  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 rate through measurements and simulations. The
findings confirm that the main sources of the large pulses are natural
radioactivity and muons striking the PMT glass. By analyzing the PMT dark count
rate spectrum, it becomes possible to roughly estimate the radioactivity levels
in the surrounding environment.
