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Stability tests of GEMs with cylindrical holes for ALICE upgrade

Behalf of the ALICE TPC upgrade collaboration

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The ALICE TPC is the main device in the ALICE central barrel for tracking of charged particlesand particle identification

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Design of the present ALICE TPC

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Requirement to gating grids: ion back flow is below 10-4

5Chilo Garabatos

Counting rate will be increased 100 times!

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A very attractive option is GEM…

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ν

Chilo Garabatos

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GEM-1

GEM-2

X-ray gunor Sr

Vdr

Vt1Vb2Vt2

Gas chamber

Experimental setup

6mm

3mm

7mm

2-3-mm

4-6mm

Shielding box

Monitoringsingle-wireCounter (SWC)

Gas outN2 in

55Fe

Vb23 mm

Vbp

Gas in

N2 out

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Some related images…

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Data acquisition(J. Reinink)

Labviewpresentation

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…”this is an important question. I ill try to answer it in terms of expected current per cm2 expected in Pb-Pb at 50 kHz at GEM gain of 1000. I put alice-tpc-upgrade in cc to make sure everybody agrees.I start with the charge expected in the TDR era ( dN/dy=8000) per central collisions event in the innermost region: 2.3 10^-13 mC/cmI correct for the expected true multiplicity (2200 per CC at 14 TeV) and convert into charge per Minimum Bias (not central collision) event: 1.2x10-12 mC/cmAt 50 kHz event rate this results in: 6.2x10^-10 mA/cmThis was for readout chamber with 2.5 mm pitch anode wires, so I divide by this and multiply by gain of 1000: 2.5x10^-6 mA/cm2 = 2.5 nA/cm2…”

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At what conditions the test should be done?

Chilo

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Results:

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Measurements were performed with single and double and triple GEMs in

Ne+10%CO2 and Ar+10%CO2 at p=1atmand humidity range 1000-50 ppm

A few examples are given below:

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1. Single GEM

Gas gain 200Current ≈2nA/cm2

Long-term runs as well as intensity and voltage variations were done

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0 10000 20000 30000 40000 50000 60000 70000 800000

100

200

300

400

500

600

700

800

900

Sing

le w

ire c

ount

er (S

WC)

0 10000 20000 30000 40000 50000 60000 70000 80000

-100

0

100

200

300

400

500

600

GEM

0 10000 20000 30000 40000 50000 60000 70000 800000

2

4

6

8

10

12

GEM

/SW

CRa

w d

ata

0 10000 20000 30000 40000 50000 60000 70000 800000

2

4

6

8

10

12

GEM

/SW

CCo

rrec

ted

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Intensity variations

Time (Sec)

170 10000 20000 30000 40000 50000 60000 70000 80000

0.55

0.57

0.59

0.61

0.63

0.65

Zoomed area of the corrected data

740 760 780 800 820 840 8600

2

4

6

8

10

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Correction

SWC

GEM

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0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

2

4

6

8

10

12

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000.55

0.57

0.59

0.61

0.63

0.65Zoomed

Various tests with intensity variations

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0 10000 20000 30000 40000 50000 60000-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Series1

GEM/SWC corrected

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167 min

9000 9500 10000 10500 11000 11500 120000

0.2

0.4

0.6

0.8

1

1.2

Series18min

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X-ray intensity modulations

Corrected

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 200000

0.1

0.2

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0.6

1 2

3

0 1000 2000 3000 4000 5000 6000 70000.45

0.455

0.46

0.465

0.47

0.475

0.48

0.485

0.49

0.495

0.5

8000 9000 10000 110000.45

0.455

0.46

0.465

0.47

0.475

0.48

0.485

0.49

0.495

0.5

11000 13000 15000 17000 190000.35

0.355

0.36

0.365

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0.375

0.38

0.385

0.39

0.395

0.4

Zoomed areas

1 2 3

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0

100000

200000

300000

400000

500000

600000

700000

800000

-0.2

0

0.2

0.4

0.6

0.8

1

Series1

0 200000 400000 600000 8000000.700000000000001

0.710000000000001

0.720000000000001

0.730000000000001

0.740000000000001

0.750000000000001

0.760000000000001

0.770000000000001

0.780000000000001

0.790000000000001

0.800000000000001

Series1

GEM voltage variations

The same, but zoomed±2%!In the plateau region± 1.3%

GEM/SWCcorrected on offset

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Summary of measurements with a singe GEM:

GEM stable within ±1-2% (continuous test time was 7 weeks)

Humidity level was 500-50ppm

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Will be interesting to compare with simulation

Discussion with Rob Veenhof:

His calculations show:With real conical holes 70μm in diameter no charging up effect. Some short-term (10-30min) variation with time are predicted:with 70 μm holes having and inner diameter 60 μm some initial gain loss is predicted,with inner diameters less than 60 μm gain may increases with time

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2. Double GEM

The same type of measurements: long-term runs, intensity and voltage

variations

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18000 18100 18200 18300 18400 18500 18600 18700 18800 18900 190000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

