Charge dynamics in electron-beam irradiated polypropylene

The charge dynamics in 34-40 μm thick polypropylene films during and after injection of 10-40 keV electrons is investigated with the split-Faraday cup and laser-induced pressure-pulse methods. For an open-front electrode, the depth of the charge centroid increases with time during and after irradiation and attains eventually a stationary value. It also depends on the 1.7-th power of the electron-beam energy. The centroid depth further increases with increasing injected charge density but does not significantly depend on current density for the same injected charge density. If the front and rear electrodes are short-circuited after charging in open circuit, the motion of the charge centroid is halted. For relatively small injected charge densities, the charge is trapped in a narrow region around the range of the electrons; for large charge densities, the distribution broadens. The space-charge field, radiation-induced conductivity, and limited trap density are responsible for the motion of the charge in the dielectric