In order to reduce the measurement lower limit of the primary pressure boundary leakage monitoring,the coincidence detection efficiency based on positron decay nuclides has been studied.In order to improve the coincidence detection efficiency,the particle transport process in the coincidence detection device has been simulated and analyzed by Monte Carlo code Geant4,and the effects of coincidence detection device structure and detector properties on the coincidence detection efficiency have been studied.The research results show that:adding β+absorbing layers on both sides of the filter paper can significantly improve the coincidence detection efficiency,and the optimal coincidence detection efficiency is obtained when 0.5mm Al or 0.2mm Fe is used as β+absorbing layer;due to the difference in energy resolution of different types of detectors,different types of detectors have different energy window coefficients for reaching the optimal detection efficiency.The optimal energy window coefficients of NaI(Tl),BGO and LaBr3(Ce)are 14％,26％and 7％respectively.The results of this research can provide a reference for the structural design of the detection device and the coincidence judgment logical design of the primary pressure boundary leakage monitoring system with 18F nuclide as the radioactive tracer.
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