Secondary emission

Secondary emission

Secondary emission is a phenomenon where 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 incident particle is called secondary emission yield.

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), particularly early types with anodes not treated to reduce secondary emission, exhibiting a 'negative resistance' characteristic. This side-effect could be put to use by using some older valves (e.g., type 77 pentode) as dynatron oscillators.


Photo multipliers and similar devices

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 through the Auger effect. 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.

For the first multiplication the electron is accelerated by 100 to 200 eV and hits the surface in grazing incidence so that a mean 10 secondary electrons are emitted and the chance that at least 2 electrons are emitted is very high. In this way every electron can be detected and the efficiency of about 0.3 is mostly governed by the generation of photoelectrons (one kind of secondary electron) and their ejection into the vacuum. Ions are detected by accelerating them onto an a separate dynode, which suffers from sputtering, and detecting their secondary electrons. Ions at keV kinetic energy generate about 30 secondary electrons.

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.

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.

See also

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