Micromegas for the ATLAS Muon System Upgrade Joerg Wotschack (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

Outline  Introduction  Micromegas  Making micromegas spark-resistant  Two-dimensional readout  Development of large-area muon chambers  First data from ATLAS  Other projects Hefei, 5 Sept. 2011Joerg Wotschack (CERN)2

The LHC & ATLAS Hefei, 5 Sept. 2011Joerg Wotschack (CERN)3 ATLAS CMS

The ATLAS detector Hefei, 5 Sept. 2011Joerg Wotschack (CERN)4

LHC operation & luminosity upgrade  LHC is working at √s = 7 TeV and performs very well  Fills routinely L ≥ 2 x cm -2 s -1  Longest fill lasted 24 hours  LHC upgrade schedule:  Physics run until end 2012  Shutdown 2013/14 to prepare for √s = 14 TeV  Physics run 2015–17; hope to reach L = 1 x cm -2 s -1  Shutdown 2018 to prepare for L = 2–3 x cm -2 s -1 + experiments upgrade  Physics run at L = 2–3 x cm -2 s -1  Shutdown 2021 or 2022 (?) to prepare for L = 5 x cm -2 s -1 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)5

The ATLAS upgrade for 2018ff The prospect of reaching luminosity larger than 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 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)6

Count rates in ATLAS for L=10 34 cm -2 s -1 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)7 Small Wheel Rates in Hz/cm 2 Rates at inner rim are close to 2 kHz/cm 2

Why new Small Wheels  Small Wheel muon chambers were designed for a luminosity L = 1 x 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 p T > 20 GeV Triggers At higher luminosity p T thresholds GeV are a MUST Currently over 90% of high p T triggers are fake  Improve p T resolution to sharpen thresholds Needs ≤1 mrad pointing resolution Hefei, 5 Sept. 2011Joerg Wotschack (CERN)8

The problem with the fake tracks Hefei, 5 Sept. 2011Joerg Wotschack (CERN)9 ProposedTrigger  Provide vector A at Small Wheel  Powerful constraint for real tracks  With a pointing resolution of 1 mrad it will also improve p T resolution  Currently 96% of High p T triggers have no track associated with them 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

Performance requirements  Spatial resolution ≈100  m (Θ track < 30°)  Good double track resolution  Efficiency > 98%  Trigger capability (time resolution ≈5 ns)  Rate capability ≥ 10 kHz/cm 2  Radiation resistance  Good ageing properties Joerg Wotschack (CERN) 10 Hefei, 5 Sept. 2011

2.4 m The ATLAS Small Wheel upgrade Hefei, 5 Sept. 2011Joerg Wotschack (CERN)11 CSC chambers Today: MDT chambers (drift tubes) + TGCs for 2 nd coordinate (not visible) Our proposal  Replace the muon chambers of the Small Wheels with 128 micromegas chambers (0.5–2.5 m 2 )  These chambers will fulfil both precision measurement and triggering functionality  Each chamber will have eight active layers, arranged in two multilayers  a total of about 1200 m 2 of detection layers  2M readout channels

Hefei, 5 Sept. 2011Joerg 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

Hefei, 5 Sept. 2011Joerg Wotschack (CERN)13 A possible segmentation of Large and Small Sectors Segmentation in radius is indicative

The micromegas technology Hefei, 5 Sept. 2011Joerg Wotschack (CERN)14

-800 V -550 V Micromegas operating principle  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)15 The principle of operation of a micromegas chamber Hefei, 5 Sept Conversion & drift space Mesh Amplification Gap 128 µm (few mm)

Pillars (  ≈ 300 µm) The bulk-micromegas* technique PCB Photoresist (64 µm) r/o strips Mesh *) I. Giomataris et al., NIM A 560 (2006) 405 The bulk-micromegas technique, developed at CERN, opens the door to industrial fabrication Hefei, 5 Sept. 2011Joerg Wotschack (CERN)16

