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Surface events suppression in the germanium bolometers EDELWEISS experiment. Xavier-François Navick (CEA Dapnia) TAUP2007 - Sendai September 07. +++. - - -. Ionization-heat detectors. T = 20 mK. Thermal Sensor (NTD Ge). Al electrode. WIMP neutron X or rays. Biais amplifier. - PowerPoint PPT Presentation
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Surface events suppression in the germanium
bolometers EDELWEISS experiment
Xavier-François Navick (CEA Dapnia)
TAUP2007 - Sendai September 07
Ionization-heat detectors
Biais amplifier
Charge amplifier
Thermal Sensor (NTD Ge)
Al electrodes (charge collection ) HPGe single crystal
WIMPneutron
X or rays
D = 2 cm
Al electrode
V = 1-6 Volts
T = 20 mK
- - -+++
TAUP2007 X-F Navick September 2007
Simultaneous measurement of ionization and heat => Evt per evt identification Q = Eionisation / Erecul
Q = 1 for electronic recoil (ambiant radioactivity) Q 0.3 for nuclear recoil (WIMP and neutron)
discrimination /n > 99.9% pour Er> 15keV
Rejection power
TAUP2007 X-F Navick September 2007
• Developed by CEA Saclay and Canberra
• Optimized NTD size in collaboration with LBNL for sub keV resolution
• New holder and connectors (Teflon and copper only)
Edelweiss-II Ge/NTD detectors
TAUP2007 X-F Navick September 2007
Incomplete charge collection
• Amorphous layers (Ge or Si) improve the charge collection
• At very low T and V : electric field is screened => diffusion phase
• Some carriers are trapped in the “wrong” electrode => incomplete collection
TAUP2007 X-F Navick September 2007
Effect of the amorphous layers
TAUP2007 X-F Navick September 2007
2003I
Edelweiss-I data with phonon trigger
TAUP2007 X-F Navick September 2007
E = 5.3 MeV, Q = 0.3
’s from 210Po (E=5.3 MeV)
• Q=0.3 decays near surfaces
• Rate ~ 400 /m²/d
• As expected, non-fiducial part more exposed
210Pb on Cu covers or Ge surfaces
Should see Pb recoils and ’s
No 206Pb recoil peak at 100 keV observed as heat-only
events: 210Pb implanted in Cu, not Ge.
Rate of 0.3 < Q < 1.0 events at low energy consistent
with expected surface ’s
does not exclude contribution from 14C
By removing Cu between detectors, these events
should disappear, or ID by coincidences
Surface contamination
TAUP2007 X-F Navick September 2007
Edelweiss-II first data taking
TAUP2007 X-F Navick September 2007
Edelweiss-II first data taking
TAUP2007 X-F Navick September 2007
100 keV recoils, no ionization5.3 MeV alpha
Initial effort : improve the cupper treatment (Rn contamination study)and reduce the surface exposure to radon
Passive rejection : improve the charge collection for surface event Physics of the Ge and Si amorphous underlayer Detectors with thick electrodes
Active rejection : identification of the surface events Interleaved electrodes
localization of the event Pulse shape analysis of the charge signals Detectors sensitive to athermal phonons Ge/NbSi detectors
Surface events suppression
TAUP2007 X-F Navick September 2007
Passive protection
• Amorphous layers: Al / aGe:H / cGe / aGe:H / Al => GGA
• Thick electrodes (high entrance window)
20mKE
---
+++
Al
aGe:H
GeAl
aGe:H
TAUP2007 X-F Navick September 2007
electrodesNTD Ge
thermometer
guard ring
Top view Cross-section
24 mm
Interleaved electrodes design
TAUP2007 X-F Navick September 2007
- 200 g Ge crystal- Hydrogenated a-Ge underlayer for improved charge collection- Annular aluminum electrodes, interconnected by ultrasonic bonding (strip width: 200 mm; pitch: 2 mm)- 7 measurement channels: 6 charge (7 MHz bandwidth) + heat (Ge NTD)
