A Time Projection Chamber for High-Rate Experiments: Towards an Upgrade of the ALICE TPC

摘要

A Time Projection Chamber (TPC) is a powerful detector for 3-dimensionaltracking and particle identification for ultra-high multiplicity events. It isthe central tracking device of many experiments, e.g. the ALICE experiment atCERN. The necessity of a switching electrostatic gate, which prevents ionsproduced in the amplification region o MWPCs from entering the drift volume,however, restricts its application to trigger rates of the order of 1 kHz. Charge amplification by Gas Electron Multiplier (GEM) foils instead ofproportional wires offers an intrinsic suppression of the ion backflow,although not to the same level as a gating grid. Detailed Monte Carlosimulations have shown that the distortions due to residual space charge fromback-drifting ions can be limited to a few cm, and thus can be corrected usingstandard calibration techniques. A prototype GEM-TPC has been built with thelargest active volume to date for a detector of this type. It has beencommissioned with cosmics and particle beams at the FOPI experiment at GSI, andwas employed for a physics measurement with pion beams. For future operation of the ALICE TPC at the CERN LHC beyond 2019, wherePb-Pb collision rates of 50 kHz are expected, it is planned to replace theexisting MWPCs by GEM detectors, operated in a continuous, triggerless readoutmode, thus allowing an increase in event rate by a factor of 100. As a firststep of the R&D program, a prototype of an Inner Readout Chamber was equippedwith large-size GEM foils and exposed to beams of protons, pions and electronsfrom the CERN PS. In this paper, new results are shown concerning ion backflow, spatial andmomentum resolution of the FOPI GEM-TPC, detector calibration, and dE/dxresolution with both detector prototypes. The perspectives of a GEM-TPC forALICE with continuous readout will be discussed.