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. 2011
Joerg Wotschack (CERN)
2
The LHC & ATLAS
ATLAS
CMS
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
3
The ATLAS detector
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
4
LHC operation & luminosity upgrade
 LHC is working at √s = 7 TeV and
performs very well
 Fills routinely L ≥ 2 x 1033 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 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
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
5
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 subdetectors
 The trigger system
 Replacement of the first station of the end-cap
muon system: the Small Wheel
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
6
Count rates in ATLAS for L=1034cm-2s-1
Small Wheel
Rates in Hz/cm2
Rates at
inner rim
are close to
2 kHz/cm2
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
7
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 GeV Triggers
At higher luminosity pT thresholds 20-25 GeV are a MUST
Currently over 90% of high pT triggers are fake
 Improve pT resolution to sharpen thresholds
Needs ≤1 mrad pointing resolution
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
8
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
Hefei, 5 Sept. 2011
ProposedTrigger
 Provide vector A at Small Wheel
 Powerful constraint for real tracks
 With a pointing resolution of 1 mrad
it will also improve pT resolution
 Currently 96% of High pT triggers
have no track associated with them
Joerg Wotschack (CERN)
9
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
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
10
The ATLAS Small Wheel upgrade
Today:
MDT chambers
(drift tubes) +
TGCs for 2nd
coordinate (not
visible)
CSC chambers
Hefei, 5 Sept. 2011
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 have eight
active layers, arranged in two
multilayers
 a total of about 1200 m2
of detection layers
 2M readout channels
Joerg Wotschack (CERN)
11
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. 2011
Joerg Wotschack (CERN)
12
A possible segmentation of Large and Small Sectors
Segmentation in
radius is indicative
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
13
The micromegas technology
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
14
Micromegas operating principle
 Micromegas (I. Giomataris et al.,
NIM A 376 (1996) 29) are parallelplate 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
Hefei, 5 Sept. 2011
-800 V
Conversion & drift space
(few mm)
-550 V
Mesh
Amplification
Gap 128 µm
The principle of operation
of a micromegas chamber
Joerg Wotschack (CERN)
15
The bulk-micromegas* technique
The bulk-micromegas technique, developed at CERN, opens
the door to industrial fabrication
Pillars ( ≈ 300 µm)
Mesh
Photoresist (64 µm)
r/o strips
PCB
*) I. Giomataris et al., NIM A 560 (2006) 405
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
16
Bulk-micromegas structure
Pillars (here: distance = 2.5 mm)
Standard configuration





Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
Pillars every 2.5 – 10 mm
Pillar diameter ≈300 µm
Dead area ≈1%
Amplification gap 128 µm
Mesh: 325 wires/inch
17
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. 2011
Joerg 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. 2011
Joerg Wotschack (CERN)
19
2008: Demonstrated performance
Ar:CF4:iC4H10 (88:10:2)
events/(0.02 mm)
 Standard micromegas
 Safe operating point with
excellent efficiency
 Gas gain: 3–5 x 103
 Superb spatial resolution
250
200
Mean = (3.5 ± 1.3) mm
Sigma = (70.7 ± 1.3) mm
y (mm)
(MM + Si telescope)
150
σMM = 36 ± 7 µm
100
Inefficient areas
250 µm
strip pitch
50
0
-1
-0.8
-0.6
Hefei, 5 Sept. 2011
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
xsi-xmm (mm)
Joerg Wotschack (CERN)
X (mm)
20
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
Hefei, 5 Sept. 2011
Joerg 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 (55Fe
source, Cu X-ray gun, cosmics), H6 pion & muon
beam, and with 5.5 MeV neutrons
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
22
The resistive-strip protection concept
Hefei, 5 Sept. 2011
Joerg 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. 2011
Joerg Wotschack (CERN)
24
Several resistive-strip detectors tested
 Small 10 x 10 cm2 chambers with
250 µm readout strip pitch
 Various resistance values
Chamber
NR:Nro
RGND
Rstrip
(MΩ)
(MΩ/cm)
R11
15
2
1:1
R12
45
5
1:1
R13
20
0.5
1:1
R14
100
10
1:1,2,3,4,72
R15
250
50
1:1,2,3,4,72
R16
55
35
x-y readout
R17
100
45
x-y readout
R18
200
100
x-y readout
R19
50
50
xuv readout
Hefei, 5 Sept. 2011
 Gas mixtures
 Ar:CO2 (85:15 and 93:7)
 Gas gains
Joerg Wotschack (CERN)
 2–3 x 104
 104 for stable operation
R16
25
Detector response
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
26
Performance in neutron beam
Standard MM
Resistive MM
 Standard MM could not be
operated in neutron beam
 HV break-down and currents
exceeding several µA already for
gains of order 1000–2000
Hefei, 5 Sept. 2011
 MM with resistive strips operated
perfectly well,
 No HV drops, small spark currents
up to gas gains of 2 x 104
Joerg Wotschack (CERN)
27
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
Hefei, 5 Sept. 2011



