Townsend coefficient

The first Townsend coefficient ( α ), also known as first Townsend avalanche coefficient is a term used in an ionization process where there is a possibility of secondary ionization because the primary ionization electrons gain sufficient energy from the accelerating electric field or from the original ionizing particle. It is equal to the reciprocal of the mean free path ( γ ) of the primary electrons in i.e., α = 1/γ . The coefficient gives the probability of a secondary ionization per unit path length.