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
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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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@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 Lachlan _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Hi all, I want only to add something to your discussion first - are we talking about edge step in the linear or logarithmic scale - ln(3)=~1, so jump ~3 can be ok - if it is in linear scale second - for higher absorption you can expect the self-absorption effect, which also decreases the amplitude of EXAFS oscillations. To correct it you can try to use Menu/ Data/ Self Absorption darek -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of LACHLAN MACLEAN Sent: Monday, April 28, 2008 6:28 PM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Dodgy Edge Steps 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
Lachlan, Sorry for jumping in late on this discussion, but I was out of town. I did want to add something to this discussion: I think an edge step as large as 3.4 could very well give fine data. I've seen many cases of good data on standards with edge jumps > 3. The issues addressed so far (that so many x-rays are absorbed that you'd get noisy data from counting statistics, and thickness / pinhole effects) are good points, but not necessarily fatal. If you were really running out of x-ray intensity, the data would probably look very noisy anyway. You didn't mention the detectors you used, so I'll assume it was a standard ion chamber. These tend to not work very well when the x-ray shot noise is an important noise source (even for fluorescence "Lytle" chambers), so I'd doubt this is a problem and assume you had enough x-rays. The rest of the counting chain could cause problems. Generally, current amplifiers aren't linear over many decades, so we're often fighting trying to find the "best" gain settings for a particular *total absorption*. An edge jump of 3.4 (~0.03 transmitted intensity above edge as below edge) could be OK, but it's starting to get close to amplifier limits. The signal from current amplifiers are often sent through a V2F (Voltage to Frequency Converter) which can have remarkably low resolution, so that interdigitation becomes a problem for small signals: <side track> (AKA I learned this the hard way) The system I use (and common), uses a current-to-voltage amplifier that saturates around ~6 Volts, so I try to not have voltages higher than 5 Volts. This signal goes to a V2F converter where 1 V gives 100000 counts per sec (this is the integer recorded as Intensity). If the voltage is 0.01Volts, I'd get 1000 counts per second. At that voltage, it doesn't matter what the x-ray intensity is or the current amplifier settings, the "noise level" WILL BE 1e-3 (I can record 999, 1000, or 1001, but not anything in between. I could count longer than a second, but that isn't a huge win). So, to be safe, I really want the voltage (never mind the x-ray flux) to be between 0.05 and 5 Volts. That's only two decades (so, a max edge step of 4.6 = ln(100)), and I have to fiddle with the amplifier gains so it doesn't saturate below the edge or go too low above the edge. But: if the data looks OK, the counting chain is probably not killing your data. The main issues with large edge steps are a) pinholes and b) harmonics. If the sample has pinholes and you measure an edge step of 3.4, it's possible that your sample is much much thicker, and that all the transmitted intensity you do see is going through the pinholes. This might be trickier to diagnosis after the fact, but if the data looks like real EXAFS data, it is probably OK, though it may need amplitude corrections. Similarly, if the x-ray beam had significant harmonics (which are seen fairly efficiently by gas-filled ion chambers), it's possible that your sample is much much thicker and all the transmitted intensity you do see is harmonics getting through your sample. Again, if this was the case, I'd expect the data to look completely useless. So, my advice would be: if data with an edge step of 3.4 *looks* OK, it probably is OK. There may be pinhole / thickness effects. If you're using the standard to extract S02 or amplitude factors, you may want to check the literature on how to correct for this. If other standards can give you S02, and you're using this one to see if you can model the distances, I'd guess the data was still useful. --Matt
participants (5)
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Kelly, Shelly D.
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LACHLAN MACLEAN
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Matt Newville
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Stefan Mangold
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Zajac, Dariusz