The EDELWEISS-II experiment

Silvia SCORZAUniversité Claude Bernard- Institut de Physique nucléaire de Lyon

CEA-Saclay DAPNIA/DRECAM (FRANCE), CNRS/CRTBT Grenoble (FRANCE),CNRS/IN2P3/CSNSM Orsay (FRANCE), CNRS/IN2P3/IPN Lyon (FRANCE), CNRS/INSU/IAP Paris (FRANCE), CNRS-CEA/Laboratoire Souterrain de Modane (FRANCE), JINR Dubnia (RUSSIA), FZK/Universtat Karlsruhe (GERMANY)

Outline

Limits of EDELWEISS-I

EDELWEISS-II setup Current situation (3 detector types)

Outlook

Direct Search Principle

Detection of the energy deposited due to elastic scattering off target nuclei

• Event Rate :

< 1 ev /kg/week

• Recoil Energy :

1 – 100 keV

• Low energy threshold

◊ Large detector mass

• Low background

Radio – purity

Active/passive shielding

Deep underground sites

EDELWEISS @ Laboratoire Souterrain de Modane

Ge Heat-Ionization Detectors

Simultaneous measurements: Ionization @ few V/cm with Al

electrodes Heat @ 17 mK with Ge/NTD

sensor Different Ionization/Heat

energy ratio for nuclear and electronic recoils (dominate bkg)

Event by event background rejection

=> discrimination /n > 99.9% for Er>15keV

Edw-I limiting backgroundPRD71, 122002 (2005)

Background in the physics data taking

Leakage from the to the recoil band Neutrons: - 1 n-n coincidence observed

- Neutron shield not thick enough

Surface electron recoils:

- Bad charge collection (trapping and recombination)

- Rate compatible with 210Pb contamination:

rate “e-” 5/kg.day

E=5.3MeV Q=0.3

206Pb

210Pb 210Po

Best Sensitivity up to 2003, but…

EDW – II set-up radiopurity

dedicated HPGe detectors for systematic checks of all materials

clean room (class 100 around the cryostat, class 10000 for the full shielding)

deradonized air (from NEMO-3) neutron shielding

20 cm Pb shielding 50 cm PE and better coverage

active veto (>98% coverage)

up to 110 detectors Ge/NTD + Ge/NbSi + new ID

electrodes

expected sensitivity: EDW-I × 100 -n 10-8 pb (phase 100) = 0.002 evts/kg/day (Er>10keV)

23*320g Ge/NTD : Developed by CEA Saclay and Canberra-Eurisys Amorphous Ge and Si sublayer: better charge collection for surface evts Optimized NTD size and homogeneous working T (16-18 mK) : goal keV resolution New holder and connectors (Teflon and copper only)

All @ LSM

Ge-NTD detectors

EDW-I rate: ~ 700 evts/kg/day for E>100keVAfter the most recent improvement EDW-II rate: ~ 300 evts/kg/day

EDW-I rate on central electrode : 2.5 - 5.5 evts/kg/day (400-

1000 /m2/day)EDW-II rate

~ 3 evts/kg/day

first low bg runs in 2006:

old bg

new bg

7*400g Ge/NbSi detectors :• developed by CSNSM Orsay• NbSi thin film thermometers for active surface evt rejection

2 @ LSM

Ge/NbSi detectorsidentification of surface events with Ge/NbSi detector

data taken with 1 NbSi detector May&June 2007: 1.5kg.d (fiducial)

200

150

100

50

0

806040200

x10-3

200

150

100

50

0

806040200

x10-3

Transient

Thermal

Heat signal of thermometer NbSiA :Surface eventBulk event

Thermometer Thermometer NbSiNbSiBB

Thermometer Thermometer NbSi NbSi AA Thermometer Thermometer NbSi NbSi AA

Thermometer Thermometer NbSiNbSiBB

Time (ms)

200

150

100

50

0

806040200

x10-3

200

150

100

50

0

806040200

x10-3

Transient

Thermal

Heat signal of thermometer NbSiA :Surface eventBulk event

Thermometer Thermometer NbSiNbSiBB

Thermometer Thermometer NbSi NbSi AA Thermometer Thermometer NbSi NbSi AA

Thermometer Thermometer NbSiNbSiBB

200

150

100

50

0

806040200

x10-3

200

150

100

50

0

806040200

x10-3

Transient

Thermal

Heat signal of thermometer NbSiA :Surface eventBulk event

Thermometer Thermometer NbSiNbSiBB

Thermometer Thermometer NbSi NbSi AA Thermometer Thermometer NbSi NbSi AA

Thermometer Thermometer NbSiNbSiBB

Time (ms)Rejection works, resolutions needs tuning

ID detector

Radius (cm)

Z (

cm

)

guard ‘g’: + 1V

guard ‘h’: - 1V

‘a’ electrodes: +2V ‘b’ electrodes:

+1V

‘c’ electrodes:-2V‘d’ electrodes: -1V

Electron trajectories

holetrajectories

‘a&c’ bulk event

‘a&b’ near- surface event

‘a,b&c’ event in low-field area

200g detector, concentric electrodes

2mm gaps, 200µm width

trigger thresh. 24keV241Am source with 60keV ´s

all eventsafter cut “b” < 5keV(30% of evts. survive)

recoil energy [keV] recoil energy [keV]

lab data

preliminary!

status ID bolometer

~1.7 kg.d

summary:first detector (200g) withinterleaved electrodes in LSM performance as expected

New 400g ID´s to be installedend of March

EDELWEISS-II next steps

Goal 2x10-8 pb (1200 kg.d no event) (~1 evt/kg/y)

acquire physics data with 23 NTD build 6 400 g NbSi

Goal 2x10-10 pb (~10 evt/t/y)

build 16 NbSi (same/tuned if needed) build 24 NTD with reduced contamination and/or with ID electrodes