Secondary emission is a phenomenon that occurs when high-energy particles strike a surface or a material and induce the emission of secondary particles, such as electrons or ions. For example, when electrons or ions in a vacuum tube hit a metal surface, they can knock out more electrons from the metal. These are called secondary electrons. The number of secondary electrons emitted per incident particle is called the secondary emission yield.
Secondary emission is used in many devices that can detect and amplify single particles or photons, such as photomultiplier tubes, image intensifier tubes, and particle detectors . By using a device called a microchannel plate (MCP), which has many tiny tubes that multiply electrons through secondary emission, the input signal can be amplified by millions of times .
Secondary emission can also occur as an undesirable side effect in electronic vacuum tubes, when electrons from the cathode strike the anode and cause parasitic oscillation. To prevent this, special materials or coatings with low secondary emission yield are used for the anode.
