[Ifeffit] Peaks in MCA spectra

Tue Apr 1 10:45:31 CDT 2014

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>> 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>> 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>> 630-252-0431 <tel:630-252-0431>
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