What is an Electron Multiplier?

This article describes the fundamental concept of the workings of an electron multiplier.

Surface analysis instruments use electron multipliers to increase the detected signal to a level where it can be amplified and then processed into data. Electrons are the detected signal in X-ray photoelectron analyzers and Auger electron spectrometers, whereas ions are detected by secondary ion spectrometers.

Electron Multipliers

Earlier in the 1960s, electron multipliers were developed from a range of copper beryllium (CuBe) plates treated with oxygen. Copper containing 3%–4% beryllium, which is heat-treated with oxygen, has a secondary electron yield of about 3 (differs somewhat for kinetic energies from 100 up to 1500 V).

The following diagram demonstrates the fundamental concept. A single electron strikes the first plate, following which a few more secondary electrons are produced. A positive voltage is then applied across the multiplier array, which is split by a sequence of vacuum-compatible resistors. Every plate becomes gradually more positive, and therefore, the emitted electrons are pulled toward the next plate. The ensuing avalanche of electrons is pulled toward the last collector plate, where the signal is eventually decoupled from the electron multiplier. The gain of the multiplier is determined by an overall number of plates. A majority of the CuBe electron multipliers utilized on Auger electron spectrometers had a gain of 2 x 10E6.

Image Credit: RBD Instruments.

Image Credit: RBD Instruments.

X-Ray Electron Spectrometers

During the initial development of X-ray electron spectrometers, electron multipliers with higher gains were needed to realize better signal to noise. At that time, continuous dynode electron multipliers (Channeltrons) were created.

A Channeltron electron multiplier utilizes a high-resistance semiconductor material rather than a series of discrete plates. This semiconductor material also has high secondary electron emissivity. Usually, gains of a Channeltron range between 2 x 10E7 and 2 x 10E8.

The following diagram demonstrates the gain concept. At present, a majority of the Channeltrons are spiral-shaped rather than horn-shaped to offer a uniform higher gain.

Image Credit: RBD Instruments.

Image Credit: RBD Instruments.

Micro Channel plate is the third type of electron multiplier developed to achieve a larger detector surface area along with multi-channel detectors. Channel plates are actually a series of very small Channeltron multipliers linked in parallel.

Two plates are stacked on top of one another to increase the gain. The following diagram demonstrates the gain concept. Channel plate electron multipliers are often utilized on X-ray photoelectron spectrometers.

Image Credit: RBD Instruments.

Image Credit: RBD Instruments.

Conclusion

Usually, electron multipliers can last for a number of years if used normally but can last for several decades if used occasionally. Ultimately, the electron multiplier becomes contaminated or the high secondary electron emissivity materials in the electron multiplier get depleted. Following this, the signal to noise degrades at which time the electron multiplier has to be replaced.

This information has been sourced, reviewed and adapted from materials provided by RBD Instruments, Inc.

For more information on this source, please visit RBD Instruments, Inc.

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