Development of silica aerogel with refractive index gradient for RICH radiator

A proximity focusing RICH using aerogel as a Cherenkov radiator has been developed for the upgrade of the Belle detector. We have considered the idea to accumulate multiple aerogel radiators so that Cherenkov photons from each tile organize a unified ring image on a photo-detector plane. Advancing this idea, we have developed aerogel in which the refractive index varies continuously. We report the production method of the silica aerogel with refractive index gradient and he result of test beam experiment.