Detailed performance evaluation of a new 20-inch photomultiplier tube with a Box and Line dynode

For Hyper-Kamiokande, which is a future planned experiment using a 1 Mton water Cherenkov detector, several types of new photodetectors are under development. Box & Line photomultiplier tube is the leading candidate with high resolution and efficiency. By the improvement on quantum efficiency and photoelectron collection efficiency, the Cherenkov photon detection efficiency is twice as high as that of the photomultiplier tube used for Super-Kamiokande. The detailed performance of the Box & Line photomultiplier tube was evaluated to check its appropriateness for Hyper-Kamiokande, and it was confirmed to be sufficiently good. At single photoelectron level, a high resolution of 35% σ/μ in charge and 4.1 ns at FWHM in transit time was confirmed, superior to 53% and 7.3 ns, respectively, in case of the Super-K photomultiplier tube. The linearity range within 340 photoelectrons and high rate tolerance up to 87 MHz at single photoelectron are evaluated. The dark rate at room temperature is about 10 kHz typically.