Progress in the development of silica aerogel as a RICH radiator in the Belle II experiment

This paper describes recent progress in the development of large-area hydrophobic silica aerogel tiles for use as a radiator in the aerogel-based ring-imaging Cherenkov (A-RICH) counter that will be installed in the forward end cap of the Belle II detector. The proximity-focusing A-RICH counter is designed to efficiently identify charged pions and kaons; our goal is a separation capability of more than 4σ at momenta up to 4 GeV/c. We plan to fill the large end-cap area of 3.5 m² with 124 segmented dual-layer-focusing aerogel combinations with different refractive indices (n ~ 1.05). It is crucial to minimize the number of aerogel tiles to reduce tile boundaries because the number of detected photoelectrons decreases at the boundaries. As a step toward achieving high performance in the actual detector, we performed test productions of large-area (over 18 × 18 × 2 cm³) aerogel tiles using both conventional and pin-drying methods. In view of the crack-free production yield, we have decided to mass-produce both upstream and downstream aerogel tiles by the conventional method in the Belle II program, although the pin-dried aerogels showed excellent transparency. We also used an electron beam at DESY to conduct a beam test for investigating the performance of a prototype A-RICH counter. When we used a basic counter configuration with dual-layer-focusing aerogels having n ~ 1.045 and 1.055 produced by the conventional method, we confirmed a π/K separation capability better than 4σ at a momentum of 4 GeV/c.