Secondary Electron Emission from Targets of Barium-Strontium Oxide

The secondary electron yield of (BaSr)O has been studied, as induced by microsecond pulses of primary electrons with energy up to 2000 ev. The $\delta \mathrm{vs}.\mathrm{} V_p$ curves have the usual form, with maximum $\delta$ near 1200 ev. At room temperature, and before surface charges build up, the ${\delta }_{max}$ is of the order 12, but it may be reduced to 6 by less than 0.1 atomic layer of Ba evaporated from a nearby thermionic cathode. With increasing temperature $\delta$ decreases to an apparent minimum at Å600°C. With the onset of d.c. thermionic emission the total yield increases during each pulse, in rough proportion to the thermionic current. The increase is thought to represent a transient change in thermionic activity caused by the bombardment. No change with temperature is observed for the energy distribution of the true secondaries. The possibility of field-enhanced secondary emission at low temperatures is considered.