Observed changes in electrical materials properties of ZrO2 in Megapie leak detectors during irradiation

Reliable leak detection is of high importance for liquid metal targets. For the PSI Megapie target two novel approaches had been pursued: a thermocouple-based leak detector and one employing capacity sensors with impedance read out. Whereas the temperature based leak detector performed according to expectations, severe degradation of the impedance probes occurred during the irradiation period of Megapie. The electrical characteristics of the chosen insulator material ZrO2 stabilized by 1% Y2O3 showed massive changes over time and they exhibited strong dependence on the prevailing environment conditions. Three different contributions to effective electrical conductivity in the supposed insulator could be identified: a prompt radiation induced, a temperature induced, and most probably a gas induced change in conductivity. These components, both increases and decreases in bulk conductivity of the insulating slabs in the impedance probes, demonstrated some cross-dependence on each other. The alterations in conductivity and the mutual influence of the identified factors changed with the irradiation time and the probable exposure to gaseous radiolysis products at the location close to the beam entrance window of the liquid metal target. A phenomenological and qualitative description of the observed effects as they evolved during Megapie operation is presented.