Secondary emission

Commonly used secondary emissive materials include In a photomultiplier tube,[2] 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.

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.

In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode (plate).

This side effect could be put to use by using some older valves (e.g., type 77 pentode) as dynatron oscillators.

Visualisation of a Townsend avalanche , which is sustained by the generation of secondary electrons in an electric field
Secondary emission used in a photomultiplier tube . The initial electrons emitted when light strikes a photocathode are made to strike a dynode electrode, knocking out more electrons, which strike a second dynode. Each incident electron produces multiple secondary electrons, so the cascaded dynode chain amplifies the initial electrons.
The display on an oscilloscope at normal intensity.
The same tube with greater intensity. The disk around the dot in the center is due to secondary emission. Electrons are struck off the screen and travel backwards into the tube. The voltage across the tube causes them to be accelerated forward again, striking the screen over a wide area. [ citation needed ]