An example of double GEM reaction on intensity variation

18000 18200 18400 18600 18800 190000.750000000000001

0.760000000000001

0.770000000000001

0.780000000000001

0.790000000000001

0.800000000000001

0.810000000000001

0.820000000000001

0.830000000000001

0.840000000000001

0.850000000000001

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Conclusion from double GEM stability studies

double GEM, irradiated in its middle area (current ~1.8nA/cm2, gain 900) is stable within ~±2%.After the voltage variation it is still not so bad: ±2.6%

When the gain was increase 2 times (from 900 to ~ 2000) and intensity 4 times, so that the current range ~ 7-15nA/cm2, stability is ±3%(which is inside the expected √2 degradation)

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Triple GEM-the most important case

Measurements were done at 180 and 70ppm

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Results at humidity 180 ppm

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0 200000 400000 600000 800000 1000000 12000000

200

400

600

800

1000

1200

Single-wire, raw data

0 200000 400000 600000 800000 1000000 12000000

100

200

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600

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GEM, raw data

Overview of raw data

Gain 1200

Gain 1800

180ppm

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0 50000 100000 150000 200000 250000 300000 350000 4000000.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

0.46

0.48

0.5

Corrected GEM data at gain 1200

±3%, If pikes excluded ± 2%-both are consistent with old measurements

Noise came for humiditymonitorThen it was switched off

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425000 445000 465000 485000 505000 525000 5450000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1

23 4

Step by step gain increase from 1200 to 1800

27 hours

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427500 428000 428500 429000 429500 4300000

0.1

0.2

0.3

0.4

0.5

0.6

Region 1

427750 427950 428150 428350 428550 428750 4289500.3

0.31

0.32

0.33

0.34

0.35

0.36

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0.39

0.4

± 1.5%

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425000 427000 429000 431000 433000 435000 437000 4390000

0.1

0.2

0.3

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0.7

0.8

Region 2

428000 429000 430000 431000 432000 433000 434000 435000 4360000.5

0.52

0.54

0.56

0.58

0.6

± 1.4%

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Region 3

435000 445000 455000 465000 475000 485000 495000 505000 5150000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

435000 445000 455000 465000 475000 485000 495000 505000 5150000.700000000000001

0.710000000000001

0.720000000000001

0.730000000000001

0.740000000000001

0.750000000000001

0.760000000000001

0.770000000000001

0.780000000000001

0.790000000000001

0.800000000000001

±1.3%

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515000 565000 615000 665000 715000 765000 815000 865000 915000 965000 10150000.725

0.73

0.735

0.74

0.745

0.75

0.755

0.76

0.765

0.77

515000 565000 615000 665000 715000 765000 815000 865000 915000 965000 10150000.700000000000001

0.710000000000001

0.720000000000001

0.730000000000001

0.740000000000001

0.750000000000001

0.760000000000001

0.770000000000001

0.780000000000001

0.790000000000001

0.800000000000001

±1.3%

Region 4 (gain 1800)

~6 days

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…70 ppm

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20000 30000 40000 50000 60000 70000 80000 90000 1000000

2

4

6

8

10

12

20000 30000 40000 50000 60000 70000 80000 900001000000.7

0.71

0.72

0.73

0.74

0.75

0.760000000000001

0.770000000000001

0.78

0.79

0.8

± 1.4%Sr was removed

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80000 82000 84000 86000 88000 90000 92000 94000 96000 98000 1000000

2

4

6

8

10

12

8000082000

8400086000

8800090000

9200094000

9600098000

1000000.7

0.71

0.72

0.73

0.74

0.75

0.760000000000001

0.770000000000001

0.78

0.79

0.8± 1.4%

20min

Zoomed area

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700000 720000 740000 760000 780000 800000 820000 8400000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

700000 720000 740000 760000 780000 800000 820000 8400000.45

0.5

0.55

0.6

0.65

± 2%

Various voltage variations

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770000

772000

774000

776000

778000

780000

782000

784000

786000

788000

7900000.45

0.5

0.55

0.6

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777000 779000 781000 783000 785000 787000 7890000.45

0.47

0.49

0.51

0.53

0.55

20min

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Conclusion for triple GEM:

Stability between ± 2% (gain 1200) and ± 1.4% (gain 1800)was observed with triple GEMs

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General conclusions:

1. Long-term measurements (total time more than 6 months) were performed with GEMs having cylindrical holes

2. At expected LHC conditions (corresponding to GEM’s current ~2nA a gain of ~ 1000) and humidity 100-50pp the gain variation were (over all tests performed) below ± 3%

3. Probably, better results could be achieved if we implement better compensation on environmental variations

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Further plans:

1) Try to compensate on environmental variations even better (using a detector which has a working

voltage and a gain vs. voltage close to our GEMs; in the case of the single wire detectors these

parameters are very far away)2) Perform tests very “dry” GEMs (in preparation by

Leszek and Eraldo RD-51 Lab)

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Made of liquid crystal (LCP) The geometry of them is exactly same as the standard CERN-GEM: hole diameter = 70 microns, hole pitch = 140 microns, thickness of LCP = 50 microns, thickness of Cu layers = 5 microns.

One of the possibilities is to use as a refernce detector Japanese GEMs:

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…hence,more adventures are coming

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