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

Figure 27: GE1/1-II efficiency performance at high and low η ends. Efficiency reaches 96.5% at gain 7000.
Figure 28: Results for GE1/1-II using pulse height measurements with APV25/SRS during the Aug. 2011 test
beam campaign: (a) strip cluster size distribution; (b) difference∆x between hit positions measured with GE1/1-II
and with a reference GEM in the tracker.
when the prototype was read out with electronics that recorded pulse height information. Specifically, the detector
was read out with APV25[14] chips in combination with the Scalable Readout System (SRS) [15] that has been
developed by the RD51 Collaboration. Two small 10cm×10cm GEM detectors TR5 and TR1 (see Fig. 23)
were used to provide a reference measurement for the tracks. With the GE1/1-II prototype detector flowing an
Ar/CO2/CF4 (45:15:40) gas mixture, data were taken with the H4 pion beam pointing at the detector region with
the smallest strip pitch, i.e. 573 µm.
An average cluster size of about 3 strips was found in this measurement and for the space resolution an upper limit
of 103 µm can be derived as shown below using the additional information on the charge sharing among adjacent
strips that is available with a pulse height measurement. This result is significantly better than the resolution
expected from a purely binary readout, i.e. strip pitch/ √ 12 = 573/ √ 12 = 165 µm.
y
Figure 29: Left: ∆y distribution for TR5 and TR1. Right: ∆x distribution for TR5 and GE1/1-II using central
tracks in a pion beam.
To minimize the impact of beam divergences in this spatial resolution measurement, tracks were selected to be
from a 2×2mm 2 spot in the center of a pion beam which is much narrower than the muon beam. Assuming
that both TR5 and TR1, which are constructed in the same way, have the same spatial resolutions (σy5 = σy1) and
that the beam divergence in y is negligible in the center, Fig.29 shows that we have for the width σ∆y of the ∆y
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