1. Micromegas — Properties and Applications — David Attié
I’m here to present a TPC review
GET Workshop – Caen – March 12th, 2009
Astrophysics Detector Workshop – Nice – November 18th, 2008

2. GET Workshop – Caen – March 12th, 2009
Outline
Introduction, Micromegas as a MPGD for High Energy Physics
Description
Technology, meshes
Bulk Micromegas
Performance
Gain measurements, stability and uniformity
Energy resolution
Sparking probability
Micromegas with resitive anode
Purposes and history
Last results
Applications and examples
Conclusion
GET Workshop – Caen – March 12th, 2009

3. Nuclear Physics/Particle Physics needs
Tracking of charged particles in high energy (NP/PP) and high intensity beam (NP) (ex: 3×107 particles/s from 200 GeV muon beam)
High spatial resolution better than 100 μm in (NP: small/PP: large) volume
Ageing of the detector limited (NP/PP)
Low noise and full efficiency (NP/PP)
High rate (NP)
Limited incident track scattering (NP/PP)
High mean atomic number <Z> to use the gas as a target (NP)
Fast readout signal and fast electronics (NP/PP)
Low material and low cost (NP/PP)
GET Workshop – Caen – March 12th, 2009

4. GET Workshop – Caen – March 12th, 2009
Micromegas (MPGD)
Best technology for gaseous detector readout: Micro Pattern Gaseous Detector
more robust than wires
no E×B effect
fast signal & high gain
low ion backdrift
better ageing properties
easier to manufacture
Micromegas
MICROMEsh GAseous Structure Y. Giomataris et al., NIM A 376 (1996) 29
metallic micromesh (typical pitch 50μm)
sustained by μm pillars
cathode
~50 µm
~50 kV/cm
~1 kV/cm
simplicity
single stage of amplification
fast and natural ion collection
discharges non destructive
Giomataris et.al., NIM A376 (1996) 29
F. Saul, NIM A386 (1997) 531
GET Workshop – Caen – March 12th, 2009

5. Micromegas concept
The multiplication takes place in high E field (> 20 kV/cm)
between the anode (strips, pads) and the mesh in one stage
small size ( μm)
fast signals (< 1 ns)
short recovery time (~150 ns)
high rate capabilities (> MHz)
high gain (up to 105 or more)
A GARFIELD simulation
of a Micromegas avalanche
(Lanzhou university)
micromesh signal
strip 1
strip 2
strip 3
GET Workshop – Caen – March 12th, 2009

6. Micromegas technology
Meshes
Many different technologies have been developped for making meshes (Back-buymers, CERN, 3M-Purdue, Gantois, Twente…)
Exist in many metals: nickel, copper, stainless steel, Al,… also gold, titanium, nanocristalline copper are possible.
Deposited by vaporization
Electroformed
Chemically etched
Wowen
Laser etching, Plasma etching…
Pillars
200 mm
Can be on the mesh (chemical etching) or on the anode (PCB technique with a photoimageable coverlay).
Diameter 40 to 400 μm
GET Workshop – Caen – March 12th, 2009

7. Bulk Micromegas technology
Result of a CERN-Saclay collaboration (2004)
Process to encapsulate the mesh on a PCB (mesh = stretched wires)
Motivations for using bulk Micromegas
the mesh is held everywhere:
no dead space, no frame
robustness (closed to dust)
can be segmented
repairable
Copper segmented anode
Base Material
FR4
Lamination of Vacrel
Photo-imageable
polyamide film
Positioning of Mesh
Stainless steel
woven mesh
Encapsulation
Border frame
Development
Spacer
Contact to Mesh
I. Giomataris et.al., NIM A560 (2006) 405
INSTR08 – BINP, Novosibirsk – March 1st, 2008

8. GET Workshop – Caen – March 12th, 2009
Performances
GET Workshop – Caen – March 12th, 2009

9. Mixtures of gases containing argon: gain curves
iC4H10
CO2, CH4
C2H6
Micromegas Mesh : 50 mm gap of 10x10 cm² size
GET Workshop – Caen – March 12th, 2009

10. Gain and efficiency in Ar and Ne
D.Thers et al. NIM A 469 (2001 )133
GET Workshop – Caen – March 12th, 2009

11. GET Workshop – Caen – March 12th, 2009
Gain stability
Very good gain stability G. Puill et al.
Optimization in progress for CAST
GET Workshop – Caen – March 12th, 2009

12. Micromegas gain studies
Three examples of field structure for 75 μm of gap:
M. Chefdeville
Gain
Vgrid (V)
460
500
540
420
103
104
102
103
104 10 20 30 40 60
FWHM (%)
Gain
Gain dependency with the optical transparency
Resolution minimum (13 %) at:
G ~ 5.103
No grid geometry dependency
GET Workshop – Caen – March 12th, 2009

