New 50 cm photo-detectors for Hyper-Kamiokande

Two types of 50 cm-diameter photo-detectors were developed for Hyper-Kamiokande, a large water Cherenkov detector planned in Japan for neutrino and nucleon decay researches. A box-and-line dynode enabled further improvements on a 50 cm photomultiplier tube (PMT) used in Super-Kamiokande, which was developed to be an R12860 type by Hamamatsu Photonics K.K. (Hamamatsu). The collection efficiency reached 95%, superior to 67% of the PMT in Super-Kamiokande. It also gave a higher quantum efficiency of 30% typical at peak than 22% in case of the Super-Kamiokande. Another one is a 50 cm-diameter hybrid photo-detector (HPD), R12850 by Hamamatsu, using an avalanche diode for the amplification system. The detection efficiency is as high as the improved PMT, while the resolutions are highest among the existing 50 cm photo-detectors. At single photoelectron, charge resolutions were evaluated to be 35% and 15% in standard deviation and time resolutions to be 4.1 and 3.6 nsec in FWHM for the improved PMT and HPD, respectively. The glass bulb of both new photo-detectors sustained high pressure water up to 1.25 MPa. A newly developed cover for a long-term safe operation in Hyper-Kamiokande was validated at the 80 m deep water level even with a bulb implosion.