Bulk-micromegas structure Hefei, 5 Sept. 2011Joerg Wotschack (CERN)17 Standard configuration  Pillars every 2.5 – 10 mm  Pillar diameter ≈300 µm  Dead area ≈1%  Amplification gap 128 µm  Mesh: 325 wires/inch Pillars (here: distance = 2.5 mm)

The MAMMA R&D project  ATLAS MM Upgrade Project: started 2008 Since then, we produced and tested a large number of prototype micromegas chambers  By end of 2009 their excellent performance and potential for large-area muon detectors 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 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)18

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 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)19

2008: Demonstrated performance Hefei, 5 Sept. 2011Joerg Wotschack (CERN)20  Standard micromegas  Safe operating point with excellent efficiency  Gas gain: 3–5 x 10 3  Superb spatial resolution 250 µm strip pitch σ MM = 36 ± 7 µm Ar:CF 4 :iC 4 H 10 (88:10:2) (MM + Si telescope) X (mm) y (mm) Inefficient areas

Conclusions by end of 2009  Micromegas (standard) work  Clean signals  Stable operation for detector gains of 3–5 x 10 3  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 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)21

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 ( 55 Fe source, Cu X-ray gun, cosmics), H6 pion & muon beam, and with 5.5 MeV neutrons Hefei, 5 Sept. 2011Joerg Wotschack (CERN)22

The resistive-strip protection concept Hefei, 5 Sept. 2011Joerg Wotschack (CERN)23

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 100 V)  Spark signals fast (<100 ns), recovery time a few µs, slightly shorter for R12 with strips with higher resistance  Frequently multiple sparks Hefei, 5 Sept. 2011Joerg Wotschack (CERN)24

Several resistive-strip detectors tested  Small 10 x 10 cm 2 chambers with 250 µm readout strip pitch  Various resistance values Hefei, 5 Sept. 2011Joerg Wotschack (CERN)25  Gas mixtures  Ar:CO 2 (85:15 and 93:7)  Gas gains  2–3 x 10 4  10 4 for stable operation R16

Hefei, 5 Sept. 2011Joerg Wotschack (CERN)26 Detector response

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 10 4 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)27 Standard MMResistive MM

Spark rates in neutron beam (R11)  Typically a few sparks/s for gain 10 4  About 4 x more sparks with 80:20 than with 93:7 Ar:CO 2 mixture  Neutron interaction rate independent of gas  Spark rate/n is a few for gain 10 4  Larger spark rate in 80:20 gas mixture is explained by smaller electron diffusion, i.e. higher charge concentration Hefei, 5 Sept. 2011Joerg Wotschack (CERN)28

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 > 10 4 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)29 Gain ≈ 10 4 Gain ≈

Spatial resolution & efficiency Hefei, 5 Sept. 2011Joerg Wotschack (CERN)30 More details in talk by M. Villa in RD51 Collaboration meeting (WG2) 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 σ MM ≈ 30–35 µm Efficiency measured in H6 pion beam (120 GeV/c); S3 is a non-resistive MM, R12 has resistive-strip protection R12 (resistive strips) S3 (non-resistive)

Homogeneity and Charge-up  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 ? Hefei, 5 Sept. 2011Joerg Wotschack (CERN)31 R ≈ 45 MΩ R ≈ 85 MΩ

R11 rate studies Hefei, 5 Sept. 2011Joerg Wotschack (CERN)32 Clean signals up to >1 MHz/cm 2, but some loss of gain Gain ≈ 5000

Test beam Nov 2010 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)33 Active area 10 x 10 cm 2 Four chambers with resistive strips aligned along the beam NEW: Scaleable Readout System (SRS) APV25 hybrid cards

Hefei, 5 Sept. 2011Joerg Wotschack (CERN)34 R11 R12 R13 R15 Time bins (25 ns) Charge (200 e - ) Strips (250 µm pitch)

Hefei, 5 Sept. 2011Joerg Wotschack (CERN)35 R11 R12 R13 R15 Delta ray Time bins (25 ns) Charge (200 e - )

