Townsend discharge

The discharge requires a source of free electrons and a significant electric field; without both, the phenomenon does not occur.

If the mean free path is too short, then the electron gives up its acquired energy in a series of non-ionising collisions.

The electric field is applied across a gaseous medium; initial ions are created with ionising radiation (for example, cosmic rays).

The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause further impact ionisations, and so on.

In common gas-filled tubes, such as those used as gaseous ionisation detectors, magnitudes of currents flowing during this process can range from about 10−18 to 10−5 amperes.

[citation needed] Townsend's early experimental apparatus consisted of planar parallel plates forming two sides of a chamber filled with a gas.

He 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.

between the plates is equal to the breakdown voltage needed to create a self-sustaining avalanche: it decreases when the current reaches the glow discharge regime.

[clarification needed] Subsequent experiments revealed that the current I rises faster than predicted by the above formula as the distance d increases; two different effects were considered in order to better model the discharge: positive ions and cathode emission.

Townsend, Holst and Oosterhuis also put forward an alternative hypothesis, considering the augmented emission of electrons by the cathode caused by impact of positive ions.

The accompanying plot shows the variation of voltage drop and the different operating regions for a gas-filled tube with a constant pressure, but a varying current between its electrodes.

The sawtooth shaped oscillation generated has frequency where Since temperature and time stability of the characteristics of gas diodes and neon lamps is low, and also the statistical dispersion of breakdown voltages is high, the above formula can only give a qualitative indication of what the real frequency of oscillation is.

Avalanche multiplication during Townsend discharge is naturally used in gas phototubes, to amplify the photoelectric charge generated by incident radiation (visible light or not) on the cathode: achievable current is typically 10~20 times greater respect to that generated by vacuum phototubes.

The incident radiation will ionise atoms or molecules in the gaseous medium to produce ion pairs, but different use is made by each detector type of the resultant avalanche effects.

In the case of a GM tube, the high electric field strength is sufficient to cause complete ionisation of the fill gas surrounding the anode from the initial creation of just one ion pair.

The electric field and chamber geometries are selected so that an "avalanche region" is created in the immediate proximity of the anode.