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I want to collect charge from an electron beam source. For that, I'm proposing to put a floating capacitor with its bottom terminal floating and directly exposed to the electron beam gun. The top terminal would be connected to a transconductance amplifier.

Do you think that it will work? What considerations or modifications would you suggest?

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JRE
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Jorge Johanny Sáenz Noval
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    A transimpedance amplifier with capacitor instead a resistor is known as "charge amplifier"... – Circuit fantasist Apr 13 '21 at 08:57
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    How energetic are your electrons? Is this in a vacuum? A typical tool for such an application might be some type of [electron multiplier](https://en.wikipedia.org/wiki/Electron_multiplier), perhaps an [MCP](https://en.wikipedia.org/wiki/Microchannel_plate_detector) but it really depends on what you're talking about, exactly, and what you're trying to measure... exactly. – J... Apr 13 '21 at 16:58
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    If most of the above questions are relevant you may get more traction on the Physics stack with this question... in either case, I think we need a lot more information about what, exactly, you're trying to achieve - and why. – J... Apr 13 '21 at 18:13
  • My electrons have energies between 1keV up to 30keV. I'm trying to measure how many electrons are hitting the capacitor per area unit. – Jorge Johanny Sáenz Noval Apr 13 '21 at 20:55
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    so you *have* to use this capacitor because its part of the device under test ? – tobalt Apr 13 '21 at 21:29
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    the standard term for what you're trying to make here is a Faraday Cup. All you need is a bare metal surface in vacuum and something outside vacuum to turn the current into a measurable signal (usually a transimpedance amplifier). https://en.wikipedia.org/wiki/Faraday_cup – llama Apr 13 '21 at 22:15
  • @JorgeJohannySáenzNoval ...presumably in a vacuum chamber? – J... Apr 13 '21 at 22:18
  • @JorgeJohannySáenzNoval And what of the intensity of the beam? If you're trying to count a very small number of electrons it's much different than trying to characterize an intense, high-power beam. Definitely this should go on the physics stack - there are all kinds of instruments and techniques for measuring these types of things and how, exactly, you would best go about doing it will depend heavily on details we don't know. – J... Apr 13 '21 at 22:26
  • @tobalt The capacitor is a part of the sensor. – Jorge Johanny Sáenz Noval Apr 15 '21 at 14:51
  • @JorgeJohannySáenzNoval Does it need to be, or is the capacitor just part of *your idea* for **a** sensor for this purpose? Would a different sensor that accurately counted the electron flux be acceptable? *Is this in a vacuum chamber?* – J... Apr 15 '21 at 19:22
  • @JorgeJohannySáenzNoval if you cannot direct the e-beam to the driven electrode, but *have* to use the open electrode, be wary of the charging. The more electrons you deposit in the floating electrode, the more negative it will become. Already way before this potential reaches your electron energy, you will see strong deflection towards other grounded parts of the chamber and will read a reduced current than is actually present. – tobalt Apr 16 '21 at 06:45
  • @J... Obviously it is in a vacuum. e-beams are strongly absorbed in atmosphere and kV acceleration voltage would arc through air. – tobalt Apr 16 '21 at 06:48
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    @tobalt Not obvious - there are techniques to allow an electron beam to escape the vacuum such that they are incident upon a sample in atmosphere. See : [plasma window](https://en.wikipedia.org/wiki/Plasma_window) – J... Apr 16 '21 at 13:02
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    @tobalt Also [High Performance Micropane Electron BeamWindow](https://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=1003&context=elct_facpub), as another example, different technology. – J... Apr 16 '21 at 13:10
  • @J... thanks for those examples. Interesting :) – tobalt Apr 16 '21 at 13:17

1 Answers1

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It will be probably much easier without the capacitor. Shining the e-beam on the steady potential electrode will generate a much more stable current reading using a conventional transimpedance amp.

The problem with the capacitor terminal is that it will charge; and eventually this charge will deflect the e-beam from hitting the electrode.

Another issue is that you will generate secondary electrons emission losses from your target, so if you want to measure the beam current accurately, the target should have a hollow geometry with the beam entering through a hole. That way, stray emission will be recaptured.

Depending on the beam intensity, target heating can become an issue.

tobalt
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