Development and measurement of new large-aperture photodetectors for Hyper-Kamiokande

Two types of new photodetectors are currently being developed with a large aperture, for use in the large water Cherenkov detector, Hyper-Kamiokande planned in Japan [1]: Hybrid Photo-Detector (HPD) of a 20-inch diameter size with an avalanche diode and a 20-inch Photomultiplier Tube (PMT) with a dynode upgrade to a box-and-line type. These are expected to have an excellent performance about timing and collection efficiency compared to a 20-inch PMT with a venetian blind dynode used in Super-Kamiokande. Also there is a possibility to improve quantum efficiency (QE) of these photodetectors from 22% to around 30% using the super-bialkali technique. A proof test in a smaller water Cherenkov detector is planned in advance for these photodetectors to determine the best candidate for Hyper-Kamiokande. Two prototypes of new photodetectors have already been manufactured for testing: the 8-inch HPD, and a 20-inch PMT with a higher QE of 30% compared with 22% of the 20-inch PMT used in Super-Kamiokande. These photodetectors were installed in a 200-ton water tank in summer 2013 for a proof of using test in a water Cherenkov detector. We report the specification of these photodetectors, a status and plan of the development and proof test.