Stability tests of GEMs with cylindrical holes for ALICE upgrade
description
Transcript of Stability tests of GEMs with cylindrical holes for ALICE upgrade
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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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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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…”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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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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Intensity variations
Time (Sec)
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Zoomed area of the corrected data
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Correction
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Various tests with intensity variations
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X-ray intensity modulations
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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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An example of double GEM reaction on intensity variation
18000 18200 18400 18600 18800 190000.750000000000001
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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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Single-wire, raw data
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GEM, raw data
Overview of raw data
Gain 1200
Gain 1800
180ppm
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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
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Step by step gain increase from 1200 to 1800
27 hours
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± 1.5%
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± 1.4%
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Region 3
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Region 4 (gain 1800)
~6 days
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± 1.4%Sr was removed
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80000 82000 84000 86000 88000 90000 92000 94000 96000 98000 1000000
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20min
Zoomed area
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Various voltage variations
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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: