MICROMEGAS
per l’upgrade delle Muon Chambers
di ATLAS per SLHC
Arizona, Athens (U, NTU, Demokritos), Brookhaven, CERN,
Harvard, Istanbul (Bogaziçi, Doğuş), Naples, Seattle, USTC
Hefei, South Carolina, St. Petersburg, Shandong,
Thessaloniki
M.Alviggi, GR1-Napoli, 16 dicembre 2008
Thanks to P.Iengo per la maggior parte dei plot
2
Micromegas as candidate technology
• Combine triggering and tracking
functions
• Matches required performances:
– Spatial resolution ~ 100 m
(track< 45°)
– Good double track resolution
– Time resolution ~ few ns
– Efficiency > 98%
– Rate capability > 5 kHz/cm2
• Potential for going to large areas
~1m x 2m with industrial processes
3
Prototype P1
• Standard bulk micromegas
fabricated at CERN-TS/DEM
• Homogeneous stainless steel
mesh
• 325 line/inch = 78 m pitch
• Wire diameter ~25 m
• Amplification gap = 128 m
• 450mm x 350mm active area
• Different strip patterns (250, 500,
1000, 2000 µm pitch; 450mm and
225 mm long)
• Drift gap: 2-5 mm
4
Test beam set up
2007 Test beam set up
2008 Test beam set up





P1 tested @ CERN H6 beam line in November 2007 & June to August 2008
120 GeV pion beam
Scintillator trigger
External tracking with three Si detector modules (Bonn Univ.); independent DAQ
Three non-flammable gas mixtures with small isobutane admixture used in 2008:
Ar:CO2:iC4H10 (88:10:2), Ar:CF4:iC4H10 (88:10:2), Ar:CF4:iC4H10(95:3:2)
 Data acquired for 4 different strip patterns and 5 impact angles (0 to 40 degrees)
5
Readout
DAQ based on ALTRO CHIP
trigger
FEC
32 channels
Micro
Megas
Two inverted diodes for spark protection
Zero channels died
DAQ PC (ALICE DATE)
32 channels
200 ns integration time
64 charge samples/ch
100 ns/sample
15 pre-samples
1 ADC count ~ 1000 e-
Typical ADC spectra
 Noise subtraction (from 12
pre-samples)
 Cluster position from center of
gravity
6
6
Cluster charge distribution





Gas mixture: Ar:CF4:iC4H10 (88:10:2)
 Gain measurement from HVmesh scan
Drift gap 5 mm; drift field = 200 V/cm  Exponential gain increase
Strip pitch = 250 µm
 Stable operation (small spark rate) for gain
Horizontal axes: ADC count
of 3–5 · 103
ADC count = 1000 electrons
HVmesh = 460 V
Cluster charge (ADC counts)
7
8
Spatial resolution
• Si tracker σextr≈ 50 µm
• MM cluster position
Convoluted
Strip pitch: 250 µm
Gas: Ar:CF4:iC4H10 (88:10:2)
Track impact angle: 90°
Convoluted resolution of Si
tracker + extrapolation
σ(Si+MM) = 63 µm
MM intrinsic resolution
 σ(MM) ≤ 40 µm
9
Spatial resolution … more
‘x-raying’ the micromegas
Look for areas where micromegas is inefficient, i.e. tracks in Si tracker
with no hits in the micromegas  pattern of pillars
Si tracks
Equipped micromegas
area (8 mm)
2.54 mm
Pillar distance
10
…Micromegas as a TPC
• A time resolution of a nanosecond
results in space points with a resolution
along the drift direction of 100 µm
• Each micromegas gap delivers a set of
space points, the more the track is
inclined the more space points are
available
• Solves the problem of spatial resolution
for large track inclination
12
Therefore…
 Robust detector
 Works with non-flammable gases
 Spatial resolution is excellent with small strip
pitch
 MM as TPC will give track segments &
excellent space resolution; needs time
measurement (ns)
 Electronics should measure time, may relax on
charge
 Trigger capability to be proven with faster
electronics
13
What next ?
 Test beam stopped ; setting up cosmics test stand in
lab@CERN and in few other labs (Naples,Demokritos) 
optimize gas mixtures wrt drift velocity, diffusion,
primary ionization, sparks, ageing…
 Analysis of test beam data taken in 2008 with goals:
 Definition of readout segmentation
 Definition of requirements for r/o electronics
 Construction and test of 1300 x 400 mm2 prototype
(Rui de Oliveira)
14
The 50% prototype
 Active area: 1.3 x 0.4
 Segmented mesh (cut) to
reduce mesh capacity
 250 and 400 µm strip pitches
 Long and short strips
 Construction has started in
CERN/TS-DEM
 Expect MM board to be ready
by early 2009
 Chamber to be completed
spring 2009
 Test beam in May 2009
The stretched micromegas
mesh on its frame
15
Backup
• Richieste CSN1
• RD51
16
σ = 30 ns
σm ≈ 1.2 ns
17
ns*100
Inclined tracks
Cluster
First strip
Impact angle: 50°
Last strip
18
Micromegas as TPC (II)
Track under 50° with
relative time info
19
Software tool
• Software tool* for
quasi online and offline reconstruction
(based on ROOT)
• Permits alignment of
Si tracker modules
with MM chamber
• Combines data from
Si tracker and MM
• ‘online’ resolution
• Also: simple event
display
*) Thanks to Woo Chun Park (U. South Carolina)
20
Simple event display
Si module1
Si module3
Si module6
Micromegas
21
Spatial resolution – ‘online’
Si tracker
Micromegas
 Residuals of MM cluster position and
extrapolated track from Si
 Convolution of:
Beam
– Intrinsic MM resolution
– Tracker resolution (extrapolation)
⎬ ~60 µm
– Multiple scattering
Gas: Ar:CF4:iC4H10 (88:10:2)
Drift field: 200 V/cm
Strip pitch: 250 um
Strip width: 150 um
: 85 um
MM: 60 um
mm
Strip pitch: 500 um
Strip width: 400 um
Strip pitch: 1000 um
Strip width: 900 um
: 102 um
MM: 82 um
: 212 um
MM: 203 um
mm
mm
22