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