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 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, $β$ 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 $γ$-rays from \(^{60}\mathrm{Co}\) 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