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 4 sigma at momenta up to 4 GeV/c. We plan to fill the large end-cap area of 3.5 m(2) with 124 segmented dual-layer-focusing aerogel combinations with different refractive indices (n similar to 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 x 18 x 2 cm(3)) 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 similar to 1.045 and 1.055 produced by the conventional method, we confirmed a pi/K separation capability better than 4 sigma at a momentum of 4 GeV/c.