Large Area APDs for the PANDA EMC

Abstract

PANDA is a 4pi detector planned for the antiproton storage ring at the proposed International Accelerator Facility for Antiproton and Ion Research (FAIR) at GSI. Experiments utilizing the excellent antiproton beam of up to 15 GeV/c beam momentum require an electromagnetic calorimeter of high granularity and resolution to measure photons of several GeV down to 10 MeV in energy. High fluence and thick internal targets deliver unprecedented luminosities which require a fast scintillator. A candidate for the scintillation material for the calorimeter of PANDA is PbWO4 with significantly improved light yield. For the detection of the scintillation light adequate photo sensors are required, which operate magnetic field independently and cover a large part of the readout surface of each crystal. Avalanche photo diodes (APDs) have become attractive devices for this purpose. First APDs with an active area of 10 times 10 mm2 (LAAPDs) have been developed and were tested concerning their radiation hardness due to proton irradiation at room temperature as well as under detector operation conditions of a designated temperature of T = -25 degC. A quality assurance procedure for the approx. 44000 APDs for the PANDA-EMC has been developed during the last year, which includes the measurements of the device properties at those two temperatures. Based on the experience of the CMS collaboration with this kind of photo sensors a screening procedure of the property changes due to irradiation will be performed at GSI. The mounting of the APDs on the rear side of each crystal will be realized by using a capsule built out of PEEK, a special kind of plastic, manufactured by precision injection molding. The temperature screening during the measurements will be realized by using a special kind of radiation hard thermistor, mounted on the rear side of the APD capsule.