[Ifeffit] Frustrating monochromator glitch

[Carlo U. Segre]

Dear Mark:

A monochromator glitch is when, because of some small misalignment of one 
of the two (generally) monochromator crystals, a parasitic reflection 
comes into Bragg alignment and reflects a significant maount of intensity 
off into a direction which is not towards the sample.  These reflections 
can be shifted around or removed by tweaking the crystal in question in 
one of the two axes perpendicular to the primary axis of rotation of the 
monochromator.  Sometimes this is not possible (as with a cryogenically 
cooled first crystal where these additional motions are not readily 
available) and you just have to live with them.

You will notice the Io drops precipitously at a glitch and this will cause 
a reduction of intensity in your It or If detector as well.  If the 
detectors which you are using are well matched in their response to 
changes in intensity, the glitch is mitigated in the ratioed XAFS data. 
This is usually possible to do in a transmission experiment, where both 
detectors are ionization chambers.  You can test for this before starting 
your data collection by plotting the ratio of Io/It and deliberately 
attenuate the beam.  If the ratio remains constant, you have a good chance 
of not seeing the effects of the glitch in the data.

It is much more difficult when you are doing fluorescence experiments 
since the fluorescence detector is hardly ever a perfect match with Io. 
In these cases, you just have to try to minimize the differences in 
response by trying different things.  With a Fluorescence Ion chamber, you 
might try increasing the potential on the chamber (just as long as you 
don't get a shock!).  With a solid state detector, you just have to fuss 
around until you are satisfied that you have the best match between 
detector responses.

There is no way that I know of correcting numerically for nonlinearities 
in the two detectors.  Deglitching will have to be done.  In fact, you 
need to make sure that the nonlinearities are not effect on the shape of 
the edge itself, since that is potentailly a place where there is a 
significant change in intensity in one of the detectors.

I am sure that I haven't covered everything.

Hope this helps,

Carlo

On Thu, 9 Dec 2004 m.bondin at pgrad.unimelb.edu.au wrote:

> Dear mailing list - my last round of EXAFS experiments performed on the 
> Fe-K edge in fluorescence mode gave data extending nicely to 15 in k. 
> One problem was a nasty monochromator glitch at approx 13.5 in k which 
> is so great in some cases it causes sufficient distortion to the data 
> that even upon deglitching I must cut back the data to 13 in order to 
> gain a good fit using my models. Firstly I was wondering if someone here 
> could describe to me how a monochromator glitch arises (some systematic 
> defect in the monochromator crystal or more? - the only problem with 
> this explanation is that some data sets seem more affected than others 
> and it has not manifested itself as badly in previous trips). Also is 
> there any other way to handle the removal of a monochromator glitch 
> besides deglitching? My supervisor seems to think that there must be a 
> scale factor or offset in the I0 monitor counts which could be corrected 
> in order to overcome a glitch in the existing data - has anyone ever ! 
> thought of other ways to salvage data in this way?
>
> Regards,
>
> Mark
>
>

-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Special Projects, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
Carlo.Segre at iit.edu    http://www.iit.edu/~segre