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From the Rietveld Mailing List
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From: W.I.F.David@rl.ac.uk To: rietveld_l@ill.fr Subject: RE: As sent to the neutron mailing list Date: Fri, 24 Sep 1999 15:56:19 +0100 Dear Frank, I've just received an e-mail from the Rietveld mailing list courtesy of Paolo Radaelli that has an HRPD diffraction pattern of MgO - it's big but hopefully you will be able to plot the file. This should help you get a feel for the data and the nature of the binning. Now concerning the binning of time-of-flight data on HRPD ... As I said yesterday the delta d /d resolution is constant - the d-spacing is proportional to the time of flight and so the time width (delta t) is proportional to t. We thus bin our data so that (delta t)/t is constant - on HRPD this is usually 1 x 10-4. On a few occasions, when I have had extremely sharp peaks then I have binned more finely at 5 x 10-5. So how many points does that give - well we generally have a 100,000 or 200,000 microsecond starting somewhere between 15,000 (d = 0.25A in backscattering) and 30,000 (d = 0.25A in backscattering) microseconds. Since t(i+1) = (1+w)*t(i) where w = (delta t)/t then it's easy to show that since w<<1 then the number of points N is very closely given by Nw = ln(tmax/tmin) For w=10-4, tmin=30000 and tmax=130000, this gives N=14663 - this is typically the sort of number that we work with. For the most heroic case, w=5x10-5, tmin=15000 and tmax=215000, we get 53252 but then that's the price you pay for high resolution. When I was last at Andy Fitch's powder diffractometer at the ESRF in Grenoble, we collected datasets that contained 20000 points. Hope that clears things up. Bill |

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