Radioactive noble gas radon ( 222 Rn) is one of the major background sources below the MeV region in rare event search experiments. To precisely measure radon concentration in purified gases, a radon detector with an electrostatic collection method is widely used. In this paper, we discussed the improvements of a radon detector by installing a new PIN-photodiode (28 × 28 mm) whose surface area is 2.5 times larger than that used previously (18 × 18 mm). We evaluated the detector's performance by serially connecting two radon detectors equipped with two types of PIN-photodiodes. As a result of the calibrations, we found an improvement of (3.8 ± 2.4)% in the detection efficiencies below 1.0 g/m 3 , while a 10-20% improvement occurred above this level. The intrinsic background of the detector equipped with the large PIN-photodiode was measured as 0.24 +0.09 −0.05 mBq/m 3 . This background level is consistent with the radon detector with the small PIN-photodiode, although we installed the large one. This improvement is useful for applications in radon emanation measurements from a material, which also emits water from its surface.
Hyper-Kamiokande is a large water Cherenkov detector planned for the near future. It will be equipped with forty thousand 50 cm photomultiplier tubes (PMT), R12860 by Hamamatsu Photonics K. K. The PMT has a high quantum efficiency, faster time response, better charge resolution and a higher detection efficiency with a stable mechanical structure, compared to the PMTs in Super-Kamiokande. The detection efficiency of the R12860 is double of the Super-Kamiokande PMTs, but the dark rate is currently high compared to those. A reduction of the dark rate is essential to improve the sensitivities of Hyper-Kamiokande. Since a scintillation light derived from PMT glass could contribute to the dark rate, we investigate the light background of the PMT glass. We found that 50 cm PMTs caught the glass scintillation by measuring a coincidence signal of two PMTs by putting glass samples and radiation sources.
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