THE CO2 GAS CHERENKOV DETECTORS FOR THE JEFFERSON-LAB HALL-A SPECTROMETERS

Two threshold gas Cherenkov counters have been constructed for the ele ctron and hadron High Resolution Spectrometers (HRS) of the Jefferson Lab Experimental Hall-A. These counters are intended to separate elect rons/positrons from other particles up to 4 GeV/c. The counters are op erated at atmospheric pressure with CO,. Each counter is equipped with ten mirrors. Light weight, thin spherical mirrors (similar to 5.5 x 1 0(-3) radiation lengths) have been employed resulting in a total thick ness of similar to 1.4 x 10(-2) radiation lengths crossed by the parti cles. A prototype of the counter has been tested at CERN with a mixed beam of positrons, pions and protons from 1 to 4 GeV/c. Its detection efficiency for positrons and the rejection ratios for pions and proton s have been measured as a function of the pulse height response (or eq uivalently the number of photoelectrons). An improvement of 34% in the number of photoelectrons has been obtained by using a wavelength shif ter coated on the photocathode glass window. With such an improvement in 1 m long radiator, an inefficiency for positrons less than 10(-3) a nd rejection ratios pi/e at the level of few 10(-3) and p/e smaller th an 10-3 have been obtained for pulse heights above 2 photoelectrons. C ontaminations of particles below the Cherenkov threshold is fully unde rstood considering delta-rays production. (C) 1998 Elsevier Science B. V. All rights reserved.