Faraday mug

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Faraday beaker with suppressor electrode
Symbolic structure

A Faraday cup (also Faraday collector or Faraday detector or a Faraday cup . Or cup-electrometer, abbreviated as FC or FDC of eng F Faraday C up) is a detector for measuring ions - or electron currents .

Since the inside of a conductive hollow body (metal cup) is field-free, the charge of a charged object (e.g. a falling positively charged particle) is transferred to the container wall without touching the wall and can be fed from there to a charge measuring device (influence). So z. B. the charge of an insulating material can be measured by placing it in a Faraday cup. The conductor of a band generator can also be viewed as a large Faraday cup. This means that no charges migrate from the inner wall of an electrically charged beaker to a metal sample ball, but do migrate from the outside surface of the beaker. Michael Faraday made this discovery around 1830, after whom the structure was named.

To measure ion or electron beams, the Faraday cup is placed in the beam that absorbs the particles. If the Faraday cup is kept at a constant potential, the trapped ions must  be balanced by electrons, which can flow into or out of the Faraday cup via a connected high-resistance resistor (typically 10 8  - 10 12 Ω). A voltage therefore drops across the resistor, which is a measure of the strength of the current and z. B. can be measured with an electrometer . However, there are also measuring arrangements with Faraday cups with low resistance that achieve a temporal resolution in the nanosecond range.

If reflected ions / electrons or secondary electrons knocked out of the detector surface are prevented from leaving the Faraday cup, a Faraday catcher can be used to directly determine the number of charge carriers caught per unit of time. This can be achieved by the geometrical shape of the Faraday beaker and by suppressor electrodes which are at negative potential and which force the secondary electrons back to the detector.

Faraday collectors are used as an alternative or in addition to the secondary electron multiplier (SEM). The advantage of the Faraday collector is its reliability and robustness and the possibility of measuring the ion current or electron current absolutely. In addition, the sensitivity is constant over time and, in contrast to the SEV, is not mass-dependent. Disadvantages are the poorer detection sensitivity compared to an SEV (typically 2000 ions / s) and the lower bandwidth (ie long response time). The reason is the large time constant (typically around 0.1 s), which results from the self-capacitance in connection with the very high value of the discharge resistance. However, it is also possible to build so-called fast Faraday cups with the help of certain geometries, which have significantly smaller time constants down to 1 µs.

In the case of neutral gas Faraday interceptors, the suppressor electrode is positively biased so that the secondary electrons generated by the impact of neutral atoms are diverted away from the Faraday interceptor. To balance the charge, electrons must therefore flow in through the high-ohmic resistor, with which a signal can be detected.

Even with normal Faraday beakers, suppressor electrodes (~ 100 V voltage) are often used in order to avoid charge losses and the corresponding falsification of measured values ​​caused by the secondary electrons generated.

See also

Individual evidence

  1. ↑ a b Spektrum.de: Faradayscher Becher - Lexicon of Physics - Spectrum of Science , accessed on March 12, 2017
  2. ↑ KL Brown, GW Taut hard: Faraday Cup monitor for High-Energy Electron Beams. In: Review of Scientific Instruments. 27, 1956, p. 696, doi : 10.1063 / 1.1715674 .
  3. ↑ Günter Lüttgens: Expert-Praxislexikon static electricity 1600 terms on hazards, disturbances and applications . expert verlag, 2000, ISBN 978-3-8169-1486-0 , p. 116 ( limited preview in Google Book search).
  4. ^ Ernst-Wilhelm Otten: Repetitorium Experimentalphysik . Springer-Verlag, 2009, ISBN 978-3-540-85788-4 , pp. 452 ( limited preview in Google Book search).
  5. ↑ Diethelm Völcker: Physics, intermediate level optics, magnetism, electricity, atomic physics . Mentor, 2003, ISBN 978-3-580-63661-6 , pp. 16 ( limited preview in Google Book search).
  6. ^ A b John Lindon, John Holmes, George Tranter: Encyclopedia of Spectroscopy and Spectrometry . Academic Press, 2010, ISBN 978-0-12-374417-3 , pp. 1075 ( limited preview in Google Book search).
  7. ↑ a b M. Pollermann: Components of Physical Technology - A Guide to the Development of Research Apparatus . Springer-Verlag, 2013, ISBN 978-3-642-65284-4 , pp. 324 ( limited preview in Google Book search).
  8. ^ Peter Strehl: Beam Instrumentation and Diagnostics . Springer Science & Business Media, 2006, ISBN 978-3-540-26404-0 , pp. 23 ( limited preview in Google Book search).
  9. ↑ Karl Jousten: Wutz manual vacuum technology - theory and practice . Springer-Verlag, 2013, ISBN 978-3-322-96971-2 , pp. 527 ( limited preview in Google Book search).
  10. ↑ ntg.de: Strahldiagnosesysteme: Faraday cups , accessed on June 5, 2017
  11. ^ Robert Graham Cooks: Collision Spectroscopy . Springer Science & Business Media, 2012, ISBN 978-1-4613-3955-7 , p. 40 ( limited preview in Google Book search).