Townsend's first ionization coefficient for methane and nitrogen

Measurements have been made of Townsend's first ionization coefficient α in methane for a range of E/p 0 from 24·3 to 220 V cm -1 torr -1 and a range of pressures p 0 , referred to 0°C, from 8·3 to 9920 torr (14 atmospheres absolute). Three independent methods have been used, involving studies of the steady conduction current, the shape of the current pulse produced by an individual electron avalanche, and the frequency distribution of the magnitudes of electron avalanches. The first two methods have also been used to determine α for nitrogen for a range of E/p 0 from 31·5 to 59 V cm -1 torr -1 and a range of p 0 from 708 to 7080 torr. There is excellent agreement among the results obtained with the various techniques and also with previously published results. It has been confirmed that α/ p 0 is dependent only on the value of E/p 0 , despite large changes in the pressure and electrode separation.