Numerical Study of Transport Properties of Ar-based Ternary Gas Mixtures and Their Gain in GEM Detectors

Compact Muon Solenoid (CMS) at the Large Hadron Collider (LHC) has
installed Gas Electron Multiplier (GEM) detectors in the muon system during Long
Shutdown 2 (LS2). This step is one of the major upgrades for the CMS because of
increasing the LHC luminosity since GEM detector is capable to work at high fluxes
of radiation. As a gas detector, the chosen gas has a strong influence on the detection
performance and the gain. The currently used gas mixtures in CMS GEM detectors are
Ar/CO2 (70/30) and Ar/CO2/CF4 (45/15/40). Although the second mixture is faster, but
CF4 is hazardous and has to be limited. So in search for a replacement to CF4, a
numerical study on the transport properties of Ar-based ternary gas mixtures presented
in this work. As one of the detection requirement is to have a high gain, the gas
mixtures that show optimal properties used to calculate the gain of single and triple
GEM with different field configuration. It found that the Ar/CO2/N2 mixture is giving
a high drift velocity while the Ar/CO2/C5H12 mixture has low diffusion coefficients and
the highest Townsend coefficient, which results in higher gain in both single and triple
GEM. Mixing N2 and C5H12 with Ar is giving very low attachment coefficient.