Detectors 016
Detectors 016 : Ionization: (4.4.2) Quantitative description of the phenomenon: The basic setup of Townsend's early experiments investigating ionization discharges in gases consisted of planar parallel plates forming two sides of a chamber filled with a gas. A high voltage direct currentvoltage source was connected between the plates, the lower voltage plate being the cathode while the other was the anode. Forcing the cathode to emit electrons by the photoelectric effect, by irradiating it for example with an X-ray source, Townsend found that the current flowing through the chamber depends on the electric field between the plates in such a way that gas ions seemed to multiply as they moved between them. He observed currents varying exponentially over ten or more orders of magnitude with a constant applied voltage when the distance between the plates was varied. He also discovered the importance of the pressure of the gaseous medium, and was able to generate ions in gases at low pressure with a much lower voltage than that required to generate a spark. This overturned conventional thinking about the amount of current that an irradiated gas could conduct. The experimental data obtained from his experiments are described by the following formula where is the current flowing in the device, is the photoelectric current generated at the cathode surface, is the Euler number is the first Townsend ionisation coefficient, expressing the number of ion pairs generated per unit length (e. g. meter) by a negative ion (anion) moving from cathode to anode, is the distance between the plates of the device. The almost constant voltage 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. Subsequent experiments revealed that the current rises faster than predicted by the above formula as the distance increases: two different effects were considered in order to explain the physics of the phenomenon and to be able to do a precise quantitative calculation
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