[Ifeffit] Dodgy Edge Steps

[Kelly, Shelly D.]

Hi Lachlan,

 

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 at millenia.cars.aps.anl.gov [mailto:ifeffit-bounces at 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
Lachlan





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