P o S(TIPP2014)292

We are developing a low mass, cylindrical GEM detector with analog readout for the inner tracker upgrade of the BESIII experiment at the BEPC-II e+e − collider. The GEM detector will replace the current inner drift chamber that is suffering early aging due to the increase of the machine luminosity. The new inner tracker is expected to match the momentum resolution (σpt/pt 0.55% at 1 GeV) and radial resolution (σxy ∼ 100µm) of the drift chamber and to improve significantly the spatial resolution along the beam direction (σz ∼ 150µm) with very small material budget (about 1 X0). The inner tracker will be composed by three layers of triple cylindrical GEM with an angular coverage of 93 % of the solid angle. Each layer is composed by five cylindrical structures: the cathode, three GEMs and the anode readout. To minimize the amount of material, no support frames are used inside the active area and the GEM foils are mechanically stretched being glued to fiberglass rings at their ends. The anode configuration is studied by means o