Performance and commissioning of HAPDs in the Aerogel RICH counter

The Belle II Aerogel Ring Imaging Cherenkov (ARICH) counter uses the angular distribution of Cherenkov photons emitted in silica aerogel to discriminate between charged pions and kaons. The Hybrid Avalanche Photo Detector (HAPD) is used as the photo-sensor, and is clearly the most critical component of the detector. HAPDs were installed into ARICH in July 2017, and ARICH was installed in the Belle II spectrometer in the end of 2017. During the Belle II beam commissioning in spring 2018, the performance of HAPDs was evaluated through measurements of the offset value, noise level and pulse height. Long term stability of the signal-to-noise ratio and the fraction of dead channel was also monitored. The signal-to-noise ratio exceeded 6, and the fraction of dead channels was less than 1%. The results of the performance evaluation and of the commissioning showed that the ARICH counter fulfills the requirements.