Studies of a proximity focusing RICH with aerogel radiator for Belle II experiment

For the Belle II experiment, we are developing a proximity focusing ring imaging Cherenkov counter using a silica aerogel as a radiator. Recently, a new aerogel fabrication method, called “pin-hole drying”, was invented and remarkably improved optical transparency has been realized in the region of refractive index larger than 1.05. For the photo-detector, we have developed a 144-channel Hybrid Avalanche Photo-Detector with Hamamatsu Photonics K. K. The HAPD has excellent single photon detection performance, 5 × 5 mm² position resolution and immunity to magnetic field. The tolerance to the neutron irradiation up to 5 × 10¹¹ neutron/cm² has been confirmed. In November 2009, we have performed a test beam experiment at KEK for a prototype A-RICH counter with 6 HAPDs and aerogel tiles produced by the “pin-hole drying” method. We successfully observed a clear Cherenkov ring image and obtained the single photon Cherenkov angle resolution of 13.5 mrad and 15.3 detected photoelectrons. This result corresponds to more than 6σ K/π separation at 4 GeV/c and fulfills the requirement for the Belle II.