Hi George,

On Mon, May 29, 2017 at 5:57 PM, George Sterbinsky <GeorgeSterbinsky@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