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Description
The Aerogel Ring Imaging Cherenkov (ARICH) counter is a particle identification device located in the endcap region of the Belle II detector. The main components of the ARICH counter are 248 silica aerogel tiles built into the Cherenkov radiator and 420 Hybrid Avalanche Photo Detectors (HAPDs) making up the photon detector. Angular distribution of Cherenkov photons emitted from silica aerogel and detected by HAPDs is used to identify charged particles. Therefore, HAPD performance is essential for the ARICH counter.
The HAPD is a vacuum tube made of ceramic body, quartz window with semitransparent photocathode and 4 segmented Avalanche Photo Diode (APD) chips. APD chips consist of 36 pixels covering $4.9 \times 4.9\, mm^2$ each. Multiplication in an HAPD is a combination of impact ionization of a photo-electron accelerated by the high voltage applied between photocathode and APD, bombardment gain, and APD avalanche process, avalanche gain.
All the HAPDs were tested before the installation into ARICH detector. One of the issues of HAPDs is a very large signal observed when operated inside the magnetic field therefore each HAPD was also tested in the magnetic field. Final results of the tests before the installation will be reported.
Installation of HAPDs to the ARICH detector finished in July 2017, and the ARICH was installed in the Belle II detector at the end of 2017. The commissioning of the ARICH counter was performed in the spring 2018 with cosmic rays and beam collision events. Although all HAPDs passed several tests before the assembly, few of them suffered issues as frequent large signals and inability to apply bias voltage properly. The behavior of these HAPDs will also be discussed. Performance of HAPDs based on the number of detected Cherenkov photons and Cherenkov angle distributions was evaluated and the results will be reported.
