Studies for the upgrade of Belle II Aerogel RICH

The Aerogel Ring Imaging CHerenkov (ARICH) counter of the Belle II spectrometer performs particle identification by detecting Cherenkov photons emitted when particles pass through aerogels. Hybrid Avalanche Photo Detectors (HAPDs) detect ring images formed on their surfaces by Cherenkov light cones. Since Belle II will extend its operation into the 2040s, we plan an upgrade of ARICH in the early 2030s to ensure the longer-term operation and to improve particle identification efficiency. Although HAPDs are expected to retain sufficient performance for about ten years despite radiation damage, they are unsuitable for extended use, and their production has ended. We therefore evaluate alternative photodetectors, such as Multi-Pixel Photon Counters (MPPCs) and Large Area Picosecond Photo Detectors (LAPPDs), together with compatible readout systems. We assessed various MPPC models from Hamamatsu Photonics, considering particularly operation under high-background environment of Belle II, which increases dark pulses. After we irradiated MPPC samples at a neutron beam line, we tested them under cooling or after annealing, both of which substantially mitigated dark count rates. For signal readout from MPPCs, we developed a dedicated readout ASIC, TF01A64, and 64-channel readout system incorporating TF01A64. To prepare for an upcoming beam test using Cherenkov ring imaging at an electron beam facility, we confirmed laser spot imaging at single-photon level. In parallel, we evaluated LAPPDs from Incom with a TOFPET2-ASIC-based readout system as a preparation of planned beam magnetic-field tests.