The Aerogel Ring-Imaging Cherenkov detector (ARICH) was developed to provide a particle identification in the forward endcap of Belle II detector at a SuperKEKB electron–positron collider at the High Energy Accelerator Research Organization (KEK), Japan. It is used to identify charged $\pi $ and K mesons at momenta between 0.5 and 4.0 GeV/c. The ARICH system is a proximity-focusing RICH counter with an expansion length of 20 cm that uses silica aerogels as radiators and hybrid avalanche photo-detectors as position-sensitive photo-sensors, which was installed in the Belle II spectrometer in late 2017. The design objective is a $\pi $/K separation capability exceeding 4$\sigma $ at a momentum of 4 GeV/c.
To detect more photons without degrading the Cherenkov angle resolution, we introduced a double layer aerogel radiator in a novel focusing configuration. It consists of 2 layers of 2 cm thick aerogel tiles with different refractive indices of 1.045 and 1.055. It is important to reduce the number of the aerogel tiles used to cover the large radiator area (~3.3 m$^2$) because particles cannot be clearly identified in these gaps. Therefore, large, crack-free aerogel tiles are preferred. Installing the tiles onto the module by trimming them with a water-jet cutter and avoiding optical degradation of the aerogels via moisture adsorption during long-term experiments should ultimately result in highly hydrophobic conditions.
Approximately 450 large-area (18 $\times $ 18 $\times $ 2 cm) aerogel tiles were manufactured, and their optical characteristics (refractive index and transmission length) were confirmed to meet the ARICH design requirements. Each tile was cut into wedge shapes using a water-jet cutter to fit the cylindrical support structure. A total of 248 aerogel tiles were successfully installed in the 124 segmented containers of the support structure. Here, results from the development of the dual-layer aerogel radiator module for the Belle II ARICH system are presented.