Secondary emission
![Visualisation of a Townsend avalanche, which is sustained by the generation of secondary electrons in an electric field](/Images/godic/202502/09/Electron_avalanche2716.gif")
![Secondary emission used in a photomultiplier tube. The initial electrons emitted when light strikes a photocathode are made to strike a dynode electrode, knocking out more electrons, which strike a second dynode. Each incident electron produces multiple secondary electrons, so the cascaded dynode chain amplifies the initial electrons.](/Images/godic/202502/09/Photomultiplier_schema_en2716.png")
![The display on an oscilloscope at normal intensity.](/Images/godic/202502/09/Oscilloscopeabc2716.jpg")
![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.](/Images/godic/202502/09/Oscilloscopeabcde2716.jpg")
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 term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. 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. Secondary electron emission is used in photomultiplier tubes and image intensifier tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation.