HYPOLIT, a high resolution fast beam RICH

A new Cherenkov counter has been designed for on-line tagging of high energy particles, more specifically 270 GeV hyperons in WA89 experiment at CERN. It is equipped with a 6 m focal length spherical mirror and a chromatic corrector. The ring size is reduced by a factor of 8 (adjustable) by means of a set of two conical mirrors. The photon detector is an image intensifier. Its output window is divided in 160 pixels read with photomultipliers via optical fibres. The ring radius is determined electronically and a tagging signal is available in less than 2.5 μs. HYPOLIT has been tested in a 220 GeV pion beam (50 μrad divergence). A spatial resolution of 0.125 mm on the radius has been obtained, and the pion—kaon separation was better than 5σ.