Performance of 50 cm Photomultiplier Tube for Hyper-Kamiokande

We developed a 50 cm-diameter photomultiplier tube (PMT) for Hyper-Kamiokande, which is a large water Cherenkov detector in Japan aiming at neutrino and nucleon-decay research by using forty thousand photosensors. The PMT, R12860 manufactured by Hamamatsu Photonics K.K. (Hamamatsu), achieved a double detection efficiency, reduced timing and charge resolutions as well as sufficient mechanical strength compared with a prior 50 cm PMT, R3600 for Super-Kamiokande. In 2018 over a hundred of the R12860 PMTs were installed in Super-Kamiokande, where we evaluated the performance together with the R3600 PMTs. The collection efficiency reached 95% with a higher quantum efficiency (QE) of 33% typical at peak of wavelength around 400 nm. Charge resolution was evaluated to be 27% in standard deviation and time resolution to be 1.5 nanoseconds in FWHM. Finally, noise reduction succeeded with clean materials and low dark count rate.