A large silica aerogel Cerenkov counter for SKS

A large silica aerogel Cerenkov counter has been constructed as a kaon trigger counter to veto pions of 0.7 GeV/c for experiments studying hypernuclei by (π+, 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. Mean photoelectron number Ne-bar = 6 - 7 was obtained for 0.72 GeV/c (β = 0.984) pions with refractive index of 1.06. (author)