Radiation-Induced Conductivity in Plastic Films at High Dose Rates

Abstract

A study has been made of the radiation‐induced conductivity (RIC) in poly[ethyleneterephthalate] (Mylar) films, using pulses of electrons of energies up to 30 MeV from a linear accelerator. The absence of polarization indicates that electronic conduction is responsible for the RIC, which has typical values of ∼10⁻⁸ Ω⁻¹ cm⁻¹ at 5×10⁹ rad(Mylar)sec⁻¹. The RIC is a bulk effect and shows both a ``prompt'' portion which falls very rapidly at the end of an electron pulse and a ``delayed'' portion which persists for milliseconds. The prompt portion varies as the 0.8 power of the dose rate and is independent of pulse width. The delayed portion varies as a lower power, saturating at dose rates of ∼10¹⁰ rad(Mylar)sec⁻¹. The decay of the delayed portion is hyperbolic, and its rate is independent of temperature. The RIC increases monotonically with temperature in the range 100°–300°K. This behavior corresponds to a model in which one sign of carrier is rapidly and permanently trapped, the delayed conductivity being due to the other sign of carrier remaining free until it recombines. The saturation of the delayed conductivity is ascribed to a finite number of such traps.