Behaviour of non-conducting plastics under e-beam irradiation

When a non-conducting plastic block is irradiated with high-energy electrons, the thermalised electrons are trapped in the block if the range of the incident electrons is less than its thickness. These stored charges set up an electrostatic field in the block that can affect the trajectories of the incident electrons. The strength and the shape of the field depend on the intrinsic properties of the material as well as on the irradiation geometry. For this study, we selected various commercial electrical insulators such as polymethyl methacrylate (PMMA), polycarbonate (PC), polyethylene (PE), Teflon (PTFE), polyvinylchloride (PVC) and nylon. We measured the influence of the electrostatic field on depth-dose distribution in these materials using GafChromic© film dosimeters, and the subsequent leakage of the stored charge. This is an extension of our previous work on PMMA. The effect on the depth-dose distribution was most pronounced in PMMA, while in PTFE and PE it was minimal. On the other hand, PC showed an intermediate behaviour. Nylon and PVC electrically broke down at low doses. We have attempted an explanation of this behaviour of the various plastics based on radiation-induced conductivity, the measured leakage charge and also on their intrinsic electrical properties such as dielectric constant and dielectric strength.