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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 03 - 05 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) 03 - 05 February 2014RD51 Electronics School3 The bulk MicroMegas technique

4 Bulk MicroMegas structure 03 - 05 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 03 - 05 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 40-50 kV/cm)  Drift-electrode: -800 V (~600 V/cm)  Currents in nA range 03 - 05 February 2014RD51 Electronics School6

7 Performance requirements  Spatial resolution: ~60  m  Angular resolution:~0.3 mrad  Good double track resolution  Trigger capability  Efficiency:> 98% 03 - 05 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 03 - 05 February 2014RD51 Electronics School8

9 Read-out stripInsulator Resistive strip 0.5-5 MΩ/cm Read-out strip Embedded resistor 15-45 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) 154520 Resistance along strip (MΩ/cm) 250.5 Resistive characteristics of the chambers 03 - 05 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 03 - 05 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 03 - 05 February 2014RD51 Electronics School11

12 Spark signals resistive vs standard 03 - 05 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 -600 -400 -200 0

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 03 - 05 February 2014RD51 Electronics School13


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