[Ifeffit] Peaks in MCA spectra

Matthew Marcus mamarcus at lbl.gov
Tue Apr 1 17:07:14 CDT 2014


I don't think so.  This may be a better question for M. Newville, or whoever wrote the program.
	mam

On 4/1/2014 9:56 AM, George Sterbinsky wrote:
> Hi Matthew,
>
> Hephaestus shows the strength of the Ll emission to be about 10% of the La2 emission. When fitting the spectrum as a sum of Gaussians, I find that the area of of the Gaussian used to fit the Ll is 24% of that used to fit the La2. What would cause the ratio determined from tabulated data to differ from that found in the data I collected? Am I misunderstanding the meaning of the "strength" value in Hephaestus?
>
> Thanks,
> George
>
>
> On Tue, Apr 1, 2014 at 11:45 AM, Matthew Marcus <mamarcus at lbl.gov <mailto:mamarcus at lbl.gov>> wrote:
>
>     OK, I think the "other Matt" has solved it - it's the L1 peak.  I was confused by an inaccurate attempt at reading the energy scale.
>     Also, I somehow didn't read correctly the branching ratio from Hephaestus.
>              mam
>
>
>     On 3/31/2014 8:57 PM, George Sterbinsky wrote:
>
>         Hi Matt,
>
>         Thanks for your reply. Please see below.
>
>         On Mon, Mar 31, 2014 at 8:10 PM, Matt Newville <newville at cars.uchicago.edu <mailto:newville at cars.uchicago.edu> <mailto:newville at cars.__uchicago.edu <mailto:newville at cars.uchicago.edu>>> wrote:
>
>              Hi George,
>
>              Calibrated spectra would help, but if we guess the calibration is 0.56
>              bins/ eV, then we'd have
>
>
>         I've attached a plot of the data showing the x-axis in keV. Also, as requested by Zack, I've attached a two column data file.
>
>
>                 line             E (eV)       bin #
>              ------------------------------__---------------
>              Co La1,2       775          434
>              O Ka1,2         525          294
>              C Ka1,2         277          155
>
>              which looks  pretty good.     This puts the the unknown peak near
>              380/0.56 which is 678 eV.   Tthat's very close to Co Ll (M1->L3),
>              which is at 677 eV, and should be a bit less than 10%  of Co La1 and
>              La2, which is roughly right.
>
>
>         Based on fitting the spectrum with Gaussians, the Co LI is 24% of the La2. As you mention, a value of roughly 10% is expected. What could cause such a discrepancy?
>
>
>              So, I think it's Co Ll.  That says the sample is just Co, C, and O.
>              Is that reasonable?
>
>
>         Yes, it is. I think maybe a little fluorine too, but it is very weak as I mention in my response to Matthew.
>
>
>              What surprises me is that there is no signal from the elastic peak.
>              Was that somehow filtered out?
>
>
>         No, if there is an elastic peak it is probably lost under the Co La2.
>
>
>         Thanks,
>         George
>
>              The fact that the counts don't go to
>              zero between C and O could be many factors, including incomplete
>              charge collection.  This (and Compton scattering) generally make peaks
>              have a slightly non-Gaussian shape, with a low-energy tail.
>
>
>
>
>
>
>              Cheers,
>
>              --Matt
>
>              On Mon, Mar 31, 2014 at 5:01 PM, George Sterbinsky
>              <GeorgeSterbinsky at u.__northwestern.edu <mailto:GeorgeSterbinsky at u.northwestern.edu> <mailto:GeorgeSterbinsky at u.__northwestern.edu <mailto:GeorgeSterbinsky at u.northwestern.edu>>> wrote:
>               > Hello,
>               >
>               > I am writing with a general XAS question. It does not necessarily pertain to
>               > Ifeffit, however, I think the topic is something some, maybe most, list
>               > members will be knowledgeable about. So it seems like this list is a good
>               > place to post this question.
>               >
>               > On to the question. I have attached a plot of a MCA spectrum collected with
>               > a vortex silicon drift detector. The spectrum is actually the average of
>               > several spectra, all collected in the post edge region of the Co L-edge. The
>               > spectra were averaged to reduce noise. The three peaks result from
>               > fluorescence from carbon, oxygen, and cobalt. Low-energy shoulders on the Co
>               > and O peaks are also observed. These can be seen as the regions of the
>               > spectrum that are not well reproduced by the fit. The main reason I included
>               > the fit in the plot is to illustrate the presence of these shoulders,
>               > particularly in the oxygen florescence, where the additional intensity is
>               > not so obvious.
>               >
>               > I am writing to see if anyone has any suggestion as to what the origins of
>               > these peaks might be. They are not due to additional elements, as they
>               > appear at the same incident energies as the main florescence peaks, i.e. the
>               > Co shoulder appears at the same incident energies as the main Co peak, and
>               > the O shoulder appears at the same incident energies as the main O peak. It
>               > is possible that the peaks result form other transitions. Considering Co,
>               > the main peak is due to L3/L2-M4 transitions, and the shoulder is in a
>               > position that could be consistent with L3/L2-M1 transitions. However, by
>               > fitting the peaks with Gaussians, one finds an area for the shoulder that is
>               > about 25% of the area of the main peak. This is significantly larger than
>               > what one might expect from tabulated transition strengths like those given
>               > in Hephaestus.
>               >
>               > To summarize, does anyone know what these shoulders might result from if not
>               > lower energy transitions? If they are low energy transitions, why would the
>               > relative transition strengths differ from tabulated values?
>               >
>               > Thank you,
>               > George
>               >
>               >
>               >
>               >
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>              --
>              --Matt Newville <newville at cars.uchicago.edu <http://cars.uchicago.edu> <http://cars.uchicago.edu>> 630-252-0431 <tel:630-252-0431> <tel:630-252-0431 <tel:630-252-0431>>
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