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

Figure 34: Exploded view of a Triple-GEM arrangement (Distances are not to scale)
4.2 Study of the transport parameters
In this section, we present studies of the transport parameters for two gas mixtures, (Ar/CO2/CF4) and (Ar/CO2)
in the ratios 45 : 15 : 40 and 70 : 30 respectively. Some discussions on transport properties in gaseous detectors
can be found here[20]. Recently GEM detectors have been operated withAr/CO2/CF4 successfully in a high rate
environment in the LHCb experiment[18], and with Ar/CO2 in a 70 : 30 ratio in the TOTEM experiment[13].We
are investigating the usage of Ar/CO2/CF4 as this gas combines a high drift velocity along with a small Lorentz
angle (almost comparable toAr/CO2), which will be useful for triggering and other physics studies in the forward
region. Also, this gas was found to give a better time resolution of ∼ 5 ns as compared to Ar/CO2 which gave a
time resolution of∼ 10 ns [18]. We do a feasibility study of these gas mixtures for the CMS scenario. Since CMS
has a magnetic field of 4 T in particular, we would like to study the effect of the magnetic field and the effect of
the angle between the E-field and B-field.
When electrons and ions in a gas are subjected to an electric field, they move on an average along the electric field,
but individual electrons deviate from the average due to scattering on the atoms and molecules of the gas. Scattering
leads to variations in velocity, called longitudinal diffusion, and to lateral displacements, called transverse
diffusion. The scattering process in each direction can to a good approximation be considered Gaussian on a microscopic
scale. Electric field affects the transverse and longitudinal diffusion differently and so the two coefficients
are plotted separately in the figures. In cold gases like carbon-dioxide for example, the diffusion is small, while
drift velocity is low and unsaturated for values of electric fields which are usually used in gas detectors. Warm
gases like argon on the other hand, have a higher diffusion, but when they are mixed with polyatomic/organic gases
having vibrational and rotational modes, diffusion is reduced in most cases, while the drift velocity is increased.
Fig. 35 shows the diffusion coefficients for two gas mixtures as a function of the electric field. As can be seen
from the plot, the diffusion in the mixture Ar/CO2/CF4 is lower, as expected, because of a higher polyatomic
gas component; both CF4 and CO2 having vibrational modes which contribute to lowering the diffusion. CF4
is advantageous to use in a high-rate environment because of its high drift velocity but it suffers from electron
attachment. Therefore CO2 is used to “cool” the electrons and reduce the electron attachment which occurs in
CF4.
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