Simulation of start electron distribution for insulation material testing using PXIPD

Pulsed X-ray Induced Partial Discharge (PXIPD) is a technique to test the quality of insulation materials by making use of the generation of start electrons through X-ray irradiation of the material. These start electrons accelerate the time needed for the generation of partial discharges within voids. An important question is the radiation dose necessary in order to be able to detect voids of a specific volume. We make use of a state-of-the-art Monte Carlo Code (Geant4) to simulate the generation of these start electrons for different void sizes. We explain the underlying techniques and discuss the results. We find that especially at high X-ray photon energies of several 100 keV most of the processes leading to start electrons are initiated in the insulation material surrounding the void and not in the gas of the void itself for the whole range of void sizes considered. This is due to the length of the electromagnetic shower generated from the primary interaction of the photon with the insulating material, which then reaches the void. We give limits of the void size detectable as a function of the total dose applied within the individual X-ray pulses and discuss implications for its application.