Study on the impact of radioactive background on the dark count rate of 20-inch MCP-PMTs

Abstract This study systematically investigates the impact of natural radioactive background on the dark count rate (DCR) of 20-inch microchannel plate photomultiplier tubes (MCP-PMTs). Variations in PMTs' DCR under different radiation conditions were examined via underground tests, lead shielding experiments, and irradiation with 55 Fe, 60 Co, and 90 Sr sources. The experimental results indicate that natural radioactivity from surrounding air and rock in the underground environment results in a significantly higher DCR compared to laboratory conditions. Further, lead shielding experiments confirmed that effective shielding can markedly reduce the interference from environmental background radiation. Notably, β particles from the 90 Sr source increased the DCR by approximately 14 kHz, whereas the effects of X-rays from 55 Fe and γ-rays from 60 Co were comparatively minor. In addition, Geant4 simulations provided quantitative analysis of Cherenkov radiation induced by β particles, with the simulation results closely matching the experimental data and confirming β particles as the primary contributor to the DCR increase. These findings offer both theoretical and experimental evidence for a deeper understanding of the influence of radioactive background on 20-inch MCP-PMTs' performance in underground experiments and hold significant implications for improving the energy resolution of large-scale neutrino detection systems