Light collection of the POLAR detector

POLAR is a compact polarimeter dedicated to measuring the polarization of GRBs between 50–350 keV. The light collection of 200 mm×6 mm×6 mm plastic bars has been simulated and optimized in order to get uniform response to X-rays at different points along one single bar. According to the Monte Carlo results, the amplitude uniformity strongly depends on the level of polishing of the scintillator surface and the covering. A uniformity of 89% is achieved with a prototype constructed by a non position-sensitive PMT and an array of 4×4 bars.