Surface events suppression in the germanium bolometers EDELWEISS experiment

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


• 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


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


Effect of the amorphous layers


2003I

Edelweiss-I data with phonon trigger


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


Edelweiss-II first data taking


Edelweiss-II first data taking


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


Passive protection

• Amorphous layers: Al / aGe:H / cGe / aGe:H / Al => GGA

• Thick electrodes (high entrance window)

20mKE

---

+++

Al

aGe:H

GeAl

aGe:H


electrodesNTD Ge

thermometer

guard ring

Top view Cross-section

24 mm

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)

Mode of operation


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


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


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


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)


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


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.

Surface events suppression in the germanium bolometers EDELWEISS experiment

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