Critical Primary Velocities in the Secondary Electron Emission of Tungsten

The secondary electron emission of tungsten has been studied at temperatures up to 1480°K under a vacuum of ${10}^{-8\mathrm{}}$ mm of Hg, using a tube specially designed to give a surface as free as possible from gas layers. The curve showing the number, $n$, of secondary electrons produced by one primary electron as a function of the primary velocity, taken with tungsten at 1250° to 1450°K and in the velocity range of 10 to 700 volts has a maximum at 15.3 volts, a minimum at 20 volts and 18 breaks between 25 and 600 volts which have not been observed hitherto. There is a good agreement between these breaks and the inflections found by Richardson and Chalklin in the soft x-radiation of tungsten. A comparison with computed values for the $N$ and $O$-energy levels of tungsten gives good evidence that some of the breaks are due to these levels. The maximum of the secondary emission curve is found near 630 volts primary velocity. With cold tungsten, as long as the surface has taken on but little gas, a few additional breaks are obtained besides those found with hot tungsten. Generally, with cold, well degassed tungsten the breaks are more intensive. None of these breaks appear when a large amount of gas is on the surface. With hot tungsten $n$ is reproducible to within 0.1 percent, if operating conditions are not changed and if the tungsten is heated sufficiently long before measuring. With two different targets $n$ is found to be different, having maximum values of 1.29 and 1.40. The effect of adsorption of gas is to increase $n$ for low velocities, while for higher velocities the reverse is true. Both with cold and hot target $n$ varies with time, as long as adsorption or liberation of gas takes place. The variation of $n$ with time may be thus used to follow up adsorption or liberation of gas by a metallic surface in a high vacuum. The shape of the secondary emission curve in the range of 10 to 25 volts primary velocity is due to adsorbed gases. There is a maximum at 15.5 volts with cold, well degassed tungsten, which is also observed even at the highest temperatures produced. This maximum is perhaps due to a surface layer of oxygen molecules. Judging by the secondary emission the surface of tungsten cannot be considered as entirely freed from gas at temperatures up to 1500°K.