 secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. the primary particles are often charged particles like electrons or ions. if the secondary particles are electrons, the effect is termed secondary electron emission.r. kollath, secondary electron emission of solids irradiated by electrons, encyclopedia of physics (ed. s. flÃ¼gge) vol. 21, p. 232 - 303 (1956, in german) in this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. if the secondary particles are ions, the effect is termed secondary ion emission. == 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,h. semat, j.r. albright, introduction to atomic and nuclear physics, 5th ed., ch. 4.12, chapman and hall, london (1972) 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 == thumbnail|right|the display on an oscilloscope at normal intensity. thumbnail|right|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. === 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 su
Condensed_matter_physics Electron Vacuum_tubes