Detection of internally reflected Cherenkov light, results from the BELLE DIRC prototype

The DIRC (Detector of Internally Reflected Cherenkov light) is a new type of ring imaging Cherenkov detector which utilizes precisely machined quartz bars as Cherenkov radiators. Bar quality is determined by both laser and cosmic ray measurements. The Cherenkov photons are transported several meters along the bar until they reach the detector cavity where they are proximity focussed on an array of conventional photomultiplier tubes. Results from a prototype device comprising a 2 × 4 × 240 cm3 quartz bar read by an array of 480 PMTs are presented. Sample images, which are the first observed in this type of detector, are also shown. Measurements of the light yield (approximately 20 photoelectrons per image) and the angular resolution, which are both in good agreement with Monte Carlo predictions, are shown to lie well within the expected operating tolerances of the DIRC.