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M icromegas for the ATLAS Muon System Upgrade

M icromegas for the ATLAS Muon System Upgrade

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M icromegas for the ATLAS Muon System Upgrade

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  1. Micromegas for the ATLAS Muon System Upgrade JoergWotschack (CERN) MAMMA Collaboration Arizona, Athens (U, NTU, Demokritos), Brandeis, Brookhaven, CERN, Carleton, Istanbul (Bogaziçi, Doğuş), JINR Dubna, LMU Munich, Naples, CEA Saclay, USTC Hefei, South Carolina, St. Petersburg, Thessaloniki

  2. Outline • Introduction • Micromegas • Making micromegas spark-resistant • Two-dimensional readout • Development of large-area muon chambers • First data from ATLAS • Other projects Joerg Wotschack (CERN)

  3. The LHC & ATLAS ATLAS CMS Joerg Wotschack (CERN)

  4. The ATLAS detector Joerg Wotschack (CERN)

  5. LHC operation & luminosity upgrade • LHC is working at √s = 7 TeV and performs very well • Fills routinely L ≥ 2 x 1033cm-2s-1 • Longest fill lasted 24 hours • LHC upgrade schedule: • Physics run until end 2012 • Shutdown 2013/14 to prepare for √s = 14TeV • Physics run 2015–17; hope to reach L = 1 x 1034 cm-2 s-1 • Shutdown 2018 to prepare for L = 2–3 x 1034 cm-2 s-1 + experiments upgrade • Physics run at L = 2–3 x 1034 cm-2 s-1 • Shutdown 2021 or 2022 (?) to prepare for L = 5 x 1034 cm-2 s-1 Joerg Wotschack (CERN)

  6. The ATLAS upgrade for 2018ff The prospect of reaching luminosity larger than 1034 cm-2 s-1 after the 2018 shutdown makes some upgrades of the ATLAS detector mandatory • Replacement of pixel vertex detector • Replacement of electronics in various sub-detectors • The trigger system • Replacement of the first station of the end-cap muon system: the Small Wheel Joerg Wotschack (CERN)

  7. Count rates in ATLAS for L=1034cm-2s-1 Rates in Hz/cm2 Small Wheel Rates at inner rim are close to 2 kHz/cm2 Joerg Wotschack (CERN)

  8. Why new Small Wheels • Small Wheel muon chambers were designed for a luminosity L = 1 x 1034 cm-2 s-1 The rates measured today are ≈2 x higher than estimated All detectors in the SW are expected to be at their rate limit • Eliminate fake trigger in pT> 20 GeVTriggers At higher luminosity pTthresholds 20-25 GeV are a MUST Currently over 90% of high pTtriggers are fake • Improve pTresolution to sharpen thresholds Needs ≤1 mrad pointing resolution Joerg Wotschack (CERN)

  9. The problem with the fake tracks • Current End-cap Trigger • Only a vector BC at the Big Wheels is measured • Momentum defined by implicit assumption that track originated at IP • Random background tracks can easily fake this • ProposedTrigger • Provide vector A at Small Wheel • Powerful constraint for real tracks • With a pointing resolution of 1 mradit will also improve pT resolution • Currently 96% of High pT triggers have no track associated with them Joerg Wotschack (CERN)

  10. Performance requirements • Spatial resolution ≈100 m (Θtrack< 30°) • Good double track resolution • Efficiency > 98% • Trigger capability (time resolution ≈5 ns) • Rate capability ≥ 10 kHz/cm2 • Radiation resistance • Good ageing properties Joerg Wotschack (CERN)

  11. The ATLAS Small Wheel upgrade Our proposal • Replace the muon chambers of the Small Wheels with 128 micromegas chambers (0.5–2.5 m2) • These chambers will fulfil both precision measurement and triggering functionality • Each chamber will haveeight active layers, arranged in two multilayers • a total of about 1200 m2 of detection layers • 2M readout channels Today: MDT chambers (drift tubes) + TGCs for 2nd coordinate (not visible) 2.4 m CSC chambers Joerg Wotschack (CERN)

  12. A tentative Layout of the New Small Wheels and a sketch of an 8-layer chamber built of two multilayers, of four active layers each, separated by an instrumented Al spacer for monitoring the internal chamber deformations Joerg Wotschack (CERN)

  13. A possible segmentation of Large and Small Sectors Segmentation in radius is indicative Joerg Wotschack (CERN)

  14. The micromegas technology Joerg Wotschack (CERN)

  15. Micromegasoperating principle -800 V -550 V Conversion & drift space (few mm) Mesh Amplification Gap 128 µm The principle of operation of a micromegas chamber • Micromegas (I. Giomataris et al., NIM A 376 (1996) 29) are parallel-plate chambers where the amplification takes place in a thin gap, separated from the conversion region by a fine metallic mesh • The thin amplification gap (short drift times and fast absorption of the positive ions) makes it particularly suited for high-rate applications Joerg Wotschack (CERN)

