Development of large area APDs for the P̄ANDA EMC readout

The barrel part of the P̄ANDA electromagnetic calorimeter will consist of nearly 11.000 lead tungstate crystals, read out with two Avalanche Photodiodes per crystal based on the requirement of using a magnetic field indendent operating readout device. Due to the geometry of the rear crystal face the invisaged diodes have to be rectangularely shaped. Therefore the R&D of these photodiodes was started by the development of large area APDs (LAAPDs) with an active area of (10×10)mm² to fix the device properties in terms of radiation hardness, dark current behaviour, etc. Based on the measurements done with these quadratically shaped APDs, first rectangularely shaped APDs have been delivered to the EMC group in september 2008. This paper will present the APD properties of the two tested differently shaped LAAPDs which have been under investigation so far and will describe the influence of irradiation on the device properties in detail.