[3dem] LaB6 vs FEG for electron diffraction

[Tim Gruene]

Dear all,

Maybe someone on this list has some technical background on electron 
crystallography. What benefit would a FEG have compared with a LaB6 source?

I have collected 3D electron diffraction data both with an F30 and a CM200 
(LaB6), as well as a Talos, a Polara and a CM200-FEG. 
The CM200 (LaB6) is the oldest instrument (except for the detector, a hybrid 
pixel detector). Its data are clearly the best I have collected so far, both 
in terms of stability of the rotation stage and the common data statistics.

The advantage for a LaB6 are low maintenance costs better tolerance with 
respect to the vacuum: when we mounted our detector, the instrument was 
operational in a few hours, while with the F30, we had to wait overnight.

With the CM200(LaB6), we collected data at about 0.5-1 degree /s, at spot size 
7-9, i.e. still far from its maximum intensity. As we can focus the direct 
beam on the detector in diffraction mode, a perfectly parallel beam is not 
required - X-ray instruments are run with a 10-20 times greater beam 
divergence.

I would greatly appreciate some quantitative input on the question as to what 
advantages a FEG should have for 3D electron diffraction.

Best regards,
Tim


-- 
--
Tim Gruene
Head of the Centre for X-ray Structure Analysis
Faculty of Chemistry
University of Vienna

Phone: +43-1-4277-70202

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