Breakdown of Ionic Crystals by Electron Avalanches

In gases at moderate pressures breakdown occurs when the avalanche produced by a primary electron regenerates with certainty a new starting electron (Townsend condition). In ionic crystals the breakdown strength appears to be determined by the probability of exciting lattice vibrations, without reference to the electron supply. The theory as formulated to date predicts an increase of the breakdown strength with temperature. Confirming our previous results we find for both d.c. and a.c. an increase followed by a decrease. By extending our experiments to transient voltages of rise times down to ${10}^{-8\mathrm{}}$ sec., and by adopting evaporated metal electrodes and various kinds of irradiation, the influence of field emission and of space charges on the breakdown strength and on the scatter of the breakdown values has been investigated. The occurrence of directional breakdown paths seems to be connected with the Brillouin zones of the crystal structure as the outcome either of electron diffraction or of collisions between electrons and directed lattice vibrations. The possibility of using crystals as Geiger counters is briefly discussed.