Surface events suppression in the germanium bolometers EDELWEISS experiment

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Surface events suppression in the germanium
bolometers EDELWEISS experiment

• Xavier-François Navick (CEA Dapnia)
• TAUP2007 - Sendai September 07

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Ionization-heat detectors
T 20 mK
Thermal Sensor (NTD Ge)
Al electrode
Biais amplifier
WIMPneutronX or ?? rays
V 1-6 Volts
D 2 cm
Charge amplifier
Al electrodes (charge collection )
HPGe single crystal
TAUP2007 X-F Navick September 2007
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Rejection power
? Simultaneous measurement of ionization and heat
gt Evt per evt identification ? Q Eionisation
/ Erecul ? Q 1 for electronic recoil (ambiant
radioactivity) ? Q ? 0.3 for nuclear recoil
(WIMP and neutron)? discrimination g/n gt 99.9
pour Ergt 15keV
TAUP2007 X-F Navick September 2007
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Edelweiss-II Ge/NTD detectors

• 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)

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Incomplete charge collection

• Amorphous layers (Ge or Si) improve the charge
  collection
• At very low T and V electric field is screened
  gt diffusion phase
• Some carriers are trapped in the wrong
  electrode gt incomplete collection

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Effect of the amorphous layers
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Edelweiss-I data with phonon trigger
2003
I
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Surface contamination
E 5.3 MeV, Q 0.3

• as from 210Po (Ea5.3 MeV)
• Q0.3 ? a 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 bs

• No 206Pb recoil peak at 100 keV observed as
  heat-only events 210Pb implanted in Cu, not Ge.
• Rate of 0.3 lt Q lt 1.0 events at low energy
  consistent with expected surface bs
• does not exclude contribution from 14C
• By removing Cu between detectors, these events
  should disappear, or ID by coincidences

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Edelweiss-II first data taking
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Edelweiss-II first data taking
100 keV recoils, no ionization
5.3 MeV alpha
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Surface events suppression

• ? 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

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Passive protection

• Amorphous layers Al / aGeH / cGe / aGeH / Al
  gt GGA

20mK
---
E

• Thick electrodes (high entrance window)

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Interleaved electrodes design

• 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)

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Mode of operation
channel b (Vb)
channel a (Va)

Bulk events Qa Qc 0 Qb - Qd Surface
events (top surface) Qc Qd 0 Qa?0 Qb?0
(bottom surface) Qa Qb 0 Qc?0 Qd?0
guard g (Vg)
near- surface event
low-field area
Type III low-field area
bulk event
NTD Ge thermometer
guard h (Vh)
channel c (Vc)
channel d (Vd)
Voltage biases Va 1V, Vb 2V, Vc -1V,
Vd -2V, Vg 0.5V, Vh -0.5V
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Rejection of the surface event
241Am g source Cuts Qa gt 2 keV Qb Qd
gt 2 keV (e.e.) ? event rejected
before selection 3596 evts
after selection 1134 evts
Ionisation yield
Ionisation yield
Events of incomplete charge collection
Erecoil(keV)
Erecoil(keV)
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
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Reduction of the fiducial volume
241Am g source 252Cf neutron source
before selection 3472 evts
after selection 1058 evts
Ionisation yield
Ionisation yield
38 evts
32 evts
Erecoil(keV)
Erecoil(keV)
Loss in fiducial volume (38-32) / 38 15
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Localization by ionization shape analysis
Test at LSM on a 320g Edw-I detector

Experimental setup (-1 V collection voltage)
Event location
Due to electronic noise, surface event rejection
by pulse-shape analysis is limited in practice
to high-energy events only ( gt? 50 keV e.e.)
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Ge detectors with NbSi sensors
Surface event discrimination Nb-Si thin films as
athermal phonon sensors developed in CSNSM in
Orsay
Surface event discrimination Nb-Si thin films as
athermal phonon sensors developed in CSNSM in
Orsay
See Claudia Noness talk
Surface event
Bulk event
Thermal component (energy-sensitive only)
Athermal phonon (position-sensitive) component
of signals
Signal amplitude (a.u.)
Time (s)
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200g Ge detector with NbSi sensors in the Edw-I
set-up at LSM
57Co g source
(a) Ionization yield vs. recoil energy (thermal
component of heat signals only) note the large
population of surface events of poor charge
collection
(b) With the proper cuts in the athermal
component of phonon signals to remove surface
events
Ionisation yield (a.u.)
Recoil energy (keV)
Recoil energy (keV)
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Summary
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.
TAUP2007 X-F Navick September 2007