The CO2 Gas Cherenkov Detectors for the Jefferson Lab Hall A Spectrometers

Two threshold gas Cherenkov counters have been constructed for the electron and hadron High Resolution Spectrometers (HRS) of the Jefferson Lab Experimental Hall A. These counters are intended to separate electrons/positrons from other particles up to 4 GeV/c. The counters are operated at atmospheric pressure with CO2. Each counter is equipped with ten mirrors. Lightweight, thin, spherical mirrors (5.5 + 10-3 radiation lengths) have been employed resulting in a total thickness of 1.4 + 10-2 radiation lengths crossed by the particles. 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 protons have been measured as a function of the pulse height response (or equivalently the number of photoelectrons). An improvement of 34% in the number of photoelectrons has been obtained by using a wavelength shifter coated on the photocathode glass window. With such an improvement in 1 m long radiator, an inefficiency for positrons less than 10-3 and rejection ratios pi/e at the level of few 10-3 and p/e smaller than 10-3 have been obtained for pulse heights above 2 photoelectrons. Contaminations of particles below the Cherenkov threshold is fully understood considering delta-rays production