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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)

Mesh

Amplification

Gap 128 µm

The principle of operation

of a MicroMegas chamber

RD51 Electronics School

slide3

The bulk MicroMegas technique

Drift

UV

5 mm

Pillars (Ø ~300μm)

Mask

128 µm

PCB

250 µm

150 µm

RD51 Electronics School

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

RD51 Electronics School

slide5

MicroMegas as μTPC

µ

HV = -850 V

HV = -550 V

RD51 Electronics School

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

RD51 Electronics School

performance requirements
Performance requirements
  • Spatial resolution: ~60 m
  • Angular resolution: ~0.3 mrad
  • Good double track resolution
  • Trigger capability
  • Efficiency:> 98%

RD51 Electronics School

slide8

Sparks in the chamber

Mesh support pillars

Mesh

PCB

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

RD51 Electronics School

slide9

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

PCB

Read-out strip

Insulator

Embedded resistor

15-45 MΩ 5mm long

+500V

Read-out strip

PCB

RD51 Electronics School

slide10

Resistive MicroMegas chambers

-300V

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

+500V

GND

128 µm

Y-strips

X-strips

RD51 Electronics School

slide11

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

R11:

Low currents

Despite discharges, but no HV drop

Chamber operated stably up to max HV

HV values

Mesh currents

RD51 Electronics School

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

RD51 Electronics School

slide13

Equivalent scheme of resistive

MicroMegas chambers

-HV

Mesh

Induced charge

C2

Resistive strip

C1

C3

Amp

Copper strip

R1

CA

C4

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

RD51 Electronics School

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