A LARGE-ACCEPTANCE IMAGING GAS CHERENKOV DETECTOR SYSTEM

A threshold gas Cherenkov detector for electrons was developed which combines large solid-angle acceptance with high detection efficiency, excellent imaging performance, and effective discrimination against other particles and electrons below β = 0.9997. Operating with air at atmospheric pressure, the counting system consists of an array of 12 ellipsoidal mirrors which focus the Cherenkov photons seen in target direction onto photomultipliers. The mirrors cover the full azimuth for polar angles 115° ≤ θ{symbol} ≤ 145°, thus covering 20% of 4π. In its use as a nonmagnetic spectrometer for electrons in the medium-energy region, the detector was designed to cut the main part of the scattering spectrum below E = 50 MeV, whereas the detection efficiency surpasses 90% at E = 100 MeV. The detector was used for measuring a 10-6 asymmetry in the quasielastic scattering of longitudinally polarized electrons which is caused by weak neutral currents. © 1990.