Enhanced Photoemission Capability of Bialkali Photocathodes for 20-Inch Photomultiplier Tubes

The 20-inch photomultiplier tube has been extensively applied in neutrino detection, for example, in the Super-K, Daya Bay, and JUNO detectors. In order to achieve a high quantum efficiency, two types of structures, namely variably-doped and uniformly-doped structures, are proposed in this study. It is found that the maximum QE of the variably-doped K-Cs-Sb cathode is about 33%, as compared to the maximum QE of 25% for the uniformly-doped cathode. These high QE bialkali photocathodes of the 20-inch photomultiplier tubes were delivered to JUNO, and the average detection efficiency (410 nm) of 6384 mass-produced PMTs is found to be 30.1%.