The RICH detectors (RIB  and RIF ) employ an ambitious technique to identify charged hadrons in the barrel and endcaps respectively. The velocity, , of a particle travelling faster than the local speed of light in a material medium may be determined by measuring the presence and angle of emission of Cherenkov light. This ultraviolet light is detected using the time projection technique in quartz drift tubes (48 in the barrel, 24 in each endcap) containing a small quantity of a photo-ionizing vapour (TMAE). Both the Cherenkov angle (reconstructed from the positions of individual photons crossing the drift tubes with respect to the particle's trajectory) and number of photons give a measure of the particle's velocity. With the momentum determined from the tracking detectors, this allows, in many cases, the particle's mass, and hence identity, to be determined.
Two perfluorocarbon Cherenkov radiators are used: a liquid radiator consisting of CF to identify soft particles (0.7-8 ) and a gaseous radiator of CF (RIB) or CF (RIF) to identify hard particles (2.5-25 ). The gas radiator is situated behind the drift tubes, so the Cherenkov light is reflected and focused back to them with parabolic/spherical (RIB/RIF) mirrors on the far wall of the detector. No focusing is required for light from the liquid radiator which is thin compared to its distance from the drift tube.
Tim Adye 2002-11-06