1. Developments of micromegas detector at CERN/Saclay
Shuoxing Wu

2. Outline: Introduction to Micromegas detector 1. properties
2. main problem with micromegas
November test beam
1. test beam set up
2. current-voltage monitoring
3. offline analysis
4. spark topology

3. Part 1 Introduction to micromegas

4. Working principle 1 ) Conversion stage:
cathode
Micro-grille
High voltage 1
Anode strip
Anode plane
Incident charged particle
Avalanche e-
Conversion gap
Amplification gap
Gas E1 E2
High voltage 2
1 ) Conversion stage:
Ionization of gas by the charged particle which produces electron-ion pairs. Electrons directed by E1 (0.1 to 1 kV / cm) to the micro-grid, and cross the mesh holes.
2 ) Amplification stage:
E2 (≈ 50 kV/cm) >> E1  avalanche, electrons mulitplication.

5. Widely used in particle physics experiments:
Compass:
Prototype for ILC-TPC:
Prototype for ILC-DHCAL:

6. The Upgrade of ATLAS muon spectrometer
LHC upgrade
LSLHC~ 10 LLHC
bunch crossing time: 50ns (25 ns)
Critical regions in ATLAS Muon Spectrometer:
EI layers:
CSC (27 m2)
EIS/L1 (54 m2)
EIS/L2 (68 m2)
EM >2:
EMS/L1 (85 m2) 6 6

7. Micromegas for ATLAS Muon upgrade
Combine triggering and tracking functions
Matches required performances:
Spatial resolution ~ 100 m
Good double track resolution
Time resolution ~ few ns
Efficiency > 98%
Rate capability > 5 kHz/cm2
Potential for going to large areas (1 m x 2 m) with industrial processes
Cost effective
Robustness 7

8. Spark in micromegas and resistive coating solution:
A point charge being deposited at t=0, r=0, the charge density at (r,t) is a solution of the 2D telegraph equation. Only one parameter, RC (time per unit surface), links spread in space with time. R~1 MW/□ and C~1pF per pad area matches µs signal duration:
Micro-grid
Resistive film (kapton) or ink
(1k-500MΩ/☐)
Insulator (75 µm)
Anodes
Micro-grille
Resistive strip
(few hundreds of kΩ/☐)
Anodes
Anodes
Resistive pad
(few tens of kΩ/☐)
Micro-grille
Vacrel

9. Part2 November test beam

10. Test beam set up: SPS-H6 120 Gev π+ Beam Tested detectors: Telescope:
3 X-Y detectors(10 x 10 cm2) manufactured at Saclay
Aim: Test different resistive films detectors manufactured by Rui De Oliveira at CERN and compare behaviour to non-resistive detectors
Electronics: GASSIPLEX
DAQ: realised by Demokritos
Gas: 95%Ar + 3% CF4 + 2% isobutane
SPS-H6
120 Gev π+ Y
Resistive
Non-Resistive X
Beam
1 mm
0.25 mm
Detectors in test Y
Tested detectors:
Standard bulk detectors; Resistive coating detectors; Segmented mesh detector 10

11. Summary of tested detectors:
Type
Name
Properties:
Standard bulk:
SLHC3
1mm pitch
SLHC2
2mm pitch
Resistive coating:
R3&R4
2mm,2MΩ/ ,kapton+insulator R5
2mm,250MΩ/ ,resistive paste R6
1mm,400KΩ/ , resistive strip R7
0.5mm, tens of KΩ/ , resistive pad
Segmented mesh S1
1mm pitch,8 segmentations

12. Readout and online monitoring:
Data acquistion based on Labview:
Monitoring through raw data by C++/Root code:
Maximum strip ID:
Charge spectrum: 12

13. Spark counting device:
C=50 pF
C=50 pF
Voltage-Current monitoring PC
Pre-amplifer MM
detector
R1=5,6 kΩ
R2=5,6 kΩ
HVmesh
C=470 pF
for standard ones
for resistive ones 13

14. Different sparking behaviors of standard and resistive detector:
Standard SLHC2 (2mm)
R6(1mm,400kΩ/ resistive strip)
SLHC2: HV=400 V (Gain ~3000): current when sparking < 0.4 mA
voltage drop< 5%
R6: HV=390 V (Gain ~3000): current when sparking < 0.08 mA
voltage drop<0.5%

15. Sparking behavior of resistive detector R3:
Standard SLHC2 (2mm)
Resistive R3(2mm,2MΩ/ )
SLHC2: HV=400 V (Gain ~3000): current when sparking < 0.4 mA
voltage drop< 5%
R3: HV=410 V (Gain ~3000): current when sparking < 0.2 mA
voltage drop<2%

16. Sparking behavior of S1:
Seven,eight sparking
six sparking
five sparking
four sparking
three sparking
two sparking
one sparking
Standard bulk: SLHC2
S1: 16

17. Charge vs. channel ID:
1mm
250mm R6
1mm
1mm
250mm R3
1mm
Pedestal shift in standard MM (telescope) due to sparks.
No pedestal shift in resistive detectors (R3&R6). 17

