The light collection non-uniformity of strongly tapered PWO crystals and its impact on the energy resolution of the PANDA electromagnetic calorimeter in the energy region below 1 GeV

– The barrel part of the target EMC of the PANDA detector at the future FAIR facility will consist of 11 crystal geometries with a varying degree of tapering. The tapered shape introduces a focussing effect to the light collection, which in combination with the absorption of the scintillation light within the crystal causes a non-uniformity in light collection. For the most tapered crystals, light generated in the front part of the crystal is enhanced by approximately 40% compared to light generated in the rear part of the crystal. Due to the distribution of the electromagnetic shower within the crystal, this non-uniformity leads to a smearing of the energy response, resulting in a deterioration of the energy resolution. To avoid this effect, the light collection has been made uniform by de-polishing one lateral crystal side face to an average roughness Ra of 0.3 μm. Applying this method, the non-uniformity of the most tapered crystals has been decreased to a level of 5 % with a slight decrease of the light yield from the front part and a significant increase in the rear part of the crystal. This paper will discuss the observed effects of the light collection and compare the response of a 3×3 array of crystals with one de-polished side face with an identical array of completely polished crystals in the energy region below 1 GeV. In the energy region above 200 MeV a significant improvement of the energy resolution has been achieved.