An electron multiplier is a device that increases the rate of electron emission of an electron stream. It bombards an emissive material with a stream of electrons, causing the material to emit electrons. Each electron that strikes the emissive material can cause it to emit as many as three electrons, thus multiplying the emission rate of the electron stream.
The emissive material typically is a metal plate, and an electron multiplier usually contains a series of these plates. The application of an electrical potential difference between successive plates will cause the emitted electrons from the first plate to jump to the second plate, which causes the emission of more electrons from the second plate. This process continues through the entire series of plates, resulting in the emission of many more electrons than were in the initial electron stream. An anode then collects the electrons produced by the electron multiplier.
The cascading effect of electrons requires that the electrons strike the emissive material with sufficient energy to induce a secondary emission. This requirement allows an electron multiplier to function as an ion detector. A photon stream can also cause the emission of photons, as in the case of a photomultiplier tube. This device separates each emissive surface by a series of anodes known as dynodes. A series of resistors separates the dynodes from each other, such that the electrical potential between successive dynodes is typically +100 volts to +200 volts.
This structure requires the electrode material to have a high electrical resistance to merge the voltage division function with the secondary emission function. An electron multiplier typically has the shape of a funnel and is constructed of glass. The glass funnel has a thin coating of some substance with a limited ability to conduct electricity, known as a semiconductor.
The electron multiplier receives a high negative voltage at the wide end of the funnel and a low positive voltage at the narrow end of the funnel. This configuration of voltages causes the funnel to emit electrons from its surface, which then move from the wide end to the narrow end of the funnel. The electrons travel a short distance before striking the surface of the funnel, causing the emission of more electrons.
An anode at the narrow end of the funnel collects the electrons. This arrangement is known specifically as a single-channel electron multiplier. A microchannel plate is a more complex type of electron multiplier that uses a two-dimensional array of single-channel electron multipliers.
