The Temperature Dependence of Secondary Electron Emission from Oxide-Coated Cathodes

Abstract

The secondary electron emission from alkaline-earth oxide-coated cathodes has been investigated under both continuous and pulsed bombardment. Experiments have been performed with three types of apparatus. Yield vs. energy data reveal values of $\delta$ of 4-7 at room temperature, with a more or less flat maximum at approximately 1000 volts primary energy. The yield increases with temperature in an exponential manner, and plots of $log \Delta \delta (\mathrm{i}.\mathrm{e}.\mathrm{},{\delta }_{K°}-{\delta }_{300°K}) \mathrm{vs}.\mathrm{} \frac{1}{T}$ give straight lines. Values of $Q_1$ between 0.9-1.5 ev are generally indicated, and from extrapolation of these curves, yields exceeding 100 at 850°C are deduced. The secondary emission depends upon the degree of activation, and increases with enhancement of the thermionic emission characteristics. Short time effects such as growth or decay of secondary current after the onset of primary bombardment or persistence after the cessation of bombardment have not been observed, and values of yield obtained by pulsed methods are in accord with those obtained under d.c. conditions. Tail phenomena reported by J. B. Johnson and interpreted as "enhanced thermionic emission" from oxide-coated cathodes become manifest only under experimental conditions characterized by certain space-charge effects, and have been effectively simulated by bombarding a tantalum target adjacent to an electron-emitting tungsten filament. Various measurements of the energy distribution of secondary electrons as a function of primary voltage and temperature have been obtained. It was observed that the average energy of the secondary electrons decreases with temperature at a rate which more than compensates for the increase in the number of secondaries emitted per incident primary. The mechanism of the observed dependence of yield upon temperature is not well understood. Various alternative explanations are discussed and, in the light of the present state of our knowledge, regarded as untenable.