The Micromegas Project for the ATLAS Upgrade
PPreprint typeset in JINST style - HYPER VERSION
The Micromegas Project for the ATLAS Upgrade
G. Iakovidis a , b ∗ a National Technical University of AthensZografou Campus, GR15773, Athens, Greece b Brookhaven National LaboratoryUpton, NY 11973, USAE-mail: [email protected] A bstract : Micromegas is one of the detector technologies (along with the small Thin Gap Cham-bers) that has been chosen for precision tracking and triggering purposes of the ATLAS muonforward detectors in the view of LHC luminosity increase. To fulfill the requirements of such up-grade, several prototype micromegas detectors were tested in recent test beam campaigns with highenergy hadron beams at CERN. Performance studies and results on spatial resolution for perpendic-ular and inclined tracks, e ffi ciency, as well detector performance and comparison to simulation in amagnetic field are presented .Moreover, an overview of detector performance after neutron, X-ray,gammas and alphas exposure and construction achievements of large area micromegas detectorsare presented.K eywords : Micromegas; Gaseous Detectors; ATLAS Upgrade; Muon Spectrometer; Trigger. ∗ On behalf of the MAMMA collaboration. a r X i v : . [ phy s i c s . i n s - d e t ] O c t ontents
1. Introduction 12. Micromegas Detectors 2
3. Performance 4 µ TPC Scheme 53.2 Micromegas Inside Magnetic Field 53.3 Triggering with micromegas 53.4 Ageing 73.5 Testing Micromegas in ATLAS Cavern 7
4. Conclusions 8
1. Introduction
The Large Hadron Collider (LHC) at CERN, after the scheduled shutdown of 2017-2018, willresume its operation with a luminosity increase of five times its original design luminosity of L = cm − s − . For the ATLAS detector (ref. [1]), such a luminosity increase means higher particlerates. While in most of the ATLAS muon system the detectors have enough safety margin tohandle these rates, the first forward station of the muon spectrometer, called the Small Wheel, willexceed its design capabilities. At pseudorapidity η = ± .
7, rates up to 15 kHz / cm are expected,far higher than what the currently installed detectors can handle. Furthermore, the upgraded SmallWheel is expected to take part in the Level 1 trigger decision, something that the present systemwas not designed for. The physics objective is to sharpen the trigger threshold turn-on as well asdiscriminate against background while maintaining the low transverse momentum ( p T ) thresholdfor single leptons ( e and µ ) and keeping the Level-1 rate at a manageable level.The Muon ATLAS MicroMegas Activity (MAMMA) R&D explored the potential of themicromegas technology for its use in LHC detectors and finally proposed to equip the New SmallWheel (NSW) with micromegas detectors, combining trigger and precision tracking functionalityin a single device. In middle 2013 the ATLAS Collaboration endorsed the proposal (ref. [2]).In total, eight planes of micromegas (ref. [3]) detectors covering the full NSW will be installed,corresponding to a total detector area of 1200 m . In addition to the micromegas, the NSW shouldalso be equipped with eight planes of thin-gap multiwire detectors, called sTGC, such as to createa fully redundant system, both for trigger and tracking.– 1 – igure 1. On the left, a sketch (not in scale) of the resistive-strip protection principle with a view along andorthogonal to the strip direction; on the right, oscilloscope screen shot of about 10 spark signals as seen onthe readout strips with 50 Ω termination.
2. Micromegas Detectors
The first two years (2008 / ff erent layouts, with respect to strip pitch and strip length and gas mixtures were tested.Detectors with strip length up to 1 m were build while the operating gas mixture was settled on aAr : CO ff erent approaches were tried. The final approach wassettled on a scheme with a layer of resistive strips above the readout strips that matches geomet-rically them. This technique transformed micromegas detectors spark resistant while maintainingtheir ability to measure minimum-ionizing particles with excellent precision in high-rate environ-ments. Resistive strips layout rather than a continuous resistive layer was chosen mainly to avoidcharge spreading across several readout strips while keeping the area a ff ected by a discharge assmall as possible and thus maintaining a high rate capability of the detector. The principle of theresistive-strip protection scheme is illustrated in figure 1 and described in more detail in ref. [4].The power of the resistive-strip spark protection scheme is illustrated in figure 2. It shows the mon-itored HV and the currents for a standard micromegas and one with the resistive-strip protectionunder neutron irradiation (ref. [5][6]) for di ff erent mesh HV settings. Figure 3 shows the NSW (ref. [2]) layout divided into small and large sectors. Every sector consistseight layers of PCB boards each one divided into two micromegas. In total 512 micromegas up toa surface of 3.1 m should be build covering an area of 1200 m . The strip pitch of ∼ µ m resultsin a system of a total of 2.1 M channels. Strips on the four out of the eight layers will be underan angle of ± . ◦ providing second coordinate measurement while they contribute to the precisioncoordinate measurement with the other four at the same time.– 2 – igure 2. Monitored HV (continuous line) and current (points) as a function of HV mesh under neutronirradiation, left a non-resistive micromegas; right a micromegas with resistive-strip protection layer.
