[Ifeffit] Peaks in MCA spectra
GeorgeSterbinsky at u.northwestern.edu
Tue Apr 1 11:56:25 CDT 2014
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?
On Tue, Apr 1, 2014 at 11:45 AM, Matthew Marcus <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.
> 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.
>> 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.
>> 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,
>> > 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
>> > 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
>> > 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
>> > 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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