Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons.
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10 Facts About Townsend avalanche
2.
Townsend avalanche discharge is named after John Sealy Townsend avalanche, who discovered the fundamental ionisation mechanism by his work circa 1897 at the Cavendish Laboratory, Cambridge.
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3.
Townsend avalanche forced the cathode to emit electrons using the photoelectric effect by irradiating it with X-rays, and he found that the current flowing through the chamber depended on the electric field between the plates.
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4.
Townsend avalanche observed currents varying exponentially over ten or more orders of magnitude with a constant applied voltage when the distance between the plates was varied.
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5.
Townsend avalanche discovered that gas pressure influenced conduction: he was able to generate ions in gases at low pressure with a much lower voltage than that required to generate a spark.
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6.
Townsend avalanche put forward the hypothesis that positive ions produce ion pairs, introducing a coefficient expressing the number of ion pairs generated per unit length by a positive ion moving from anode to cathode.
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7.
Occurrence of Townsend avalanche discharge, leading to glow discharge breakdown shapes the current–voltage characteristic of a gas-discharge tube such as a neon lamp in a way such that it has a negative differential resistance region of the S-type.
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8.
Avalanche multiplication during Townsend discharge is naturally used in gas phototubes, to amplify the photoelectric charge generated by incident radiation on the cathode: achievable current is typically 10~20 times greater respect to that generated by vacuum phototubes.
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9.
Townsend avalanche discharges are fundamental to the operation of gaseous ionisation detectors such as the Geiger–Muller tube and the proportional counter in either detecting ionising radiation or measuring its energy.
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10.
The electric field and chamber geometries are selected so that an "Townsend avalanche region" is created in the immediate proximity of the anode.
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