Hi George,
On Mon, May 29, 2017 at 5:57 PM, George Sterbinsky <
georgesterbin...@u.northwestern.edu> wrote:
> Hello,
>
> Can anyone explain why the values given by the ion chamber utility in
> Hephaestus differ from those given for a 10 cm He filled ion chamber in
> figure 3.14 of Grant Bunker's Intro to XAFS book at high energy? For
> instance, figure 3.14 indicates an absorption fraction of ~3E-7 at 40 keV,
> whereas Hephaestus indicates 2E-4 at the same energy.
>
>
I believe the difference is probably in whether one considers only the
photo-electric cross-section or also includes the incoherent and coherent
scattering. For very light elements at high enough energy, the incoherent
scattering becomes much more likely than the photo-electric absorption,
Using the Elam et al tables, give (with mu in cm^2/gr):
larch> mu_elam('He', 40000.0, kind='total')
0.1763091369476893
larch> mu_elam('He', 40000.0, kind='photo')
0.00025649564143021724
The question then becomes how are you using this value. To estimate what
fraction of the 40 keV X-ray intensity will exit a 10 cm ion chamber filled
with He, use the total attenuation. But to estimate the photo-current
generated in the ion chamber from that beam, I think you should use the
photo-electric cross-section only.
Hope that helps, or that someone else has better insight,
--Matt
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