[Ifeffit] Brain teaser

Thu Jun 15 15:27:35 CDT 2023

HI Mike, it is dilute Pd in Au nanoparticles deposited on SiO2 with about
5% metal weight loading.
In my view, the lack of curvature in transmission (the original problem in
my post) is a simple consequence of two opposite trends, 1) the curving up
of the transmission signal by the same reason (yet to be identified, which
is something I will try to, after getting useful hints from many people
here) as the curving up of the incident beam and 2) the opposite trend due
to the greater removal of the photons at higher energy by I0 detector. The
two opposite trends tend to cancel each other.

Anatoly

On Thu, Jun 15, 2023 at 4:14 PM Mike Massey <mmassey at gmail.com> wrote:

> A question because I'm not really super clear on how harmonics work:
>
> (I'll get to the question in a moment...) I note that I0 starts to go
> non-linear around 12200-12400 eV and stays that way. So an increase
> (proportional?) in the amount of signal in It might be happening at around
> the same point.
>
> Could Pd fluorescence into It be impacting the signal in It? I realize
> that the Pd emission lines are very low in energy but they are around 3k eV.
>
> I'm not super familiar with alloy systems but I did have an issue once
> upon a time with back fluorescence into an ion chamber and things went a
> bit wonky. This wouldn't exactly be "back" fluorescence, but if the system
> is mostly Pd with a little bit of Au, I guess...? And it might be easy
> enough to test, just put a high-pass filter in front of It?
>
> I got nothin' but it's kinda fun to take a shot. Cheers,
>
>
>
> Mike
>
>
>
>
> On Jun 16, 2023, at 03:26, Matt Newville <newville at cars.uchicago.edu>
> wrote:
>
> 
> 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 at 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 <newville at cars.uchicago.edu>
>> 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/1000/51000/50/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 at 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
>>>
>>> <image.png>
>>>
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>>
>>
>> --
>> --Matt Newville <newville at cars.uchicago.edu> 630-327-7411
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>
>
> --
> --Matt Newville <newville at cars.uchicago.edu> 630-327-7411
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