The dielectric response of polyethylene irradiated at 5 K

An investigation has been made of the effects of nuclear radiation on dielectric breakdown and charge transport in low-density polyethylene (LDPE) at the temperature of liquid helium. The practical application of this work is the integrity of the electrical insulation in the superconducting coils necessary for the magnetic confinement of fusion reactor plasmas. Test specimens of 30- mu m-thick LDPE film were irradiated at 5 K to radiation absorbed doses of up to 5*10/sup 6/ Gy in a thermal nuclear reactor. Following irradiation they were maintained at 5 K and subjected to various electrical tests. Isochronal current/voltage measurements were made of charging current, and associated short-circuit discharging current, for applied fields up to approximately 100 MV/m. Dielectric breakdown strength tests were also carried out using a 10-KV/ mu s ramp from a Marx impulse generator. The results show that charge transport in LDPE under the above conditions is some form of polarization. The electric conductivity increases as a result of exposure to radiation. It is postulated that these effects are due to fast neutrons.<>