I'm not sure why the intensity would go up unless the ion chamber was poorly set up. But, as others have pointed out, the mirror reflectivity for a Pt mirror should not change significantly over this energy range - the energy range is not that close to the Pt L3 or L2 edges. Depending on where it was located, fluorescence from the Pt mirror might pollute the signal in the I0 ion chamber, but that would also likely be a fairly constant background.

But, why would you fill the I0 ion chamber with Argon? A 10-cm ion chamber filled with Ar would absorb about 50% of the beam at 12 keV. Even at 24 keV, that would absorb 8% of the beam - not necessarily a problem but also probably generating at least a micro-Amp, so way more signal than you would need.

For mirror reflectivity curves, allow me to humbly remind everyone of

https://xraydb.xrayabsorption.org/reflectivity/Pt/21.45/2.5/10/s/1000/51000/50/platinum/linear

which is both interactive and works with X-ray energies above 30 keV.

On Wed, Jun 14, 2023 at 7:28 PM Anatoly Frenkel <anatoly.frenkel@stonybrook.edu> wrote:

Hello, all. It is a low- to medium- level brain teaser.

Pt-coated collimating mirror was in place for Pd K-edge measurement, but Au L3-edge of Pd-Au alloy was measured (for testing purposes). I0 and It detectors were both Ar filled ionization chambers. Because of the energy dependence of reflectivity of the Pt mirror, I0 intensity was strongly nonlinear (blue curve). However, the transmission intensity in the It detector was almost linear (red curve). Why?

Anatoly

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