[Ifeffit] Self-Absorption Corrections

[Francois Farges]

>Peter, Francois
>
>Thanks for the thought on self-absorption corrections.  I agree that
>Corwin's work is an excellent place to start.  Incorporating
>self-absorption corrections that handle XANES (as from Haskel's FLUO
>or Sam Webb's SixPack) is also important.
>
>Peter Pfalzer wrote:
>>  Corwin Booths approach to selfabsorption correction seems to be very
>>  nice. I think that especially its possibility to give up the "infinite
>>  sample thickness" limitation could be an important improvement over the
>>  previous approaches.
>>  Still, it makes two (more or less implicit) assumptions:
>>  * the detector surface has to be parallel to the x-ray beam (phi + theta
>>  = 90 deg)
>>  * the detector has to have a neglectable solid angle
>>
>>  I'm not sure if these two assumptions hold for most fluorescence
>>  experiments?
>
>Francois Farges wrote:
>>>  surely no !  (cf ID21 at ESRF) and most future expeirments won't
>>>  be that "ideal" for sure.
>
>I would expect that Peter's assumptions do, and will, hold for most
>measurements.  Perhaps I'm misunderstanding Francois, but I thought
>that ID21 (a micro-fluorescence line) uses solid-state detectors,
>and nominally at phi+theta=90.  Is that not so?

not always, Matt. that's even rarely because of steric effects. 
pin-diods are the thing to use at ESRF because that's the rule (...). 
so pin-diods are used most of the time, on either micro BL (ID 21 and 
22).

and anyway one needs a code with all the angles allowed to vary. when 
you use furnaces, cells and so forth you always have some weird 
designs that are not the "case-study" like in books. future will be 
full of such devices just because the ideal experiment will never 
exists.

>
>Anyway, the 'phi+theta=90' approximation is still the norm for
>fluorescence ion and solid state detectors, simply because reducing
>elastic scatter is important.  It is not always the case, but for
>solid-state detectors and ion chambers it is definitely most common.

I never get such geometry because, inside the Lytle for instance, 
even using the transmission setup while collecting the fluo the 
average solid angle is not 90 degrees but 88 degrees - based on 
self-absorption correction of the fluo vs. the transmission. and it 
makes a different when you want some accuracy on pre-edge intensities 
of number of neighbors.

I never had to use sold state detectors because when my 
self-absorption arises, it's because the local concentration of the 
studied element is high and then such detectors saturates by a factor 
of so much so we need to get rid of it. So using pin diods or Lytle 
with some lighter gas.

>
>An important counter-example (and possibly one to become more widely
>used) is when using crystal analyzers (either in Bragg or Laue
>geometries) to select a fluorescence line.  For these, eliminating
>the elastic scattering with geometry is not so important and other
>considerations determine the analyzer/detector geometry.
>
>The 'small solid angle' argument seems mostly safe to me too.  If I
>understand the papers by you, Corwin, Troger, etal, and Brewe, etal,
>this is not a huge effect near 'phi ~= theta ~= 45' (where ~= means
>+/- 15degrees'), and becomes most important near phi~=0 or phi~=90

this does not work for cations above, say, Fe/Ni and when the local 
concentration (i.e., at the impact of the spot) is above, say 30 
mol.%. I've tons of data that I ve collected to try understanding 
these effects (because no code really works in the detail). Even at 
90°, you get significant self-abs. effects, in ZrO2 at the Zr K-edge 
for instance. Troger et al is highly tuned for light elements (Si, 
namely).


>(grazing incidence or grazing exit). Corwin wrote: '... for detector
>geometries where phi+theta=90, we find the maximum error in
>(sin(phi)/sin(beta)) is on the order of 1-2% even for
>delta_theta=5degrees at theta=80degrees'.  I interpret that to mean
>that even for fairly large opening angle of the detector the effect
>should be small, except for the grazing incident/exit geometry.  Is
>that your understanding too?
>
>If so, I'm willing to neglect the grazing incident/exit geometries
>(at least for now), and expect that people who use grazing incident
>or exit usually know what they're doing and how to make these
>correction themselves.
>
>Peter wrote:
>>  When I collected my last fluorescence data a couple of years ago, large
>>  solid angle detectors (like Lytle-detectors) were still in use. I have
>>  shown that Troegers approach to selfabsorption correction can be
>>  generalized for large detector surfaces (Phys. Rev. B 60, 9335 (1999)).
>>  In principle this should be also possible for Corwin Booths formula.
>
>Francois wrote:
>>>  except for cations above than Zr.
>
>Maybe I'm misunderstanding Francois on this, but I don't see how the
>self-absorption correction depends on Z, except for implicit (and
>known) Z dependence of mu.  Do Corwin's approximation break down at
>high Z?  I would have guessed self-absorption got worse at lower
>energies.  Am I missing something?

I do not do any theory because I tjust don't know it. Also, as a 
modest scientists, I would always think that we might not fully 
understands everything. So to skip my ignorances, I just collected 
Mo's of data at various edges (Ti, Fe, Mn, Zr, Mo, Th, U etc) that I 
collected in fluo mode at many angles for many compounds with various 
concentrations and compared to transmission (and collected at 
different beamlines with different detectors to get a clue on 
"localized effects" too). and I used (and abused !!) the Troeger 
trick (as it used to call it when I did all these test in the early 
90's for our high temperature furnace).  So I can really tell you 
that the Troeger trick does not work anymore above Zr (at mpderate 
concentrations such as in ZrSiO4). at Ni it fails for higher conc. of 
Ni (such as in NiO, but Ni2SiO4 is fine). etc.

if this is not due to self-absorption (despite it was clearly 
affected by the angles and FLUO did an fairly good job to correct but 
not perfect, esp. in the pre-edge region), I would be pleased to 
learn the origin !

anyway I always had a very empirical approach to it (because Goulon's 
paper is just indigestable to me at this was before Troeger's work). 
but I could never really detect a single theory that works (just too 
much for me !). And i think we should all be open-minded as we might 
not have understood everything.

But we will agree, I think, to say that what matters for the user is 
not the theory (or the theories) but something that can correct 
efficiently and correctly. and any attempt is great. but the task 
might be huge. and I would say that experiments should drive and help 
theoreticians (and not vice-versa).

>
>Peter wrote:
>>  But when integrating over large solid angles, the exact geometry of the
>>  experimental setup plays a crucial role in determining the
>>  selfabsorption correction and I doubt that a useful implementation into
>>  iFeffit would be possible.
>>
>>  If, however, everyone is using solid state detectors now, I would say
>>  that implementing Corwin Booths code into iFeffit could be worth the effort.
>
>How large of a solid angle do you mean?  I'd expect a few percent of
>4pi to be typical for both ion chambers and solid-state detectors. 
>Everyone is definitely *not* using solid state detectors, but
>between those and relatively small fluorescence ion chambers (e.g.,
>Lytle chambers), that does seem like most fluorescence work done. 
>
>Anyway, I agree (I think with both you and Francois??) that the
>'large solid angle' correction can be postponed at least until
>something works reasonably well.
>
>I also definitely agree with Francois that correcting XANES is very
>important.  It sure would be nice to have a complete self-absorption
>correction for both XANES and EXAFS....

thanks Matt !

-- 
Francois FARGES

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