An experimental study of charge distributions in electron-beam irradiated polypropylene films

Mean depths and profiles of charge distributions in 34 to 40 μm thick polypropylene films during and after injection of 10 to 40 keV electrons are investigated with the split Faraday cup and the laser-induced pressure pulse methods. For an open front (irradiated) electrode, the mean charge depth increases with time during an after irradiation and eventually attains a stationary value. This value depends on the 1.7-th power of the electron beam energy. The charge depth also increases with increasing injected charge density, but does not significantly depend on current density at the same injected charge density. If the front and rear electrodes are short circuited after charging in open circuit, the motion of the charge is halted. For relatively small injected charge densities, the charge eventually is trapped in a narrow region just beyond the continuous slowing down approximation range of the electrons. For larger charge densities, the distribution broadens toward the rear electrode. The charge dynamics are caused by the space charge field, the radiation-induced conductivity, and the limited trap density