Towards a PMT based optical readout GEM TPC—First results

We report the first results on the operation of an optical double GEM TPC, read out by four photomultipliers in a square array. The detector, operated with Ar+5%CF4Ar+5%CF4 at atmospheric pressure, was tested with 241Am alpha particles emitted in various orientations. We obtained a typical track angle resolution of 2∘2∘(σ)(σ) at 45∘45∘ and achieved a point position resolution of better than 1 mm (FWHM) for the X- and Y-directions in the GEM plane.