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

Table 1: Performance requirements in the various η sectors.
Region Rates (Hz/cm 2 ) High Luminosity LHC Phase II
LHC (10 34 cm −2 /s) 2-3 × LHC (10 35 cm −2 /s)
Barrel RPC 30 Few 100 ≈ kHz (tbc)
Endcap RPC 1, 2, 3,4 η < 1.6 30 Few 100 ≈ kHz (tbc)
Expected Charge in 10 years 0.05 C/cm 2 0.15 C/cm 2 ≈ C/cm 2
Endcap RPC 1, 2, 3,4 η > 1.6 500Hz ≈kHz few kHz few 10s kHz
Total Expected Charge in 10 years 0.05-1 C/cm 2 few C/cm 2 few 10s C/cm 2
Over the past two years, an effort has been focused on looking for options based on Micro-Pattern Gas Detectors
(MPGDs). A dedicated R&D program was launched in 2009 to study the feasibility of using micro-pattern gas
detectors for the instrumentation of the vacant |η| > 1.6 region in the present RPC endcap system. Micro-pattern
gas detectors can offer an excellent spatial resolution of order 100µm, a time resolution below 5 ns, a good overall
detector efficiency above 98% and a rate capability of order 10 6 Hz/mm 2 that is sufficient to handle the expected
particle fluxes in the LHC environment. In the case of the existing RPC system, the large volume, the cost of
the gas mixture, and the need to constantly remove impurities from the gas circuit to guarantee a stable detector
operation, make the use of a rather complex closed-loop gas system including filtering mandatory. For MPGDs,
their operation with a non-flammable gas mixture, e.g. Ar/CO2/CF4, is therefore also advantageous compared to
the present RPC system.
With the enhanced(η,φ) readout granularity and rate capability of the MPGDs, one could effectively improve the
Level-1 muon trigger efficiency and even offer both triggering and tracking functions at the same time. In this
case, we consider to extend the pseudo-rapidity range of the system up to |η| = 2.4, to match the coverage of the
Cathode Strip Chambers in the endcaps.
1.6 Present experience with MPGDs in experiments
An investigation of MPGDs as candidate technology to instrument the vacant zone in the high eta RPC part of
the CMS detector, namely 1.6 < |η| < 2.4, is undertaken. The objective is to develop the CMS detector with
enhanced and optimized readout granularity (η − φ), and a rate capability improved by two orders of magnitude
compared to RPCs, to improve the muon trigger efficiency and combine triggering and tracking functions.
Two types of micropattern detectors have been considered: the MICROMEGAs and the gas electron multiplier
(GEM). The MICROMEGAs is a gaseous detector made with a metallic mesh exploiting the exponentially increasing
Townsend Coefficient at very high electric fields. The gas electron multiplier on the other hand is a
thin metal-coated polymer foil perforated with a high density of holes (50−100/mm 2 ); each hole acting as the
multiplication region. Single GEMs can operate up to gains of a several thousands and GEMs can be used in
tandem. Thus a double or Triple-GEM detector delimiting the gas volume with a drift cathode and customized
readout anode can be assembled. The operating point of each single GEM in a Triple-GEM is thereby at a lower
gain with safe operating margin from the discharge point; multiple structures provide equal gain at lower voltage.
Both technologies have the potential for production of large area detectors (1m×2m) with cost effective industrial
processes. MICROMEGAs and Triple-GEMs have been installed at the COMPASS experiment in 2002 and were
also operated for one week with a 25 ns LHC-like hadron beam with the intensity on the detector of 5·10 6 pions
per spill of 5s on a surface of approximately 1cm 2 , similar to the one expected in CMS for an LHC luminosity of
10 33 cm −2 s −1 . The COMPASS collaboration reports good performance of the GEM detectors, with no evidence
of deterioration. Triple-GEMs have been installed in the first LHCb muon station, while ATLAS is considering
the micromegas for its muon upgrade. On a COMPASS Triple-GEM operated at the gain of 2·10 4 , a charge of
2C/cm 2 has been integrated on the readout board. GEM detectors are rather insensitive to ageing under sustained
irradiation, at a gain of ≈ 10 4 with Ar/CO2 (70:30), corresponding to one year of continuous run at a maximum
rate of 10 5 /cm 2 . No sign of ageing has ever been observed which implies that the detectors could be operated
without degradation at even higher integrated charges. Operation at lower gain (for example ≈ 8000) can further
enhance the robustness of the detector for high particle rates, in view of the high luminosities that may be achieved
with the LHC upgrades. For Triple-GEMs, with a gain of ≈ 2·10 4 , very good gain stability was measured up
to a photon flux of about 5·10 7 Hz/cm 2 , and extensive ageing measurements have been performed in the past.
Considerable improvement with respect to the Ar/CO2 (70/30) gas mixture, which exhibits a poor time resolution
of about 10ns rms, is obtained with the new CF4 and iso−C4H10 based gas mixtures, which allow to reach
time resolutions better than 5ns rms with the optimum choice which is the fast & non-flammable gas mixture
Ar/CO2/CF4 (45:15:40). As all gaseous detectors, GEMs have a finite probability of exhibiting a breakdown
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