A TPC with Tiple-GEM Gas Amplification and TimePix Readout

We report on measurements with a GEM based TPC and the TimePix CMOS ASIC as
charge collecting anode. 

Tests with a small prototype having  6 mm drift space were performed in a 
5 GeV electron beam at DESY. A Si-strip-hodoscope installed in the beam also
recorded the electron track position. Different GEM orientations and geometries
were tested. It is shown that the point resolution for short drift distances is
better than 20 micrometer. The achieved time resolution is better than 10 ns
at  100 MHz clock frequency. Furthermore, experimental studies with an
enlarged pixel size are addressed. 

To study the performance for longer drift distances a chamber with 26 cm drift
length is operated in Bonn. Data with cosmic muons and in the ELSA test
beam have been recorded. The spatial resolution and cluster proberties are
studied in dependence on the drift distance.