A Large Acceptance Threshold Cherenkov Counter For Experiment-760 At Fermilab

A threshold gas Cherenkov counter is used in Fermilab Experiment 760 to tag electrons in the detection of charmonium states. Mechanical structures and mirrors were built with carbon-fiber-epoxy composites, resulting in a light weight detector covering 2-pi in azimuthal angle. The counter is operated at atmospheric pressure with different gases, CO2 and Freon 13, respectively in the two cells at small and large polar angles, to maximize pion rejection. Design considerations, construction details, and performance of the counter are described.