A test of new large aperture photodetectors in a water Cherenkov detector

Several new photodetectors with high resolution and efficiency have been developed for a one megaton water Cherenkov detector, Hyper-Kamiokande, planned in Japan. A reliable photodetector is essential for such a long-term experiment aiming at the neutrino physics and the nucleon decay search. In 2013, a first proof test started using a 200 ton water Cherenkov detector in order to evaluate the feasibility of new photodetector techniques for Hyper-Kamiokande: high quantum efficiency (QE), a hybrid photodetector (HPD) and a new photomultiplier tube (PMT) with a box-and-line dynode. Eight HPDs with a 20 cm diameter were installed to the 200-ton water tank in advance of a 50 cm HPD. The QE of 50 cm Venetian-blind-dynode PMT used in SuperKamiokande was improved to 30% from 22%. Five of such high-QE PMTs were attached as well in 2013. Three 50 cm high-QE box-and-line PMTs joined in 2014 and showed high detection performance. In the 200-ton tank, in total 240 photodetectors including the 50 cm conventional Super-Kamiokande PMTs have been tested over a year. Sufficient performance to detect the single photoelectron was confirmed in the tank; 40-45% charge resolution in sigma and 5.1 nsec timing resolution at FWHM for box-and-line PMTs. New photodetectors were monitored over a year, and a stability of gain is confirmed in 2% level. The test allows to select the best candidate photodetector for Hyper-Kamiokande, with coming 50 cm high-QE HPDs.