  16. The bulk-micromegas* technique The bulk-micromegas technique, developed at CERN, opens the door to industrial fabrication Pillars (≈ 300 µm) Mesh r/o strips Photoresist (64 µm) PCB *) I. Giomataris et al., NIM A 560 (2006) 405 Joerg Wotschack (CERN)

  17. Bulk-micromegas structure Pillars (here: distance = 2.5 mm) • Standard configuration • Pillars every 2.5 – 10 mm • Pillar diameter ≈300 µm • Dead area ≈1% • Amplification gap 128 µm • Mesh: 325 wires/inch Joerg Wotschack (CERN)

  18. The MAMMA R&D project • ATLAS MM Upgrade Project: started 2008 Since then, we produced and tested a large number of prototype micromegaschambers • By end of 2009 their excellent performance and potential for large-area muondetectors was demonstrated • 2010 was dedicated to make chambers spark resistant • 2011 moving to large-area chambers • Growing interest in the community (now ≈20 institutes) • Major role in the RD51 Collaboration Joerg Wotschack (CERN)

  19. Performance studies • All initial performance studies were done with ‘standard’ micromegas chambers • We used the ALICE Date system with the ALTRO chip, limited to 64 channels • End 2010 we switched to new readout electronics (APV25, 128 ch/chip) and a new ‘Scalable Readout System’ (SRS) developed in the context of RD51 Joerg Wotschack (CERN)

  20. 2008: Demonstrated performance • Ar:CF4:iC4H10 (88:10:2) • Standard micromegas • Safe operating point with excellent efficiency • Gas gain: 3–5 x 103 • Superb spatial resolution 250 µm strip pitch Inefficient areas (MM + Si telescope) y (mm) σMM= 36 ± 7 µm X (mm) Joerg Wotschack (CERN)

  21. Conclusions by end of 2009 • Micromegas(standard) work • Clean signals • Stable operation for detector gains of 3–5 x 103 • Efficiency of 99%, only limited by the dead area from pillars • Required spatial resolution can easily be achieved with strip pitches between 0.5 and 1 mm • Timing looks Ok, but performance could not be measured with our electronics • Sparks are a problem • Sparks leads to a partial discharge of the amplification mesh => HV drop & inefficiency during charge-up • But: no damage on chambers, despite many sparks Joerg Wotschack (CERN)

  22. 2010: Making MMs spark resistant • Several protection/suppression schemes tested • A large variety of resistive coatings of anode • Double/triple amplification stages to disperse charge, as used in GEMs (MM+MM, GEM+MM) • Finally settled on a protection layer with resistive strips • Tested the concept successfully in the lab (55Fe source, Cu X-ray gun, cosmics), H6 pion & muon beam, and with 5.5 MeV neutrons Joerg Wotschack (CERN)

  23. The resistive-strip protection concept Joerg Wotschack (CERN)

  24. Sparks in resistive chambers • Spark signals (currents) for resistive chambers are about a factor 1000 lower than for standard micromegas (spark pulse in non-resistive MMs: few 100V) • Spark signals fast (<100 ns), recovery time a few µs, slightly shorter for R12 with strips with higher resistance • Frequently multiple sparks Joerg Wotschack (CERN)

  25. Several resistive-strip detectors tested • Gas mixtures • Ar:CO2 (85:15 and 93:7) • Gas gains • 2–3 x 104 • 104 for stable operation • Small 10 x 10 cm2 chambers with 250 µm readout strip pitch • Various resistance values R16 Joerg Wotschack (CERN)

  26. Detector response Joerg Wotschack (CERN)

  27. Performance in neutron beam • Standard MM could not be operated in neutron beam • HV break-down and currents exceeding several µA already for gains of order 1000–2000 • MM with resistive strips operated perfectly well, • No HV drops, small spark currents up to gas gains of 2 x 104 Standard MM Resistive MM Joerg Wotschack (CERN)

  28. Spark rates in neutron beam (R11) • Typically a few sparks/s for gain 104 • About 4 x more sparks with 80:20 than with 93:7 Ar:CO2 mixture • Neutron interaction rate independent of gas • Spark rate/n is a few 10-8 for gain 104 • Larger spark rate in 80:20 gas mixture is explained by smaller electron diffusion, i.e. higher charge concentration Joerg Wotschack (CERN)

  29. Sparks in 120 GeV pion & muon beams • Pions, no beam, muons • Chamber inefficient for O(1s) when sparks occur • Stable, no HV drops, low currents for resistive MM • Same behaviour up to gas gains of > 104 8000 Gain ≈ 4000 Gain ≈ 104 Joerg Wotschack (CERN)

  30. Spatial resolution & efficiency R12 (resistive strips) σMM ≈ 30–35 µm S3 (non-resistive) Spatial resolution measured with an external Si telescope, shown is convoluted resolutions of Si telescope + extrapol. (≈30 µm) and MM with 250 µm strip pitch Efficiency measured in H6 pion beam (120 GeV/c); S3 is a non-resistive MM, R12 has resistive-strip protection More details in talk by M. Villa in RD51 Collaboration meeting (WG2) Joerg Wotschack (CERN)

