A GEM Detector System for an Upgrade of the CMS Muon Endcaps

Figure 37: Lorentz angles for the gas mixtureAr/CO2/CF4 for the anglesθ(E,B) = 8 ◦ (left) andθ(E,B) = 90 ◦
(right) for a magnetic field of 3 T.
The diffusion effect leads to variations in drift velocity. In Fig. 38, one can see that the simulation results for
different gas mixtures compared with experimental LHCb test beam results[18]. It can be seen that the mixture
Ar/CO2/CF4 is a faster gas on account of addition of the CF4 gas.
Figure 38: The drift velocities for various gas mixtures from simulation and the experimental values from LHCb
studies. The simulation shows a good agreement with the experimental results
4.3 Gain studies
When an electron enters the GEM hole, under the influence of the high electric field it gains enough kinetic energy
between two successive collisions to further ionize the argon and more and more free charges are produced rapidly.
The primary electron therefore produces an “avalanche” under the influence of the high electric field. This increase
in the number of electrons is also called gain. In this section we study the gain and losses in a single GEM for
Ar/CO2/CF4. We then compare it with Ar/CO2. We then study the gain for a Triple-GEM and compare it with
test beam data.
Even after 10≈years since the invention of the GEM detector, gas gain is still not very well understood. However,
in a very recent approach[16], microscopic tracking has been used, which tracks electrons from one collision
with a gas molecule to the other using Monte Carlo methods. The electron-gas interaction takes into account the
processes of elastic scattering, attachment,excitation and ionization. Electrons with an energy above the excitation
threshold can lift an electron of an argon atom to a higher energy state. The excited states undergo radiative decay
but excited noble gas atoms are more aggressive and can alternatively de-excite by transferring their excess energy
to a quencher molecule which may even get ionized if the energy transfer exceeds its ionization potential. We study
the effect of this process, called Penning transfer [21],[22],[?]. in theAr/CO2 gas. A previous paper[19] has done
a detailed gain study which includes the effect of Penning transfer in Ar/CO2 gas. It was shown that the Penning
transfer can play a big role. The issue under study here was whether it plays a similar role in Ar/CO2/CF4 gas,
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