Inclined tracks (40°) – µTPC Hefei, 5 Sept. 2011Joerg Wotschack (CERN)36 R12 R11 Time bins (25 ns)Charge (200 e - )

… and a two-track event Hefei, 5 Sept. 2011Joerg Wotschack (CERN)37 R11 R12 Time bins (25 ns) Charge (200 e - )

Two-dimensional readout Hefei, 5 Sept. 2011Joerg Wotschack (CERN)38

2D readout (R16 & R19)  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 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)39 x strips: 250/150 µm r/o and resistive strips y: 250/80 µm only r/o strips PCB Mesh Resistivity values R G ≈ 55 MΩ R strip ≈ 35 MΩ/cm Resistive strips x strips y strips

R16 x-y event display ( 55 Fe γ) Hefei, 5 Sept. 2011Joerg Wotschack (CERN)40 R16 x R16 y Charge (200 e - ) Time bins (25 ns)

R19 with xuv readout strips  x strips parallel to R strips  u,v strips ±60 degree Hefei, 5 Sept. 2011Joerg Wotschack (CERN)41 R strips v strips u strips x strips Mesh  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 σ = 94/√2 µm

Ageing Hefei, 5 Sept. 2011Joerg Wotschack (CERN)42

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 cm 2  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/cm 2 over 5 years at sLHC Hefei, 5 Sept. 2011Joerg Wotschack (CERN)43

Towards large-area MM chambers Hefei, 5 Sept. 2011Joerg Wotschack (CERN)44

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) Hefei, 5 Sept. 2011Joerg Wotschack (CERN)45

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

Mechanics – detector housing Hefei, 5 Sept. 2011Joerg Wotschack (CERN)47 PCB with micromegas structure To be inserted here Foam/FR4 sandwich with aluminium frame Stiffening panel Spacer frame, defines drift gap Cover & drift electrode

Assembly of large resistive MM (1.2 x 0.6 m 2 )  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 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)48 Dummy PCB

Cover and drift electrode Hefei, 5 Sept. 2011Joerg Wotschack (CERN)49

Drift electrode HV connection Hefei, 5 Sept. 2011Joerg Wotschack (CERN)50 HV connection spring O-ring seal Al spacer frame

Chamber closed  Assembly extremely simple, takes a few minutes  Signals routed out without soldered connectors Hefei, 5 Sept. 2011Joerg Wotschack (CERN)51

Chamber in H6 test beam (July 2011) Hefei, 5 Sept. 2011Joerg Wotschack (CERN)52 Large resistive MM R19 with xuv readout (seen from the back)

Experience with large (1.2 x 0.6 m 2 ) MM  A first large MM with resistive strips and 0.5 mm readout strip pitch has been successfully tested this July in the H6 test beam  It has been operating very stably and produced very nice data (analysis just started)  Construction took a few iterations and helped to understand and cure the weak points (see talk by R. de Oliveira)  Will implement what we learned in the next chamber of the same size, hopefully ready for our next test beam run in Oct Hefei, 5 Sept. 2011Joerg Wotschack (CERN)53 Event display showing a track traversing the CR2 chamber under 20 degree

Micromegas in ATLAS cavern Hefei, 5 Sept. 2011Joerg Wotschack (CERN)54

MMs in ATLAS cavern  Four 10 x 10 cm 2 MMs are installed since beginning of 2011 in the ATLAS cavern on the HO structure behind EOL2A7 …. they work flawlessly Hefei, 5 Sept. 2011Joerg Wotschack (CERN)55 2 trigger chambers  R11, R12 2 chambers are read-out  R13, R16(xy-strips)  3 x 3 APV chips (960 ch) R11R12R13 R16xy Trigger (strips) DCS mmDAQ DCS mmDAQ Laptop in USA15 ≈120 mm

MM location on HO structure side A 12/08/2011J. Wotschack56 R11 R12 R13 R16xy Trigger (strips) DCS mmDAQ DCS mmDAQ Laptop in USA15 ≈120 mm R16

