The SAD story

SAD is a Cherenkov detector using silica aerogel as radiator. Silica aerogel is a porous material with structural inhomogenities which are small compared to the wavelength of light. To look at, aerogel is rather transparent, but in the ultraviolet wavelength region the absorption is important. The refractive index of the silica aerogel used in SAD is 1.03. This gives threshold momenta for pions, kaons and protons of 0.6, 2.0 and 3.8 GeV/c respectively. SAD is consequently used for identification of rather low momentum particles in the range 0.6 - 4.5 GeV/c. The silica aerogel detector is situated in the European Hybrid Spectrometer at the CERN Super Proton Synchrotron (SPS). It has been used in several experiments during the last few years. Section 2 describes the performance of SAD under experimental conditions, Section 3 tells the story of the detector, from the first idea to the final detector, and in Section 4 conclusions are drawn.