Measurements and simulation of the spatial charge distribution in electron-beam-irradiated polymers

The amount and profile of the charge distribution in a polymethyl methacrylate plate of 7.9-mm thickness were measured after electron beam irradiation. When the plate was irradiated by a beam whose energy was 1 MeV and the total dose was 3 kGy, a spatial charged layer was produced at approximately 2.5 mm from the irradiated side. The maximum charge density was 1.7 μC/cm3 . The charges of the irradiated side decayed faster than the charges which accumulated at positions further away from the irradiated side in the specimen. Computer simulations were carried out to study the charge decay phenomena, assuming that shallow trap levels were created at the irradiated side by the electron beam. The time dependence of the accumulated charge distribution agreed well with the experimental results.