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Micromegas-TPC development for rare event detection
Micromegas-TPC development for rare event detection
Leila OunalliNeuchâtel University
3rd symposium on Large TPCs for low energy rare event detection,
Paris, 11-12 December 2006.
Restrictive conditions for rare event detection
Big detector mass (high pressure), Radioactive background as low as
possible (underground laboratory + radio-pure components),
Good energy resolution (FWHM), High gas gain (collected charge / initial
charge).
Cones Sticks
Optic nerve
Brain (analyze, classify, memorize)
The eye and the retinaLight
Micromegas « Compact »
The TPC and Micromegas
1mm
spacers
Woven wires http://www.bopp.ch/(gamma, RX, UV …)
Cathode
GridAmplification (> 50 m)
Micromesh
Ed ~ 200V/cm
Ea ~ 40-100kV/cmAnode
Conversion + Drift e-
• analyze,• classify, • memorize.
Dave Nygren (1970)
The Neuchâtel mini-TPC
miniTPC(10X20cm) • The source position. max of count @ dC-g=18cm
Edrift
• The drift electric field choice: Ed= 200 V.cm-1.bar-1
• The gap dimension (dgrille-anode ): (75-100-250 µm) High pressure + low voltagesHigh pressure + low voltages
• The quencher choice and %: - Xe + (CF4, isobutene):
double beta decaydouble beta decay - CF4 + (Xe, Ar):
solar solar @ low E @ low E
Why we replace the MWPC by the Micromegas micro-pattern?
Contours of V near the amplification gap
x10-3
Y-A
xis
[cm
]
X-Axis[cm]
x1
0-3
http://www.bopp.ch
Comparison with MWPC’s
Y-A
xis
[cm
]
X-Axis[cm]
rectilinear:E uniform: // of electrons.(RE + ξ ) good (35% @ 6keV with 1 bar of CF4)
Circular form:Charge deviation from their trajectories.(RE + ξ ) bad (50% @ 6 keV)
The increase of the gap amplification permits a good
charge collection at high pressure
C
GAmplification (75, 250) m
Micromesh
A
Conversion + Drift
How to operate the Micromegas-TPC at higher pressures?
Why we choose a gap of 250 µm?
104 @ 4 bar75 µm
250 µm
How to improve the charge collection in Xe?
A small CF4 addition is sufficient
• increases the electron drift velocity in Xe.
CF4 is the best additive for Xe
CF4 addition:
• reduces longitudinal and transversal diffusions.
← improves the charge collection.
P: 1.00 atm, Ed=200 V.cm-1.atm-1,Gap: 100 µm
■ Xe-CF4 (2, 5, 10, 50%)▲ Xe-isobutene (2%)
Optimal parameters
EEdd= 200 V.cm= 200 V.cm-1-1.atm.atm-1-1.. Gap: 250 Gap: 250 µm.µm. Gas: Xe(98)CFGas: Xe(98)CF44(2)(2) Make preliminary tests in the mini-TPC of Make preliminary tests in the mini-TPC of
Neuchâtel (Neuchâtel (241241Am).Am).
The Xe(98)CF4(2) gain and the energy resolution @ 60 keV at different pressures
1.05 atm
1.05 atm
2.01 atm
2.01 atm
3.00 atm
3.00 atm
4.00 atm
4.00 atm
Gas: Xe(98)CF4(2)Ed= 200V.cm-1.atm-1
Source: 241Am (37kBq)
▲ 8.05 keV Cu-Kα
29.779 keV Xe-Kα
103
Pulse height spectra of 241Am source in Xe(98)CF4(2) with a Micromegas-TPC
63% @ 8 keV G ≈ 1340
19% @ 30 keVG ≈ 1670
36% @ 30 keVG ≈ 1670
68% @ 8 keV G ≈ 1340
Cu-Kα Cu-Kα
Xe-Kα
Xe-Kα
Pulser
Pulser
1 bar 3 bar
Radio-pure and radio-active components
The Germanium detector :gamma spectrometry
Ge (400 cm3)
“Vue-des-Alpes”
Kevlar
Resin-epoxy
2614
2614
x104
• Lead. • Copper (TPC+rings+cathode).• Glue (araldite).• Grid (Stainless steel).• insulators (delrin, teflon)
Radio-pure
• Printed-circuit (resin-epoxy).• Resistances (ceramic).• Solder (210Pb)
Radio-active
Gotthard results
The Gotthard TPC
TPC (60X70cm)
- Find the sources of noise: Measure the radioactivity of components using a Ge detector “Vue-des-Alpes”.
• Estimate the radioactive background of the TPC.
• Gotthard-TPC calibration @ low energies (241Am, 133Ba).
diameter: 50 cmGap: 250 µm
Micromegas
The compact Micromegas is tested before being installed
Micromegas (50 cm of diameter):(Am and Ba) sources effect
53% @ 60 keV (241Am)81 keV (133Ba)
The Compton plateau (133Ba)60 keV (241Am)
The behavior of the background registered in the Gotthard TPC
1 bar of P10 gas
3 bar of P10 gas
46 keV
46 keV
Conclusions
We improve the energy resolution when we replace the MWPC with a Micromegas.
Xe(98)CF4(2): ideal for double beta search: high gains, good efficiency, good (energy, spatial
and time) resolutions. Increase the gap (amplification): permits a good charge collection in Xe and go up at higher pressures. Micromegas in compact: (50 cm) showed high
efficiency and good energy resolution.
