the edelweiss-ii experiment

The EDELWEISS-II experiment

Silvia SCORZA Université 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 Dubna (RUSSIA), FZK/Universtat Karlsruhe (GERMANY)

2

Direct Search PrincipleDetection 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

3

EDELWEISS @ LSM

4

EDW–II set-up Radiopurity

dedicated HPGe detectors for systematic checks of all materials

Strict control of bkg: material selection/cleaning procedure/Environment

Shielding 20 cm Pb shielding 50 cm PE and better coverage active μ veto (> 98% coverage)

Up to 110 detectors -> 40kg Ge detector

Ge/NTD Develop new ID detectors

Goal: EDW-I × 100 σW-n ≤ 10-8 pb<0.003 evts/kg/day (Er>10keV)

5

µ-veto

Candidates for coincident veto-bolometer events

Accidental coincidences

Expected from Geant4 simulation:

~0.03 events/kg.dMeasured:

0.04 events/kg.d

Pb

Polyethylene

Veto

roc

k

p,π,α

´

n Cu(Cryostat)

Ge

<300GeV>

nVeto

Pb

Gd-loaded scintillator

6

Ge Heat-Ionization Detectors

• Simultaneous measurements:• Ionization @ few V/cm with

Al electrodes• Heat @ 20 mK with Ge/NTD

sensor

• Different Ionization/Heat energy ratio for nuclear and electronic recoils (dominate bkg)

7

Event by event background rejection

8

Edw-I limiting backgroundPRD71, 122002 (2005)

Problem:Surface electron recoils

Interpretation: Bad charge collection

(trapping and recombination) Indications of 210Pb

contamination (exposition to Radon):

α rate ~ e rate ~ 4 /kg.day

9

206Pbβ

210Pb

210Po

210Po

210Pb

210Pb source

10

GeNTD data: improved bkg Gamma background:

reduction of x3 relative to EDELWEISS-I

210Pb-chain background: reduction of x2 relative to EDELWEISS-I

EDW-I~4 /kgd

EDW-II~2 /kgd

5 kgd

95 kgd

preliminary

Full volume

Further bkg reductions after fiducial+ coincidence cuts

Conclusion: Reduction of background from EDW-I to EDW-II

11

Physics run: GeNTD• 11 detectors with <30 keV

threshold• Threshold chosen before

start of run(EDW-I results expected

bkg)• 93.5 kg.day• 3 events observed in nuclear

recoil band• 31, 31 and 42 keV

• Evidence for events with deficient charge collection from 210Pb

Preliminary

EDELWEISS93.5 kgd

Question:How to reach < 10-8 pb ?

NEED >1000 kgd at 15 keV threshold

>105 rejection for gammas to reject expected >4000 from 210Pb

IDea:Develop detectors with surface event rejection

using interleaved electrode design (ID)

12

A A A A AB B B B

C C C C CD D D D

G

H

GuardFiducial volume

→E

→E

→E

→E

‘A’ electrodes: +2V

ID detector

Radius (cm)

Z (c

m)

guard ‘G’: + 1V

guard ‘H’: - 1V

‘C’ electrodes:-2V ‘D’ electrodes: -1V

Electron trajectoriesholetrajectories

‘A&C’ bulk event

‘A&B’ near- surface event

‘A,B&C’ event in low-field area

NTD heat sensor• E-field modified near surface with interleaved electrodes• B+D signals -> vetos % surface• 1x200g + 3x400g tested in2008• 10x400g running‘B’ electrodes:

+1V

13

ID detector rejection Gamma rejection of 400g

~1 month calibrations Beta rejection of 200g

-equivalentto 3x104 kgd

6x104

6x104210Bi

6x104210Po

0 events

-equivalentto ~103 kgd

14

Physics run: ID detectors 1 x 400g + 1 x 350g

detectors, 86 live days

<15 keV threshold achieved for exposure of 18.6 kg.days

50% efficiency at 10 keV

No events in (or around) nuclear

recoil band

Conclusion: This is the good technology for 10-8 pb and beyond

15

Limits 93.5 kgd GeNTD

• 11 detectors x 4 months• 30 keV threshold• 3 events observed in

nuclear recoil band 18.3 kgd ID

• 2 detectors x 4 months• 15 keV threshold• No nuclear recoils• No evts outside band

2009: 10 ID detectors• improvement in

sensitivity: 4x10-8 pb• More detectors build in

2009

Preliminary

16

Conclusions/Outlook• Significant reduction in , β and γ backgrounds relative to

EDELWEISS-I• Improved understanding of backgrounds and of response of

detectors to backgrounds• Improved limit relative to EDELWEISS-I• Passive background reduction alone not sufficient to reach

< 10-8 pb• ID detectors have the surface rejection needed to reach this

goal Running in 2009 with 10 x 400g detectors Prototype of ID detectors with larger fiducial volumes currently in test EURECA = 1 ton scale experiment (CRESST, EDELWEISS, CERN, …) @ LSM extension

This is the end…

1818

Decay Chain

210Po

206Pb

5.3 MeV100 keV

Al 70nm

amGe 70nm

Ge 2cm

206Pb

e-

10keV - 100keV - 1MeV

Cu few mm

5 mm

~50 nm

350 nm

20 m

700 m

210Bie- 1.16 MeV max

210Pbe- 61 keV max

46.5 keV (4%)

46 keV

3 mm

NO ionisation

22 yr

1919

EDELWEISS-II 210Pb source calibration

Confirms interpretation of EDW-I bkg as 210Pb surface . Response of detectors to this important background

210Pb

206Pb

β 210Po

E=5.3 MeVQ~0.3

Edw-Idata

EDELWEISS-II210Pb source

210Pb

210Po

206Pb recoilscoincident with 210Po

210Bi

210Bi

Edw-II

Edw-II

Edw-II

Implantation depth of 210Pb

2020

EURECA - II

2121

EURECA-I

2222

2323

France-Italy Fréjus tunnel :new safety gallery planned

Existing road tunnel

Existing lab

s ?

2 projectsLimited extensionVery large cavity

2424G Gerbier EURECA ULISSE meeting- Lyon july 2008

24

Implantation of new lab

First drawings nov 2006 (Lombardi eng company)

2525

Compatibility DAMA other experiments SI

2626

Compatibility DAMA other experiments SD

2727

Present limits

2828

Number of events in WIMP region: q< 0.5, 25 < E < 60 кэВregistered23 events

Experimental data

Alpha rate 2.5 events

2929

N alphas in calibration spectrum = 3040 eventsNumber of events in WIMP region: q< 0.5, 25 < E < 60 кэВregistered = 460 events

460 events

Expected number events from calibration with 210Pb =37 events

Estimation of background from 210Pb