High Precision Measurements for the PANDA Barrel DIRC Radiators

The PANDA experiment at FAIR (Facility for Antiproton and Ion Research in Europe) at GSI, Darmstadt, will study fundamental questions of hadron physics and QCD. A compact, fast focusing Ring Imaging Cherenkov counter using the DIRC (Detection of Internally Reflected Cherenkov light) technology, will provide charged particle identification (PID) in the barrel region of the PANDA experiment. To meet the PANDA PID requirements, the Barrel DIRC has to provide precise measurements of the Cherenkov angle, which is conserved for Cherenkov photons propagating through the radiator by total internal reflection. The radiators, rectangular bars made from synthetic fused silica, have to fulfill strict optical and mechanical requirements. This includes squareness and parallelism of the bar sides, sharp corners, and smoothly polished surfaces in the order of 10 Å, ensuring very little angular distortions and high transport efficiency. An optical setup, consisting of a computer-controlled positioning and multi-wavelength laser system, is used to evaluate the bars to obtain critical values like transmittance and reflectivity. Prototypes from various manufacturers using different production techniques have been tested to qualify vendors for the Barrel DIRC bar production. Setup, measuring procedure and measurement results will be presented in this contribution