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)
28
Sparks in 120 GeV pion & muon beams
Gain ≈ 4000
8000
Gain ≈ 104
 Pions, no beam, muons
 Chamber inefficient for O(1s)
when sparks occur
Hefei, 5 Sept. 2011
 Stable, no HV drops, low currents
for resistive MM
 Same behaviour up to gas gains
of > 104
Joerg Wotschack (CERN)
29
Spatial resolution & efficiency
R12 (resistive strips)
! " #$%&#' (0. (/- $&(11(23456 7(89: 4; <((
( ""#
' $#
20
18
16
14
12
10
8
6
0
10
' "#
MM efficiency
y [mm]
30
40
50
60
Channel #
! " #$%&#' ()* +(
20
σMM ≈ 30–35 µm
&$#
&"#
%$#
+( ) #, "- . /0122#
%"#
3*#( %- . /0122#
! $#
3*#, ) - . /0122#
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)
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
30
Homogeneity and Charge-up
R ≈ 45 MΩ

R ≈ 85 MΩ
No strong dependence of effective
gain on resistance values (within
measured range)


Hefei, 5 Sept. 2011
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)
31
R11 rate studies
Gain ≈ 5000
Clean signals up to >1 MHz/cm2,
but some loss of gain
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
32
Test beam
Nov 2010
Four chambers with
resistive strips aligned
along the beam
NEW: Scaleable Readout
System (SRS)
APV25 hybrid cards
Active area
10 x 10 cm2
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
33
Charge (200 e-)
R12
R13
Time bins (25 ns)
R11
R15
Strips (250 µm pitch)
Hefei, 5 Sept. 2011
Strips (250 µm pitch)
Joerg Wotschack (CERN)
34
Charge (200 e-)
R12
R13
Time bins (25 ns)
R11
Delta ray
R15
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
35
Inclined tracks (40°) – µTPC
Charge (200 e-)
Time bins (25 ns)
R11
R12
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
36
… and a two-track event
R12
Hefei, 5 Sept. 2011
Time bins (25 ns)
Charge (200 e-)
R11
Joerg Wotschack (CERN)
37
Two-dimensional readout
Hefei, 5 Sept. 2011
Joerg 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
Mesh
Resistive strips
y: 250/80 µm
only r/o strips
PCB
Resistivity values
RG ≈ 55 MΩ
Rstrip ≈ 35 MΩ/cm
x strips: 250/150 µm
r/o and resistive strips
Hefei, 5 Sept. 2011
y strips
Joerg Wotschack (CERN)
x strips
39
R16 x-y event display (55Fe γ)
R16 y
Hefei, 5 Sept. 2011
Time bins (25 ns)
Charge (200 e-)
R16 x
Joerg Wotschack (CERN)
40
R19 with xuv readout strips
Mesh
R strips
v strips
u strips
x strips


