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
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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