Figure 3.
On the left the NSW layout is shown. On the middle and right the segmentation of the modules ofsmall and large sector respectively.
Over the last few years several micromegas prototypes of an active area of ∼ . were build (ref.[7][8]) using the bulk technique. Experience with these detectors shows that mechanically the meshdeposition process is well under control resulting in a very homogeneous detector response over thefull area. The weak point lies in the risk of enclosing some impurities (dust or dirt) under the mesh,leading to a high-ohmic current bridge between the mesh and the resistive strips. Cleaning out suchdust particles is di ffi cult or impossible. Overcoming this issue, a novel construction technique wasdeveloped over the last year. The chambers were constructed with a removable mesh. It consistsof the readout panel and the drift electrode panel, the latter incorporates the mesh. Details on thisconstruction method can be found in ref. [8].In the beginning of 2013 two large 1 × . (0 . × .
12 m active area) micromegas detectorswere build. The readout panel is composed by four micromegas boards of 0.5 mm thick gluedtogether to an aluminum plate that served as a sti ff ening panel. The total number of strips are2 × µ m high pillars were deposited every 4 mm, followingthe same procedure as used in the bulk process. On the drift panel the “inner” skin serves as drift– 3 – igure 4. On the left, the assembly of the large 1 × . micromegas prototype. On the right, the largeprototype sitting on the cosmic stand at the CERN RD51 lab. Figure 5.
The reconstructed cluster charge (left) and position (right) over the full detector surface. electrode while on the same skin, a 5 mm high frame is fixed to which the mesh is glued. When thechamber is closed the mesh, that makes the ground contact, sits on the 128 µ m high pillars. WhenHV is applied to the resistive strips the electrostatic force between mesh and resistive strips ensuresthat the mesh is in good contact with the pillars. Data taken with the chamber in the test beam showan excellent performance with clean signals and a homogeneous response over the full area of thedetector. Figure 4 shows the detector on the assembly and testing phases while the figure 5 showsthe homogeneity over its surface.
3. Performance
Micromegas detectors of an active surface of 10 ×
10 cm have been tested during test beam cam-paigns at CERN with high momentum hadron beams. Using those detectors with a strip pitchof 400 µ m a spatial resolution of 65 µ m was easily achieved for perpendicular tracks. Since theNSW will be located in the ATLAS experiment tracks between 10 ◦ –30 ◦ are expected, studying theperformance with inclined tracks is of particular interest.– 4 – trip number A P V fi t ti m e ( n s ) Figure 6.
On the left a reconstructed track in the 5 mm drift gap under 30 ◦ . On the right plot the spatialresolution versus the incident track angle with di ff erent reconstruction techniques. µ TPC Scheme
Measuring the arrival time of the ionized electrons with a time resolution of a few nanosecondsallows reconstructing the position of the ionization process and thus reconstruction of the particletrack in the drift gap of the detector. With the Ar:CO (93:7) mixture and an electrical drift fieldof 600 V / cm the drift velocity is 4.7 cm / µ s, corresponding to a maximum drift time of about 100 nsfor a 5 mm drift gap. Figure 6 shows an example for a reconstructed track traversing the detectorunder 30 ◦ and the spatial resolution as a function of the incident track angle using the µ TPC mode.An analysis technique that combines the reconstructed µ TPC and charge centroid points, improvesthe spatial resolution especially for particle tracks of around 10 ◦ , while results in a homogeneousspatial resolution under 100 µ m along di ff erent track angles. The NSW will operate under a multi-directional and non constant magnetic field up to 0.5 T. Sev-eral micromegas detectors of 10 ×
10 cm active were installed inside a superconducting magnet atCERN (figure 7) and tested under the influence of a magnetic field up to 2 T. No degradation of theperformance was observed while we were able to measure the Lorentz angle and the drift velocity.Figure 8 shows those measurements in comparison with Monte Carlo simulation. Micromegas detector system will contribute to the formation of the Level-1 muon end-cap trigger,forming a very powerful, redundant trigger system along with sTGC detectors. Exploiting thiscapability, however, requires a large number of electronics channels, about two million for an eightlayer detector system. The electronics of the NSW must provide both a high resolution vector, inreal time, to be used in the formation of the muon Level-1 trigger in addition to the amplitude andtime information provided on the reception of a Level-1 accept.– 5 – igure 7.
Eight migromegas chambers inside the superconducting magnet in H2 CERN on June 2012 testbeam.
Figure 8.
The measured drift velocity (left plot, red points) and the lorentz angle measured (right plot, blackpoints) versus the magnetic field. Both measurements were found to be in agreement with Garfield (ref. [9])simulation.
In order to reduce the number of trigger channels the design takes advantage of the micromegasdetector’s fine readout pitch to reduce the number of channels by a factor of 64 resulting in a totalchannel count for the trigger logic of about 33,000. This is accomplished by considering only thefirst arriving hit in each 64-channel front-end ASIC for a given bunch crossing (25 ns), resultingin a system with granularity of 32 mm (64 × ngle [Degrees]0 5 10 15 20 25 30 A ngu l a r R e s o l u t i on [ m r ad ] Trigger on thresholdTrigger on peak
Figure 9.
