Jonathan Bortfeldt

LS Schaile, Ludwig-Maximilians-Universität München

DPG Frühjahrstagung Bonn, 18. März 2010

Development of High-Resolution Micromesh based Gas Detectors

Atlas-MDT Tests @ GIF / CERN

MΩ tracking system →larger Ω, less sensitivity to space charge effects

MΩres.< 40μm30x50cm²

MΩ

Gamma IrradiationFacility

2/10

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

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

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

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

Sparking Rate @ Ar:CO2 93:7● sparks registered by HV-supply● Umesh = -555V: τspark ≈ 200s

7/10

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

Cosmics

● measurement period 14h

● 3fold coincidence, triggered by 2 scintillators + MICROMEGAS

9/10

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

backup slides – 55Fe, double Gauß-fit

backup slides - cosmics

● Measurement period 12,5h

● 2fold coincidence, triggered by 2 scintillators

backup slides – e- transparency