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Micro MEsh GASeous Detectors (MicroMegas) RD51 Electronics school CERN 3 – 5 February 2014.

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Presentation on theme: "Micro MEsh GASeous Detectors (MicroMegas) RD51 Electronics school CERN 3 – 5 February 2014."— Presentation transcript:

1 Micro MEsh GASeous Detectors (MicroMegas) RD51 Electronics school CERN 3 – 5 February 2014

2 -800 V -550 V 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 February 2014RD51 Electronics School2 The principle of operation of a MicroMegas chamber Conversion & drift space Mesh Amplification Gap 128 µm (few mm)

3 UV Mask PCB 250 µm150 µm Drift 128 µm 5 mm Pillars (Ø ~300μm) February 2014RD51 Electronics School3 The bulk MicroMegas technique

4 Bulk MicroMegas structure February 2014RD51 Electronics School4 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

5 µ HV = -550 V HV = -850 V MicroMegas as μTPC February 2014RD51 Electronics School5

6 Operating parameters  Chambers are operating with an Ar:CO 2 (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 February 2014RD51 Electronics School6

7 Performance requirements  Spatial resolution: ~60  m  Angular resolution:~0.3 mrad  Good double track resolution  Trigger capability  Efficiency:> 98% February 2014RD51 Electronics School 7

8 Read-out strip Mesh support pillars PCB Sparks in the chamber Mesh 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 February 2014RD51 Electronics School8

9 Read-out stripInsulator Resistive strip MΩ/cm Read-out strip Embedded resistor MΩ 5mm long +500V PCB 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 MicroMegas chambers CHAMBERR11R12R13 HV resistor (MΩ) Resistance along strip (MΩ/cm) Resistive characteristics of the chambers February 2014RD51 Electronics School9

10 -300V 128 µm 5 mm 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 X-strips Y-strips +500V GND Resistive MicroMegas chambers February 2014RD51 Electronics School10

11 MicroMegas mesh currents and HV drop in neutron beam 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 Gas: Ar:CO 2 (85:15) Neutron flux: ≈ 10 6 n/cm 2 /sec Standard chamber vs resistive HV values Mesh currents February 2014RD51 Electronics School11

12 Spark signals resistive vs standard February 2014RD51 Electronics School12 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

13 Induced charge CACA Amp Copper strip Resistive strip C1 C3 C2 C4 R1 -HVMesh C1 – capacitance Mesh to ground C2 – capacitance R-strip to ground C3 – capacitance R-strip to readout strip C4 – capacitance readout strip to ground C A – input capacitance of preamplifier Equivalent scheme of resistive MicroMegas chambers February 2014RD51 Electronics School13


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