using GSAS. The refined structural and thermal parameters are fine, but I get strange r-factors, with wRp of 25% and crystallographic R also near 25%, chi**2 of 0.5. Does anyone know why this might happen? With my old Siemens D5000 I easily get down to 10% or better in the r-factors right away, so something obvious must be wrong. Thank you, - Kurt Leinenweber William Bisson -- Webmaster and Administrator - CCP14 and BCA http://www.ccp14.ac.uk http://bca.chem.ucl.ac.uk/
Re: advice on new powder diffractometer
Dear Kurt, The only thing that I remember is to change cos(2theta) of monochromator in polarization factor or precisely calculate the polarization factor for n-reflection in the monochromator. Regards Sasaki Kurt Leinenweber escreveu: Hi All, I am attempting to assist others to choose a new powder x-ray diffractometer. We are currently looking at a Panalytical X'Pert Pro in alpha-1 configuration, and I would like to know if anyone has any experience with this for Rietveld refinements? For other reasons we would like to keep the sample flat, but then we could not use the alpha-1 configuration, so instead we would consider a hybrid monochromator - I would like to ask if this monochromator makes it impossible to do Rietveld? Also, any other insights and suggestions for systems that are competitive with these would be welcome. We are at an early stage and have time to think about this, will be looking at other possibilities, and I would appreciate any suggestions. Incidentally, I received some data on quartz (supplied by me) from the alpha-1, and refined it using GSAS. The refined structural and thermal parameters are fine, but I get strange r-factors, with wRp of 25% and crystallographic R also near 25%, chi**2 of 0.5. Does anyone know why this might happen? With my old Siemens D5000 I easily get down to 10% or better in the r-factors right away, so something obvious must be wrong. Thank you, - Kurt Leinenweber -- ___ Dr. José Marcos Sasaki (professor associado) Departamento de Física Universidade Federal do Ceará - UFC Campus do Pici CEP 60455-760 Caixa Postal 6030 Fortaleza - Ceará - Brazil Telefone: 55 (0xx85) 3366 9013 (sala 34) Telefone: 55 (0xx85) 3366 9917 (lab.) FAX: 55 (0xx85) 3366 9450 http://www.fisica.ufc.br/raiosx VoIP (Skype): marcossasaki E-mail: [EMAIL PROTECTED], [EMAIL PROTECTED]
Re: advice on new powder diffractometer
Kurt Leinenweber wrote: ... with wRp of 25% and crystallographic R also near 25% .. Maybe the background is too low :-? Jon
RE: advice on new powder diffractometer
Hi Kurt, If you plot the results in POWPLOT do the statistical analysis what does the delt/sig vs 2-theta plot look like? If weights are right then the band of delt/sig ought to be roughly 2-3 sig wide. Is your data in cpm? Some instruments make that by default. That will mess up the R-factors. Bob From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: Fri 2/15/2008 12:44 AM To: rietveld_l@ill.fr Subject: advice on new powder diffractometer Hi All, I am attempting to assist others to choose a new powder x-ray diffractometer. We are currently looking at a Panalytical X'Pert Pro in alpha-1 configuration, and I would like to know if anyone has any experience with this for Rietveld refinements? For other reasons we would like to keep the sample flat, but then we could not use the alpha-1 configuration, so instead we would consider a hybrid monochromator - I would like to ask if this monochromator makes it impossible to do Rietveld? Also, any other insights and suggestions for systems that are competitive with these would be welcome. We are at an early stage and have time to think about this, will be looking at other possibilities, and I would appreciate any suggestions. Incidentally, I received some data on quartz (supplied by me) from the alpha-1, and refined it using GSAS. The refined structural and thermal parameters are fine, but I get strange r-factors, with wRp of 25% and crystallographic R also near 25%, chi**2 of 0.5. Does anyone know why this might happen? With my old Siemens D5000 I easily get down to 10% or better in the r-factors right away, so something obvious must be wrong. Thank you, - Kurt Leinenweber
PS RE: advice on new powder diffractometer
Dear Rietvelders: P.S. I should mention that with that background of only 2 counts, the maximum signal was around 2000 counts. Thanks to everyone for all the help so far. - Kurt -Original Message- From: Jon Wright [mailto:[EMAIL PROTECTED] Sent: Friday, February 15, 2008 4:05 AM To: rietveld_l@ill.fr Subject: Re: advice on new powder diffractometer Kurt Leinenweber wrote: ... with wRp of 25% and crystallographic R also near 25% .. Maybe the background is too low :-? Jon
