Physics:Aerogel detectors
Aerogels are transparent, highly porous materials of low density, ranging from 0.05 to 0.15 g/cm3. Silica aerogel consists of amorphous grains of silica with diameter 4-10 nm, produced by hydrolysis of Silane (Si (OCH3)4) in the presence of a solvent, which is subsequently evaporated at high temperature and under pressure, i.e. replaced by air. Typical remaining bubble sizes are of the order of 60 nm; the porosity (air content) can reach 98%.
The amount of solvent determines the final refractive index n of silica aerogel, which can be adjusted to be between that of glasses (n > 1.2) and that of light gases (n < 1.002 at atmospheric pressure and room temperature). Aerogel thus can be used for particle identification by Cherenkov radiation in the momentum domain of a few GeV/c. The use as Cherenkov counters was first demonstrated by M. Cantin Cantin74, who observed 6 to 12 photoelectrons in a volume of 
for hadrons with 
. Today, aerogel is produced commercially, mostly as superinsulator, and has also demonstrated its practical applicability in large detectors (e.g. Carlson86, Poelz86). For a reference book, 
Fricke86.
The refractive index of aerogel has been measured to obey, in rough agreement with the formula of Clausius-Mosotti, the relation
where 
is the density of the material. The density of compact silica being about 2 g/cm3, the ratio of pore and silica volumes can be calculated by measuring n
Ratios of 60 with n as small as 1.007 have been achieved. The upper limit is given by the minimum of solvent (n = 1.06), but by baking, n = 1.094 has been reached. Typically, aerogel can be produced in blocks of 100 mm sides.
The optical quality of aerogel is determined by the homogeneity of the gel. If pores are larger than some 20% of the wavelength of radiated light, Rayleigh scattering will affect the light transmission. Absorption hence dominates at 
nm. At 
nm, the diffusion length is of the order of 10 mm.
Due to Rayleigh scattering, the directionality of the radiated Cherenkov light is somewhat lost, and classical mirror focusing techniques for light collection become difficult or inadequate. Light diffusion in walls surrounding the radiator can be used for light collection; an efficiency of 50% can be achieved. For more details, INTE94.
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