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Development of m icromegas for the ATLAS Muon System Upgrade

Development of m icromegas for the ATLAS Muon System Upgrade . Jörg Wotschack (CERN) MAMMA Collaboration

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Development of m icromegas for the ATLAS Muon System Upgrade

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  1. Development of micromegas for the ATLAS Muon System Upgrade JörgWotschack (CERN) MAMMA Collaboration Arizona, Athens (U, NTU, Demokritos), Brandeis, Brookhaven, CERN, Carleton, Istanbul (Bogaziçi, Doğuş), JINR Dubna, MEPHI Moscow, LMU Munich, Naples, CEA Saclay, USTC Hefei, South Carolina, Thessaloniki Work performed in close collaboration with CERN/TE-MPE PCB workshop (Rui de Oliveira) Lots of synergy from the RD51 Collaboration

  2. Outline • The ATLAS Muon System upgrade • The micromegas technology • Making micromegas spark-resistant • Performance & ageing studies • Two-dimensional readout structure • Towards large-area micromegas chambers • First data from ATLAS • Next steps Not covered: electronics Joerg Wotschack (CERN)

  3. The MAMMA*) R&D activity • Using the micromegas technology to build muon chambers for the ATLAS upgrade was first suggested in an ATLAS Muon Collaboration brainstorming meeting back in 2007 by I. Giomataris (CEA Saclay) • By this time MMs had been successfully used in several experiments at very high rates (COMPASS, NA48) but the largest chambers did not exceed 0.4 x 0.4 m2 • The idea looked intriguing to some of us • Profiting from the know-how of the CERN PCB workshop, the first prototype chamber, 0.4 x 0.5 m2 in size, was built still in 2007; it worked very nicely • By 2009 the excellent performance of MMs and their potential for large-area muondetectors was demonstrated • 2010 was dedicated to making micromegas spark resistant • 2011 the first resistive large-area chambers successfully built and tested *) Muon ATLAS MicroMegas Activity Joerg Wotschack (CERN)

  4. The ATLAS Muon System Upgrade Joerg Wotschack (CERN)

  5. Count rates*)in the ATLAS Muon System at √s = 14TeV for L = 1034 cm-2s-1 Small Wheel Rates in Hz/cm2 Rates at inner rim 1–2 kHz/cm2 *) ATLAS Detector paper, 2008 JINST 3 S08003 Joerg Wotschack (CERN)

  6. The ATLAS detector • Small Wheels Joerg Wotschack (CERN)

  7. Rates: measurement vs simulation Measured and expected count rates and in the Small Wheel detectors Data correspond to L = 0.9 x 1033 cm-2s-1 at √s = 7 TeV Ratio between MDT data and FLUGG*) simulation (CSC region not shown) Rate in Hz/cm2 as a function of radius in the large sectors *) FLUGG simulation gives rates about factor 1.5 – 2 higher than old simulation Joerg Wotschack (CERN)

  8. Three reasons for new Small Wheels • Small Wheel muon chambers were designed for a luminosity of L = 1 x 1034 cm-2 s-1 The rates measured today are 2–3 x higher than estimated; all detectors in the SW will be at their rate limit at 5 x 1034cm-2 s-1 • Eliminate fakes in high-pT(> 20 GeV) triggers At higher luminosity pT thresholds of 20-25 GeV are a MUST Currently over 95 % of forward high pTtriggers are fake • Improve pT resolution to sharpen thresholds Needs ≤1 mrad pointing resolution Joerg Wotschack (CERN)

  9. The problem with the fake tracks • Current LVL1 end-cap trigger • Only the vector BC at the Big Wheels is measured • Momentum defined by assumption that track originated at IP • Random background tracks can easily fake this • Currently 96% of forward high-pTtriggers (at LVL1) have no track associated with them • Proposed LVL1 trigger • Add vector A at Small Wheel • Powerful constraint for real tracks • A pointing resolution of 1 mradwill also improve pTresolution Joerg Wotschack (CERN)

  10. Small Wheel performance requirements • Rate capability 15 kHz/cm2 (L ≈ 5 x 1034 cm-2s-1) • Efficiency > 98% • Spatial resolution ≈100 m (Θtrack< 30°) • Good double track resolution • Trigger capability (BCID, time resolution ≤ 5–10 ns) • Radiation resistance • Good ageing properties All requirements can be met with micromegas Joerg Wotschack (CERN)

