Secondary Electron Emission Produced by Relativistic Primary Electrons

Abstract

Investigations of the primary energy dependence of the rate of production of secondary electrons by relativistic primary electrons, and of the nature of the secondary energy distributions, were conducted. The "sandwich" arrangement, characterized by the feature that the primary beam is essentially monoenergetic while traversing the target, was utilized. Spectra of electrons emitted from C, Ni, and Al bombarded by primaries having energies over the range 0.3-1.6 MeV were determined. The distributions were essentially identical with those previously measured at much lower energies. The secondary electron yield was determined as a function of primary energy for C, Ni, and Al. With thick targets, and at lower energies, an enhancement occurs as a consequence of elastic scattering. The application of calculated corrections for this effect provides yield values which, for a given material, are independent of target thickness. The result that the data then follow the theoretical energy-loss curves indicates that the yield is proportional to $\frac{\mathrm{dE}}{\mathrm{dx}}$. Earlier measurements of the variation of yield with primary angle of incidence were extended to lower primary energies. An apparent saturation of the yield at steep angles of incidence occurs as a consequence of a balance between the enhanced production rate ($sec\theta$ factor) and the loss of primaries which are scattered out of the surface. It is concluded from the various results of these experiments that the secondary emission process can be represented by the relationship: $\delta ={\epsilon }^{-1\mathrm{}}\left(\frac{\mathrm{dE}}{\mathrm{dx}}\right)\Delta xsec\theta$, where $\delta$ is the number of emitted secondary electrons per incident primary, $\epsilon$ is the average energy required to produce one emergent secondary electron, $\frac{\mathrm{dE}}{\mathrm{dx}}$ is the rate of energy loss of the primary electrons, $\Delta x$ is the thickness of the region in which escaping secondary electrons are produced, and $\theta$ is the angle of incidence of the primary electrons. Values of the ratio $\frac{\epsilon }{\Delta x}=150\mathrm{}$ keV ${\mathrm{cm}}^2$/mg for C, 90 keV ${\mathrm{cm}}^2$/mg for Al, and 100 keV ${\mathrm{cm}}^2$/mg for Ni, are in agreement with those previously determined with 1-10 keV primaries. For bulk Al targets, the ratio of the measured yield at 1 keV to that at 1 MeV, approximately 55, is equal to the corresponding ratio of the theoretical rates of energy loss.