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

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
 The thin amplification gap (short
drift times and fast absorption of
the positive ions) makes it
particularly suited for high-rate
applications
-800 V
Conversion & drift space
(few mm)
-550 V
Mesh
Amplification
Gap 128 µm
The principle of operation
of a MicroMegas chamber
RD51 Electronics School
03 - 05 February 2014
2
The bulk MicroMegas technique
Drift
UV
5 mm
Pillars (Ø ~300μm)
Mask
128 µm
250 µm
RD51 Electronics School
150 µm
PCB
03 - 05 February 2014
3
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
03 - 05 February 2014
4
MicroMegas as μTPC
µ
HV = -850 V
HV = -550 V
RD51 Electronics School
03 - 05 February 2014
5
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 nA range
RD51 Electronics School
03 - 05 February 2014
6
Performance requirements





Spatial resolution: ~60 m
Angular resolution:~0.3 mrad
Good double track resolution
Trigger capability
Efficiency: > 98%
RD51 Electronics School
03 - 05 February 2014
7
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
03 - 05 February 2014
8
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
CHAMBER
R11
R12
R13
HV resistor
(MΩ)
15
45
20
2
5
0.5
Insulator
Resistance
along strip
(MΩ/cm)
+500V
Embedded resistor
15-45 MΩ 5mm long
PCB
RD51 Electronics School
Read-out strip
03 - 05 February 2014
9
Resistive MicroMegas chambers
-300V
First 2D chambers with the “spark protection” resistive strips
R16, R17, R18
5 mm
Pitch: all strips – 250 µm;
Width: resistive – 60 µm
Y-strips – 100 µm
X-strips – 200 µm
+500V
GND
128 µm
Y-strips
RD51 Electronics School
X-strips
03 - 05 February 2014
10
Standard chamber vs resistive
MicroMegas mesh currents and HV drop
in neutron beam
Gas: Ar:CO2 (85:15)
Standard MM:
Large currents
Large HV drops, recovery time O(1s)
Chamber could not be operated stably
Neutron flux: ≈ 106 n/cm2/sec
R11:
Low currents
Despite discharges, but no HV drop
Chamber operated stably up to max HV
HV values
Mesh currents
RD51 Electronics School
03 - 05 February 2014
11
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
10 µs
RD51 Electronics School
-600
-400
-200
0
R12 shows 2-3 order of magnitude less signal and shorter recovery time
than standard MM
03 - 05 February 2014
12
Equivalent scheme of resistive
MicroMegas chambers
-HV
Mesh
C2
C1
Induced charge
Resistive strip
C3
Copper strip
R1
Amp
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
03 - 05 February 2014
13