Development of new silica aerogel for the RICH radiator of the Super Belle detector

Currently, we are working on the program of the Belle detector upgrade, and the Cherenkov ring imaging counter with silica aerogel radiator (aerogel-RICH) is the most promising candidate for the particle identification device (PID) at the endcap section of the BELLE detector, where a threshold-type aerogel Cherenkov counter is occupied at present, in order to further expand pi-K separation capabilities at higher momentum region. Refractive index of silica aerogel should be around ∼1.05 to match our PID requirement and must possess high optical transparency. Based upon our experience, new production technique has been developed to make high transparent aerogels with hydrophobicity. In this method, new chemical solvent, DMF (di-methyl formamide), is introduced and various conditions such as a mixing ratio of solvent as well as catalyst are optimized. As a result, aerogels with transmission length longer than 40 mm at 400 nm wavelength are successfully obtained. The optical quality is also confirmed in a series of beam tests, and we have observed Cherenkov photon yields significantly increased. We also produced multi-layer aerogels, where two layers with different refractive index are stacked as one sample. This can be used as a radiator to increase Cherenkov photons without stacking radiators by focusing Cherenkov photons. In this talk, details of the aerogel production method are described and high optical performance is also demonstrated.