Secondary emission
In
Applications
Secondary emissive materials
Commonly used secondary emissive materials include
- alkali antimonide
- Beryllium oxide (BeO)
- Magnesium oxide (MgO)
- Gallium phosphide (GaP)
- Gallium arsenide phosphide (GaAsP)
- Lead oxide (PbO)
Photo multipliers and similar devices
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. 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.
Historic applications
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.[3]
Early computer memory tubes
The first
Undesirable effects - the tetrode
Secondary emission can be undesirable such as in the
See also
References
- ^ R. Kollath, Secondary electron emission of solids irradiated by electrons, Encyclopedia of Physics (ed. S. Flügge) Vol. 21, p. 232 - 303 (1956, in German)
- ^ H. Semat, J.R. Albright, Introduction to Atomic and Nuclear Physics, 5th ed., ch. 4.12, Chapman and Hall, London (1972)
- ^ "1630, Tube 1630; Röhre 1630 ID17477, HEXODE".