Dark count of 20-inch PMTs generated by natural radioactivity

Abstract The primary objective of the JUNO experiment is to determine the ordering of neutrino masses using a 20-kton liquid-scintillator detector. The 20-inch photomultiplier tube (PMT) plays a crucial role in achieving excellent energy resolution of at least 3 % at 1 MeV. Understanding the characteristics and features of the PMT is vital for comprehending the detector's performance, particularly regarding the occurrence of large pulses in PMT dark counts. This research paper aims to further investigate the origin of these large pulses in the 20-inch PMT dark count through measurements and simulations. Our results confirm that natural radioactivity and muons striking the PMT glass are the main sources of the large pulses. We evaluate their contribution quantitatively by performing spectrum fitting. By analyzing the PMT dark count spectrum, it becomes possible to roughly estimate the radioactivity levels in the surrounding environment.