Response of the shashlyk forward spectrometer of PANDA to photons below 800 MeV energy

In order to reach a nearly 4π detection efficiency for electromagnetic calorimetry in PANDA at the FAIR facility, the target calorimeter is complemented by a calorimeter in the Forward Spectrometer designed as a sampling calorimeter covering 4.6 m² surface and comprising 54×28 cells of a shashlyk type design. Each cell has a cross section of 5.5×5.5cm² (~ 1 Moliere radius) composed of 380 layers of lead absorbers (0.275mm thick) and plastic scintillator tiles (1.5mm thick) with a total length corresponding to 19.6 X₀. The scintillation light is collected and accumulated via 18 wavelength shifting fibers, which are inserted through holes and form loops on the front side of the module. Photomultipliers (18 mmØ) are used as photo sensors since the calorimeter is placed at sufficient distance to the tracking magnets. The design is optimized for electromagnetic probes at very low detection threshold to cover a photon energy range down to about 10 MeV.