Re: [Ifeffit] Brain teaser

Hi Anatoly, I think Robert or Matthew made this point, but if set up for Pd, the mirror angle may have been chosen to reject ~70 keV, but possibly not 36 keV -- the harmonic at the Au edge. Do you know what the mirror angle was? The Ar-filled I0 would be very efficient at absorbing 12 keV, and only pretty efficient at absorbing 36 keV. That would leave a more harmonic-rich beam exiting I0 and hitting the sample than entering I0. The good news is that the dense Pd/Au sample would be efficient at absorbing 36 keV too (but it was ~1 absorption length at 24 keV?) too. Mono reflectivity of 36 keV vs 12 keV might also factor in. If you were not deliberately detuning but the crystals were slightly misaligned, the harmonic content may change significantly over the scan range. I would not guess that to dominate, but maybe it factors in. On Thu, Jun 15, 2023 at 9:54 AM Anatoly Frenkel < anatoly.frenkel@stonybrook.edu> wrote:

Thank you, Matt. Ion chambers were filled with 90% Ar, and Pt coating was used, because we were measuring Pd K edge for the project, but we decided to look at the Au edge for testing purposes.

Anatoly

On Jun 15, 2023, at 10:45 AM, Matt Newville wrote:

 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/...

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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