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 10000 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 100000 and the background becomes 10000, 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 B peak height PH 0.02 95 8580 0.04 320 18900 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 70000 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 ****************************************************************************
