Photon background in DIRC fused silica bars

The detector of internally reflected Cherenkov (DIRC) radiation is the ring imaging Cherenkov detector of the BABAR detector at the Pep-II ring of SLAC. The Cherenkov radiators consist of 4.9-m-long rectangular fused silica bars each glued together from four equal pieces. The photon detector is a water tank equipped with an array of 10752 conventional photomultipliers. The current study attempts to identify sources of photonic background generated in the DIRC bars. A conclusion of this work is that there are two major sources: one such component consists of photons created by the delta-ray electrons in the fused silica. The second component comes from the reflections of photons from the EPOTEK-301-2 glue-fused silica interface while they are traveling in the bars. The reflection occurs because of a slight mismatch of the refractive indices.