Townsend's Ionization Coefficient for Helium

ACCURATE experimental data are lacking for Townsend's ionization coefficient α/p for electrons drifting through helium, when the ratio X/p of electric field to pressure is small. As has been pointed out by Loeb¹, however, it is possible in this instance to calculate α/p from the relation² where W is the drift velocity of the electrons, v the speed of an electron which has energy V in its random motion, f(V) the energy distribution function in the random motion, I the ionization energy, and P(V) the probability of ionization for an electron with energy V per unit random path at unit pressure, since the form of f(V) has been determined with considerable accuracy by Smit³.