Gas Electron Multiplier (GEM) as readout for the ALICE TPC

We present and discuss the results of systematic measurements carried out with a prototype TPC, in order to investigate the performance of a double GEM amplification structure with a pad readout.<P>List of figures<UL><LI>Figure 1 <A HREF="Image244.gif"> Scheme of the test setup, a 50 cm long field cage coupled to a double GEM readout structure</A><LI>Figure 2 <A HREF="Image245.gif"> Schematics showing the geometrical distances and the applied fields for the measurements</A><LI>Figure 3 <A HREF="Image246.jpg"> Printout from the oscilloscope of the GEM readout (channel 1) and the trigger counter (channel 2) for a single event</A><LI>Figure 4 <A HREF="Image247.gif"> Scheme of the pad configuration for the charge distribution measurements. Each of the adjacent pads is connected to a single amplifier</A><LI>Figure 5 <A HREF="Image248.gif"> The collected charge on 3 adjacent pads of the double GEM readout structure, which has its origin from one triggered track</A><LI>Figure 6 <A HREF="Image249.gif"> Measured charge distribution for an Ar-CO<SUB>2</SUB> gas mixture, drift distance of 32 cm and pad width of 2 mm</A><LI>Figure 7 <A HREF="Image250.gif"> Preliminary result from the sigma of the charge distribution versus the square root of the drift distance</A><LI>Figure 8 <A HREF="Image251.gif"> Scheme of the pad plane settings for the first tracking measurements</A><LI>Figure 9 <A HREF="Image252.gif"> Residuals from tracking using the centre of gravity distribution of three rows</A></UL>