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Novel Gaseous Detector and Technology R&D
Preliminary considerations
The choice and the characteristics of the detectors that will be used for the phase 2 upgrades of the muon
systems of the LHC experiments will be steered by the tasks that they will called to accomplish.
Given the regions they will be likely to be installed, located in the endcaps and close to the beam pipe, full
efficiency despite a high rate of impinging particles (charged and neutral) is mandatory. Even if this varies
significantly in the various regions, a value around 10 kHz/cm2 should be assured. Time and space
resolutions comparable to the present - of the order of 1 ns and 1 cm, respectively – are also a minimum
request.
However, most of the added value for the upgrade of the muon system might derive from the fact that
novel detectors could provide a space or a time resolution much better – typically and order of magnitude than the present one. A hypothetical detector combining excellent characteristic for reconstruction (a
space resolution around 100 µm) and an around 100 ps time resolution would be, in principle, quite
attractive and allow very interesting solutions.
Finally, these detectors should be placed in a high radiation environment and operate for around 10 years,
so strict requirements about longevity in these harsh conditions are also very important. Therefore each of
the possible candidate detectors should also undergo an intense program of assessments of the aging
effects, to understand if the foreseen performance deterioration would be compatible with the requested
specifics.
In the following the four main candidates (actually in some cases the decision has already been done) will
be briefly reviewed, emphasizing the reason why they could accomplish the task they are called to do, and
possible weak points of the technology chosen. Also the necessary R&D will be reviewed and other possible
options rapidly examined.
In ATLAS, decisions on the type of technology has already been taken, so this case will be reviewed first,
while CMS is still in the process of taking the decision about the best configuration and detectors to use.
Therefore, ATLAS will be reviewed first; then we will examine CMS and LHCb. The ALICE experiment does
not foresee, for what concerns the detectors, any phase-2 upgrade.
ATLAS
The major project for the upgrade of the muon system in ATLAS will consist in the installation of the New
Small Wheels (NSW), the inner end-cap wheels of the muon spectrometer, covering a pseudorapidity
intervals of [1.3-2.7] for muon tracking and [1.3-2.4] for Level-1 trigger. Some of the problems connected
with the increased luminosity and subsequent rate in the present configuration will be solved installing the
New Small Wheels; designed to meet all the requirements for the Phase-1 upgrade, but also to guarantee
stable performance of the experiment after the Phase-II upgrade.
Each one of the two NSWs consists of 16 detector planes arranged in two multilayers. Each multilayer
comprises four small-strip TGC (sTGC) and four Micromegas (MM) detector planes. sTGC are the primary
detectors for triggering, while MM are the primary detectors for tracking; the NSW is a highly redundant
system since each detector technology is able to provide both high resolution tracking and bunch crossing
identification for Level-1 muon trigger.
An intense R&D programme has been carried out for the development of sTGC and MM suited to be
employed for the upgrade of the ATLAS muon spectrometer.
Small-strip Thin Gap Chambers
The small-strip TGCs are a further development of the TGC already in use in ATLAS. They consist of a grid of
50 um gold-plated tungsten wires with a 1.8 mm pitch, sandwiched between two cathode planes at a
distance of 1.4 mm from the plane. The chatode planes are made of a graphite-epoxy mixture with a typical
surface resistivity of 100 kOhm/square sprayed on a 100 um thick G-10 plane, behind which there are on
one side strips (3.2 mm pitch, running perpendicular to the wires) and on the other pads (covering large
rectangular surface), on a 1.6 mm thick PCB with the shielding ground on the opposite side.
It is known that space-charge effects are not a limiting rate element for TGC. The rate capability is limited
by the use of the resistive coating in the cathodes, which reduces, under high irradiation, the effective
operating voltage of the detector in areas far from the ground contacts. As a consequence a low surface
resistivity coating (approximately 100 kΩ/square) has been used, and the capacitance between the
strips/pads and the cathode has been increased to keep the same transparency for fast signals.
This optimization, combined with a reduction of the HV and ground decoupling resistors is sufficient for
maintaining high efficiency for minimum ionizing particles in large surface detectors subject to detected
rates of up to 20kHz/cm2 over the full surface. Spatial resolutions below 200um have been reached both
with TDC- and ADC-based read-out for impact angles of minimun ionizing particles up to 30 degrees (the
angular range for the New Ssmall Wheels is between 8 and 30).