Mode of operation
TAUP2007 X-F Navick September 2007
Bulk events: Qa = Qc = 0 Qb = - Qd
Surface events (top surface): Qc = Qd = 0 Qa0 & Qb0
(bottom surface): Qa = Qb = 0 Qc0 & Qd0
* Voltage biases: Va = 1V, Vb = 2V, Vc = -1V, Vd = -2V, Vg = 0.5V, Vh = -0.5V
Type III: low-field area bulk
event
channel d (Vd) channel c (Vc)
guard g (Vg)
channel a (Va)
channel b (Vb)
near-surfaceevent
low-field area
guard h (Vh)
NTD Ge thermometer
Rejection of the surface event
TAUP2007 X-F Navick September 2007
Voltage biases: Va= -0.25V, Vb= 2V, Vc= 0.25V, Vd= - 2V, Vg= 0.5V, Vh= - 0.5V
Trigger threshold in ionization: 12 keV (e.e.) Baseline energy resolution of the ionization channels: 1 keV (e.e.) Heat channel resolution: 4 keV (FWHM) T = 17 mK
241Am source Cuts: |Qa| > 2 keV & |Qb + Qd| | > 2 keV (e.e.) event rejected
Ion
isat
ion
yie
ld
Erecoil(keV) Erecoil(keV)Io
nis
atio
n y
ield
before selection: 3596 evts after selection: 1134 evts
Events of incomplete charge collection
Reduction of the fiducial volume
TAUP2007 X-F Navick September 2007
241Am source + 252Cf neutron source
Erecoil(keV) Erecoil(keV)
Ion
isat
ion
yi e
ld
Ion
isat
ion
yi e
ld38 evts 32 evts
before selection: 3472 evts after selection: 1058 evts
Loss in fiducial volume: (38-32) / 38 ~ 15 %
Near-surface, 122 keV event:
centre electrode signal in redguard electrode signal in bluebest fit simulation in black
Experimental setup(-1 V collection voltage)
1000 ns
Event location
Due to electronic noise, surface event rejection by pulse-shape analysis is limited in practice to high-energy events only ( 50 keV e.e.)
Test at LSM on a 320g Edw-I detector
TAUP2007 X-F Navick September 2007
Localization by ionization shape analysis
Surface event discrimination: Nb-Si thin films as athermal phonon sensors developed in CSNSM in OrsaySensor ASensor A
Sensor BSensor B
Bulk eventSurfaceevent
Sig
nal
am
plit
ud
e (a
.u.) Thermal component
(energy-sensitive only) Athermal phonon(position-sensitive) component of signals
sensor A’s response
sensor B’s response
Responses of sensors A and B
Ge detectors with NbSi sensors
Time (s)
TAUP2007 X-F Navick September 2007
See Claudia Nones’s talk
Surface event discrimination: Nb-Si thin films as athermal phonon sensors developed in CSNSM in Orsay
Sensor BSensor B
Sensor ASensor ASensor ASensor A
(a) Ionization yield vs. recoil energy(thermal component of heat signals only):note the large population of surface events of poor charge collection
57Co source(b) With the proper cuts in the athermal component of phonon signals to remove surface events
Ioni
satio
n yi
eld
(a.u
.)
Recoil energy (keV)Recoil energy (keV)
200g Ge detector with NbSi sensors in the Edw-I set-up at LSM
TAUP2007 X-F Navick September 2007
Summary
TAUP2007 X-F Navick September 2007
The surface events appears to limit the sensitivity of ionization-heat germanium bolometers to WIMP.
Different techniques are under development in Edelweiss to reduce or reject these events.
The most promising ones lead to an active discrimination by athermal phonon measurement with NbSi thin film or by charge collection on interleaved electrodes.
We are starting to apply these techniques in LSM with prototype detectors.
In the next step of Edelweiss-II we will produce massive bolometers with active rejection.
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