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Concept
Secondary emission
Summary
In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission. Secondary electron emission is used in photomultiplier tubes and tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation.
Commonly used secondary emissive materials include
alkali antimonide
Beryllium oxide (BeO)
Magnesium oxide (MgO)
Gallium phosphide (GaP)
Gallium arsenide phosphide (GaAsP)
Lead oxide (PbO)
In a photomultiplier tube, one or more electrons are emitted from a photocathode and accelerated towards a polished metal electrode (called a dynode).
They hit the electrode surface with sufficient energy to release a number of electrons through secondary emission. These new electrons are then accelerated towards another dynode, and the process is repeated several times, resulting in an overall gain ('electron multiplication') in the order of typically one million and thus generating an electronically detectable current pulse at the last dynodes.
Similar electron multipliers can be used for detection of fast particles like electrons or ions.
In the 1930s special amplifying tubes were developed which deliberately "folded" the electron beam, by having it strike a dynode to be reflected into the anode. This had the effect of increasing the plate-grid distance for a given tube size, increasing the transconductance of the tube and reducing its noise figure.
Official source
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