A high resolution fast-RICH counter for on-line tagging of high energy particles

The design and performances of a new Cherenkov counter for on-line tagging of high energy particles are reported. The counter is equipped with a 6 m focal length spherical mirror and a chromatic corrector. The Cherenkov ring size is reduced by means of a set of two conical mirrors to match the active area of an image intensifier which output is read by a matrix of optical fibres partitioned in pixel, each pixels being coupled to a photomultiplier. Results of tests with π (plus K andp) beams are reported and performances of the apparatus are discussed. A spatial resolution of 0.125 mm (20.6 μrad angular) on the Cherenkov radius measurement has been obtained with a 50 μrad divergence 220 GeV pion beam, outlining encouraging prospects for a separation of 270 GeV Σ− and Ξ− hyperons at the 5σ level.