Kinetic Secondary Electron Ejection from Tungsten by Cesium Ions

Initial measurements of the number of electrons ejected per incident ion, ${\gamma }_i$, have been made for medium-energy (1-21 keV) cesium ions at normal incidence on a clean polycrystalline tungsten surface. The residual gas pressure was ≤5×${10}^{-9\mathrm{}}$ Torr. The tungsten surface was cleaned by prolonged bakeouts and by flashing before each measurement. A dc ion-beam pulse method of measurement was used to prevent cesium coverage of the tungsten surface. In the energy range of 3-21 keV ${\gamma }_i$ is a linear function of the ion kinetic energy with $\frac{\partial {\gamma }_i}{\partial E}\simeq 0.04$ electron per ion per keV. At 3 and 21 keV ${\gamma }_i$ is 0.02 and 0.74, respectively. If the data are extrapolated to ${\gamma }_i=0\mathrm{}$, a threshold energy of 2.5 keV is obtained. Below this energy the measured values of ${\gamma }_i$ were equal to zero within the accuracy of the measurement. The change of ${\gamma }_i$ with cesium adsorption on the tungsten was also recorded.