Electron Multiplier Section:

• When the surface of a metal is bombarded by incident electrons having high velocities, secondary emission takes place. Aluminium, as an example, can release several secondary electrons for each incident primary electron.
  • Camera tubes often include an electron multiplier structure, making use of the secondary emission effect to amplify the small amount of photoelectric current that is later employed to develop video signal.
  • The electron multiplier is a series of cold anode cathode electrodes called dynodes mounted internally, with each at a progressively higher positive potential as illustrated in diagram.
  • The few electrons emitted by the photocathode are accelerated to a more positive dynode. The primary electrons can then force the ejection of secondary emission electrons when the velocity of the incident electrons is large enough.
  • The number of electrons available is multiplied each time the secondary electrons strike the emitting surface of the next more positive dynode. The current amplification thus obtained is noise free because the electron multiplier does not have any active device or resistors.

Diagram: