Aerogel RICH for forward PID at Belle II

For the Belle II spectrometer we are preparing a proximity focusing RICH with aerogel as the radiator. It will be positioned in the forward direction of the spectrometer in the small space between the drift chamber and the electromagnetic calorimeter inside a strong magnetic field of 1.5 T. The Hybrid Avalanche Photo Diode used as a photo sensor, is able to detect single photons with a high efficiency, can operate in the magnetic field and is resistant to the expected neutron and gamma fluxes in the detector. By detecting more than 11 photons per incident 4 GeV/c pion, with 15 mrad single photon Cherenkov angle resolution, the designed aerogel RICH should enable an efficient separation of kaons from pions in the wide range of particle momenta from 0.5 GeV/c up to 4 GeV/c.