How an electron multiplier works

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 10E6discrete dynode electron multiplier gain

 

 

 

 

 

 

 

discrete dynode electron multiplier

 

 

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.continuous dynode electron multiplier gain

 

 

 

 

 

 

 

 

Channeltron multilplier

 

 

 

 

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.MCD channel plates gain

 

 

 

 

 

 

 

Micro Channel plates

 

 

 

 

 

 

 

 

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 used in PHI AES and XPS analyzers

The Physical Electronics AES cylindrical mirror (CMA), double-pass ESCA (XPS) and single channel SCA hemisphere analyzers use variations of the Channeltron® (registered trade mark of Photonis – Burle – Galileo) type of electron multiplier. The function of the electron multiplier is to increase the number of electrons that the analyzer detects by a gain of up to 2 X 108.

All of these electron multipliers have the same connection scheme – the front of the multiplier is where the electrons enter and it is biased by the negative (NEG) lead of the electron multiplier supply. With respect to ground, the multiplier NEG is actually biased positive by 50 to 300 VDC, depending on the specific analyzer. The center connector of the electron multiplier is connected to the positive (POS) lead of the electron multiplier supply. The POS is biased with a positive voltage of up to 3000 VDC depending on the specific analyzer.  Typically though, once the multiplier voltage needs to be set above 2500 VDC in order to obtain reasonable signal to noise, it is time to replace the electron multiplier. Finally, the back end of the multiplier (furthest away from the opening at the front) is connected to the collector connector on the analyzer. For most CMAs the collector wire on the electron multiplier is connected to the PC (pulse count) connector via a 50pf capacitor, and to the COL or ANA connector via a 100k ohm resistor.electron multiplier

 

 

 

 

 

There are only two things that you need to know about these multipliers:

  1. They have a limited shelf life. If you are not planning on using an electron multiplier shortly after you buy it then you need to store it under vacuum. Storing it in a desiccator will not prevent degradation. A few weeks are OK, but after a few months the gain will start to drop off noticeably.
  2. When installing the multiplier, remember that the front with the hole in it is the NEG, the middle is the POS and the back end is the COL.

Below is a table that shows the RBD Instruments Inc. part numbers for the electron multipliers used in the various Physical Electronics analyzers that we service and provide replacement parts for.

System Type Analyzer Electron Multiplier
5100 XPS 10-360 SCA 4821GRE
540 AES 10-150, 10-155 4839RE
545 AES, 15-110 4731GRE
590 AES 25-110 4731GRE
548 ESCA 15-255G 4831GRE
560 ESCA, 570  ESCA 25-260, 25-270 4831GRE
600 AES 660 AES 25-120A 4831GRE

 

RBD Instruments Inc. also provides channel plates for the 5300 and 5400 XPS analyzers, and the Chevron plates for the 5500 to 5800 series of XPS analyzers.