The TASSO gas and aerogel Cherenkov counters

We describe the gas and aerogel threshold Cherenkov counters built for the TASSO experiment at the DESY e+e- storage ring PETRA. The counters are arranged in two diametrically opposed groups as part of the two hadron arms. The gas counters contain Freon 114 and CO2 respectively; 128 ellipsoidal mirrors focus the Cherenkov light through funnels onto Philips 5 photomultipliers coated with wavelength shifter. β = 1 charged particles are detected with at least 99.9% efficiency. The aerogel counters are of the diffusing wall type. They have a sensitive area of 12 m2, devided into 32 cells and use a total of 192 RCA Quantacon 5 photomultipliers. The average aerogel refractive index is around 1.025. The efficiency for β = 1 charged particles is 98%. Design condiserations, prototype evaluation, construction details, and performance of the final counter system are described. (orig.)