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 70000 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
--
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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
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