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

Abstract

This p a p er p resents the recent p ro g ress in the develo p ment of lar g e-area hydro p hobic silica aero g els for use as radiators in the aero g el-based rin g -ima g in g Cherenkov (A RICH) counter that will be installed in the forward end ca p of the Belle II detector. The p roximity-focusin g A-RICH counter is desi g ned to identify char g ed p ions and kaons and to have a se p aration ca p ability of more than 40" at momenta u p to 4 GeV/c.In a focusin g -dual-layer radiator with different refractive indices, a p rototy p e A-RICH counter satisfied the required p erformance usin g our conventional aero g el tiles with refractive indices n of 1.045 and LOSS, transmission len g ths AT of 3�0 mm, and desi g ned final dimensions of 18 x 18 x 2 cm 3 • Moreover, we achieved better 1[/ K se p aration p erformance usin g n rv 1.06 aero g els with relatively small dimensions but lon g transmission len g th (AT> SO mm) produced by a novel p in-dryin g method.We need to fill the lar g e end ca p area of 3.5 m 2 with two layer aero g el tiles and to minimize the number of tiles with realistic dimensions to reduce the tile boundaries, at which the number of detected p hotoelectrons decreases. Therefore, we studied lar g e area aero g el p roduction by both conventional and p in-dryin g methods as a key to achievin g hi g h p erformance over the actual detector. This p a p er discusses the crack-free yield, o p tical trans p arency, and uniformity in refractive index within a monolith of recently p roduced large area aerogels. I. INT RODUCTIONT HE aerogel-based ring-imaging Cherenkov (A-RICH) counter is one of the particle identification devices for the Belle II experiment [1] that will be performed using the SuperKEKB accelerator at KEK, Japan. The Belle II is a super B factory experiment to search for new physics beyond the standard model of particle physics through flavor physics and C P violation measurements. Because the forward end cap of the Belle II detector has a limited space, the A-RICH system is designed as a proximity-focusing RICH counter. To

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

“…We previously studied the optical performance of aerogels using small samples [9], [12]. In [15], we confirmed that high optical qualities can be obtained even in large aerogel tiles. This is further confirmed in Fig.…”

“…To suppress tile cracking, we found that a very slow, stable ramp up/down of the internal pressure in the autoclave was highly significant during the supercritical drying phase. In 2012, the first 17 × 17 × 2 cm 3 tile with tag = 1.059 was obtained with no cracking by the pin-drying method [15]. In the final step, in 2013, we performed a two-batch test production of 19 × 19 × 2 cm 3 tiles with tag = 1.055-1.060 by the pin-drying method; however, no crack-free samples were obtained.…”

“…In the final step, in 2013, we performed a two-batch test production of 19 × 19 × 2 cm 3 tiles with tag = 1.055-1.060 by the pin-drying method; however, no crack-free samples were obtained. We intend to produce tiles as large as possible and then trim the tile corners because this is where the refractive index steeply increases [15].…”

“…In such a large-area aerogel, the positional dependence of the mean and resolution of the Cherenkov angle distribution should be thoroughly investigated, particularly for the pindried aerogels because those tiles exhibited significant nonuniformity in refractive index [15]. In very preliminary analyses, nonuniformities in the large-area pin-dried aerogels seem to be within our requirements and are acceptable.…”