This post will explain the basic concept of how an electron multiplier works.
Electron multipliers are used in surface analysis instruments to boost the detected signal to a level where it can be amplified and processed into data. For Auger Electron spectrometers and X-ray photo electron analyzers the detected signal are electrons. Secondary ion spectrometers detect ions.
In the 1960s electron multipliers were made out of a series of Oxygen treated copper beryllium (CuBe) plates. Copper with 3 to 4% beryllium that is heat treated with oxygen has a secondary electron yield of approximately 3 (varies slightly for kinetic energies between 100 up to 1500V)
The drawing below shows the basic concept. One electron impacts the first plate and then a few more secondary electrons are generated. A positive voltage is applied across the multiplier array which is divided by a series of vacuum compatible resistors. Each plate is progressively more positive and so emitted electrons are attracted to the next plate. The resulting avalanche of electrons is attracted to the final collector plate where the signal is decoupled from the electron multiplier. The total number of plates determines the gain of the multiplier. Most of the CuBe electron multipliers used on Auger spectrometers had a gain of 2 X 10E6
When X-ray Electron spectrometers were first developed electron multipliers with higher gains were required in order to achieve better signal to noise. During that time continuous dynode electron multipliers (Channeltrons) were developed. Instead of a series of discrete plates, a Channeltron electron multiplier uses a high resistance semiconductor material that also has high secondary electron emissivity. Gains of a Channeltron are typically 2 X 10E7 to 2 X 10E8. The drawing below shows the gain concept. Many Channeltrons today are spiral instead of horn shaped to provide an even higher gain.
A third type of electron multiplier, the Micro Channel plate, was developed in order to obtain a larger detector surface area in conjunction with multi-channel detectors. Channel plates are essentially a lot of tiny Channeltron multipliers in parallel. Two plates are stacked on top of each other to increase the gain. The drawing below shows the gain concept. Channel plate electron multipliers are commonly used on X-ray Photo electron spectrometers.
Electron multipliers typically last for several years with normal usage. With just occasional use they can last for decades. Eventually the high secondary electron emissivity materials in the multiplier are depleted or the multiplier becomes contaminated and then the signal to noise degrades at which time the multiplier needs to be replaced.
Some additional reference links are listed below. Most of these refer to ions and mass spectroscopy but it is the same principle for electron based detectors used in Auger Electron and X-ray photo electron spectrometers.
cires.colorado.edu/jimenez/CHEM-5181/Lect/MS_Detectors_AD_SNR.pdf
http://www.irjponline.com/admin/php/uploads/1414_pdf.pdf
http://www.chm.bris.ac.uk/ms/detectors.xhtml
Electron Multipliers are available from these companies –
Scientific Instrument Services
For assistance with installing electron multipliers or channel plates in older Physical Electronics XPS and AES surface analysis instruments contact RBD Instruments, Inc.
