A GEM based time projection chamber with pixel readout

The large number of measured track points and the low material budget make a Time Projection Chamber (TPC) the ideal tracking device for particle flow optimized experiments. A highly segmented readout scheme, based on GEMs (Gas Electron Multipliers) or Micromegas, offers best performance. The potential of charge collection on the bare pixels of a CMOS chip placed directly underneath a short-spaced triple GEM stack is evaluated in a TPC prototype with 26 cm drift distance. The employed Timepix ASIC features 256 x 256 pixels with 55 μm edge length. These allow for characterization of electron avalanches started by individual primary electrons. Down to small drift distances, where the pitch of the GEM holes (140 μm) becomes dominant, the spatial resolution of this pixel TPC is limited only by single electron diffusion. For the readout of next generation gaseous detectors, development of an improved ASIC with 65 k-pixels, offering simultaneous charge and arrival time measurements with nanosecond accuracy, is pursued.