Development of a RICH counter with 144-ch Hybrid Avalanche Photo-Detectors for the Belle II experiment

For the Belle II experiment at the superKEKB accelerator, we have been developing a proximity focusing ring imaging Cherenkov counter using silica aerogel as a radiator (ARICH). 144-channel Hybrid Avalanche Photo-Detectors (HAPD) developed with Hamamatsu Photonics K.K. were adopted as the Cherenkov photon detectors for the ARICH. We confirmed that the HAPD has sufficient performance for the ARICH photon detector by the bench test and carried out a test beam experiment with a prototype ARICH counter with six HAPDs, arranged as the actual ARICH configuration, at CERN in September 2011. As the result of the experiment, we succeeded in observing distinct Cherenkov images and confirmed that prototype ARICH had potentially 5.5σ K/π separation ability. One of our remaining concerns is radiation hardness of the HAPDs especially for γ-rays and neutrons. The HAPD is required the radiation hardness of the 10-year Belle II operation. From past studies of APDs for γ-ray and neutrons, we found the major cause of the radiation damage to the APD, and improved the structures of the APD. In order to confirm the improvement of HAPD for radiation hardness, we carried out a series of irradiation tests with various types of HAPDs using a
⁶⁰
Co facility and also a neutron beam facility. By understanding the reasons of the radiation damages in the APD, we have successfully improved radiation hardness of the APD by modification of electronic structure on it. In this paper, the latest status of HAPD development and the study of the radiation hardness for the HAPD are described.