Micro mesh gaseous detectors micromegas
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Micro MEsh GASeous Detectors (MicroMegas). RD51 Electronics school CERN 3 – 5 February 2014. What are Micromegas ?. Micromegas are parallel-plate chambers where the amplification takes place in a thin gap, separated from the conversion region by a fine metallic mesh

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Micro mesh gaseous detectors micromegas

Micro MEshGASeous Detectors(MicroMegas)

RD51 Electronics schoolCERN 3 – 5 February 2014

What are micromegas
What are Micromegas ?

  • Micromegas are parallel-plate chambers where the amplification takes place in a thin gap, separated from the conversion region by a fine metallic mesh

  • The thin amplification gap (short drift times and fast absorption of the positive ions) makes it particularly suited for high-rate applications

-800 V

-550 V

Conversion & drift space

(few mm)



Gap 128 µm

The principle of operation

of a MicroMegas chamber

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The bulk MicroMegas technique



5 mm

Pillars (Ø ~300μm)


128 µm


250 µm

150 µm

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Bulk micromegas structure
Bulk MicroMegas structure

  • Standard configuration

  • Pillars every 5 (or 10) mm

  • Pillar diameter ≈350 µm

  • Dead area ≈1.5 (0.4)%

  • Amplification gap 128 µm

  • Mesh: 325 lines/inch

Pillar distance on photo: 2.5 mm

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MicroMegas as μTPC


HV = -850 V

HV = -550 V

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Operating parameters
Operating parameters

  • Chambers are operating with an Ar:CO2 (93:7) gas mixture (same gas as MDTs, safe and cheap, no flammable components)

  • High Voltage (moderate HV requirements)

    • Mesh: -500 V (amplification field 40-50 kV/cm)

    • Drift-electrode: -800 V (~600 V/cm)

    • Currents in nArange

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Performance requirements
Performance requirements

  • Spatial resolution: ~60 m

  • Angular resolution: ~0.3 mrad

  • Good double track resolution

  • Trigger capability

  • Efficiency:> 98%

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Sparks in the chamber

Mesh support pillars



Read-out strip

Sparks between mesh and readout strips may damage the detector and readout electronics and/or lead to large dead times as a result of HV breakdown

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Resistive MicroMegas chambers

To avoid spark effect the readout strips were covered with the 64 µm thick insulator layer with resistive strips on top of it connected to the +HV via discharge resistor and mesh is connected to GND

Resistive characteristics

of the chambers

Resistive strip

0.5-5 MΩ/cm


Read-out strip


Embedded resistor

15-45 MΩ 5mm long


Read-out strip


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Resistive MicroMegas chambers


First 2D chambers with the “spark protection” resistive strips

R16, R17, R18

Pitch: all strips – 250 µm;

Width: resistive – 60 µm

Y-strips – 100 µm

X-strips – 200 µm

5 mm



128 µm



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Standard chamber vs resistive

MicroMegas mesh currents and HV drop

in neutron beam

Gas: Ar:CO2 (85:15) Neutron flux: ≈ 106 n/cm2/sec

Standard MM:

Large currents

Large HV drops, recovery time O(1s)

Chamber could not be operated stably


Low currents

Despite discharges, but no HV drop

Chamber operated stably up to max HV

HV values

Mesh currents

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Spark signals resistive vs standard
Spark signals resistive vs standard

Sparks measured directly on readout strips through 50 Ohm

Several spark signals plotted on top of each other to enhance the overall characteristics

R12 shows 2-3 order of magnitude less signal and shorter recovery time than standard MM

-600 -400 -200 0

10 µs

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Equivalent scheme of resistive

MicroMegas chambers



Induced charge


Resistive strip




Copper strip




C1 – capacitance Mesh to ground

C2 – capacitance R-strip to ground

C3 – capacitance R-strip to readout strip

C4 – capacitance readout strip to ground

CA – input capacitance of preamplifier

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