A trick I've used when the fluorescence data are good but affected by overabsorption, and the transmission data are noisy is to
reduce both data sets to post-edge normalized form (0 in pre-edge, oscillating about 1 post-edge), then apply a simple
overabsorption model to 'correct' the fluorescence so it matches the transmission. This is a bit risky in that some of the same
samples which exhibit overabsorption will also show hole (thickness) effect, which looks enough like overabsorption so that you
under-correct the fluorescence.
Otherwise, if the goal is to reduce noise, perhaps the best way is to reduce both to post-edge normalized form, then do a weighted
sum. What weights? Look at the noise level on each and weight it accordingly. One way to do that is to fit a polynomial and look
at the magnitude of the residuals and ask "how many counts/point would an ideal Poissonian counter need to get the residual this
low?". The "2-column Editor" program available from the ALS Beamline 10.3.2 website
http://xraysweb.lbl.gov/uxas/Beamline/Software/Software.htm -> Utilities has that as a capability. If you want to use it, download
the program and reply to me off-line for an explanation of how to use the noise analysis; the on-line doc is outdated and doesn't
include that feature.
mam
----- Original Message -----
From: "Bruce Ravel"
On Monday 10 August 2009 06:13:32 pm jrkizews@ncsu.edu wrote:
Dear XAFS community members, I have a question concerning data merge. We normally collect XAFS data in both transmission mode and fluorescence mode. We normally have to merge a few scans to enhance signal-to-noise ratio. For one particular sample, I want to know if it is legitimate to merge its transmission scans with its fluorescence scans to improve data quality?
Hi Fiona,
There is no a priori reason not to do so and certainly not a numerical reason. From a numerical perspective, you can merge anything with anything!
My one concern is that the fluorescence and transmission data are really equivalent. That is, if the fluo data are affected by significant self-absorption attenuation or if the transmission data suffer from significant pin-hole effects, then you run the risk of degrading the entire data set by doing the merge. But if the data are merely noisy, then I think you are safe doing so.
Think about it this way: when you use a multi-element detector, you are making several measurements which are presumed to be identical. We routinely merge the channels of an MED. You situation is, from a measurement theory perspective, analogous. Assuming your data do not suffer from the problems mentioned above, it seems all right to me.
B
--
Bruce Ravel ------------------------------------ bravel@bnl.gov
National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973
My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
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