Secondary emission
From Wikipedia, the free encyclopedia
Secondary emission is a phenomenon that additional electrons, called secondary electrons, are emitted from the surface of a material when an incident particle (often, charged particle such as electron or ion) impacts the material with sufficient energy. The number of secondary electrons emitted per an incident particle is called secondary emission yield.
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[edit] Undesirable effects
Secondary emission 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.
[edit] Applications
[edit] Photo multipliers
The effect can also be exploited to advantage such as in the photomultiplier tube. In this instance the electrons (or an 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.
[edit] 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.
[edit] Early computer memory tubes
The first random access computer memory used a type of cathode ray tube called the Williams tube that used secondary emission to store bits on the tube face. Another random access computer memory tube based on secondary emission was the Selectron tube. Both were made obsolete by the invention of magnetic core memory.
