Operation of a GEM-TPC with pixel readout

A prototype Time Projection Chamber with 26 cm drift length was operated with a short-spaced triple GEM stack in a setup triggering on cosmic muon tracks. A small part of the anode plane is read-out with a CMOS pixel-ASIC, the Timepix chip providing ultimate readout granularity. Clusters of charge depositions corresponding to single primary electrons are observed and analyzed to reconstruct charged particle tracks. A data set of several weeks of cosmic ray data is analyzed. The number of clusters per track length is well described by simulation. The obtained single point resolution approaches 50 μm at short drift distances and is well reproduced by a simple model of single electron diffusion.