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

3 Studies of small and full-scale GEM prototypes
3.1 Initial prototypes for choosing among MPGD technologies
As the very first step of this project, small prototypes of two different MPGD types were characterized: one
Micromegas [7] detector and one Triple-GEM [8] detector. Both prototypes were produced with an active area of
10 cm×10 cm in the CERN EN-ICE surface treatment workshop and were subsequently tested in the RD51 [4]
lab of the CERN Detector Technology Group (DT). Using standard Ar/CO2 gas mixtures, the two detectors were
characterized by measuring gain and pulse height spectra with radioactive sources and Cu x-rays from a generator.
Their efficiency plateaus were measured and the optimal operational voltages were determined.
In October 2009, the two prototypes were put into a pion/muon test beam at the CERN SPS H4 beam line [9]. In
this test, good detector performance was observed for the Triple-GEM while the Micromegas prototype showed a
substantial number of discharges and hence poorer data quality. The discharge probabilities of the two detectors
were subsequently measured in the RD51 lab. For the Triple-GEM a probability of 10 −6 was measured for gains
up to 2·10 4 , while the Micromegas was discharging with a probability of 10 −4 at a gain of less than 2000. These
results were consistent with previous MPGD studies. Based on these findings and given the existing expertise on
GEMs within the research group, the Triple-GEM MPGD was selected for detailed further studies.
3.2 Triple-GEM prototypes
GEM foils are made from 50µm thick kapton sheets with a 5µm copper cladding on both sides. The initial small
10cm×10cm Triple-GEM was constructed using the standard double-mask technique for etching GEM foils. The
GEM foils and cathode drift foils were glued onto fiberglass frames and mounted inside a gas-tight box as shown
in Figure 9. The detector had 128 readout strips with a pitch of 0.8 mm. Two different gap size configurations
were tested to study the effect on the detector performance, i.e. (drift, transfer 1, transfer 2, induction gap size):
(3/2/2/2 mm) and (3/1/2/1 mm). For this Triple-GEM prototype a rate capability of over 10kHz/mm 2 and a time
resolution of 4.5 ns (rms) were measured (see Section 3.4.2).
Figure 9: The standard double-mask Triple-GEM prototype. Left: Detector during assembly. Right: Configuration
of gaps between electrodes.
The stretching of GEM foils during detector assembly is a somewhat time-consuming procedure. An alternative
construction technique was investigated with a small Triple-GEM prototype where honeycomb spacers were inserted
into the gaps between the electrodes to main the distances between electrodes and to avoid the need for foil
stretching. A significant detection inefficiency was observed where the honeycomb “ribs” are located [10] and
consequently this technique has not been pursued further.
In addition to the standard double-mask GEM prototype, a10cm×10cm Triple-GEM prototype was constructed
in the same way but using GEM foils produced with the single-mask etching technique [11], which overcomes the
problems with the alignment of the masks on either side of the foils during the photolithographic etching of the
holes. This prototype had 256 strips in each of two perpendicular directions with a strip pitch of 0.4 mm.
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