A LARGE SILICA AEROGEL CHERENKOV COUNTER FOR SKS

A large silica aerogel Cherenkov counter has been constructed as a kaon trigger counter to veto pions of 0.7 GeV/c for experiments studying hypernuclei by the (pi+, K+) reaction with the superconducting kaon spectrometer (SKS) at the KEK 12-GeV proton synchrotron. It features a large single diffusion box with a sensitive area of 140 cm x 120 cm and a thickness of about 40 cm. Special attention was paid to the shape of a reflector in order to achieve uniform efficiency over the sensitive area. A mean photoelectron number of N(e)BAR = 6-7 was obtained for 0.72 GeV/c (beta = 0.984) pions with a refractive index of 1.06.