On the left plot, the angular resolution with a set of 6 micromegas chambers with a level armof 50 cm is shown. On the right plot, the address of the strip with the earliest arrival time outputted by theVMM1 compared to Monte Carlo simulation.
Table 1.
Radiation test with micromegas detectors.
Irradiation with Charge Deposit (mC / cm ) HL-LHC EquivalentX-Ray 225 5 HL–LHC years equivalentX-Neutron 0.5 10 HL–LHC years equivalentGamma 14.84 10 HL–LHC years equivalentAlpha 2.4 5 × sparks equivalent Extensive ageing tests of a 10 ×
10 cm resistive-strip micromegas detector were performed at CEASaclay in 2011 (ref. [10]) and 2012 with 8 keV X-rays, thermal neutrons, ∼ L = × cm − s − . A second detector, not irradiated, served as reference detec-tor. No signs of performance deterioration of the exposed detector were observed. The e ffi ciencymeasurement as a function of the absolute gain for both irradiated and non-irradiated detectors isshown in figure 10. Five small prototype micromegas detectors were installed in the ATLAS detector during LHCrunning at √ s = × . area detector (MBT) was placed in front of the electromag-netic calorimeter, 1 m radially and 3.5 m horizontally from the interaction point. On the surfaceof the electromagnetic calorimeter particle rates at the level of 70 kHz / cm at ATLAS luminosity L = × cm − s − are expected. Four 9 ×
10 cm detectors installed on the current ATLAS SmallWheels at the inner region in front of the Cathode Strip Chambers (CSC). A comparison of the cur-rents drawn by the detector installed in front of the electromagnetic calorimeter with the luminosity– 7 – igure 10. The e ffi ciency measurement as a function of the absolute gain for both irradiated and non-irradiated detectors. measurement in ATLAS experiment was done showing a strong correlation between them. Figure11 shows the MBT current together with the ATLAS luminosity for one day of data taking. Thefour micromegas on the small wheel operated under much less particle flux reconstructing particletracks without any problem. Figure 11.
The top plot shows the MBT current (red points) and the ATLAS luminosity (black line). Thelower plot gives the MBT current versus the ATLAS luminosity. The blue line is a linear fit to the data.
4. Conclusions
The extensive MAMMA R&D program has transformed micromegas detectors spark resistant al-lowing them to operate in high energy experiments like ATLAS. Construction techniques devel-oped, allowed us to build large area detectors of 2 m while involving industry. The first generationof the VMM ASIC unveiled the capabilities of micromegas as a trigger and tracking detector. All– 8 –he above achievements brought micromegas detectors ahead of the R&D phase. A full sector iden-tical to a detector that can be installed to ATLAS will be build in 2014 featuring the next generationof the VMM ASIC providing trigger and tracking information. Following the ATLAS schedule,1200 m of micromegas detectors will be constructed and assembled on 2015–2016 while the in-stallation and commission of the full system will follow on 2017–2018. Acknowledgments
The work described here is an intensive work performed from the MAMMA collaboration duringthe last years. The CERN PCB workshop team played a major role in development and constructionof almost all micromegas prototypes and a big thanks goes to them as well.The present work was co-funded by the European Union (European Social Fund ESF)and Greek national funds through the Operational Program "Education and Lifelong Learning"of the National Strategic Reference Framework (NSRF) 2007-1013. ARISTEIA-1893-ATLASMICROMEGAS.
References [1] ATLAS Collaboration,
New Small Wheel Technical Design Report , ATLAS-TDR-15 ;CERN-LHCC-99-015 .[2] ATLAS Collaboration,
New Small Wheel Technical Design Report , CDS ATLAS-TDR-020 .[3] Y. Giomataris et al.,
MICROMEGAS: a high-granularity position-sensitive gaseous detector for highparticle-flux environments , Nucl. Instr. Meth.
A 376 (1996) 29-35.[4] T. Alexopoulos et al.,
A spark-resistant bulk-Micromegas chamber for high-rate applications Nucl.Instr. Meth.
A 640 (2011) 110.[5] T. Alexopoulos et al.,
Study of resistive micromegas detectors in a mixed neutron and photon radiationenvironment , 2012
JINST C05001.[6] G. Iakovidis et al.,
Development and Performance of spark-resistant Micromegas Detectors , inproceedings of
International Europhysics Conference on High Energy Physics , July, 21–27, 2011Grenoble, Rhône-Alpes France
PoS(EPS-HEP2011)406 .[7] T. Alexopoulos et al.,
Development of large size Micromegas detector for the upgrade of the ATLASMuon system , Nucl. Instr. Meth.
A 617 (2010) 161-165.[8] J. Wotschack et al.,
The development of large–area micromegas detectors for the ATLAS upgrade , Mod. Phys. Lett. A (2013) 1340020.[9] R. Veenhof, Garfield, recent developments , Nucl. Instr. Meth.
A 419 (1998) 726.[10] J. Galán et al.,
An ageing study of resistive micromegas for the HL–LHC environment , 2013