[3dem] beam blank versus beam shutter

[Buecker, Robert]

Dear Tim,

the speed of the blanker varies a lot between TEMs and their configuration. On our TF20 STEM, where blanking is achieved by mis-adjusting the beam tilt, it was in the few seconds range (hence unusable for our ED experiments) originally. However, this was remedied by replacing a specific electronics board (with what is essentially a dummy plug) that filters the gun deflector currents, at the expense of somewhat reduced STEM resolution due to jitter. It is now of the order of 10 ms, so would be acceptable for your ED collection. We actually can trigger our Medipix detector from the shutter signal then, though mostly we go vice versa. Titans, on the other hand, have a dedicated beam blanker which is ms-range without influencing beam jitter or drift.
In our JEOL 2100 we get 10 ms or so by default, but pushed it to sub-µs using a custom electro-static blanker (which might be overkill for most use cases). There is also beautiful work by the Antwerp group on a fast magnetic blanker that sits on the C2 aperture (Appl. Phys. Lett. 108, 093103).
That said, I see no real reason against well-engineered (that is, no magnetic stray fields, charging, long-term reliability…) mechanical shutters, even though I’d rather stick to magnetic or electric.

Cheers,
Robert

--
Dr. Robert Bücker
Centre for Structural Systems Biology // University of Hamburg
Notkestraße 85 // 22607 Hamburg // Germany
robert.buecker at cssb-hamburg.de<mailto:robert.buecker at cssb-hamburg.de>
+49 157 70210628


Am 14.05.2021 um 13:50 schrieb Tim Gruene <tim.gruene at univie.ac.at<mailto:tim.gruene at univie.ac.at>>:

Dear all,

In 1975, Unwin and Henderson used a beam shutter instead of the beam
blank (JMB 1975, 94, 425-440) to avoid specimen drifts.

When I collected ED data with the EIGER detector, i.e. at 100Hz
(10.1002/anie.201811318), we used a beam blank. I had to inspect each
data set individually, because the beam was creeping towards its final
stable position, which took 50-75images, i.e. 0.5-0.75s. This seems a
lot to me. Those data were measured with a Tecnai F30, which
is less than 46 years old, but not the latest model either.

Personally, I would prefer a mechanical shutter over a beam blank. What
is the current situation with modern TEMs? Does the beam find its
position within a ms when using the beam blank? Are there reasons NOT
to use a mechanical shutter? For crystallographic purposes, the opening
of the shutter to trigger the detector for data recording seems quite
plausible.

I look forward to your experience and opinions.

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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