Development of large area silica aerogel used as RICH radiator for the Belle II experiment

This paper presents the recent progress in the development of large-area hydrophobic silica aerogels for use as radiators 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 identify charged pions and kaons and to have a separation capability of more than 4σ at momenta up to 4 GeV/c. In a focusing-dual-layer radiator with different refractive indices, a prototype A-RICH counter satisfied the required performance using our conventional aerogel tiles with refractive indices n of 1.045 and 1.055, transmission lengths Λ_T of 30-40 mm, and designed final dimensions of 18 × 18 × 2 cm³. Moreover, we achieved better π/K separation performance using n ~ 1.06 aerogels with relatively small dimensions but long transmission length (Λ_T > 50 mm) produced by a novel pin-drying method. We need to fill the large end cap area of 3.5 m² with two-layer aerogel tiles and to minimize the number of tiles with realistic dimensions to reduce the tile boundaries, at which the number of detected photoelectrons decreases. Therefore, we studied large area aerogel production by both conventional and pin-drying methods as a key to achieving high performance over the actual detector. This paper discusses the crack-free yield, optical transparency, and uniformity in refractive index within a monolith of recently produced large area aerogels.