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

(a) (b)
Figure 22: (a) The GE1/1-I detector installed in the SPS H4 beam line. (b) The GE1/1-II detector installed inside
the CMS M1 magnet at the SPS H2 beam line.
Scintillator X,Y
Tracker 1 Tracker 2 TR
1 Tracker 3 TR
5
Scintillator X
CMS_GE11_II
Figure 23: The RD51 telescope used during all test beams at the RD51 H4, H6, and H8 beam area.
3.5.3 Prototype performances in beam test
Fig. 25 shows a few results obtained with the GE1/1-I prototype in the October 2010 campaign.
With the beam pointing at the η-sector of the GE1/1-I detector with the smallest average strip pitch, i.e. 1.05 mm,
a spatial resolution of 355 µm was measured with an Ar/CO2 (70:30) gas mixture. This is close to the expected
value of 1050 µm/ √ 12 = 305 µm for a binary readout. The detector efficiency was measured to be above 98%
for this prototype. The uniformity of the detector response across the GEM active area was found to be very good.
In the 2011 measurements, the noise could be substantially reduced compared with the 2010 measurements when
operating the detector with VFAT2 binary electronics. Based on the very low observed noise in the detector without
beam, the VFAT2 threshold was set to 12 units 2) and the VFAT2 internal comparator current was set to 40 µA.
With these settings the noise was practically absent, when operating the detector with gains up to10 4 .
Fig.26 shows average strip cluster sizes of 1.6-1.9 strips (depending on the η position) and a residual of 291 µm
between extrapolated tracker tracks and hits measured in the GE1/1-II. This is about 20% better than what was
2) One VFAT2 threshold unit corresponds to a charge of ≈ 0.08 fC at the input channel comparator stage.
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