1. Micromegas-TPC development for rare event detection
3rd symposium on Large TPCs for low energy rare event detection,
Paris, December 2006.
Leila Ounalli
Neuchâtel University

2. 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).

3. The eye and the retina Cones Sticks Light Optic nerve Brain
(analyze, classify, memorize)

4. The TPC and Micromegas spacers Dave Nygren (1970) Woven wires
1mm
spacers
Cathode
Grid
Amplification (> 50 m)
Micromesh
Ed ~ 200V/cm
Ea ~ kV/cm
Anode
Conversion + Drift
(gamma, RX, UV …) e-
Dave Nygren (1970)
Woven wires
Micromegas « Compact »
analyze,
classify,
memorize.

5. The Neuchâtel mini-TPC
miniTPC(10X20cm)
The source position.
max of dC-g=18cm
Edrift
The drift electric field choice:
Ed= 200 V.cm-1.bar-1
The gap dimension (dgrille-anode ):
( µm)
High pressure + low voltages
The quencher choice and %:
- Xe + (CF4, isobutene):
double beta decay
- CF4 + (Xe, Ar):
solar low E

6. Why we replace the MWPC by the Micromegas micro-pattern?

7. Charge deviation from their trajectories. (RE + ξ ) bad (50% @ 6 keV)
Contours of V near the amplification gap
x10-3
Y-Axis[cm]
X-Axis[cm]
Comparison with MWPC’s
Y-Axis[cm]
X-Axis[cm]
Circular form:
Charge deviation from their trajectories.
(RE + ξ ) bad 6 keV)
rectilinear:
E uniform: // of electrons.
(RE + ξ ) good 6keV with 1 bar of CF4)

8. How to operate the Micromegas-TPC at higher pressures?
The increase of the gap amplification permits a good charge collection at high pressure C G
Amplification (75, 250) m
Micromesh A
Conversion + Drift

9. Why we choose a gap of 250 µm?
250 µm
4 bar
75 µm

10. How to improve the charge collection in Xe?
A small CF4 addition is sufficient

11. CF4 is the best additive for Xe
← 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%)
CF4 addition:
increases the electron
drift velocity in Xe.
reduces longitudinal and
transversal diffusions.

12. Optimal parameters Ed= 200 V.cm-1.atm-1. Gap: 250 µm.
Gas: Xe(98)CF4(2)
Make preliminary tests in the mini-TPC of Neuchâtel (241Am).

13. The Xe(98)CF4(2) gain and the energy resolution @ 60 keV at different pressures
1.05 atm
2.01 atm
3.00 atm
4.00 atm
Gas: Xe(98)CF4(2)
Ed= 200V.cm-1.atm-1
Source: 241Am (37kBq)
▲ 8.05 keV Cu-Kα
keV Xe-Kα
103

14. Pulse height spectra of 241Am source in Xe(98)CF4(2) with a Micromegas-TPC
8 keV
G ≈ 1340
30 keV
G ≈ 1670
30 keV
8 keV
Cu-Kα
Xe-Kα
Pulser
1 bar
3 bar

15. Radio-pure and radio-active components

16. The Germanium detector : gamma spectrometry
Ge (400 cm3)
“Vue-des-Alpes”

17. Copper (TPC+rings+cathode). Glue (araldite). Grid (Stainless steel).
Kevlar
Resin-epoxy
Lead.
Copper (TPC+rings+cathode).
Glue (araldite).
Grid (Stainless steel).
insulators (delrin, teflon)
Radio-pure
2614
x104
Printed-circuit (resin-epoxy).
Resistances (ceramic).
Solder (210Pb)
Radio-active

18. Gotthard results

19. The Gotthard TPC diameter: 50 cm TPC (60X70cm) Micromegas Gap: 250 µm
Gotthard-TPC low
energies (241Am, 133Ba).
Estimate the radioactive
background of the TPC.
- Find the sources of noise:
Measure the radioactivity of
components using a Ge
detector “Vue-des-Alpes”.

20. The compact Micromegas is tested before being installed

21. Micromegas (50 cm of diameter): (Am and Ba) sources effect
81 keV (133Ba)
60 keV (241Am)
60 keV (241Am)
The Compton plateau (133Ba)

22. The behavior of the background registered in the Gotthard TPC
1 bar of P10 gas
46 keV
3 bar of P10 gas

23. 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.