[Ifeffit] Ifeffit Digest, Vol 159, Issue 3
Christopher Thomas Chantler
chantler at unimelb.edu.au
Fri May 13 16:43:34 CDT 2016
Dear All, Matteo
It is difficult to do post facto, but if you have a well-defined solid, foil or crystal then mapping the thickness, measuring the mass and hence determining the mass per unit area, for the exact region illuminated by the beam, provides accurate units. Usually you will need to make a measurement of the average m/A from (i) total mass and (ii) total area, and then (iii) map the whole sample with X-ray tomography and (iv) the region illuminated by the X-ray beam.
We have a series of papers which explain more than one methodology for doing that within the XERT technique. If your sample is a solution it is a bit more tricky, and I would refer you to recent JSR and JPCC papers for our Hybrid technique. Both can work well but need a bit more care than a normal XAFS measurement.
Best wishes
Chris
------------------------------------------------------------
Christopher Chantler, Professor, FAIP
Editor-in-Chief, Radiation Physics and Chemistry
Chair, International IUCr Commission on XAFS
President, International Radiation Physics Society
School of Physics, University of Melbourne
Parkville Victoria 3010 Australia
+61-3-83445437 FAX +61-3-93474783
chantler at unimelb.edu.au chantler at me.com
http://optics.ph.unimelb.edu.au/~chantler/xrayopt/xrayopt.html
http://optics.ph.unimelb.edu.au/~chantler/home.html
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From: Ifeffit [ifeffit-bounces at millenia.cars.aps.anl.gov] on behalf of ifeffit-request at millenia.cars.aps.anl.gov [ifeffit-request at millenia.cars.aps.anl.gov]
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Subject: Ifeffit Digest, Vol 159, Issue 3
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Today's Topics:
1. How to assign measure unit to XAFS data (Matteo Busi)
2. Re: How to assign measure unit to XAFS data (Alexey Boubnov)
3. Re: How to assign measure unit to XAFS data (Matt Newville)
----------------------------------------------------------------------
Message: 1
Date: Fri, 13 May 2016 02:11:39 +0200
From: Matteo Busi <basebush at gmail.com>
To: ifeffit at millenia.cars.aps.anl.gov
Subject: [Ifeffit] How to assign measure unit to XAFS data
Message-ID:
<CAOyEiGg1N7wL-tnzPXFBCrUYQD=6=bvsRFnDNEe3ZaCeguFU5Q at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi Bruce, Matt, and mailing list readers,
I have one question regarding the XAFS data. I have collected absorption
coefficient in fluorescence regime of a CuSO4 penta hydrate sample(however
my question can be generalized to any compound).
Through Athena I then obtained the x\mu(eV) spectra as well as \chi(k) and
\chi(R). I am wondering how can I assign a unit to the x\mu values. I would
like to have it in the cm^-1 or cm^2 kg^-3 unit so than I can match it with
tabulated data for the Copper absorption coefficient and proceed with my
studies.
In case a treatment is needed for the normalized \mu , that would also suit
my situation.
My idea was to divide by the thickness of the sample but I don't find a
reasonable justificiation for this.
I really hope I made the question clear to any reader this time, and if
that is not the case I apologyze in advance.
Best Regards
Matteo
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Message: 2
Date: Fri, 13 May 2016 08:29:15 +0200
From: Alexey Boubnov <alexeyboubnov at gmail.com>
To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
Subject: Re: [Ifeffit] How to assign measure unit to XAFS data
Message-ID:
<CAJZPK0VrRQ2Ad-Mxr2q4kdZorjdT_ceaVroPO3m57LYuS_CiAg at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Dear Matteo,
?x that you measured is dimensionless: x in the thickness (in cm) and ? the
Absorption coeffiecient (per cm). Indeed, if you want to know the
Absorption coefficient ? of your sample, you should divide ?x by x.
All the best,
Alexey
2016-05-13 2:11 GMT+02:00 Matteo Busi <basebush at gmail.com>:
> Hi Bruce, Matt, and mailing list readers,
>
> I have one question regarding the XAFS data. I have collected absorption
> coefficient in fluorescence regime of a CuSO4 penta hydrate sample(however
> my question can be generalized to any compound).
> Through Athena I then obtained the x\mu(eV) spectra as well as \chi(k) and
> \chi(R). I am wondering how can I assign a unit to the x\mu values. I would
> like to have it in the cm^-1 or cm^2 kg^-3 unit so than I can match it with
> tabulated data for the Copper absorption coefficient and proceed with my
> studies.
>
> In case a treatment is needed for the normalized \mu , that would also
> suit my situation.
>
> My idea was to divide by the thickness of the sample but I don't find a
> reasonable justificiation for this.
>
> I really hope I made the question clear to any reader this time, and if
> that is not the case I apologyze in advance.
>
> Best Regards
> Matteo
>
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
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Message: 3
Date: Fri, 13 May 2016 07:13:08 -0500
From: Matt Newville <newville at cars.uchicago.edu>
To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
Subject: Re: [Ifeffit] How to assign measure unit to XAFS data
Message-ID:
<CA+7ESbq84ZpYx9DvP8JvM7yNzO0U31jFCZZ-_X43b21sM43+og at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi Matteo,
On Thu, May 12, 2016 at 7:11 PM, Matteo Busi <basebush at gmail.com> wrote:
> Hi Bruce, Matt, and mailing list readers,
>
> I have one question regarding the XAFS data. I have collected absorption
> coefficient in fluorescence regime of a CuSO4 penta hydrate sample(however
> my question can be generalized to any compound).
> Through Athena I then obtained the x\mu(eV) spectra as well as \chi(k) and
> \chi(R). I am wondering how can I assign a unit to the x\mu values. I would
> like to have it in the cm^-1 or cm^2 kg^-3 unit so than I can match it with
> tabulated data for the Copper absorption coefficient and proceed with my
> studies.
>
The analysis tools treat "xmu" as a dimensionless value. To get values
for the mu in cm^-1 or grm/cm^2, you'll have to convert the measured values
used for XAFS analysis (say, readings from ion chambers) into actual X-ray
intensities, and try to account for anything else in the beam path that
will attenuate the X-rays. For measurements made in transmission, this can
be done. In fact, for most transmission measurements, the *change* in
-ln(I0 / I1) is usually a very good measure of the change in absorption of
the sample, so that if the observed jump is a value of 2, the sample really
did change its absorption by Delta_xmu = 2. But the absolute values for
xmu are definitely not in meaningful units. Typically, the photo-current
from the ion chamber or diode is converted to a voltage and then summed
over some time period in a data acquisition system. You'd have to know
the details of that system to convert the "I0" and "I1" numbers into actual
beam intensities.
For fluorescence measurements, the situation is worse. You'd need a very
good measure of the solid angle of your detector, fluorescence efficiency,
and the attenuation of the fluorescence getting from the sample to the
detector. Depending on the detector type, you'd also need a good
assessment of the backgrounds. It's probably not impossible, but I think
it would not be easy unless that was the actual point of the experiment and
you made many careful measurements of the detection system. If you're
asking this question here, you'd didn't make those measurements.
>
> In case a treatment is needed for the normalized \mu , that would also
> suit my situation.
>
> My idea was to divide by the thickness of the sample but I don't find a
> reasonable justificiation for this.
>
If you're measuring in fluorescence, dividing by the thickness of the
sample is not what you want.
>
> I really hope I made the question clear to any reader this time, and if
> that is not the case I apologyze in advance.
>
>
Cheers,
--Matt
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