Analysis of the performance of GEM chambers for the upgrade of the LHCb muon system

Triple-GEM chambers, so far used in the inner region of the M1 muon station, are the ideal candidates to replace the MWPCs in the regions where, after the upgrade, the particle rate will be as high as 2 MHz/cm$^2$. Testing the triple-GEM efficiency and time resolution with different gaseous mixtures is therefore a significant step in evaluating the expected performance of the muon system after the upgrade scheduled in 2018, when the luminosity in LHCb will reach $2 \times 10^{33}$ cm$^2$ s$^{-1}$. In this note, the study of the GEM efficiency and time performance is presented. Two gas mixtures are used for the measurements: Ar/CO$_2$/CF$_4$ 40:55:5 and 45:15:40. A preliminar simulation of the gas mixture properties is performed by means of the software GARFIELD, focusing on the electron drift velocity and the specific ionization. Several measurements of efficiency and time performance of the triple GEM were thus made by means of cosmic rays by changing the detector electric fields and the voltages across the GEM foils. The experimental results agree with the computed simulation: with the 5 % CF$_4$ gas mixture a detection efficiency in a 25 ns time window of about 80 % is reached, while the 40 % CF$_4$ gas mixture permits the triple-GEM to reach more than 90 % at moderate voltages.