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M icromegas for the ATLAS Muon System Upgrade . Joerg Wotschack (CERN) MAMMA Collaboration

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M icromegas for the atlas muon system upgrade

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



  • Introduction

  • Micromegas

  • Making micromegas spark-resistant

  • Two-dimensional readout

  • Development of large-area muon chambers

  • First data from ATLAS

  • Other projects

Joerg Wotschack (CERN)

The lhc atlas




Joerg Wotschack (CERN)

The atlas detector

The ATLAS detector

Joerg Wotschack (CERN)

Lhc operation luminosity upgrade

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)

The atlas upgrade for 2018ff

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)

Count rates in atlas for l 10 34 cm 2 s 1

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)

Why new small wheels

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)

The problem with the fake tracks

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)

Performance requirements

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)

The atlas small wheel upgrade

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


MDT chambers (drift tubes) +

TGCs for 2nd coordinate (not visible)

2.4 m

CSC chambers

Joerg Wotschack (CERN)

M icromegas for the atlas muon system upgrade

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)

M icromegas for the atlas muon system upgrade

A possible segmentation of Large and Small Sectors

Segmentation in radius is indicative

Joerg Wotschack (CERN)

The micromegas technology

The micromegas technology

Joerg Wotschack (CERN)

Micromegas operating principle

Micromegasoperating principle

-800 V

-550 V

Conversion & drift space

(few mm)



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)

The bulk micromegas technique

The bulk-micromegas* technique

The bulk-micromegas technique, developed at CERN, opens the door to industrial fabrication

Pillars (≈ 300 µm)


r/o strips

Photoresist (64 µm)


*) I. Giomataris et al., NIM A 560 (2006) 405

Joerg Wotschack (CERN)

Bulk micromegas structure

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)

The mamma r d project

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)

Performance studies

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)

2008 demonstrated performance

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)

Conclusions by end of 2009

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)

2010 making mms spark resistant

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)

T he resistive strip protection concept

The resistive-strip protection concept

Joerg Wotschack (CERN)

Sparks in resistive chambers

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)

Several resistive strip detectors tested

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


Joerg Wotschack (CERN)

Detector response

Detector response

Joerg Wotschack (CERN)

Performance in neutron beam

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)

Spark rates in neutron beam r11

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)

Sparks in 120 gev pion muon beams

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


Gain ≈ 4000

Gain ≈ 104

Joerg Wotschack (CERN)

Spatial resolution efficiency

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)

Homogeneity and charge up

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)

R11 rate studies

R11 rate studies

Gain ≈ 5000

Clean signals up to >1 MHz/cm2,

but some loss of gain

Joerg Wotschack (CERN)

Test beam nov 2010

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)

M icromegas for the atlas muon system upgrade



Charge (200 e-)

Time bins (25 ns)



Strips (250 µm pitch)

Strips (250 µm pitch)

Joerg Wotschack (CERN)

M icromegas for the atlas muon system upgrade



Delta ray

Charge (200 e-)

Time bins (25 ns)



Joerg Wotschack (CERN)

Inclined tracks 40 tpc

Inclined tracks (40°) – µTPC


Time bins (25 ns)

Charge (200 e-)


Joerg Wotschack (CERN)

And a two track event

… and a two-track event


Charge (200 e-)

Time bins (25 ns)


Joerg Wotschack (CERN)

Two dimensional readout

Two-dimensional readout

Joerg Wotschack (CERN)

2d readout r16 r19

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

Resistive strips

y strips


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)

R16 x y event d isplay 55 fe

R16 x-y event display (55Fe γ)

R16 x

Charge (200 e-)

Time bins (25 ns)

R16 y

Joerg Wotschack (CERN)

R19 with xuv readout strips

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


σ = 94/√2 µm

R strips

v strips

u strips

x strips

Joerg Wotschack (CERN)



Joerg Wotschack (CERN)

Long time x ray exposure

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)

Towards large area mm c hambers

Towards large-area MM chambers

Joerg Wotschack (CERN)

Csc size chamber project

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)

M icromegas for the atlas muon system upgrade

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


Gas inlet

FE card

(2 APV25)

20 mm


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)

Mechanics detector housing

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)

Assembly of large resistive mm 1 2 x 0 6 m 2

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)

Cover and drift electrode

Cover and drift electrode

Joerg Wotschack (CERN)

Drift electrode hv connection

Drift electrode HV connection

HV connection


Al spacer frame



Joerg Wotschack (CERN)

Chamber closed

Chamber closed

  • Assembly extremely simple, takes a few minutes

  • Signals routed out without soldered connectors

Joerg Wotschack (CERN)

Chamber in h6 test beam july 2011

Chamber in H6 test beam (July 2011)

Large resistive MM

R19 with xuv readout

(seen from the back)

Joerg Wotschack (CERN)

Experience with large 1 2 x 0 6 m 2 mm

Experience with large (1.2 x 0.6 m2) MM

  • A first large MM with resistive strips and 0.5mm 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. 2011

Event display showing a track traversing the CR2 chamber under 20 degree

Joerg Wotschack (CERN)

Micromegas in atlas cavern

Micromegas in ATLAS cavern

Joerg Wotschack (CERN)

Mms in atlas cavern

MMs in ATLAS cavern

  • Four 10 x 10 cm2 MMs are installed since beginning of 2011 in the ATLAS cavern on the HO structure behind EOL2A7 …. they work flawlessly





2 trigger chambers

  • R11, R12

    2 chambers are read-out

  • R13, R16(xy-strips)

  • 3 x 3 APV chips (960 ch)


in USA15

≈120 mm



Trigger (strips)

Joerg Wotschack (CERN)

Mm location on ho structure side a

MM location on HO structure side A






in USA15

≈120 mm




J. Wotschack

Trigger (strips)

M icromegas for the atlas muon system upgrade

ATLAS cavern

Joerg Wotschack (CERN)

Measuring the cavern background

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 + 6.2 M

  • For each trigger the detector activity was measured for 28 time bins of 25 ns, i.e. 700ns.

  • 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 cm2

Joerg Wotschack (CERN)

Two types of background events

Two types of background events

Total charge: 1700 ADC counts

Total charge: >10000 ADC counts

Photon ?

Neutron ? induced nuclear break-up

Joerg Wotschack (CERN)

R 2 7 0 2 hz cm 2 at l 10 34 cm 2 s 1

R ≈ 2.7±0.2 Hz/cm2 at L=1034 cm-2s-1

(Measured rate in close-by EOL2A07 MDT ≈ 8 Hz/cm2)

Joerg Wotschack (CERN)

Readout electronics trigger

Readout electronics & trigger

Joerg Wotschack (CERN)

Trigger readout

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

Joerg Wotschack (CERN)

Bnl chip specifications prelim

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 DerandomizingBuffers

  • 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

Joerg Wotschack (CERN)

M icromegas for the atlas muon system upgrade

Trigger/DAQ Block Diagram

GBTx Gigabit Tranceiver

Chipset being developed at CERN, will combine

Data, TTC, DCS on a single fiber

Joerg Wotschack (CERN)



Joerg Wotschack (CERN)

What have we learned so far

What have we learned so far ?

  • Micromegasfulfil 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

Joerg Wotschack (CERN)

What still needs to be done

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 1m 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

Joerg Wotschack (CERN)

Thank you for your invitation to speak here and your attention

Thank you !for your invitation to speak here and your attention

Joerg Wotschack (CERN)

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