The BRAHMS ring imaging Cherenkov detector

A Ring Imaging Čerenkov detector built for the BRAHMS experiment at the Brookhaven RHIC is described. This detector has a high index of refraction gas radiator. Čerenkov light is focused on a photo-multiplier based photon detector with a large spherical mirror. The combination of momentum and ring radius measurement provides particle identification from 2.5 GeV/c up to 35 GeV/c for pions and kaons and well above 40 GeV/c for protons during runs that had the radiator index of refraction set at $n-1=1700 \times 10^{-6}$.