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

(a)
(b)
Figure 19: (a) Exploded view of the first full-size Triple-GEM prototypes for CMS. (b) Electrode layout and gap
size configurations of the two full-size prototypes GE1/1-I (left) and GE1/1-II (right).
readout planes provide a one-dimensional readout using radial strips running in long direction of the chamber,
with a pitch varying from 0.8 mm (short side) to 1.6 mm (long side) for GE1/1-I and from 0.6 mm to 1.2 mm for
GE1/1-II. If the chambers were installed in CMS, the radial strips would point to the LHC beam line. The first
prototype was divided into four η-partitions with 256 strips per partition, while the second prototype had eight
partitions with 384 strips each. Due to the improvements in GEM timing performance obtained with the small
prototypes, the gap size configuration for the second large prototype was changed to 3/1/2/1 mm (drift, transf.1,
transf.2, induction) from the 3/2/2/2 mm configuration for the first prototype. Different steps of the assembly
procedure of the large prototypes are depicted in Fig. 20. Fig. 21 shows the gain curve for GE1/1-I measured with
an x-ray gun in the RD51 lab.
3.5.2 Test beam setups
The full-scale prototypes were tested with 150 GeV muon/pion beams at the CERN SPS H4, H6, and H8 beam
lines during several RD51 test beam campaigns in 2010 and 2011. The RD51 standard double-mask Triple-GEM
beam telescope (Fig. 23) as described in Section 3.4 was used as a reference tracking device also in these tests.
The GE1/1-II was installed near the RD51 telescope on a vertically movable table for scanning different points to
account for the varying pitch of the full-scale detector along its long axis.
Fig. 24 shows a typical profile of the muon beam as reconstructed with the GEM telescope. After alignment, the
reconstruction algorithm fits telescope hits in both dimensions providing a track space resolution of around50 µm.
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