Transcript Double beta decay using the EDELWEISS detectors

Double beta decay using the
EDELWEISS detectors ?
G. Chardin
DAPNIA, CEA/saclay
Motivations
• Why use charge-phonon detectors ?
• Aim: zero background experiment
• Three improvements:
– Energy resolution (two measurements,
controlling each other)
– Alpha surface rejection
– Single site events vs. Multiple site events
• R&D program and timetable
Double b decay: backgrounds
• Surface alpha tails: bad charge collection, leads to
continuous spectrum (also surface implantation)
Example: 210Po 
206
Pb
206Pb
210
+ a (radon pollution)
Po
a-particle
• Multiple Compton interactions for gammas with
E > 2.038 MeV
bb decay: background suppression
• Reject surface alpha tails:
– Identify surface interactions by anomalous charge/phonon
ratio: 100% rejection at MeV energies
– Identify in addition alpha interactions by pulse shape
– Volume alpha interactions will be seen as discrete lines
– Note: main background for CUORE bb-decay experiment
• Reject multiple Compton interactions:
– Measure (z and r information) position in single detector (≈
1 kg) through pulse shape analysis
– Compact structure of detectors (see CUORICINO/CUORE)
Energy resolution
• FWHM energy
resolution 250 eV
obtained on one 320
gram Ge detector (500
eV for run of several
months)
• Charge and phonon
measurements can be
used to calibrate one
another
• Sub-keV resolution
seems achievable
EDELWEISS: Perfect identification of alphas
by their anomalous quenching factor
EDELWEISS: Perfect identification of alphas
by their anomalous quenching factor
10
57
Signal (mV)
8
6
Co 122keV  event
center signal
guard signal
center fit
guard fit
Time resolved measurements:
Experiment
4
At 122 keV, position
determination in z is ≈1mm !
2
0
-800
-400
0
400
800
Time (ns)
Hole
trajectory
(a)
z
(b)
V= + 6.2V
T= 16mK
Event localization in the detector :
• (a) : 122 keV  (bulk) event
(r = 4mm, z = 1.6mm)
• (b) : 85 keV e- (surface) event
(r = 10mm, z = 3mm)
r
120
100
Counts
Electron
trajectory
Voltage bias = +1.55V
T=16mK
80
60
40
≈1mm
20
Histogram of electron
impacts points
0
-0.4
-0.2
0.0
0.2
0.4
Impact point location along z axis (cm)
Time-amplitude alpha vs. Gamma identification
1.33 MeV s
alphas
gammas
Time-amplitude alpha vs. Gamma identification (ƒƒ)
Alpha amplitude
drift as a function
of surface charge…
1.33 MeV s
alphas
gammas
Two-Dimensional Electron Gas (2DEG)
Research group « Mesoscopic Physics »:
donor
electron
GaAlAs
~ 40nm
2DEG
GaAs
Advantages of 2DEG:
 Low temperature
 Low impurities
 High mobility (mean free path Le of several microns)
We can study:
1. Mesoscopic Physics
Q.P.C. (Quantum Point Contact)
2. Very high mobility, diffusive transport
HEMT
Why HEMTs for EDELWEISS ?
1. EF>Ec: HEMT works at low temperature contrary to the Si JFET.
2. High mobility (electrons separated of the donors) => low noise.
3. Low power dissipation.
4. Weak capacity of entry (Improved Signal/Noise in preamplifier).
*
*
Caracteristics of PHEMT
*
NEW MBE !!!!
New Molecular Beam Epitaxy: VEECO GEN II
Improvement of the machine:

- Vacuum
- Thermal insulation of elements
Improvement:
- 2DEG quality
- Reliability
 New MBE installation : huge work of epitaxy team for one year:
A. Cavanna,, B. Etienne, U. Gennser
In C.E.A….
Power supply
part
Circuit for
HEMT @ LT
HEMT @ LT
Conclusions
• Charge-phonon measurements allow energy
resolution improvement down to <≈ 1 keV (improvement
x ≈5 compared to Heidelberg-Moscow)
• Alphas (at surface) are clearly identified and rejected
by charge phonon measurement
• A lot of work remains to be done on fast charge
measurements, but first fast charge and charge-phonon
measurements are encouraging: reach mm scale
• A charge-phonon bb-decay experiment represents the
« sophisticated » approach
• It should probably be used only if the more
conventional approaches fail
• Its justification is zero-background at the tonne-scale
Pulse-Shape analysis of ionization Events in LowTemperature Germanium Detectors
A. Broniatowski et al. (CSNSM Orsay)
Guard electrode signal
Guard voltage bias
Center electrode signal
Center voltage bias
57Co
57Co  source
(122 & 136 keV)
109Cd
10 mm
109Cd
T=16mK
20 mm
2 measurement channels (center+guard)
• cold FETs
• low noise, wide band electronics
• -> 50ns risetime
• -> baseline noise (wide band) ≈3keV FWMH
e- source
(62 & 84keV)