Report on Measurements in
the Lab with
R11, R12, R13
Alexandra Moskaleva
What is a resistive chamber?
Non-resistive Micromegas
Resistive Micromegas
(R11, R12, R13)
Resistive Characteristics
CHAMBER
R11
R12
R13
Resistance to
Ground (MΩ)
15
45
20
Resistance
along strip
(MΩ/cm)
2
5
0.5
Types of Measurements
 Properties





Gain
Transparency
Response to cosmics
Charge-up
High rate behavior
 Tools
 5 keV 55Fe source
 8 keV X-ray gun at various rates
 120 GeV pion test beam
Gain & Transparency
Measurement – Set up
Gas out
Read out connectors
Protective cover with holes
55Fe
tablet
HV Drift
HV Mesh
Gas in
18 mm
 Each read-out connector connected to 72 strips
 Covers a cross-section of 18 mm
 Strips that are not read out are grounded
Example Spectrum
55Fe
Ar escape peak
(3 keV lower)
(5.9 keV) Peak
 Peak of 55Fe taken in arbitrary units from spectrum
 Rate, counts, live time, start time also displayed
 Converted to gain via calibration signal
 Calibration pulse = 104 gain
 Gain measurements taken for:





R11, 93% Ar 7% CO2
R12, 93% Ar 7% CO2
R12, 85% Ar 15% CO2
R13, 93% Ar 7% CO2
R13, 85% Ar 15% CO2
Gain Measurements - Results
Transparency Measurements
 Transparency =
(# of e– that make it to the amplification field)
(Total # of e– produced)
 Transparency measurements taken for:




R12, 93% Ar 7% CO2
R12, 85% Ar 15% CO2
R13, 93% Ar 7% CO2
R13, 85% Ar 15% CO2
Transparency Measurements – Set up
 Measured gain as a function of Eamplification/Edrift
 Kept Vmesh constant
 Varied Vdrift
 E=V/d, where V is the voltage difference and d is the
distance
 Edrift = (Vmesh-Vdrift)/d ≈ 300V/5mm ≈ 600 V/cm
 Eamplification = (Vmesh-Vground)/d ≈ 500V/.128mm ≈ 40 kV/cm
 Normalized highest gain = 1
Cosmic Measurements – Set
up
 Stacked R11, R12, R13 with 15mm spacers
 Used R11 and R13 to trigger, R12 to acquire
R11
HV Mesh top
Top
HV
Drift
R12
HV Mesh middle
MCA
R13
HV Mesh bottom
Bottom
Cosmics – Logic
Top
Threshold
Bottom
Threshold
NIM 
TTL
Gate
for MCA
Cosmics - Results
Cosmic most
probable
value
55Fe

55Fe
(5.9 keV) Peak
produces 225 e– in drift region, with a peak ≈ 420
 Cosmics produce a peak ≈ 100 (a factor of ¼)
 Therefore, cosmics produce ≈ 50 e– in 5mm drift region
 Agrees with 90 e–/cm for Ar and for CO2
Charge Up – Theory
 Charge is deposited on the detector in <2mm radius for
every photon
 Due to dispersion in electron avalanche
 Some charge deposits on resistive strips
 Runs to ground due to relatively low resistance
 Some charge deposits on insulative area
 Cannot drain away as quickly  builds up an excess of
negative charge
 No longer at ground
≈2mm
 Charge is deposited on resistive and insulative strips
 Footprint of roughly 2mm in diameter
 Charge leaks away through the resistive strips
 Takes much longer to leak away charge from insulative
portion
 If the rate of deposition >> rate of discharge, then there
is a buildup of charge
 Eamplification decreases
 Charge deposited per second:
 Rate = 200 Hz
 (200 Hz) × (225 e– per photon) × (104 gain) × (1.6 × 10-19)
≈ .06 nA
 Rate = 20k Hz
 (20000 Hz) × (225 e– per photon) × (104 gain) × (1.6 × 10-19)
≈ 6 nA
 Δ Voltage (very rough estimations of what to expect)
 In resistor
 Low rate: V = IR = .06 nA × 108 Ω = .006 V
 High rate: V = IR = 6 nA × 108 Ω = .6 V
 In insulator
 Low rate: V = IR = .06 nA × 1010 Ω = .6 V
 High rate: V = IR = 6 nA × 1010 Ω = 60 V
Δgain≈1600
 Gain decreases due to decrease in Emesh
Δgain≈2000
Δgain≈1000
Δgain≈5000
Δvoltage≈15V
Conclusions
 Gain
 All chambers reach gain of ≈30k
 With 93:7 Ar:CO2, the chambers can be operated at a HV 40V lower than
for 85:15 Ar:CO2
 R12 has highest gain, R11 has lowest
 Transparency
 For good transparency, Eamp/Edrift > 80
 85:15 Ar:CO2 rises faster, but shows similar behavior to 93:7 Ar:CO2 in
steady state
 Cosmics
 Cosmic measurements show good results (Landau curve) with most
probable value showing creation of 50 e– in 5mm drift region, as expected
 Charge up
 Charge up is observed
 As function of rate (reducing gain by 10% at 200 Hz, 20% at 20 kHz)
This has been an amazing
summer!
Thank you to everyone who
has guided me in this
experience!!!