Columnar ionization in the electron bombardment conductivity of amorphous dielectrics

New measurements are presented of conduction induced in arsenic trisulphide films by beta and cathode rays as a function of film thickness and incident beam current. Films between 2 and 20 μm thick have been examined, the beta and cathode rays being sufficiently energetic to penetrate the films completely. It is concluded that the majority of the carriers are generated along the paths of the primary electrons and only those that escape recombination in these columns are available for conduction. The low probability of escape is responsible for the absence of saturation of the induced conduction with bias voltage. Under steady-state conditions the free path of carriers which escape recombination in the columns is normally long enough not to reduce the efficiency of their collection appreciably.