Development of a GEM-TPC for PANDA

The PANDA (AntiProton Annihilations at Darmstadt) experiment is an internal target experiment at the High Energy Storage Ring (HESR) for antiprotons at the new Facility for Antiproton and Ion Research (FAIR) at Darmstadt, Germany. A Time Projection Chamber (TPC) is proposed as the central tracker due to its excellent tracking and particle identification capabilities [1]. A spatial resolution of ∼150μm in the rφ projecton and ∼1 mm in z, resulting in a momentum resolution of the order of a few percents over a range of 0.1 to 8 GeV, and a low material budget are needed by the rich physics program of PANDA. The continuous antiproton beam structure at HESR, however, makes the use of a traditional ion gate, normally used to avoid space charge accumulation, impractical. Due to their intrinsic ion backflow suppression, the use of GEM (Gas Electron Multiplier) structures [2] for gas amplication opens up the possibility to operate the TPC in an ungated mode without accumulating excessive space charge. A small GEM-TPC was built and characterized with cosmic muons [3]. The test chamber of 20 cm diameter and 8 cm drift length, operates with an Ar/CO2 (70/30) gas mixture. On the readout plate, an active area of 10×10 cm 2 was filled with rectangular pads of 0.8×6.0 mm and covered by 3 GEM foils for electron amplification. An average resolution of 200μm had been achived using the PASA/ALTRO [4] readout electronics. To perform a complete characterization of the test chamber, we set up a tracking telescope on the electron accelerator ELSA in Bonn, Germany. The test bench consists of four single silicon strip detectors and 4 GEM planes, triggered by four plastic scintillators with 2×2 cm active area.