13. GET Workshop – Caen – March 12th, 2009
Gain uniformity
Gain G = ead, where the Townsend coefficient a increases with E field
E field decreases with gap d at given voltage V
 there is a maximum gain for a given gap (about 50 microns for Ar mixtures)
GET Workshop – Caen – March 12th, 2009

14. T2K Micromegas module calibration
Energy resolution of a module:
Results for a typical module (35x36 cm²):
Good energy resolution: 9% r.m.s.
with a 6% dispersion over the module
Good gain uniformity: dispersion ~3%
Similar gain curve for different
Micromegas modules (rms ~ 5%)
Gain of several modules
Gain uniformity over a module:
GET Workshop – Caen – March 12th, 2009

15. T2K module sparking rate
The sparking rate was measured with different Micromegas modules and for different high voltage values
The number of sparks measured for one module is ~ 1 per -360 V
GET Workshop – Caen – March 12th, 2009

16. Discharge probability in a hadron beam
2.5 mm conversion gap 100 μm amplification gapc
15 GeV hadron beam in spill of 2×108
<Z> ~20
<Z> ~14
Future, pion beam:
- remove CF4
- lower the gain
- increase the gap to compensate
Discharge probability
<Z> ~10
In Ne-C2H6-CF4
gain ~ 104
P = 10-6
Note that discharges are not destructive
Gain
Abbon et al. NIM A 461 (2001) 29
GET Workshop – Caen – March 12th, 2009

17. Radiation hardness
No space charge effect in radiation > 30 mC/mm² > 25 LHC years
High rate capability with 8 keV x-rays
106/mm2/s
G. Puill, et al., IEEE Trans. Nucl. Sci. NS-46 (6) (1999)1894.
GET Workshop – Caen – March 12th, 2009

18. Energy resolution vs. gain
Argon/Isobutane mixture
Best RMS for a gain
between & 6.103
Degradation increase in inverse
proportion to the quencher
GET Workshop – Caen – March 12th, 2009

19. Bulk: energy resolution
Installation of a 10×10 cm² bulk Micromegas in Aachen (Germany) in Oct made at Saclay
Just “plug and play”
Resolution: σ ~ 7.5 % r.m.s.
Spectrum of 55Fe source fitted by two Gaussians
At higher energy (> 1 MeV): σ ~ 2 %
GET Workshop – Caen – March 12th, 2009

20. InGrid: energy resolution
Energy resolution depends on the
grid geometry
Grids can be very flat
best energy resolution achieved:
 13.6 % with 55Fe source in P10
removal of Kβ 6.5 keV line:
 keV in P10
Hole pitch down to 14 μm
with various diameters
Different gaps (35-75 μm)
Until now: grid is 1 μm of Al
Escape peak Kα
13.6 % FWHM
Escape peak Kβ
Gap: 50 μm; Hole picth: 32 μm,Ø: 14 μm
11.7% FWHM
Kβ-filtered spectrum with Cr foil
GET Workshop – Caen – March 12th, 2009

21. Ion feedback measurements
Measurements with a 45 μm gap InGrids
Backflow fraction (BF) down to 1 permil at low picth and high field ratio
M. Chefdeville et al. IEEE/NSS 2007
BF = p0/FRp1
Gain ~ 200
σt = 9.5 μm
20 μm pitch
p1 = 1.01
32 μm pitch
p1 = 0.90
45 μm pitch
p1 = 0.96
58 μm pitch
p1 = 1.19
GET Workshop – Caen – March 12th, 2009

22. GET Workshop – Caen – March 12th, 2009
Micromegas
with
resistive anode
GET Workshop – Caen – March 12th, 2009

23. GET Workshop – Caen – March 12th, 2009
Purposes
Spatial resolution σxy:
limited by the pad size (s0 ~ sizepad/√12) because Micromegas charge distribution is narrow (RMSavalanche ~ 15 μm)
How to improve the spatial resolution?
Decresease the pad size (narrow strips, pixels)
Spread the charge over several pads
Other advantages:
limit the number of channel
protect the electronics
Disadvantages:
need offline computing
track separation limited
time resolution is affected
GET Workshop – Caen – March 12th, 2009

24. Resistive anode
TPC COSMo (Carleton-Orsay-Saclay-Montreal) at DESY in 2006
+ Micromegas 10 x 10 cm² (gap 50 μm)
+ resistive anode used to spread charge over
126 pads (7x18) of 2x6 mm²
15 cm drift space
25 µm mylar with Cermet (Al-Si) of 1 MW/□ glued onto the pads with 50 µm thick dry adhesive
mesh
Resistive foil
Glue
pads
PCB
Micromegas
TPC COSMo
Resistive anode
5 T magnet at DESY + TPC COSMo
GET Workshop – Caen – March 12th, 2009

