11/7/2023 0 Comments Caesium reviewAt the optimum activity there are 3.7× 10 14 atoms of caesium on a cm 2 of tungsten. From these times the amount of adsorbed caesium at various temperatures, plate potentials, and vapor pressures can be determined. If the temperature is suddenly dropped, to a low value, it takes a shorter time to reach the optimum activity. At an intermediate temperature the surface is only partly covered. The product of the arrival rate, which is given by the steady positive ion current, and the time to attain the optimum activity gives the number of caesium atoms at the optimum activity. At a sufficiently low temperature every atom that strikes the filament sticks to it, at least until the optimum activity is reached. At a sufficiently high filament temperature the surface is clean. The critical temperatures are raised by an increase in the vapor pressure or by a decrease in the plate potential.Ī method of determining the amount of adsorbed caesium is developed. At higher temperatures the ion current has its initial constant value which is limited by the arrival rate of caesium atoms. At an upper critical temperature, about 50° higher than the lower critical temperature, the filament cleans itself spontaneously, the caesium comes off as ions and registers as a sudden rush of current. If the temperature is then increased in steps the ion current retraces its path. Further decreases in temperature are followed by increased adsorption and decreased ion emission. Here the ion emission drops suddenly while some caesium sticks to the filament. The positive ion emission is constant while the temperature decreases from a high value to a low critical temperature. At still lower temperatures the surface is more than completely covered, the work function increases again, and the emission decreases rapidly. The process continues until a temperature is reached at which the tungsten is just covered with a monatomic layer when the work function has a minimum value. This lowers the electron work function and increases the emission many thousandfold. As the filament temperature is lowered more caesium is adsorbed. At high temperatures the tungsten is covered only to a slight extent with adsorbed caesium. Electron emission-Curves showing the logarithm of the electron current per cm 2 from tungsten and oxidized tungsten over a wide range of filament temperatures are given for several vapor pressures of caesium.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |