[Ifeffit] Self-Absorption Corrections

[Matt Newville]

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?

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.  
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
(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?

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 again for the insight!

--Matt