Ionization Glossary

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Definitions of common words and phrases relating to electron ionization of gas:

  • Electron Ionization: The process by which an electron scatters off a gas atom/molecule and knocks out one of its electrons.
  • Primary Electron: An electron that scatters off a gas atom/molecule and ionizes it.
  • Secondary Electron: The electron that is ejected from a gas atom/molecule during ionization. Since there is no intrinsic way of knowing which of the two free electrons is the secondary electron after ionization, the electron with lesser energy is denoted the secondary electron.
  • Scattered Electron: Refers to the primary electron after it has scattered off the gas molecule. By the definition of secondary electron above, the energy of the scattered electron is necessarily higher than the secondary electron.
  • Ion: (In the context of electron ionization) The gas atom/molecule that has lost one of its electrons due to ionization and has a net positive electric charge.
  • Residual Gas Molecule: A neutral gas molecule within the accelerator vacuum.
  • Ionization Energy: The lower limit on the amount of kinetic energy a primary electron needs to ionize a gas atom/molecule. This energy is intrinsic to the gas itself.
  • Ionization Cross Section (ICS): The probability that a primary electron will ionize a gas molecule when it scatters off it.
  • Secondary Electron Differential Cross Section (SEDCS): The probability that a secondary electron will have a certain energy after ionization.
  • Secondary Electron Yield: Refers to the number of electrons ejected when an electron or ion scatters off a material surface. Although this processes is similar to electron ionization, these secondary electrons are not to be confused with those defined above.

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