Concepts for secondary electron emission from irradiated insulators

Planar 1-dimensional samples of 2.815 mm thick polymethylmethacrylate (PMMA) were irradiated in vacuum by 1 MeV electrons at constant intensity under three conditions of electrode attachment. Electrodes were either painted on the surface or spaced in vacuum 6.3 mm from the surface. Currents to the electrodes were monitored during the irradiation. The radiation dose penetrating the insulator was also monitored. As the radiation proceeded, the insulators became charged and modified the depth of penetration of the 1 MeV electrons. A simulation code predicts the correct electrode currents and penetrating dose rate only if one allows the low energy secondary electron emission to be a strong function of the electric field just inside the insulator PMMA surface. Secondary emission is enhanced more than a factor of ten with the surfaces at 10 kV/cm.