Faraday cup

A Faraday cup ( also Faraday or Faraday detector abbreviated FC ENGL. Faraday Cup ) is a detector for the measurement of ion or electron flow.

The device consists of a metal cup ( Faraday cup ), which is placed on the to be measured ion beam ( electron beam). Since the Faraday cup is maintained at a constant potential, to the trapped ions by electrons (typically from 109 to 1011 Ω ) of a connected high-value resistor and flow to flow into the Faraday cup can be compensated. Therefore, the resistance, a voltage which is a measure of the ion current ( current of electrons ) and can be measured for example with an electrometer.

Prevents reflected ions / electrons or leave from the detector surface secondary electrons knocked out the Faraday cup, can directly the number of trapped charge carriers are determined per unit of time with a Faraday cup. This can be achieved by the geometric shape of the Faraday cup located at a negative potential and suppressor electrodes back force, the secondary electrons towards the detector.

Faraday cup can be used as an alternative or in addition to secondary electron multiplier ( SEM ). Advantage of the Faraday collector is its reliability and sturdiness and the possibility to measure the ion current and electron current absolute. In addition, the sensitivity is constant over time and, unlike the ETS is not dependent on mass. Disadvantage is the opposite a SEV poorer detection sensitivity and the lower bandwidth (ie, long reaction time). The reason is the large time constant, which results from the self-capacitance in conjunction with the very high value of the discharge resistor.

At neutral gas Faraday interceptors the suppressor electrode is biased positively, so that the secondary electrons generated by the impact of the neutral atoms will be directed away from the Faraday cup. To balance the charge must therefore electrons continue to flow through the high resistance, so a signal can be detected.