Hi
The reason that you want to keep the total absorption less than 3 is
because the number of transmitted x-rays becomes small. I=I0 exp(-mu x). So
for mu x =3 the number of transmitted x-rays are 5% of the incident x-ray
intensity. If you have a lot of x-rays then you can measure spectra with greater
total absorption. At most second generation sources we have around 10^6 x-rays
/second. That means that only 5 x10^4 x-rays are measured in It for a sample
that has a total absorption of 3. Since the statistical noise goes like the
square root of N, you will have 5% noise by measuring only 1 second per point,
so you will have to measure about 25 sec per point to get usable EXAFS data. As
the signal in It becomes smaller, you will find that the oscillatory part of
the absorption spectra becomes damped because you can not accurately measure the
fluctuations in the transmitted signal.
If you have a lot of x-rays and the total absorption is dominated by
the element that you measured, your data might be salvageable. Look for a
dampening affect. If you see that when comparing to your other data then you
are in trouble. I don’t know how to reliably fix it. You can use Hephaestus
to calculate the number of x-rays per second in It from the voltage and
detector gain.
Cheers,
Shelly
From:
ifeffit-bounces@millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Stefan Mangold
Sent: Monday, April 28, 2008 11:46
AM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Dodgy Edge
Steps
Hi,
the edge step should be less than 1; the edge step at white line
should be not higher than 1.5 and die maximum absorption should not be higher
than 2.7 (max. 3) (reason is the signal to noise ratio). The reason for
limiting the edge jump are none linearities of your detector systems. So I
would not use the data for further analysis.
best regards
Stefan
--
Dr. Stefan Mangold
Institut für Synchrotronstrahlung
Forschungszentrum Karlsruhe
Am 28.04.2008 um 18:27 schrieb LACHLAN MACLEAN:
Hi All
Long-term lurker, first time poster.
I've recently collected some EXAFS data with a large edge step (~3.4) and for
some reason didn't pick up on it until it was too late. I've been told
that ideally we would want an edge step of 1 and anything above ~1.5-2 is too
high. I'm wondering if there is a way to salvage the dataset in order to
compare it with EXAFS data that I collected from two other samples (all three
are supposed to be synthetic goethite). The first two samples that I want
to compare have edge steps of about 0.35.
I know Athena allows one to adjust the edge step but wonder how appropriate
this is and how much change this would cause the EXAFS data? Trying to
adjust the edge step from 3.4 down to ~1 or so seems to be quite a jump.
I'm curious whether there is a way to salvage this data rather than wait until
the next beamrun.
Any help would be appreciated.
Cheers
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