A GEM based TPC for the LEGS experiment

A compact time projection chamber (TPC) has been constructed for the LEGS (Laser Electron Gamma Source) experiment at BNL. The TPC uses double GEMs as the amplification stage. Position encoding is achieved through charge division using zigzag shaped anode pads. The TPC has a 35 cm diameter active area and a 50 cm long drift depth. It has more than 7000 channels of readout electronics in the form of custom designed ASICs. A novel peak sensing circuit is used to measure simultaneously the amplitude and timing of the signal peak from an anode pad. Test results from a shorter prototype version of the TPC as well as the construction of the final detector are discussed.