25. Resistive anode Télégraph equation: Simulation Data 2 x 6 mm2 pads
Q(t)
(r,t) integrate over pads
r (mm)
t (ns)
M.S.Dixit et.al., NIM A518 (2004) 721
M.S.Dixit and A. Rankin NIM A566 (2006) 281
Simulation
Data
GET Workshop – Caen – March 12th, 2009

26. Spatial resolution at 0.5 T
B = 0.5 T, resolution fitted by where
Resolution 0 ( at z = 0) ~ 50 µm still good at low gain (will minimize ion feedback)
Mean of Neff = 27 (value measured before ~ 22)
 s0 = 1/40 of pad pitch
Gain = 4700
Gain = 2500
Neff=25.2±2.1
Neff=28.8±2.2
GET Workshop – Caen – March 12th, 2009

27. Spatial resolution at 5T vs. gas mixtures
Analysis: - Curved track fit
- EP < 2 GeV
- |f| < 0.05 (~3°)
Extrapolate to B = 4T with T2K gas for 2x6 mm2 pads:
DTr = 23.3 µm/cm,
Neff ~ 27,
2 m drift distance,
 Resolution of Tr  80 m will be possible !!!
  ~ 50 µm independent of the drift distance
Ar Iso (95:5)
B = 5T
50 mm
GET Workshop – Caen – March 12th, 2009

28. GET Workshop – Caen – March 12th, 2009
Applications
and
examples
GET Workshop – Caen – March 12th, 2009

29. GET Workshop – Caen – March 12th, 2009
COMPASS
3 stations of 4 planes XYUV
Active area 40×40 cm²
Strips: pitch 360 μm /420 μm
total 30 MHz; 450 kHz/strip
0.2% X0 rad. length/plane
Spatial resolution of 65 μm
GET Workshop – Caen – March 12th, 2009

30. ILC-TPC Grand Prototype
Built by the collaboration
Financed by EUDET
Examples :
- magnet : KEK, Japon
- field cage : DESY, Allemagne
- trigger : Saclay, France
- endplate : Cornell, USA
- Micromegas : Saclay, France
- GEM : Saga, Japon
- TimePix pixel : F, D, NLc
GET Workshop – Caen – March 12th, 2009

31. ILC-TPC Grand Prototypec
Endplate ø = 80 cm of 7 panel of 23 cm with interchangeable module
Resistive anode Micromegas module using T2K electronics (1726 pads)
24 row x 72 columns
<pad size> ~ 3.2x7 mm2
80 cm
GET Workshop – Caen – March 12th, 2009

32. Two Micromegas panels tested at DESY
Two modules successively mounted in 1T magnet and Large Prototype ILC-TPC
standard bulk module
bulk with resistive anode module (carbon loaded kapton with a resistivity ~ 5-6 MΩ/□)
New resistive module with resistive ink (~1-2 MΩ/□)
Standard bulk Micromegas module
Carbon loaded kapton Micromegas module
GET Workshop – Caen – March 12th, 2009

33. GET Workshop – Caen – March 12th, 2009
Beam data in T2K gas
GET Workshop – Caen – March 12th, 2009

34. GET Workshop – Caen – March 12th, 2009
Cosmic ray data sample
Peaking time: 1 μs
Frequency
sampling: 100 MHz
GET Workshop – Caen – March 12th, 2009

35. GET Workshop – Caen – March 12th, 2009
The T2K TPC
Project for a large TPC in the off-axis Near Detector (Tokai)
8 m² sensitive area, 2 m drift
72000 channel
A new ASIC (AFTER) from Saclay for the readout
To be operational in 2009
The technological choice was mainly driven by project management considerations. However some advantages of Micromegas over GEMs are:
Minimised dead space (bulk)
6 times less supplies, feed-throughs, etc…
No destruction by discharges
GET Workshop – Caen – March 12th, 2009

36. T2K test beam at TRIUMF
Starting from September the first T2K TPC, actually equipped with 12 MicroMegas modules, has been installed in a test beam at TRIUMF
The beam provides e, μ, π with a momentum up to 400 MeV/c
A Time of flight system provides e, μ,π tagging
Stable operations for MicroMegas and electronics
Low spark rate (1 spark per hour per module at -360 V)
4 Micromegas modules mounted on the TPC
1 event display
GET Workshop – Caen – March 12th, 2009

