Progress in Cherenkov ring imagingPart 2: Identification of charged hadrons at 200 GeV/c

We have used a ring-imaging Cherenkov detector to separate π's, K's, and antiprotons in a 200 GeV/c beam at Fermilab. This device was built as a prototype for a large-aperture counter now in operation in Fermilab experiment E605. The radiator consisted of 8 m of atmospheric-pressure helium gas. The photon detector was a multistep proportional chamber. Cherenkov photons near 8 eV were detected by photoionization of triethylamine (TEA) vapor in the chamber. An average of 2.5 to 2.7 Cherenkov photons were observed per event, corresponding to a figure of merit N0 ⋍ 45 per cm. A single-photon radius uncertainty of 0.47 mm was obtained with a helium/TEA/CH4 gas mixture in the photon detector. The rms uncertainty in the determination of the Cherenkov angle was ΔΘc/Θmax = 0.006, corresponding to one-standard-deviation π/K separation at 500 GeV/c. At 200 GeV/c, the particle identification efficiency in a beam containing 95.2% π−, 4.3% K−, and 0.5% antiprotons was 92% for the π's, 83% for the K's, and 90% for the antiprotons.