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Electron Multipliers

    
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This photomultiplier has the photocathode in the wide section on the left and 11 stages of multiplication in the main tube on the right

The electron multiplier makes use of the effect of secondary emission and these devices can be found in a range of valve types.

The photomultiplier uses a photo-cathode followed by several stages of secondary emission to achieve gains of up to a million. Professional camera tubes also have electron multipliers built into them to increase sensitivity. Before the development of the EF50 high mutual conductance valve in 1938, secondary emission valves were seen as the way to achieve high stage gain coupled with high bandwidth for television RF stages. The Augetron is an example of this development route. For an explanation of why high gain was vital to the development of TV and Radar See Amplification and Gain-Bandwidth.

Secondary emission occurs when an electron strikes a surface with sufficient energy to dislodge new electrons. Most valve electrodes are carefully designed to prevent this happening, but in the electron multiplier just the opposite is wanted. Photomultipliers use several stages of electron multiplication in series so that one or more photons arriving on the photo sensitive cathode will eventually be detected because of the avalance of electrons reaching the final collector.

To work, the electrons have to be accelerated from the one emitting surface to the next and each stage is normally operated at 100 to 150 Volts. For the 11 stage photomultiplier this makes a total accelerating voltage of nearly 2000 Volts.

Single stage electron multipliers or secondary emission valves usually have a single stage of electron multiplication and the EE50 was also the first all glass valve. EE50 Operation demonstrates how it works.

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