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

4 Simulation studies
4.1 The GEM detector
The GEM detector consists of a thin metal-clad polymer foil chemically perforated by a high density of holes. The
polyimide used has a dielectric constant of 3.5 and the metal used is copper. The GEM has a bi-conical hole, with
outer and inner radii as 70 µm and 50µm respectively and the pitch is140 µm.
A very high electric field develops in the GEM hole which can go as high as ∼ 60 kV/cm, when a potential
difference is applied to the two metal layers as can be seen in Fig. 33. We use a drift field region of 3 mm, and
spaces of 1 mm and 2 mm in the transfer regions and a 1 mm space in the induction field region. At CMS, the gas
under consideration is a mixture of Ar/CO2/CF4 in a 45 : 15 : 40 combination, similar to the gas that was used
in LHCb [18].
The detector is modeled using the ANSYS software, which is a general purpose finite element modeling package
for numerically solving a wide variety of mechanical problems. It can be used for solving the electric field in
a complicated structure like the GEM. The elements are defined using the solid modeling method. In order to
calculate the electric field, we use the free meshing method in ANSYS. ANSYS improves on the meshing used in
the previous simulation by using curved elements, and is more accurate.
One element of the GEM consists of a hole in the center with four quarter holes at the edges as can be seen in
the Fig. 34. The element is then extended to make a GEM detector of any size by using the periodicity feature
in Garfield++[16]. In Fig. 34, one can see the single GEM modeled in ANSYS and the Triple-GEM arrangement
made of single GEMs, the anode and cathode plane.
Figure 33: The electric field as seen in Garfield at the center of the GEM
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