Micro Megas
The R&D activity for the ATLAS Micromegas has greatly pushed ahead this technology. The vulnerability to
sparking, a weak point common to all MPGD, was solved with the introduction of a spark-protection system
by adding a layer of resistive strips on top of a thin insulator directly above the read-out electrodes. In this
way the MM are spark-insensitive and the construction of large-size (few square meters) detectors
becomes possible. Additional technical improvements were the implementation of the floating-mesh
configuration, in which the mesh is no longer integrated in the read-out structure, and the inverted-HV
scheme, according to which positive HV is applied to the resistive strips while keeping the amplification
mesh to ground. With these technological solutions Micromegas with a sensitive area of 2 m2 were built at
CERN and chamber up to 3 m2 are foreseen in the NSW.
The spatial resolution of MM with sub-mm strip pitch and analog readout can easily go below 100μm for
perpendicular tracks by using the cluster charge centroid method. A spatial resolution of about 65 μm has
been obtained at beam tests at CERN. With the same perpendicular tracks, global detector inefficiencies
have been measured to be in the range of 1–2%, consistent with the partially dead area expected from the
presence of the 300 μm diameter pillars separated by 2.5 mm.
For impact angles greater than 10◦ the μTPC method is used for a local track segment reconstruction in the
few-millimeter wide drift gap. It exploits the measurement of the hits time and the highly segmented
readout electrodes: the position of each strip gives an x coordinate, while the z coordinate (perpendicular
to the strip plane) can be reconstructed from the time measurement of the hit after calibrating the z–t
relation (z = t × vdrift). Combining the μTPC method and the cluster centroid a spatial resolution below
100μm was obtained for impact angles up to 40 degrees.
CMS
Also CMS the most critical parts of the muon system, in terms of rate subsequent to the increased
luminosity, are located in the innermost wheels of the endcaps. Even if the R&D on possible detector
technology has been going on for years now, the final decision on the optimal configuration and detectors
for the phase 2 upgrade has still not been done. The idea, essentially, is to keep stable, despite the harsher
conditions, the muon trigger and tagging capabilities in the endcap, or, if possible, to improve them. This
will be done installing new detectors in locations where, for the moment, nothing has been installed, or
extending the system toward higher pseudorapidity regions; the existing detectors will remain where they
are. Here we will review the most interesting candidates, taking into account that how and where they will
be used will depend essentially on the choice about the upgraded muon system configuration already
outlined in Section (??? The first talk).
Gas Electron Multipliers (GEMs)
GEMs were invented in 1997 by F. Sauli and represent one of the most interesting gaseous detectors
recently appeared on the scene. Essentially, the electron multiplication takes place inside conical (or biconical) holes carved in thin kapton foils, copper-coated on both sides to apply the necessary operating
voltage. Single GEMs can operate up to gains of several thousands, and GEMs can be used in cascade, to
that high gains can be reached with a negligible sparking probability. Electron collection takes place by
means of metallic submillimetric strips, so that excellent rate capability (order of MHz/cm2) coupled with a
position resolution significantly better than the present can be reached.
The GEMs for CMS collaboration, born in the RD51 framework, carried out an intense R&D program on
these detectors, focusing not only on the detector performance, but also on the issue related to its large
scale production. For instance, they demonstrated the possibility to use the single mask technology
(chemical etching of the kapton foil performed on one side only) to reliably produce large size detectors,
drastically reducing the production times.
A proposal recently presented to the collaboration aims at instrumenting the 1.5 < eta < 2.4 region of the
CMS forward muon system with an additional position sensitive detector made from pair of triple GEM
chambers, in order to provide more robust muon tracking and trigger capabilities, in particular improving
muon momentum resolution.
(to be completed)
Improved Resistive Plate Chambers (i-RPC)
The most natural solution for the upgrade of the muon systems would be to exploit detectors similar to the
ones already in place. In this sense, RPcs could be a good candidate, since they already cover thousands of
square meters in these systems, and their performance has been quite satisfactory during the past years.
