Approval Plots for GEM Test Beam 2014

Detection efficiency of the GE1/1 for two gas mixtures at various (iη,iΦ) sectors readout sectors as a function of Egain is presented. E_gain is the average electric field in the GEM foil and is given as E_gain = (I x R_avg^GEMgap)/D, where I is current supplied to high voltage divider, R_avg^GEMgap is average of gap resistance and D is the thickness of gem foil. Efficiency, ϵ is given as ϵ = N_{GE11+Trk}/N_{Trk}, where NTrk is the number of events where a track reconstructed, using a linear fit y=mx+b fit to the tracker hit positions, in the tracker with normalised 𝜒2 < 10, and N_{GE1/1+Trk} is the number of events where a GE1/1 hit within 5mm of the extrapolated track position is found. The filled circle series is taken with Ar/CO2 (70/30) gas mixture at (iη,iΦ) = (5,2) while the triangle, square, and open circle series are taken with Ar/CO2/CF4 (45/15/40) at (iη,iΦ) = {(1,2),(5,2),(8,2)}, respectively. At a fixed high voltage operating point the effective gain with the Ar/CO2 mixture is approximately one order of magnitude higher than with Ar/CO2/CF4 mixture. As a result the efficiency distribution shown in the closed circle series, obtained with Ar/CO2, is shifted to lower values of Egain.