Silica Aerogel for Cherenkov Detectors
To identify the fast charged elementary particles by Cherenkov detectors the transparent media with the refractive indices in the range from 1.01 to 1.2 is needed. Under normal conditions only aerogel satisfies this condition.
The traditional two step procedure was used for the synthesis of gels. At the first step the mixture of tetraethoxysilane oligomers synthesis was carried out in an acidic medium under stoichiometric deficiency of water. Then the gel was synthesized under alkaline catalysis in acetonitrile with an excess of water. The resulting gels after washing by isopropanol was subjected to high temperature supercritical drying.
The light scattering is one of the most important optical characteristics of the aerogel tiles used in Cherenkov detectors. The light scattering in aerogels takes place mainly on the elements of the structure which are less than the wavelength of the visible light according to the law of Rayleigh. Thus, the synthesis of aerogels for Cherenkov detectors requires to achieve the minimum size of the particles forming the aerogel.
The novel type of particle identification detectors named Focusing Aerogel Ring Imaging Cherenkov detectors (FARICH) are based on aerogel radiators with gradient of the refractive index along the thickness of tile. The use of a four-layer aerogel tile with layers of designed thicknesses and refractive indices significantly improved the accuracy of particle velocity measurement. We have also obtained aerogel tiles with continuous variation of the refractive index.