[Ifeffit] Self-Absorption Corrections

[Peter Pfalzer]

Hi Matt,


> 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 don't see the Z dependence either.


> 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?

Sorry to say, but I'm a bit less optimistic with the small solid angle 
approximation.
Corwins 1-2% are only true for large theta, for small theta, the error 
in ( sin(phi) / sin(theta[+-delta_theta]) ) becomes large (but small 
theta also means a normal incidence/grazing exit geometry where 
selfabsorption is small and may be almost neglectable). Anyway, for 
theta = 45 and delta_theta=5 I calculate a reasonable error of >~4% in 
the correction factor.

 > 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.

I have worked with an ion chamber of R=40mm at a distance of d~60mm, 
giving a solid angle of ~ 12% * 4pi, and exit angles of 10 deg < theta < 
80 deg. The solid state detectors I know (but I don't know many) have a 
roughly estimated R~5-10mm, resulting in a solid angle of < 1% * 4pi. 
In the latter case, no integration is necessary, of course. In my first 
example, without integration, the correction will be wrong by >~10% (if 
the sample is located in front of the center of the detector).


 > 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.
 >
You're right. The "simple" correction with Corwin's approach will give a 
correction factor of (at least) the right order of magnitude with any 
detector. And as the selfabsorption doesn't do anything but smoothly 
reducing the EXAFS amplitude (at least if the sample's not too thin), 
one should be able to fit the remaining deviation with a slight change 
in S_0^2 - but keeping in mind that some cases could require more 
thorough treatment.

Peter


-- 
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Peter Pfalzer
Universitaet Augsburg                    Tel: +49-821-598-3215
Lehrstuhl fuer Experimentalphysik II     Fax: +49-821-598-3411
Universitaetsstr. 1
D-86135 Augsburg
Germany                   Peter.Pfalzer at physik.uni-augsburg.de
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