[3dem] LaB6 vs FEG for electron diffraction

[Reinhard Rachel]

>>> 18.12.2019 at 17:06:
> 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?
............. 
> 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,

Dear Tim, 
your mail with the list of your experiences in the past experiments gives a
number of nice statements. 

in diffraction, when working with this type of crystalline samples (which one?
inorganic crystals? small or large biomolecules? thickness?), the beam / gun
coherence does not matter too much. Interesting to note, but might be expected.

In our 120kV LaB6 TEM (almost 30 yrs old), I have seen crystals diffracting to
high resolution (mostly, crystals which I did not want to have ...), and far
beyond the resolution of the TEM in imaging mode. 
But, this is established: the coherence of the gun matters when doing high
resolution imaging. It has direct influence on the CTF, the transfer function.
- In diffraction, the objective lens aberrations do not matter; this lens is
not in use. 
What really matters for diffraction, is (a) a good sample, which the beam can
penetrate (you do not give any information on this; thickness?); (b) a stage
which is stable and can nicely be rotated (!!); (c) a really good detector ....
the latter is true for both imaging and diffraction, although, the detectors'
characteristics might be quite different. 

some theory may help. But your experience is valuable to know. 
Merry Christmas, and a wonderful and happy New Year 2020. With lots of nice
samples diffracting well .... 
kind regards,
Reinhard

-- 
Prof. Dr. Reinhard Rachel
University of Regensburg
Centre for EM / Anatomy
Faculty of Biology & Preclin. Med.
Universitaetsstrasse 31
D-93053 Regensburg - Germany
tel +49 941 943 -2837, -1720
mail reinhard.rachel at biologie.uni-regensburg.de 
office: VKL 3.1.29 
member of the IFSM board

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