Faraday cup

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A faraday cup is a metal (conductive) cup meant to recatch secondary particles. (See Black body (details)). When a beam or packet of ions hits the metal it will be charged while the ions are neutralized. The metal can then be discharged to measure a small current equivalent to the number of discharged ions. Essentially the faraday cup is part of a circuit where ions in the gas phase act as charge carriers during part of the circuit and the faraday cup is the interface to the solid phase where electrons act as the charge carriers (as in most circuits). By measuring the electrical current (the number of electrons flowing through the circuit per second) on the metal part of the circuit the number of charges being carried by the ions in the gas phase part of the circuit can be determined. Similarly, a Faraday cup can act as a collector for electrons in a vacuum (for instance from an electron beam). In this case electrons simply hit the metal plate/cup and a current is produced.

Faraday cups are not known for sensitivity but are highly regarded for accuracy because of the one-to-one relation between the two parts of the circuit and our ability to measure current accurately.

The Faraday cup is named after Michael Faraday who first theorized ions around 1830.