The most important limiting factor for the use of RPCs in this case is related to their limited rate capability,
which ranges around 1 kHz/cm2 for the devices already in use. As already stated, even if in some of the
regions of the proposed upgrades this should be already enough, R&D has to proceed in the direction of
improving this value up to 5-10 kHz/cm2, to be sure that all regions could be covered in this way.
Rate capability of RPCs is limited by the time that the local region on the electrodes discharged after the
passage of an ionizing particle and the subsequent avalanche takes to charge up again; this is directly
proportional to the electrode resistivity. Therefore, using bakelite, or alternative materials, characterized
by a lower resistivity represent a promising way to improve RPCs rate capability.
This is not the only possibility; for instance, if the voltage drop subsequent to the avalanche could be
somehow reduced, the effect would be similar to reducing electrode resistivity. This was already done in
the beginning of ‘90s when passing from the streamer operation mode to the avalanche mode, transferring
part of the needed amplification to the frontend electronics. In principle one could push forward the same
method, using more and more sophisticated electronics; the limiting factor in this case is the detector
intrinsic noise, and the relative signal to noise ratio.
An intense R&D is going on exploiting all these possibilities, sometimes combining the methods and
combining the relative advantages.
(to be completed)
LHCb
The need for new muon detectors, able to deal with high particle fluxes, was already considered by the
LHCb Collaboration since the beginning of the experiment. The LHCb muon system covers a large solid
angle in the forward direction and huge differences in particle fluxes are present, ranging from hundredth
of Hz/cm2 In the most external regions up to almost 0.5 MHz/cm2 in the innermost regions.
For this reason, while most of the LHCb muon system is made using MWPC technology, the innermost
region of the first muon station (M1) is using Triple--‐GEM detectors with pad readout. These 24 Triple-‐GEM detectors are arranged in pairs to increase efficiency and redundancy, and cover an area of 0.7 m2
around the LHC beam pipe.
A long R&D program allowed the optimization of these detectors, and a lot of care was taken to optimize
the gas mixture, the internal geometry, the readout electronics to guarantee the required detector
performance. The installed Triple--‐GEM detector use a ternary gas mixture (Ar:CO2:CF4 45:15:40) that
provides the required efficiency (>96%) within the LHC bunch--‐crossing (25 ns)
GEMs for LHCb
LHCb is now planning an upgrade to be able to operate efficiently at instantaneous luminosities as high as
2E33/cm2/2, a x5 increase with the LHC Run 1 running conditions (4e32/cm2/s). The LHCb experiment has
performed some high luminosity tests at the end of 2012, reaching instantaneous luminosities of
6E32/cm2/s with the full detector in operation and as high as 1E33/cm2/s with only a few detectors actives
(VELO, calorimeters and muon systems).
Preliminary results indicate that the muon system should operate with a satisfactory efficiency at the
increased luminosity, excluding the first muon stations and the innermost regions of M2 (R1 and R2) and
M2 (R1) where some efficiency reductions are expected. LHCb plan is to fully remove the first muon station
(due to the very high luminosity it will not provide anymore useful information for the trigger) and to start
the high--‐luminosity phase with the current detectors, because while some reduction in efficiency are
expected these should not affect in an important way the global muon triggering and reconstruction
efficiencies.
However, In order to be ready in case of possible problems (for example detector aging in the most
irradiated regions) an R&D for new radiation--‐hard detectors for the most critical regions has started.
The plan is to develop new Triple--‐GEM detectors with digital pad readout for M2 and M3 Innermost
regions, together with a new readout electronics, to be possibly installed at LS3. These detectors will have
similar internal geometry as the one successfully used now in M1R1 and will be built with the embedded
GEM foil stretching technique developed by Rui de Oliveira and his group at CERN, which is a further
development of the stretching technique originally developed by LHCb for the current M1R1 Triple--‐GEM
detectors. Preliminary prototypes are in preparation. Some effort is also put in the study of new gas
mixtures, in particular ongoing studies have the purpose of assess the performance of the gas mixture used
in all the muon MWPC (Ar:CO2:CF4 40:55:5) on Triple--‐GEM detectors, together with an optimization of
GEM operating voltages and drift fields.
Other possible detectors
(text in preparation)
Further R&D needed
(text in preparation)
Conclusions
(text in preparation)