Design

One layer of the water Cherenkov counter is consisted of 12 segments. For each segment, 35${}^H \times$ 15${}^W \times$ 7.5${}^T$ cm${}^3$ effective volume of the Cherenkov radiator i.e., water(index: 1.33) is directly contained in a white acrylic diffusion box(3 mm thick., Mitsubishi Acrylite # 402) and 2 $\times$ 2'' photomultipuliers(HAMAMATSU H7195$-$MOD) are attached on the two long sides through 3 mm thick colorless acrylic window(Mitsubishi # 000). Maximum diffuse reflectance of the diffusion box is 94.8 % on 458.5 nm incident ray and typical transmittance of the window plate is $\sim$ 90 % on the PMT quantum effective wave length region. 50 mg/$l$ Amino G-salt(2-amino-6, 8-naphthalene-disulfonic acid) was dissolved in $\sim$18 M$\Omega\cdot$cm distiled water as a wave length shifter. This chemical and its handling is well controlled by Safety Lab(Chemical assistance team) chemical expert.