Study of the GEM detector for the LHCb experiment upgrade

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 $2\times10^{33} cm^{-2}s^{-1}$. In this thesis, a study of the GEM efficiency and time performance is conducted using a cosmic rays station at the Emilio Segrè laboratory, in the Sapienza university of Rome. 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 mixtures properties is performed using the software GARFIELD, focusing on electron drift velocity and specific ionization. Using 2 plastic scintillator as a muon trigger system, several measurements of efficiency and time performance of the triple GEM are made, in various electric configurations of the detector, i.e. changing the GEM electric fields and the voltage across the GEM foils. The experimental results agree with the computed simulation: the large CF$_4$ component permits higher electronic drift velocity and therefore better time performance of the GEM chamber. A final summary confronts the performance in the 2 gas mixtures: in the 5$\%$ CF$_4$ gas mixture the measured 25 ns efficiency is greater than 80 $\%$, but in an electric working region close to instability (i.e. electric discharges), while the 40$\%$ CF$_4$ gas mixture permits the triple-GEM to reach 25 ns efficiency of 90$\%$ at moderate voltages. Moreover, systematic measurements of the mean multiplicity and cluster size of the events in the chamber lead to the conclusion that higher gain is observed in the 40$\%$ CF$_4$ gas mixture.