Successful test of a prototype hadron blind detector

We have constructed and tested a threshold Cherenkov detector inspired by the conceptual design of Giomataris and Charpak (Nucl. Instr. and Meth. A 310 (1991) 589). The basic design consists of a gas radiator followed by a photosensitive wire chamber using a solid CsI photocathode. The photon detector lies directly in the particle path and is required to have single photo-electron sensitivity and yet be insensitive to the passage of a charged particle. We have altered the design by adding a thin LiF window. This window allows one to separate the radiator and avalanche volumes, giving greater flexibility in the gas choices. Our detector was operated with C₂F₆ as the radiator gas at lengths of 120, 80, and 40 cm. The avalanche gas was 95% He + 5% CH₄. At 95% electron efficiency, the pion efficiency was 1/333,1/150, and 1/100 for each of the lengths listed above.