Abstract
THE direct ionization of atoms by impact of positive ions, as postulated by Townsend in his theory of the electrical breakdown of gases, has been the subject of much controversy1. Direct observation of ionization by ion impact is difficult, because the effect is small and is easily obscured by ionization due to secondary electrons or to photo-ionization by the short-wave radiations emitted by the source of ions. However, there is a growing body of evidence2 that positive ions with energies of a few hundred electron-volts can ionize gases by impact. Recent work by Llewellyn-Jones and his co-workers in Swansea3 has revived belief in the validity of the Townsend hypothesis.
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