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 are available from these companies –

Photonis

Scientific Instrument Services

Restek

For assistance with installing electron multipliers or channel plates in older Physical Electronics XPS and AES surface analysis instruments contact RBD Instruments, Inc.

32-100 electron multiplier supply digital mode

The model 32-100 electron multiplier supply is used on older Physical Electronics Auger electron spectroscopy and X-ray photoelectron spectroscopy surface analysis systems to control the electron multiplier voltage.

When using the 32-100 electron multiplier supply in the digital mode (the software automatically sets the electron multiplier voltage)  the auto-ems box in the Auger Scan software AES electron multiplier properties dialog box needs to be checked, and the CMA multiplier switch on the front panel of the 32-100 must be set to digital.

But what if the 32-100 still does not work in the digital mode?  In that case, there may be a problem with one of the digital ICs.  The following procedure explains how to try and repair this problem.

  1. Turn off the 32-100 and remove the cover.
  2. Most 32-100s have three ICs for the SED digital side of the control which are not used. You can move those ICs over to the CMA side of the control and see if that solves the problem.
  3. Remove ICs U2, U4 and U6. These are the CMA digital ICs. Then, move over U3, U5 and U7 from the SED side if those chips are available. If they are missing, then you will need to order some of those ICs from Digikey, Newark or RBD Instruments. U2 and U4 are 74LS174s and U6 is an AD7521.
  4. If you did have those ICs and that did not solve the problem, then it most likely means that one of the encoder circuit ICs are defective. Those are U16 (74123) and U1 (AM25LS2538). RBD Instruments provides these parts.
  5. You do not need to replace the SED digital ICs as they are not used.

It is assumed that the 32-100 is working in the analog mode. If the 32-100 is not working in the analog mode it will not work in the digital mode either.

Refer to the pictures below for the locations of the ICs on the 32-100 motherboard.

Note that the switch positions need to be set as shown below.

 

32100-switch-positions

32100-switch-positions

 

If you need further help troubleshooting your 32-100 electron multiplier supply please contact RBD Instruments dot com

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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.