Electron emission from electron-irradiated dielectrics

Two methods for the measurement of the electron emission currents and the secondary electron yields for electron-irradiated dielectrics are discussed. Experimental results indicate that the dynamic measurement method for the secondary emission yield δ provides yield curves that are significantly lower than those obtained by conventional methods, at least in the region in which δ goes through a maximum. This effect is believed to be due to a recombination between the electrons undergoing emission and the positive surface charges which the dielectric acquires during the continuous irradiation to which it is exposed in the dynamic method. A modified method that replaces the continuous radiation by pulse radiation and eliminates the formation of charges on the dielectric is described. It is shown that, at beam energies well beyond the energy E_m of the maximum yield, both methods give similar results. The pulse method was used to determine the yield curves of a variety of polymers and to confirm that these curves can be represented in the form of a universal curve when δ/δ_m is plotted against E/E_m. It was found that the positive and negative charges generated on the dielectric by the electron irradiation remain surface charges and do not drift into the bulk of the material