[Ifeffit] Hephaestus ion chamber absorption

[George Sterbinsky]

Thank you Matt.

George

On Mon, May 29, 2017 at 10:02 PM, Matt Newville <newville at cars.uchicago.edu>
wrote:

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