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Micro MEsh GASeous Detectors (MicroMegas)
RD51 Electronics school CERN 3 – 5 February 2014
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What are Micromegas ? The principle of operation
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 Mesh Amplification Gap 128 µm (few mm) The principle of operation of a MicroMegas chamber RD51 Electronics School February 2014
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The bulk MicroMegas technique
Drift UV 5 mm Pillars (Ø ~300μm) Mask 128 µm PCB 250 µm 150 µm RD51 Electronics School February 2014
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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 February 2014
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MicroMegas as μTPC µ HV = -850 V HV = -550 V RD51 Electronics School
February 2014
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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 kV/cm) Drift-electrode: -800 V (~600 V/cm) Currents in nA range RD51 Electronics School February 2014
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Performance requirements
Spatial resolution: ~60 m Angular resolution: ~0.3 mrad Good double track resolution Trigger capability Efficiency: > 98% RD51 Electronics School February 2014
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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 February 2014
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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 CHAMBER R11 R12 R13 HV resistor (MΩ) 15 45 20 Resistance along strip (MΩ/cm) 2 5 0.5 PCB Read-out strip Insulator Embedded resistor 15-45 MΩ 5mm long +500V Read-out strip PCB RD51 Electronics School February 2014
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First 2D chambers with the “spark protection” resistive strips
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 February 2014
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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 R11: Low currents Despite discharges, but no HV drop Chamber operated stably up to max HV HV values Mesh currents RD51 Electronics School February 2014
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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 10 µs RD51 Electronics School February 2014
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Equivalent scheme of resistive
MicroMegas chambers -HV Mesh C2 Induced charge Resistive strip C1 C3 Amp R1 Copper strip 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 February 2014
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