x strips parallel to R strips
u,v strips ±60 degree
R19
R
v
u
x
Depth (µm)
0
-50
-100
-150
Strip width (mm)
0.25
0.1
0.1
0.25
Strip pitch (mm)
0.35
0.9
0.9
0.35
0.84
0.3
1
Q collected (rel.)
Hefei, 5 Sept. 2011
 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
Joerg Wotschack (CERN)
σ = 94/√2 µm
41
Ageing
Hefei, 5 Sept. 2011
Joerg 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 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
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
43
Towards large-area MM chambers
Hefei, 5 Sept. 2011
Joerg 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. 2011
Joerg Wotschack (CERN)
45
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 °
FE card
(2 APV25)
Gas inlet 20 mm
Micromegas
Cover + drift electrode
GN
D
10 mm
5 mm
20 mm
Stiffening panel
530 mm
(520 mm active)
50 mm
5 mm
20 mm
Connection
pad
Max width of PCB for production = 645 mm
Joerg Wotschack (CERN)
Hefei, 5 Sept. 2011
46
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
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
47
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
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
48
Cover and drift electrode
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
49
Drift electrode HV connection
HV connection
spring
Al spacer
frame
O-ring
seal
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
50
Chamber closed
 Assembly extremely
simple, takes a few
minutes
 Signals routed out
without soldered
connectors
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
51
Chamber in H6 test beam (July 2011)
Large resistive MM
R19 with xuv readout
(seen from the back)
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
52
Experience with large (1.2 x 0.6 m2) 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. 2011
Event display showing a track traversing
the CR2 chamber under 20 degree
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
53
Micromegas in ATLAS cavern
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
54
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
 R13, R16(xy-strips)
 3 x 3 APV chips (960 ch)
2 trigger chambers
 R11, R12
R11
R13
R16xy R12
2 chambers are read-out
Laptop
in USA15
≈120 mm
DCS
mmDAQ
Trigger
(strips)
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
55
MM location on HO structure side A
R13
R16xy
R11
≈120 mm
R12
Laptop
in USA15
DCS
mmDAQ
Trigger
(strips)
R16
12/08/2011
J. Wotschack
56
ATLAS cavern
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
57
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. 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 cm2
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
58
Two types of background events
Total charge:
1700 ADC
counts
Photon ?
Hefei, 5 Sept. 2011
Total charge:
>10000 ADC
counts
Neutron ? induced nuclear break-up
Joerg Wotschack (CERN)
59
R ≈ 2.7±0.2 Hz/cm2 at L=1034 cm-2s-1
(Measured rate in close-by EOL2A07 MDT ≈ 8 Hz/cm2)
5.00
100
4.50
90
4.00
80
3.50
70
3.00
60
2.50
50
R13+R16 rate (LHC Fill 2000)
2.00
40
R13+R16 rate (LHC Fill 2009)
1.50
30
R13 number of events/1 M
triggers (Fill 2000)
R16 number of events/1 M
triggers (Fill 2000)
1.00
0.50
Number of events
Rate (Hz/cm2)
Background rate @ L = 1034 cm-2 s-1
20
10
0.00
0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
Instant luminosity (1033 cm-2 s-1)
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
60
Readout electronics & trigger
Hefei, 5 Sept. 2011
Joerg 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. 2011
Joerg 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. 2011
Joerg Wotschack (CERN)
63
Trigger/DAQ Block Diagram
GBTx Gigabit Tranceiver
Chipset being developed at
CERN, will combine
Data, TTC, DCS on a single fiber
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
64
Conclusions
Hefei, 5 Sept. 2011
Joerg 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
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
66
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. 2011
Joerg Wotschack (CERN)
67
Thank you !
for your invitation to speak here
and
your attention
Hefei, 5 Sept. 2011
Joerg Wotschack (CERN)
68