Study on Radiation-induced Conductivity of Insulation Materials under High Electric Field

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In order to examine the radiation-induced conductivity (RIG) of insulators, some sapphire samples were irradiated at room temperature with DT neutrons and gamma-rays. The radiation-induced current of the sample was proportional to the radiation flux under a given bias voltage. And the RIG coefficient of the sapphire was about 1 x 10^<-10>/Ω/m/Gy/s. Also, a large transient current was observed at the start of the irradiation. Behavior of radiation-induced charge in the insulator was discussed together with results of pulsed X-ray irradiation experiments on a mineral-insulated (MI) cable. The reduction of the electric resistance of the cable was well measured by use of the pulse-charge measuring system.The charge induced in the central wire of the cable was approximately proportional to the bias voltage and the X-ray dose per pulse, which means that the drift and production rate of charge carriers (i.e., electrons) dominates the degradation of the cable. The average drift length of electrons in the cable insulator (MgO) was estimated to be about 15 nm (bias voltage! 100V and insulator thickness! 1.5 mm). For the detailed analysis of the degradation of the insulator and the MI cable, we need further discussions with numerical calculations based on the Poisson's equation with a precise transport model of electrons in the insulator.