Secondary Electron Emission from Metal Surfaces byH+,H0,He+, andHe0Bombardments

Abstract

In this paper, secondary electron emission coefficients for metal surfaces bombarded by ${\mathrm{H}}^{+}$, ${\mathrm{H}}^{0}$, ${\mathrm{He}}^{+}$, and ${\mathrm{He}}^{0}$, and the ratios $\frac{\ensuremath{\gamma}_{\mathrm{H}}^{0}}{\ensuremath{\gamma}_{\mathrm{H}}^{+}}$ and $\frac{\ensuremath{\gamma}_{\mathrm{H}}^{0}}{\ensuremath{\gamma}_{\mathrm{He}}^{+}}$ have been calculated. As in a previous paper, it is assumed that internal secondaries are produced by ionization. For the energy range 20 keV-2 MeV, Bethe's formula for ionization cross section has been modified to the form $\frac{{Q}_{B}}{(1+\frac{\ensuremath{\beta}}{T})}$, where ${Q}_{B}$ is the ionization cross section given by Bethe's original expression, $\ensuremath{\beta}$ is a constant, and T is the energy of the incident ion. For a proton beam, the capture cross section becomes appreciable for energy less than 200 keV. Hence, the beam is considered as a two-component system consisting of protons and hydrogen atoms both producing internal secondaries. Above this energy range, the beam is considered as a one-component system. Similarly, a hydrogen atomic beam has been considered as a three-component system consisting of ${\mathrm{H}}^{+}$, ${\mathrm{H}}^{0}$, and electrons, and a helium beam for energy greater than 80 keV as a four-component system consisting of ${\mathrm{He}}^{0}$, ${\mathrm{He}}^{+}$, ${\mathrm{He}}^{++}$, and electrons. Below 80 keV, the helium beam is regarded as a mixture of ${\mathrm{He}}^{0}$, ${\mathrm{He}}^{+}$, and electrons which produce secondaries by ionization. The calculated secondary electron emission coefficients are compared with experimental data. The agreements are satisfactory as the percentage deviation is only about 10%.