37. Description of the TimePix chip
Chip (CMOS ASIC) upgraded in the EUDET framework from the Medipix chip developed first for
medical applications
IBM technology 0.25 µm
Characteristics:
surface: 1.4 x 1.6 cm2
Matrix of 256 x 256
pixel size: 55 x 55 µm2
For each pixel:
preamp/shaper
threshold discriminator
register for configuration
TimePix synchronization logic
14-bit counter
55 mm
GET Workshop – Caen – March 12th, 2009

38. TimePix/Micromegas chambers
Windows for X-ray sources
Saclay
Micro-TPC
standard Micromegas
amorphous-Silicon protection
against discharges
6 cm height field cage
Cover
Windows for
β sources
Field cage
Micromegas
mesh
Medipix2/TimePix chip
INSTR08 – BINP, Novosibirsk – March 1st, 2008

39. TimePix/Micromegas Micro-TPC of Saclay
Timepix chip
+ SiProt 20 μm
+ Micromegas
90Sr
Ar/Iso (95:5)
Time Mode
z ~ 40 mm
Vmesh = -340 V
GET Workshop – Caen – March 12th, 2009

40. Flip-chip electronic for ILC-DHCAL prototype
DIRAC1 readout (to be chanied)
64 channels ASIC (R. Gaglione)
Possibility to chain detectors
Threshold at 19 fC
First chamber with embedded electronics working on beam line!
PCB backside
DIRAC chip
PCB backside
Mask for bulk laying
~ events in total
PCB topside
8x8 pads with bulk
GET Workshop – Caen – March 12th, 2009

41. GET Workshop – Caen – March 12th, 2009
Conclusions
Long standing experience with Micromegas has been gathered
Lots of new ideas and technological breakthroughs are opening
Large area is possible with good performances
Single electron efficiency achievable
Many records have been broken.
- Latest to date: 50 microns space resolution at all distances
- Energy resolution: 5% r.m.s.
Applications are extending: neutron detection, photo-detection, dark matter search, X-ray polarization measurement…
GET Workshop – Caen – March 12th, 2009

42. GET Workshop – Caen – March 12th, 2009
Backup slides
GET Workshop – Caen – March 12th, 2009

43. InGrid: Integrated GEM/Micromegas Grid
Integrate a GEM or Micromegas detector directly on a chip …
Grid
Walls
Si wafer + GEM
Grid
Pillars
Si wafer + Micromegas
NIKHEF (MESA+, Univ. Twente)
GET Workshop – Caen – March 12th, 2009

44. InGrid: Integrated GEM/Micromegas Grid
Integrate a GEM or Micromegas detector directly on a chip …
… using wafer post-processing :
The substrate could be:
naked or patterned wafer of Si
readout chip
substrate
photo-resist
Deposit photo-resist & UV exposure
define the amplification gap (tens of μm)
define the grid support (pillars/walls)
Deposit metal & pattern
define the grid geometry
Develop unexposed photo-resist
cleaning
20 μm pitch
50 μm pitch
NIKHEF (MESA+, Univ. Twente)
GET Workshop – Caen – March 12th, 2009

45. Charge dispersion pulses & pad response function
The Pad Response Function (PRF) is a measure of signal size as a function of track position relative to the pad.
The pulse shape is variable and non-standard because of
both the rise time & pulse amplitude depend on track position.
The PRF amplitude for longer drift distances is lower due to Z dependent normalization.
GET Workshop – Caen – March 12th, 2009

46. GET Workshop – Caen – March 12th, 2009
Track fit using the PRF
For a given track: xtrack= x0 + tan() yrow
Yrow is the y position of the row and x0 &  the track fitting parameters
Determination of x0 &  by fitting the PRF to the pad amplitude by minimizing 2 for the entire event
Definitions of the different stages:
- residual: xrow-xtrack
- bias: mean of residual xrow-xtrack = f(xtrack)
- resolution: geometric mean of the standard
deviations of track residuals
2 mm
6 mm x y x0 f
GET Workshop – Caen – March 12th, 2009

47. Pad Response Function (PRF)
0 < z < 1 cm
1 < z < 2 cm
2 < z < 3 cm
3 < z < 4 cm
4 < z < 5 cm
normalized amplitude
T2K gas
B = 5 T
15 z regions / 1 cm step
5 < z < 6 cm
6 < z < 7 cm
7 < z < 8 cm
8 < z < 9 cm
9 < z < 10 cm
10 < z < 11 cm
11 < z < 12 cm
12 < z < 13 cm
13 < z < 14 cm
14 < z < 15 cm
4 pads / ±4 mm
xtrack – xpad (mm)
GET Workshop – Caen – March 12th, 2009

48. Residuals: xrow-xtrack
0 < z < 1 cm
1 < z < 2 cm
2 < z < 3 cm
12 < z < 13 cm
13 < z < 14 cm
14 < z < 15 cm
GET Workshop – Caen – March 12th, 2009