Electron Optical Transfer : A Study of Electron Bombardment of Surfaces †

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Abstract
Electron bombardment of thin targets of barium, strontium, sodium and caesium using a standard electron gun shows that it is possible to transfer these materials from the anode to the cathode by simple electron bombardment. Their presence was revealed by the increase in thin film thermionic activity of a pure strontium oxide emitter and the subsequent changes in slope of its modulation characteristic. Threshold bombarding energies lie in the region of 200–300 volts and corresponding current densities at the target of the order of 1 mA/cm2. Increase in beam energy to 400–600 volts markedly speeds up the process. Features of such thin-film build-up include (i) large temperature dependence, (ii) high tenacity. Emission microscope studies in the case of barium and caesium give visual evidence of the arrival at the emitter of active material only during the process of target bombardment. A mechanism to account for the transfer is proposed in which L, M or N ionization occurs on the surface of the target, followed by thermal evaporation. Such positive ions as are emitted, are directed by the electron-optical field, back towards the emitter. An application of these effects can be made to the renewal of loss of active material during operation life of oxide or other thin film emitters.