Electron Multiplier


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electron multiplier

[i′lek‚trän ′məl·tə‚plī·ər]
(electronics)
An electron-tube structure which produces current amplification; an electron beam containing the desired signal is reflected in turn from the surfaces of each of a series of dynodes, and at each reflection an impinging electron releases two or more secondary electrons, so that the beam builds up in strength. Also known as multiplier.

Electron Multiplier

 

an electronic device for intensifying a stream of electrons by means of secondary electron emission. It may be a part of various electron devices (multiplier phototubes, image converters, and some television camera tubes, such as dissector tubes and image orthicons, as well as receiving tubes), or it may be used as a self-contained device for detecting electromagnetic radiation (in the range of wavelengths λ from 0.1 to 150 nanometers) or particles (electrons with energies up to several tens of kiloelectron volts and ions or neutral particles with energies up to several megaelectron volts). Such detectors, which are usually constructed with an exposed input window, may be termed open-type electron multipliers. They are used in equipment operated in a natural vacuum (for space research) and in high-vacuum measuring instruments, such as scanning electron microscopes, manometers, and mass spectrometers.

Electron multipliers are grouped into the following basic types: multiplier systems having discrete electrodes, or dynodes; channel-type electron multipliers having a continuous dynode with a distributed resistance; and systems having a set of channel electron multipliers in parallel, based on microchannel plates. In the 1960’s a vacuum-tube-semiconductor, or hybrid, electron multiplier was developed that uses the multiplying effect of electrons in electron-hole junctions when semiconductor crystals containing such junctions are bombarded with electrons having energies high enough to form electron-hole charge pairs in the crystal.

In electron multipliers with discrete dynodes (seePHOTOTUBE, MULTIPLIER: Figure 1), electrons that are accelerated and focused by an electrostatic or magnetostatic field strike the dynode surfaces and produce secondary electron emission (the secondary-emission coefficient σ ≈ 3–30). Channel-type electron multipliers (Figure 1) comprise a tube (the channel) made of glass with a high lead content or a ceramic; the tube may be straight or curving. A voltage of several kilovolts is applied to the tube to create an electrostatic field inside. Electrons entering the tube are accelerated by the field; upon striking the tube walls they produce secondary electron emission (σ ≈ 2). The number of multiplying events of the secondary electrons and the total amplification factor of a channel type electron multiplier depend on the voltage, the length of the tube, and the tube’s inside diameter (for example, with a tube having a length of 20–75 mm and an inside diameter of 0.5–1.5 mm, the amplification factor reaches 105 for straight tubes and 107 for curving tubes). Electron multipliers with microchannel plates consist of a glass plate that is perforated with 104–106 parallel holes (channels) with diameters of 10–150 micrometers to form a honeycomb structure; the amplification factor is 104–106.

Figure 1. Electron multiplication in a channel-type electron multiplier. The plus and minus signs indicate the polarity of the voltage applied to the tube. The arrows represent electron trajectories.

One of the requirements imposed on open-type electron multipliers is the ability to maintain operating parameters when the emitting surfaces come into contact with the air. The protective properties of a thin (2.5–5 nanometers) oxide emission film (BeO, Al2O3) can serve to accomplish this goal. The cathode, which is located in the input section, is usually an alloy (CuBe, AgMgO). The cathode efficiency is evaluated by the number of electrons emitted per 100 quanta of incident electromagnetic radiation (the quantum efficiency) or per single bombarding particle (the emission coefficient). The quantum efficiency for radiation having λ = 70 nanometers is approximately 20 (dropping to 0.1 for λ. = 200 nanometers); for soft X rays it is approximately 1–5. As an example, the emission coefficient for AgMgO cathodes increases with the energy of the ions in the range from 2 to 10 kiloelectron volts by about 1–5; saturation occurs with a further increase in energy.

REFERENCES

Tiutikov, A. M. “Elektronnye umnozhiteli otkrytogo tipa.” Uspekhi fizicheskikh nauk, 1970, vol. 100, issue 3.
Berkovskii, A. G., V. A. Gavanin, and I. N. Zaidel’. Vakuumnye fotoelektronnyepribory. Moscow, 1976.

V. A. GAVANIN

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