  11. ATLAS Small Wheel upgrade proposal*) Replace the muon chambers of the Small Wheels with 128 micromegas chambers (0.5–2.5 m2) • Combine precision and 2ndcoord. measurement as well as trigger functionality in a single device • Each chamber comprises eight 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 • *) other candidates: combined systems • sMDT+TGCand sMDT+RPC Joerg Wotschack (CERN)

  12. The micromegas technology Joerg Wotschack (CERN)

  13. 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)

  14. Micromegas characteristics • Drift region of a few mm with moderate electric field of 100–1000 V/cm, function of the gas • Narrow amplification gap with high electrical field (40–50 kV); typically a factor 70–100 is required to make the mesh fully transparent for electrons • Charged particles ionize the gas in the drift region, the electrons move with typically 5 cm/µs (or 20 ns/mm) to and through the mesh and are amplified in the amplification gap • By measuring the arrival time of the signals a MM functions like a TPC Joerg Wotschack (CERN)

  15. An established technology • Micromegas were (and are) used successfully in the COMPASS experiment as vertex chambers in a high-rate muon beam • 12 planes with dimensions 400 x 400 mm2 • Strip pitch: 360 µm; 1024 channels/plane • Superb performance in high rates (>100 kHz/cm2) Spatial resolution ≈70 µm; time resolution 9 ns • T2K near-detector TPC readout • 72 chambers of 350 x 360 mm2 • ‘Mass production’ using bulk micromegas technique 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 ≈1–2% • Amplification gap 128 µm • Mesh: 350 wires/inch • Wire diameter 20 µm Joerg Wotschack (CERN)

  18. Early performance studies • All initial performance studies were done with ‘standard’ micromegas chambers • We used different gas mixtures, initially Ar:CF4:iC4H10 (88:10:2, 95:3:2), moved later to Ar:CO2 (80:20, 85:15, 93:7) • The chambers were read out with the ALTRO chip (up to a maximum of 64 channels) and the ALICE Date readout system (Thanks to H. Muller and ALICE!) • End 2010 we switched to a new readout electronics (APV25, 128 ch/chip) and a new ‘Scalable Readout System’ (SRS) developed by H. Muller et al. in the context of RD51 P1 (40 X 50 cm2), H6 Nov 2007 Joerg Wotschack (CERN)

  19. 2008: Demonstrated performance • Ar:CF4:iC4H10 (88:10:2) • Standard micromegas (P1) • Safe operating point with efficiency ≥99% • Gas gain: 3–5 x 103 • Very good spatial resolution 250 µm strip pitch Inefficient areas (MM + Si telescope) y (mm) σMM= 36 ± 7 µm X (mm) Joerg Wotschack (CERN)

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

  21. 2010: Making MMs spark resistant • Tested several protection/suppression schemes • A large variety of resistive coatings of anode • Double/triple amplification stages to disperse charge, as used in GEMs (MM+MM, GEM+MM) • Settled on a protection scheme with resistive strips • Tested the concept successfully in the lab (55Fe source, Cu X-ray gun, cosmics), H6 pion & muon beam, and with 2.3 MeV and 5.5 MeV neutrons Joerg Wotschack (CERN)

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

  23. Large number of resistive-strip detectors tested • Small chambers with 9x 9cm2active area • Large range of resistance values • Number of different designs • Gas mixtures • Ar:CO2 (85:15 and 93:7) • Gas gains • 2–3 x 104 • 104 for stable operation R16 R16, first chamber with 2D readout Joerg Wotschack (CERN)

  24. Small resistive-strip detectors Joerg Wotschack (CERN)

  25. Detector response Joerg Wotschack (CERN)

  26. 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)

  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(0.1s) 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 in hadron and muon beamsS3 and R12 (250 µm strips)Analysis of data taken in July 2010 (by M. Villa) Efficiency measured in H6 pion and muon beam (120 GeV/c); S3 is a non-resistive MM, R12 has resistive-strip protection Spatial resolution as function of gain for S3 (non resistive) and R12 with resistive-strip protection Resistive strip chambers fully efficient in a wide gain window Resolution improves with resistive strips Joerg Wotschack (CERN)

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

  32. Test beamNov 2010 Four chambers with resistive strips aligned along the beam NEW: Scaleable Readout System (SRS) – RD51 (H. Muller at al.) APV25 hybrid cards Active area 9 x 9 cm2 Joerg Wotschack (CERN)

