Re: [ccp4bb] Reason for Neglected X-ray Fluorescence

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CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

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Subject: Re: Reason for Neglected X-ray Fluorescence
From: marc {- dot -} schiltz {- at -} EPFL {- dot -} CH marc {- dot -} schiltz {- at -} EPFL {- dot -} CH
Date: 2009-04-22

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Subject: Re: How small is a microbeam?
From: "Sanishvili, Ruslan" rsanishvili {- at -} ANL {- dot -} GOV
Date: 2009-04-22

Subject: Re: Reason for Neglected X-ray Fluorescence
From: Ethan Merritt merritt {- at -} U {- dot -} WASHINGTON {- dot -} EDU
Date: 2009-04-22

On Wednesday 22 April 2009 13:22:41 marc.schiltz@epfl.ch wrote:
> Quoting Ethan Merritt :
>
> > On Wednesday 22 April 2009 09:23:19 Jacob Keller wrote:
> >> Hello All,
> >>
> >> What is the reason that x-ray fluorescence is neglected in our experiments?
> >> Obviously it is measureable, as in EXAFS experiments to determine
> >> anomalous edges,
> >> but should it not play a role in the intensities as well? What am I missing?
> >
> > Fluorescence is directly proportional to f", so in one sense we do account
> > for it in any calculation that includes the anomalous scattering terms.
> >
> > If you were thinking of direct contribution of the fluorescent X-rays to the
> > measured Bragg peak - that is negligible. Those photons do not retain the
> > momentum vector of the original incident photon, and are emitted in all
>
> I am not sure whether this is a good explanation. The elastically
> scattered photons (which make up the Bragg peaks) also do not not
> retain the momentum of the incident photon.

Poorly phrased, I guess. My thought was that elastic scattering from air
favors smaller deflection angles, which means more of the photons continue
forward to strike the detector than scatter at large angles and miss it.
Is this not correct?
There is no such bias for the fluorescent photons, so they are as likely
to travel back towards the source as towards the detector.

Ethan

> > directions. I.e., they contribute even less to the diffraction image than
> > air-scatter from the direct beam or from the diffracted beam.
>
> Well, this clearly depends on the sample content and on the X-ray
> wavelength. There are many examples of data collected at an absorption
> edge, where fluorescence is the dominating contributor to the
> background, i.e. it is much larger than air-scatter from the direct
> beam or from the diffracted beams. For an extreme case, see fig. 4 in
> Shepard et al.(2000). Acta Cryst. D56, 1288-1303.
>
> Marc
>
>
>
>
>
> >
> > Ethan
> >
> >>
> >> Jacob

--
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742

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CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

Previous message:
Subject: Re: Reason for Neglected X-ray Fluorescence
From: marc {- dot -} schiltz {- at -} EPFL {- dot -} CH marc {- dot -} schiltz {- at -} EPFL {- dot -} CH
Date: 2009-04-22

Next message:
Subject: Re: How small is a microbeam?
From: "Sanishvili, Ruslan" rsanishvili {- at -} ANL {- dot -} GOV
Date: 2009-04-22