[Ifeffit] Self-Absorption Corrections
farges at univ-mlv.fr
Tue Jul 29 02:39:59 CDT 2003
>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
>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
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
>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,
>(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
>> 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.
>>> 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).
>> 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 !
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