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Jonathan Bortfeldt
LS Schaile, Ludwig-Maximilians-Universität München
DPG Frühjahrstagung Bonn, 18. März 2010
Development of High-Resolution Micromesh based Gas Detectors
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Atlas-MDT Tests @ GIF / CERN
MΩ tracking system →larger Ω, less sensitivity to space charge effects
MΩres.< 40μm30x50cm²
MΩ
Gamma IrradiationFacility
2/10
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Setup and Functional Principle of MICROMEGAS● MICROMEsh GAseous
Structure● drift gap 6mm, ≈ 400V● amplification gap 128μm,
≈ 550V● 90mm x 100mm● readout pitch 250μm,
single plane readout!● Ar:CO2 93:7, p = 1012mbar,
flux = 0.9ln/h
10MΩ
10MΩ
12Ω
1Ω
10kΩ
HV
Signal
33kΩ
5.6kΩ
1.5nF
cathode
mesh
anode
3/10
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Test Setup and Readout Chain
● cosmic muons– 2fold coinc.,
sc.-trigger– 3fold coinc.
● 55Fe-source– MΩ-trigger
4/10
scintillator low
sc. up
signal
photomultiplier
discr.
discr.coinc. &
discr. coinc. &
counter1#Sc
counter2#MΩ
FADC
90 mm
MΩ trigger
2fold coinc.
3fold coinc.
MΩ
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Signalanalysis
● FADC: 2560 points @ 1GHz●
● mean rise time, fits: 116ns● theoret. ion drift time: 170ns● mean signal rise time: 190ns● typ. amplitude: 3 – 10mV● possible: mean2-mean1 > 3 σ
f x=p0
exp p1−x / p21p3
5/10
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Efficiency
● ε = #MΩ/(#Sc*fgeometry)
● limited by:
– high Edrift low e→ - - transparency
– low Edrift poor charge →collection
● ε = 92% for Edrift = 0.67 kV/cm and Emesh = 43.4 kV/cm
6/10
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Sparking Rate @ Ar:CO2 93:7● sparks registered by HV-supply● Umesh = -555V: τspark ≈ 200s
7/10
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55Fe-source● electron capture● 5.899keV (green)● 6.49keV (blue)● 4.12keV (red)● dE/dx|MIP ≈1.6keV/0.6cm
● softwarecut to eliminate discharges and noise
● ΔE/E ≈ 11.1/91.7 ≈ 0.12● gas gain ≈ 600, approx. of
Townsend coefficient →gain ≈ 450
8/10
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Cosmics
● measurement period 14h
● 3fold coincidence, triggered by 2 scintillators + MICROMEGAS
9/10
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Summary and Outlook
● MICROMEGAS is running with Ar:CO2 93:7 @ 1012mbar →investigate different mixtures of Ar:CO2 and Ar:CH4 to optimize discharge behaviour
● 6 keV x-rays: energy resolution ΔE/E ≈ 12%
● cosmic muons: ε ≈ 92% @ Edrift = 0.67kV/cm and Emesh = 43.4kV/cm →increase with slightly higher signals possible
● strip readout based on GASSIPLEX chips in preparation (M. Böhmer, TU München)
10/10
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backup slides – 55Fe, double Gauß-fit
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backup slides - cosmics
● Measurement period 12,5h
● 2fold coincidence, triggered by 2 scintillators
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backup slides – e- transparency