A Clock Synchronization System for Large Volume with Sub-ns Resolution Designed for Hyper-Kamiokande Experiment

Hyper-Kamiokande (HK) is a next generation Water-Cherenkov detector for neutrino physics and nucleon decay search. It will have a cylindrical shape of 68 m in diameter, 71 m in height and 8 times larger effective volume than Super-Kamiokande (SK) which is the current largest underground Water-Cherenkov detector. In order to achieve the physics goal, excellent detection efficiency and timing resolution of Cherenkov photons are essential. We plan to install about 53,000 photomultiplier tubes (PMTs) in the detector. The 50 cm PMTs for HK have been developed, and its performance got twice better than that of the SK, the photo-detection efficiency is about 30 % and the timing resolution is about 1 ns. In order to maximally utilize the improved PMTs, it is necessary to develop signal digitizers and their synchronization system which have better performance than those of PMTs. We plan to install front-end electronics (FEEs) in the water tank because we would like to avoid degradation and attenuation of the PMT signal in HK. Data are communicated between inside and outside of the tank via optical fibers, whose length is up to 200 m. In this study, we have developed a prototype of the system consisting of a digitizer and a synchronization system. We designed the electronics so that a module generates a reference clock and distributes it, and then FEEs run in synchronized mode with the supplied clock. We confirmed this design has sub-ns timing resolution, and this scheme is applicable for the DAQ system in HK.