  33. APV25 data Joerg Wotschack (CERN)

  34. R11 R12 Charge (200 e-) Time bins (25 ns) R13 R15 ve ≈ 4.7 cm/µs 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-) ve ≈ 2 cm/µs R12 Joerg Wotschack (CERN)

  37. … and a two-track event R11 ve ≈ 2 cm/µs Charge (200 e-) Time bins (25 ns) R12 Joerg Wotschack (CERN)

  38. Ageing Joerg Wotschack (CERN)

  39. Ageing studies • Micromegas have shown excellent ageing properties in the past, no long-term effects were observed • However, no results are reported for MMs with resistive coating • We have started an ageing test campaign at CEA Saclay that involves high-rate exposure of resistive chambers to few MeV X-rays and thermal neutrons Joerg Wotschack (CERN)

  40. Long-time X-ray exposure 1 • A resistive-strip MM (R17a) has been exposed at CEA Saclay to 5.28 keV X-rays for ≈12 days Accumulated charge: 765 mC/4 cm2 • Expected accumulated charge at the smallest radius in the ATLAS Small Wheel: 30 mC/cm2 over 5 years at sLHC No degradation of detector response in the irradiated area (nor elsewhere) observed; rather the contrary (still to be understood) Joerg Wotschack (CERN)

  41. Long-time X-ray exposure 2 • In a second measurement the same chamber (R17a) was exposed again to the X-ray source, irradiating a non-irradiated area of the chamber. In parallel an ‘identical’ chamber (R17b) was measured without being irradiated continuously. Exposure time: 21.3 days Accumulated charge: 918 mC/4cm2 Joerg Wotschack (CERN)

  42. Exposure to thermal neutrons • R17a was then moved to the Orphee reactor at Saclayand has been under radiation from 7 – 17 Nov 2011 • Neutron flux is ≈0.8 x 109n/cm2/s with energies of 5–10 x 10-3eV • Total exposure on-time: 40 hrs equivalent to ≈20 years LHC at 5 x 1034 cm-2s-1 (including a safety factor of 3) • Exposure finished yesterday evening • Detector response perfectly stable over full duration of irradiation Joerg Wotschack (CERN)

  43. Two-dimensional readout &other structural issues Joerg Wotschack (CERN)

  44. 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 R16 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)

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

  46. 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)

  47. Simplify construction of micromegas • Mesh • Mesh stretching over large-area is possible but complicates the production process • Mesh HV insulation requires special attention, work intensive • Mesh segmentation is difficult to achieve, dead areas, labour intensive • Two variants tried • Replace the mesh with a GEM foil • Connect the mesh to ground and the R strips to HV • Readout structure RD51 Coll. Mtg, Joerg Wotschack (CERN)

  48. Replacing the mesh by a GEM foilGEM is simply placed on 128 µm high pillars on the resistive strips • 2. Standard GEM foil • Works similarly as R19G • Did not find with this configuration a HV setting to profit from gain in GEM • Fragile, sparks in GEM destroyed GEM • 3. Stripped GEM foil (upper Cu layer removed) • Did not find good operating point • Works as MM but problems with GEM transparency • 1. Stripped GEM foil (lower Cu layer removed) • R19G, successfully used in test beam in July • Works well, no striking difference to 19M (with micromesh) • High rate looks OK Effort not pursued any further MPGD2011, Joerg Wotschack (CERN)

  49. Inverting the HV connection scheme Drift electrode (-HV) Resistive Strips (+HV) • Idea by A. Ochi (Kobe) ‘Why not connect the R strips to HV and the mesh to ground ?’ • R20 was built along these lines • Simplifies considerably the detector production • Clean signals, good energy resolution • High gain • Little charge-up • Good high-rate performance R20 Mesh PCB Insulator Copper readout strip Works very well, all recent detectors were built along these lines MPGD2011, Joerg Wotschack (CERN)

  50. Performance of R20 (inverted HV) 55Fe γ (5.9 keV) Ar:CO2 93:7 G≈5000 G≈10000 Signal drop: 5 % (compared to 10–15% for R11, R12,…) 8 keV Cu X-rays Signal drop compatible with HV drop over resistive strips (0.5–1 GΩ) MPGD2011, Joerg Wotschack (CERN)

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