Optical Quality of PANDA Barrel DIRC Prototype Radiators

The PANDA experiment at the facility for anti-proton and ion research (FAIR) will study the strong interaction by precision spectroscopy. A detector system with excellent particle identification (PID) is therefore required. Charged hadron identification in the barrel region will be performed by ka compact ring imaging Cherenkov detector based on the DIRC (Detection of Internally Reflected Cherenkov light) principle. The design of the PANDA Barrel DIRC [1] is based on the BABAR DIRC [2] with several important improvements, such as focusing optics and fast photon timing. The radiators are long, rectangular bars made from synthetic fused silica. The efficiency of the photon transport inside the radiator bar depends on the quality of the bar surface polish, in particular on the surface roughness.