  31. Homogeneity and Charge-up R ≈ 45 MΩ R ≈ 85 MΩ • No strong dependence of effective gain on resistance values (within measured range) • Systematical gain drop of 10–15% for resistive & standard chambers; stabilizes after a few minutes • Charge-up of insulator b/w strips ? Joerg Wotschack (CERN)

  32. R11 rate studies Gain ≈ 5000 Clean signals up to >1 MHz/cm2, but some loss of gain Joerg Wotschack (CERN)

  33. Test beamNov 2010 Four chambers with resistive strips aligned along the beam NEW: Scaleable Readout System (SRS) APV25 hybrid cards Active area 10 x 10 cm2 Joerg Wotschack (CERN)

  34. R11 R12 Charge (200 e-) Time bins (25 ns) R13 R15 Strips (250 µm pitch) Strips (250 µm pitch) Joerg Wotschack (CERN)

  35. R11 R12 Delta ray Charge (200 e-) Time bins (25 ns) R13 R15 Joerg Wotschack (CERN)

  36. Inclined tracks (40°) – µTPC R11 Time bins (25 ns) Charge (200 e-) R12 Joerg Wotschack (CERN)

  37. … and a two-track event R11 Charge (200 e-) Time bins (25 ns) R12 Joerg Wotschack (CERN)

  38. Two-dimensional readout Joerg Wotschack (CERN)

  39. 2D readout (R16 & R19) Mesh • Readout structure that gives two readout coordinates from the same gas gap; crossed strips (R16) or xuv with three strip layers (R19) • Several chambers successfully tested Resistive strips y strips PCB y: 250/80 µm only r/o strips x strips Resistivity values RG ≈ 55 MΩ Rstrip ≈ 35 MΩ/cm x strips: 250/150 µm r/o and resistive strips Joerg Wotschack (CERN)

  40. R16 x-y event display (55Fe γ) R16 x Charge (200 e-) Time bins (25 ns) R16 y Joerg Wotschack (CERN)

  41. R19 with xuv readout strips • Tested two chambers with same readout structure (R19M and R19G) in a pion beam (H6) in July • Clean signals from all three readout coordinates, no cross-talk • Strips of v and x layers well matched, u strips low signal, too narrow • Excellent spatial resolution, even with v and u strips • x strips parallel to R strips • u,v strips ±60 degree Mesh σ = 94/√2 µm R strips v strips u strips x strips Joerg Wotschack (CERN)

  42. Ageing Joerg Wotschack (CERN)

  43. Long-time X-ray exposure • A resistive-strip MM has been exposed at CEA Saclay to 5.28 keV X-rays for ≈12 days Accumulated charge: 765 mC/4 cm2 • No degradation of detector response in irradiated area (nor elsewhere) observed; rather the contrary (to be understood) • Expected accumulated charge at the smallest radius in the ATLAS Small Wheel: 30 mC/cm2 over 5 years at sLHC Joerg Wotschack (CERN)

  44. Towards large-area MM chambers Joerg Wotschack (CERN)

  45. CSC-size chamber project • The plan • Start with a standard (non-resistive), half-size MM (fall 2010) • Then a half-size MM chamber with resistive strips (end 2010) • Construction of a 4-layer chamber (fall 2011); installation in ATLAS during X-mas shutdown 2011/12, if possible • Full-size layer, when new machines in CERN/TE-MPE workshop available (spring 2012) Joerg Wotschack (CERN)

  46. Width of final PCB = 605 mm Gas outlet HV mesh + drift (2 x SHV) Connection pad Number of strips = 2048 Strip pitch = 0.5 mm Strip width = 0.25 mm 8 FE cards F/E card 50 x 120 mm2 1024 mm Distance b/w screws 128 mm 76.3 ° Cover + drift electrode Stiffening panel Micromegas Gas inlet FE card (2 APV25) 20 mm GND 10 mm 5 mm 20 mm 50 mm 20 mm 530 mm (520 mm active) Connection pad 5 mm Max width of PCB for production = 645 mm Joerg Wotschack (CERN)

  47. Mechanics – detector housing Foam/FR4 sandwich with aluminium frame PCB with micromegas structure To be inserted here Cover & drift electrode Spacer frame, defines drift gap Stiffening panel Joerg Wotschack (CERN)

  48. Assembly of large resistive MM (1.2 x 0.6 m2) • 2048 circular strips • Strip pitch: 0.5 mm • 8 connectors with 256 contacts each • Mesh: 400 lines/inch • 5 mm high frame defines drift space • O-ring for gas seal • Closed by a 10 mm foam sandwich panel serving at the same time as drift electrode Dummy PCB Joerg Wotschack (CERN)

  49. Cover and drift electrode Joerg Wotschack (CERN)

  50. Drift electrode HV connection HV connection spring Al spacer frame O-ring seal Joerg Wotschack (CERN)