A large solid angle Cerenkov counter for the DELCO experiment at spear

Abstract

We describe the design, construction and performance of a 70% of 4 π atmospheric pressure gas Cerenkov detector built in twelve sectors. Economical methods to achieve the necessary light gathering performance of the mirrors and other optical components are presented. One full scale sector of the detector was built and operated in an electron/pion beam at SLAC to secure data on the photoelectron yield as a function of beam electron direction and origin. A one meter radiator length of ethane yielded ≈10.5 photoelectrons in an RCA 8854 photomultiplier tube coated with a p-terphenyl wavelength shifter. The efficiency of the counter, obtained using energetic electrons from Bhabha events and averaged over the twelve counter sectors and the initial running period at SPEAR, is (97±1)%. Assuming that there are no real single prompt electrons that result from ψ-decay, the counter trigger rate of 0.35% per hadron event establishes a trigger rate of <0.1% per incident pion. The counter was found to have a momentum cut off for P<0.2 GeV/ c at which value the efficiency falls to 50%.