The instrumentation in Hall A at the Thomas Jefferson National Accelerator Facility was designed to study electro-and photo-induced reactions at very high luminosity and good momentum and angular resolution for at least one of the reaction products. The central components of Hall A are two identical high resolution spectrometers, which allow the vertical drift chambers in the focal plane to provide a momentum resolution of better than 2 x 10(-4). A variety of Cherenkov counters, scintillators and lead-glass calorimeters provide excellent particle identification. The facility has been operated successfully at a luminosity well in excess of 10(38) CM-2 s(-1). The research program is aimed at a variety of subjects, including nucleon structure functions, nucleon form factors and properties of the nuclear medium. (C) 2003 Elsevier B.V. All rights reserved
Two threshold gas Cherenkov counters have been constructed for the electron and hadron High Resolution Spectrometers HRS of the Je erson 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 CO 2 . Each counter is equipped with ten mirrors. Light w eight, 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 e ciency 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 ine ciency for positrons less than 10 ,3 and rejection ratios =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 -rays production.
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