Hefei, 5 Sept. 2011Joerg Wotschack (CERN)57 ATLAS cavern

Measuring the cavern background  We recorded events taken with a random trigger, with a rate of 156 Hz, during LHC Fill 2000 and 2009, for about 20 hours and 11 hours  Total number of triggers: 11.4 M M  For each trigger the detector activity was measured for 28 time bins of 25 ns, i.e. 700 ns.  Events were accepted in a time window from 5 to 25 time bins, i.e. over 500 ns.  Total time covered: ≈ 6+3 s, total area: 2 x 81 cm 2 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)58

Two types of background events Photon ? Neutron ? induced nuclear break-up Total charge: 1700 ADC counts Total charge: >10000 ADC counts Hefei, 5 Sept. 2011Joerg Wotschack (CERN)59

R ≈ 2.7±0.2 Hz/cm 2 at L=10 34 cm -2 s -1 Hefei, 5 Sept. 2011Joerg Wotschack (CERN)60 (Measured rate in close-by EOL2A07 MDT ≈ 8 Hz/cm 2 )

Readout electronics & trigger Hefei, 5 Sept. 2011Joerg Wotschack (CERN)61

Trigger & readout  New BNL chip: 64 channels; on-chip zero suppression, amplitude and peak time finding  Trigger out: address of first-in-time channel with signal above threshold within BX  Data out: digital output of charge & time for channels above threshold + neighbour channels  Trigger signals and data driven out through one (same) GBTx link/layer (one board/layer)  Trigger: track-finding algorithm in Content-Addressable Memory (as FTK) or in FPGA in USA15; latency estimated 25–32 BXs  Small data volumes thanks to on-chip zero-suppression and digitization Hefei, 5 Sept. 2011Joerg Wotschack (CERN)62

BNL chip specifications (prelim.) 64 channels/chip (preamplifier, shaper, peak amplitude detector, ADC)  Real time peak amplitude and time detection with on-chip zero suppression  Simultaneous read/write with built-in Derandomizing Buffers  Peaking time 20–100 ns; dynamic range: 200 fC  Fast trigger signal of all and/or group of channels  Rate: 100 kHz  SEU tolerant logic A similar BNL chip (with longer integration time and smaller rate capability) has been tested with MMs and works Hefei, 5 Sept. 2011Joerg Wotschack (CERN)63

Trigger/DAQ Block Diagram 64Hefei, 5 Sept. 2011Joerg Wotschack (CERN) GBTx Gigabit Tranceiver Chipset being developed at CERN, will combine Data, TTC, DCS on a single fiber GBTx Gigabit Tranceiver Chipset being developed at CERN, will combine Data, TTC, DCS on a single fiber

Conclusions Hefei, 5 Sept. 2011Joerg Wotschack (CERN)65

What have we learned so far ?  Micromegas fulfil all (of our) requirements  Excellent rate capability, spatial resolution, and efficiency  Potential to deliver track vectors in a single plane for track reconstruction and LV1 trigger  We found an efficient spark-protection system that is easy to implement; sparks are no longer a show-stopper  MMs are very robust and (relatively) easy to construct (once one knows how to do it)  Large-area resistive-strip chambers can be built … and work very well 66Hefei, 5 Sept. 2011Joerg Wotschack (CERN)

What still needs to be done?  Optimize the resistance values (not critical)  Demonstrate 2D readout for large chambers  Demonstrate radiation hardness of all materials & their ageing properties (partly done)  Go to 1 m wide chambers (after the completion of the upgrade of the CERN PCB workshop)  Move to industrial processes for  Resistive strip deposition  Mesh placement … and then we are ready to build MMs for ATLAS Hefei, 5 Sept. 2011Joerg Wotschack (CERN)67

Thank you ! for your invitation to speak here and your attention Hefei, 5 Sept. 2011Joerg Wotschack (CERN)68