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 PMT DCR under different radiation conditions were examined via underground tests, lead shielding experiments, and irradiation with \(^{ 55}\mathrm{ Fe}\), \(^{ 60}\mathrm{ Co}\), and \(^{ 90}\mathrm{ Sr}\) 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, $\beta$ particles from the \(^{ 90}\mathrm{ Sr}\) source increased the DCR by approximately 14 kHz, whereas the effects of $\mathrm{ X}$-rays from \(^{ 55}\mathrm{ Fe}\) and $\gamma$-rays from \(^{ 60}\mathrm{ Co}\) were comparatively minor. In addition, Geant4 simulations provided quantitative analysis of Cherenkov radiation induced by $\beta$ particles, with the simulation results closely matching the experimental data and confirming $\beta$ 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
Categories: hep-ex