Total Secondary Electron Emission from Tungsten and Thorium-Coated Tungsten

The secondary electron emission from polycrystalline tungsten covered with monomolecular films of thorium evaporated onto it from a thoriated tungsten filament, has been investigated over an energy range for primary electrons of 100 to 1000 volts. The state of the target surface was ascertained from thermionic emission measurements. For clean thorium on clean tungsten apparently the secondary emission coefficient did not change with thoriation for primary energies below 200 volts, but decreased at higher voltages with increasing amounts of thorium on the target. No observed increase in secondary emission takes place with a reduction in the work function of tungsten by a monomolecular layer of pure thorium; apparently there is a decrease in secondary emission in this case. When the thorium-coated tungsten was treated with oxygen released from the thoriated tungsten filament the work function increased, but there obtained also an increase in the secondary emission coefficient; further change in work function by evaporation of thorium caused a variation of the secondary emission coefficient that other experimenters also have observed.