Photoelectric and Thermionic Investigations of Thoriated Tungsten Surfaces

Photoelectric and thermionic data were obtained over a wide range of temperatures for several activations of a thoriated tungsten filament. The temperature dependence of the thermionic and photoelectric work functions are quite different. It is shown that this is to be expected for a nonuniform surface. The measured current density in either case is $i=\left(\frac{1}{A}\right)\int s^{}{i}_s\mathrm{ds}$, in which $i_s$ is given by either the Richardson-Dushman or the Fowler-DuBridge equation and $A$ is the total emitting area. On the basis of a simple patch theory the integrations are carried out and show that (1) the saturation electron emission is independent of the size of the patches; (2) the observed or characteristic thermionic and photoelectric work functions for composite surfaces are different at low temperatures and approach each other at high temperatures; (3) the maximum and minimum values of the work function for a given surface do not vary linearly with temperature.