Detector (ionization)

A gas-filled ionization detector measures the ionization charge created within a gas caused by incident radiation. It consists of a gas-filled chamber with two electrodes, known as anode and cathode, between which a voltage potential is applied. Positive ions and negative electrons move to the electrodes of the opposite polarity under the influence of the electric field. In air, it requires an energy of 34 electronvolts to create an ion. The three basic types of gas-filled ionization detectors are:
  1. ionization chamber
  2. proportional counter
  3. Geiger–Müller tube
An ionization chamber operates at a low electric field strength, such that no gas multiplication takes place. A proportional counter operates at a slightly higher voltage, selected such that each primary ion pair produces a single charge avalanche and the output signal is proportional to the energy deposited in the gas. A Geiger–Müller tube operates at an even higher voltage, selected such that multiple avalanches are created from each primary electron and the (large) output signal no longer depends on the deposited energy.

In a semiconductor detector, ionizing radiation produces free electrons and electron holes. It requires an energy of only about 3 electronvolts to create an electron-hole pair.

Related concepts

  • detector (gas amplification)
  • detector (photographic emulsion)
  • detector (scintillation)

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