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

obtained. The angular resolution is≈ 10mrad.
The wire signals give fast and precise time information (5 ns) with coarser spatial resolution 16µm-54µm. For
high-η chambers, anode wires are rotated to compensate the Lorentz force under the 3.8 T magnetic field, avoiding
electron charge to be spread along the wires. The nominal gas mixture isAr/CO2/CF4 (45:15:40).
1.3 RPC (Resistive Plate Chambers)
The Resistive Plate Chambers[2] used in CMS are made of 2 gas gaps read out by a unique set of copper strips
placed in between the two gaps. Each gas gap is made out of two Bakelite plates filled with a gas mixture of
C2H2F4/ iC4H10/ SF6 (95.2:4.5:0.3) with a 50 % relative humidity to keep the Bakelite resistivity stable. Chambers
are operated in avalanche mode, ensuring proper operation at rates of up to 100 Hz/cm 2 .
RPCs guarantee a precise bunch crossing assignment thanks to their fast response and good time resolution. 3 rings
of 4 stations each are present in the endcaps, as shown in Fig. 1. For the low luminosity phase the innermost ring
(RE1/1) and the outermost stations (RE4) had been staged (|η| > 1.6), while RE4 stations have been up-scoped
and are under construction for installation in the Long Shutdown LS1, during 2013-2014.
The high-η rings of all endcap stations have not yet been under consideration. For high momenta muons, it is
imperative that the muon system functions well so that it contributes substantially towards momentum resolution.
For forward muons, however, CMS redundancy is compromised, due to the missing high-η muon station and to
the high background rate in the existing ME1/1 stations, already at the limit of the acceptable.
1.4 Muon system redundancy and extension up to η = 2.4
In Table 1 the particle rates and expected accumulated charges for different phases of LHC operation and its
luminosity upgrades are shown. The RPCs radiation hardness is at the limit, and the large strip pitch (1-2 cm) may
affect high-rate system performance, even with an adequate time resolution.
Micro-pattern gaseous detectors (MPGD[3],[4]) can reach rate capabilities up to 10MHz/cm 2 and provide high
spatial (100µm) and time (≈ 5ns) resolutions, with a≈ 98% detection efficiency. They can be operated with nonflammable
component gas mixtures and finer readout granularity along both η and φ allows for both triggering
and/or tracking.
1.5 Performance requirements for high-η muon detectors
For the forward Muon RPC low eta region, extensive tests were performed over several years in order to validate
the RPC technology and the gas mixture for particle rates of ≈ 10Hz/cm 2 . Bakelite RPCs are well suited for
operation at moderate rates (< 1kHz/cm 2 ). The international RPC community has devoted a tremendous effort
on aging studies, and mainly due to the LHC RPC R&D work at GIF[5] (Gamma Irradiation Facility - CERN),
we now have much better understanding for the mechanism of RPC operation, aging, rate capability. It has been
clearly shown that RPC performance and degradation are determined by complex interactions among the operating
conditions and the materials of the RPCs, in which the current, integrated charge, humidity, production of
hydrofluoric acid, etc. affect the linseed oil, graphite coating, and the bakelite itself, in complex ways that result in
degraded performance, increased dark current, reduced efficiency and increased resistivity[5]. A sophisticated gas
system was commissioned in order to recuperate the expensive components of the gas and to filter the pollutants
and contaminants produced during chamber operation. The tests carried out showed that the detectors are suitable
for operation in the low-η region, while concerns remained about the possibility of achieving stable operation with
the radiation conditions expected at η > 1.6. Thus the presently vacant high eta region of CMS RPC presents
an opportunity to instrument it with a detector technology that could sustain the environment and be suitable for
operation at the LHC and its future upgrades and the targeted installation period would be the long shutdown LS2,
in the years 2017/18. At that point detectors installed should be able to withstand the hostile environment and
high luminosity rates at the LHC upgrade, and sustain operation for a minimum of ≈ ten years after installation
namely until after CMS Phase II (≈ 2020−2030). The RE high-η region presents hostile conditions, with a particle
fluence of several 100Hz/cm 2 for an LHC luminosity of 10 34 cm −2 s −1 , which may go up to several kHz/cm 2
depending on the upgrade scenarios. In addition the rates of thermal neutrons, low energy protons and γ must
be taken into consideration. Hence the most stringent requirements for a detector at high η which can sustain
operation in the upgraded LHC are summarized in the Table 1.
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