18. Spark rate vs mesh voltage and beam intensity:
R3(2mm, 2MΩ/ ):
Spark rate= #sparks/#incident hadron 18

19. Detector performance at same gas gain (~3000):
type
Spark rate
Spark current /mA
Voltage drop
SLHC2
Standard bulk
7*E-5
0.4 5% R3
2 M Ω/
resistive kapton
9.6*E-6
0.2 2% R6
400K Ω/
resistive strip
6.4*E-6
0.08
0.5% R5
250 M Ω/
resistive paste
1.6*E-4
0.1
1.5% R7
tens of K Ω/
resistive pad
5.9*E-4
0.35
4.5% 19

20. Process to analysis the data:
1. Covert raw data into root tree 1. Decoding 2. Pedestal calculation and subtraction 3. Event filtering 4. Clusterization 5. Track reconstruction 6. Efficiency and spatial resolution studies based on the track

21. Binary data format: 1.run header (run ID, time, type, …..)
Ovf Vld strip data strip ID
2. event loop: 31 30 29 … 12 11
Loop over 8 detectors:
.number of active strips
Strip loop:
Word 1: 31 30 … 2 1
Word 2: 31 30 … 2 1 . …. . .
Word NAS[det]: 31 30 … 2 1
3. run footer (event number, run end time….)

22. Conversion from raw data into root tree:
‘Detector’ class: {
NofStrips;
stripID;
stripCharge; }

23. Channel-pin
relation
Decoding matrix:

24. 1. Decoding:
Before decoding:
After decoding:
Check decoding:

25. 2.Pedestal calculation and subtraction
Charge-ID distribution:
Strip out of event:
Strip within event:
Case of ‘noisy’ strip:
Charge-ID distribution:
Channel noise level:

26. 3. Filter event:
Based on the cut on pedestal sigma, only strips with charge larger than the threshold are kept.
threshold

27. 4. Clusterization:
Strips with charge above the threshold form a ‘Cluster’( 1 gap is allowed). The cluster center is the charge-weighted barycentre of the strips, the cluster charge is the sum of strips charge within the cluster, cluster size is the number of strips within the cluster.
Number of clusters in one event:
Cluster size:
1mm
0.25mm R6
1mm
1mm
0.25mm R6
1mm
1mm
0.25mm R3
1mm
1mm
0.25mm R3
1mm

28. y=a*x+b, a and b are determined by minimizing the chi-square.
5. Track fitting:
y=a*x+b, a and b are determined by minimizing the chi-square.
Track angle X:
Track angle Y:

29. R6 (1mm, 400KΩ/ ) efficiency : @beam intensity of 11kHz/cm2:
@voltage of 400V:
>98%
drop<3% 29 29

30. Resolution: MM MM MM
•Residuals of MM cluster position and extrapolated track from MM telescope:
•Convolution of:
- Intrinsic MM resolution
- Track resolution (extrapolated) ~68mm
beam
MM under test
1mm
0.25 mm
1mm
R6(1mm pitch, 400kΩ/ )
R3(2mm pitch, 2mΩ/ )
δ=199mm
δmm= 187±1.9mm
δ=112mm
δmm= 90±0.8mm 30

31. Resolution vs mesh voltage:
R6(1mm, 400kΩ/ resistive strip):
R3(2mm, 2MΩ/ resistive kapton): 31

32. Pillars: Track position( with hit in test detector):
Track position( without hit in test detector):
Combination: 32

33. R3 : 2mm , 2 M Ω/ Resistive kapton +insulator
Cluster size:
R3 : 2mm , 2 M Ω/ Resistive kapton +insulator
R6 : 1mm , 400 k Ω/ Resistive strip
SLHC2: 2mm standard bulk 33

34. Number of missing strips in cluster:
R6( 1mm, 400 KΩ/ ):
R3( 2mm, 2 mΩ/ ): 34

35. Probability to have missing strips in cluster and number of missing strips:35

36. Sparks in R7(few tens of kΩ/ Resistive pads):
All the ‘sparks’ in R7:
Three ‘spark’ type:
‘Spark’ amplitude distribution: 36

37. Sparks in R5(250 MΩ/ Resistive kapton):
All the ‘sparks’ in R5:
Four ‘spark’ type:
‘Spark’ amplitude distribution: 37

38. Sparks in SLHC2(standard bulk):
All the ‘sparks’ in SLHC2:
One ‘spark’ type:
‘Sparks’ amplitude distribution: 38

39. Sparks in R3(2 MΩ/ resistive kapton):
One ‘spark’ type:
All ‘sparks’ in R3:
‘sparks’ amplitude distribution: 39

40. Sparks in R6(400 kΩ/ resistive strip):
All ‘sparks’ in R6:
One ‘spark’ type:
‘sparks’ amplitude distribution: 40

41. Spark amplitude vs voltage and beam intensity:
SLHC2 voltage:
R5 voltage:
R7 voltage:
R6 voltage:
R3 intensity:
R3 voltage: 41

42. Conclusion:
Today’s micromegas detector is facing the spark problem, resistive coating is a successful way to solve it.
High efficiency achieved with a resistive strip detector R6.
Good spatial resolution.