Effect of Gas Composition on the Performance of Cathode Strip Chambers for the CMS Endcap Muon System

The composition of the gas inside the cathode strip chambers (CSCs) defines many important performance parameters, including the gas gain and operational voltage range. Differences in these parameters were examined under variations in concentration of argon, carbon dioxide, and carbon tetrafluoride around the baseline mixture of Ar+CO2+CF4 = 40+50+10. The gas gain was found to be dependent primarily on the Ar concentration and nearly insensitive of the CF4/CO2 relative concentrations. The operational voltage range of ~370V had only a weak dependence on Ar concentration in the range of 40%-60% (-30 V for +20% change) at fixed amount of CF4, but showed about 3 times stronger dependence on the concentration of CF4 (+90 V for the concentration change from 0% to 20%) at fixed amount of Ar. We discuss 2 alternative gas mixtures Ar+CO2+CF4 = 60+30+10 and 60+35+5, which both offer performance (gas gain and plateau) very similar to that of the baseline mixture, but at 400 V lower absolute voltages.