Secondary emission is a phenomenon that occurs in electron tubes where electrons impact an electrode with sufficient energy to 'knock' additional electrons from the surface of that electrode. Generally, one electron gives rise to several secondary electrons.

Undesirable effects

This effect can be undesirable such as in the tetrode thermionic valve (tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode (plate). This can give rise to excessive screen grid current. It is also partly responsible for this type of valve (tube) exhibiting a 'negative resistance' characteristic. A tetrode is a two-grid vacuum tube. ... In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ... A grid introduced into a thermionic valve or tube to greatly reduce the capacitance between two other parts of the electrode structure. ... Diagram of a zinc anode in a Daniells cell. ... Plate has several meanings: A plate electrode in a vacuum tube. ... In electrical circuits, static resistance is the ratio of the voltage across a circuit element to the current through it. ...

Applications

Photo multipliers

The effect can also be exploited to advantage such as in the photomultiplier tube. In this instance the electrons (or indeed electron) emitted from a photocathode are accelerated towards a polished metal electrode (called a dynode). This electron or electrons strike with sufficient energy to 'knock' many more electrons from its surface. These new electrons are then accelerated towards another dynode where even more electrons are emitted. This process occurs (typically) 10 or so times. The result is that the tiny and normally undetectable current from the photocathode becomes a much larger and easily measurable current flowing in the final anode circuit. The current gain is typically many hundreds of millions. Photomultipliers, or photomultiplier tubes (PMT) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. ... In an electron tube, a negatively charged electrode coated with a photosensitive compound. ... One of a series of electrodes in a photomultiplier tube. ...

Special amplifying tubes

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. A typical such "orbital beam hexode" was the RCA 1630, introduced in 1939. Because the heavy electron current in such tubes damaged the dynode surface rapidly, their lifetime tended to be very short compared to conventional tubes.