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**Developments of micromegas detector at CERN/Saclay**

Shuoxing Wu

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**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

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**Part 1 Introduction to micromegas**

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**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.

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**Widely used in particle physics experiments:**

Compass: Prototype for ILC-TPC: Prototype for ILC-DHCAL:

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**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

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**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

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**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

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**Part2 November test beam**

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**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

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**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

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**Readout and online monitoring:**

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

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**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

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**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%

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**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%

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**Sparking behavior of S1:**

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

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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

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**Spark rate vs mesh voltage and beam intensity:**

R3(2mm, 2MΩ/ ): Spark rate= #sparks/#incident hadron 18

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**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

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**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

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**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….)

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**Conversion from raw data into root tree:**

‘Detector’ class: { NofStrips; stripID; stripCharge; }

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Channel-pin relation Decoding matrix:

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

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**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:

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

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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

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**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:

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**R6 (1mm, 400KΩ/ ) efficiency : @beam intensity of 11kHz/cm2:**

@voltage of 400V: >98% drop<3% 29 29

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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

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**Resolution vs mesh voltage:**

R6(1mm, 400kΩ/ resistive strip): R3(2mm, 2MΩ/ resistive kapton): 31

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**Pillars: Track position( with hit in test detector):**

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

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**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

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**Number of missing strips in cluster:**

R6( 1mm, 400 KΩ/ ): R3( 2mm, 2 mΩ/ ): 34

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**Probability to have missing strips in cluster and number of missing strips:**

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**Sparks in R7(few tens of kΩ/ Resistive pads):**

All the ‘sparks’ in R7: Three ‘spark’ type: ‘Spark’ amplitude distribution: 36

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**Sparks in R5(250 MΩ/ Resistive kapton):**

All the ‘sparks’ in R5: Four ‘spark’ type: ‘Spark’ amplitude distribution: 37

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**Sparks in SLHC2(standard bulk):**

All the ‘sparks’ in SLHC2: One ‘spark’ type: ‘Sparks’ amplitude distribution: 38

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**Sparks in R3(2 MΩ/ resistive kapton):**

One ‘spark’ type: All ‘sparks’ in R3: ‘sparks’ amplitude distribution: 39

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**Sparks in R6(400 kΩ/ resistive strip):**

All ‘sparks’ in R6: One ‘spark’ type: ‘sparks’ amplitude distribution: 40

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**Spark amplitude vs voltage and beam intensity:**

SLHC2 voltage: R5 voltage: R7 voltage: R6 voltage: R3 intensity: R3 voltage: 41

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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.

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