Tau Neutrino Appearance via Neutrino Oscillations in Atmospheric ...

29• Cartridge polisher:Eliminates metal heavy ions such as Na + , Cl − , Ca +2 .• UV sterilizer:Kills any bacteria in the water.• Radon-free air dissolving tank:Dissolves air from the radon-free system into the water to improve theradon removal capabilities of the vacuum degasifier.• Reverse osmosis filter:Futher removes small contaminants.• Vacuum degasifier:Removes dissolved gases (oxygen and radon) from the water, with efficiencyof ∼96% for the dissolved radon gas. Oxygen in the water cancause growth of bacteria.• Ultra filter:Removes small contaminants down to sizes of 10 nm.• Membrane degasifier:Further removes dissolved radon gas with a removal efficiency of 83%.Before the purification process, a typical number of particles of size greaterthan 0.2 µm in the water is about 1,000 particles/cc, which is reduced to6 particles/cc after the purification. The resistivity of water entering thepurification system from Super-K is about 11 MΩ·cm. After the purification,the water has an average resistivity of 18.20 MΩ·cm, which is very close tothe chemical limit of 18.24 MΩ·cm. The concentration of 222 Rn in the waterafter the purification is reduced to less than 1 mBq/m 3 [58] and is monitoredin real-time by several radon detectors [59, 61]. The light attenuation lengthis achieved to be ∼100 m (Section 3.2).2.5 Radon Hut and Air Purification SystemIt is also essential to have clean air in the detector and the experimentalarea to minimize the radon level in the air. The radon concentration of themine air in the access tunnel to the experimental site has a strong seasonalvariation of 2,000 ∼ 3,000 Bq/m 3 during summer and 100 ∼ 300 Bq/m 3 duringwinter as shown in Figure 2.12. This is caused by the seasonal variation of the