Study of a Charge Distribution on a Readout Board with a Triple GEM Chamber

We constructed a triple GEM chamber using 10cm times 10cm size foils, which were made by a Japanese company in plasma and laser combined etching method. The obtained effective gas gain is comparable with the CERN standard GEM. Charge spreads for various parameters were measured as a sigma in a Gaussian fit for the charge distribution. The result shows 360mum and 180mum spreads with a triple GEM configuration for Ar-CH4(90/10) and Ar-CO2(70/30) gas mixtures, respectively. Basically, measurement values are consistent with Magboltz simulation, which consider the diffusion only. It indicates the GEM structure and the the electric field in the holes do not affect on the charge distribution so much for the triple GEM chamber.