Study on the Impact of Radioactive Background on the Dark Count Rate of 20-inch MCP-PMTs

TL;DR

This paper investigates how natural radioactive backgrounds affect the dark count rate of 20-inch MCP-PMTs, combining experiments and simulations to identify primary radiation sources and mitigation strategies for underground neutrino detectors.

Contribution

It provides a comprehensive experimental and simulation analysis of radioactive background effects on MCP-PMTs, highlighting the dominant role of $eta$ particles and effective shielding methods.

Findings

Radioactive background significantly increases DCR in underground environments.

Lead shielding effectively reduces background-induced DCR.

$eta$ particles from $^{90}$Sr$ are the main contributors to DCR increase.

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, $\beta$ 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