Study of various anode pad readout geometries in a GEM-TPC

The use of a time projection chamber (TPC) based on gas electron multipliers (GEMs) as a central tracker in particle-physics experiments is being studied by several groups. Compared to the conventional multi-wire readout this combination offers a number of advantages such as intrinsically suppressed ion feedback, high granularity and decoupling of gas amplification stage and readout geometry. The fast signal of GEMs and the reduction of the transverse diffusion in parallel magnetic fields give a good spatial resolution. However, for short drift distances and high magnetic fields, this may lead to signal sizes much narrower than the pad size and thus to a degradation of the spatial resolution. We have studied the use of different pad geometries to improve the performance of the detector in the small diffusion limit.