The GENIUS dark matter project

Laura Baudis

Stanford University

The GENIUS collaboration

MPIK HeidelbergKurchatov Institute MoscowDubnaNijnii Novgorod...

Direct detection of WIMPs

elastic scattering off nuclei (Goodman, Witten 1985):

look for the energy deposited by the recoiling nucleus

0 ~ 0.3 - 0.5 GeV/cm3

v ~ 220 km/s, MB-distrib

good candidate from SUSY: LSPm = 10 - 1000 GeV< 10-41 cm2

event rate < 1 event/kg drecoil energy < 100 keV

dn/dEr

Er

Expected recoil spectrum

muon g-2 measurement

If due to SUSY:

upper bound on the neutralino mass: 500 GeV

minimum cross section is large: 10-9pb

CDMS I

CDMS II

Baltz-Gondolo 01

GENIUS

GErmanium in liquid NItrogen Setup

L. Baudis et al., NIMA 99Proposal: hep-ph/9910205

array of 40 natural HPGe detectors in Li N (100kg)

goal: reach the extremely lowbackground of 1 ev/100 kg y keVbelow 100 keV

test a large part of MSSM parameter space for neutralinos

GENIUS: detection technique

ionization in p-type HPGe detectorsno recoil discriminationsingle/multiple events give somebackground reduction

Main background reduction:

by operating naked Ge crystals directly in liquid nitrogen

Naked Ge detectors

feel well in liquid nitrogen

contacts established usingonly 3 g of material(stainless steel to fix innerand outer contacts)

here: test detector (425 g)held on kevlar wires

Performance in LiN

no electrical interferenceno microphonicsup to 6m cables betweencrystals and FETs

resolution:1 keV @ 300 keV

threshold:2.5 keV

133Babackground

Possible location: Gran Sasso

Possible layout

study in collab. with Messer-Griesheim, Germany

Design of the holder system

1,2 or moreconcentric layersof Ge crystals

holder made ofhigh molecular PE

Background expectations

Monte Carlo simulations based on GEANT3.21 + GCALOR

Geometry: 12 x 12 m tank, 2 m PE foam, 2 mm steel walls, holder system of HD1000, 40 Ge detectors, muon veto on top of the tank

External background: photon and neutron flux from rock muon interactions + induced activities

Internal background: intrins. contam. of LiN, steel vessel, holder system, Ge-crystals activation of Ge and LiN at sea level

External background

simulation of measured and n-flux in GS

Component Events/100 kg y keV

(<100 keV)

gammas 0.4

neutrons 0.04

muon showers 0.2

muon -> n 0.1

muon capture << 0.01

goal 1

12 m tank

2m borated PE foam

muon veto: 96 % eff.

Internal background

Source Radionuclide Purity Count rate/

100 kg yr keV

Liquid

nitrogen

238U232Th40K222Rn

3.5 x 10-16 g/g

4.4 x 10-16 g/g

1.0 x 10-15 g/g

3 Bq/m3

0.07

0.04

0.01

0.03

Holder

system 238U/232Th 1.0 x 10-12 g/g 0.0015

Steel

vessel 238U/232Th 5.0 x 10-9 g/g 0.08

GOAL 1

Cosmogenics in Germanium

assumptions: 10 days @ sea level 68Ge: saturration 3 yr below ground activity

3H, 49V, 54Mn, 55Fe, 57Co, 60Co, 65Zn, 68Ge

3H Q = 18.6 keV T1/2 = 12.35 yr

68Ge EC E(Ka) = 10.37 keVQEC(68Ga) = 2921 keV

T1/2= 270 d

3H, 68Ge

assumptions: 10 days @ sea level 68Ge: saturration 3 yr below ground activity

3H 3.6 Bq/kg

~113 decays/yr

200 ev/100 kg yr keV (12-19 keV)

68Ge EC 28 Bq/kg 1.1 ev/100 kg yr keV

solutions?3H: shielded production 1m heavy concrete ~ 5 mwe: factor 30 1 d transportation: factor 10 better: produce detectors in undeground lab!68Ge: decays after another 2 yr, subtract 10.37 keV line

start early with detector production, store underground

Cosmogenics in LiN

Isotope T1/2 Energy

[keV]

Activity

[Bq/g]

Rate/

100kg yr keV3H 12.3 y 18.6 (-) 3.8 x 10-8 negligible

7Be 53.3 d 477.6 () 3.7 x 10-9 0.8

10Be 1.6 x 106y 555 (-) 8 x 10-15 negligible

14C 5.7 x 104y 156 (-) 1.4 x 10-4 0.01

Goal 1

10 d transportation @ sea level

Other background sources

radionuclide production in Ge/LiN by induced sec. n:

14N(n,x)aB, 70,72,73,74,76Ge(n,x)aB

negativ muon capture in LiN -> rates and decays ofproduced isotopes:

- + (7,14) -> (6, 14)* +

(6,14)* -> (Z‘,A‘) + (1-4)n,,p

inelastic muon scattering:

+ N -> ‘ + X*

contributions are notrelevant

Expectations for GENIUS

GENIUS

CDMS Soudan

CRESST

HDMS

CDMS 2000DAMA 3

V. Mandic, R. Gaitskell

with 100 kg Ge:

detect WIMP signaturevia annual modulation

current status?

GENIUS Test Facilityis approved and being installed @ GS

Genius Test Facility

borated PE 14 natural (40 kg) HPGein a volume of 0.064 m3

of ultra-pure LiN

0.5 mm steel vessel 0.9 x 0.9 x 0.9 m polystyrene 5 cm inner shield of HPGe 10 cm low level Cu30 cm of Pb15 cm borated PE

L. Baudis et al., hep-ex/0012022

2 m x 2m

Genius-TF goals

background: 2 events/ kg y keV: 0 - 50 keV:factor of 10 better than Heidelberg-Moscow

test material purities down to this leveldesign of crystal support system new DAQ and electronicsreliability of calculation of cosmogenic productionlong-term stability of naked HPGe in LiN

Background estimations

MC simulations:

photon, neutron, muon flux in GS

intrinsic radioactivity of:liquid nitrogensteel vesselpolystyrene foamGe, Cu shieldscrystal holder systemGe crystals+ cosmogenic activation of Ge and Cu

Assumptions for material purities

Ge crystals upper limit HM

LiN 1000 x Borexino

Rn in LiN 325 Bq/m3 (Heusser 2000)

Steel Hd (Heusser 2000)

Ge bricks 100 x limit HM

Polystyrene first measurements in HD

Copper HM (cosmo + anthro)

Holder system SNO meas. for acryl

Cosmogenics in Ge30 days production + transportation @ sea level

1 year de-activation below ground

Sumspectrum

Genius-TF: expectation

based on:

2 events/kg y keV

1 yr exposure

2 keV/11 keV threshold

Genius-TF & WIMP signature

Eth= 2 keV (11 keV)B = 4 events/kg y keV M = 40 kg (nat) Ge

0.4 - 1.4 yr (1.3 - 5 yr) exposure required to see the DAMA signal with 50% - 90% probability and 99.5 % CL (S. Cebrian et al., Astrop. Phys. 14 (2001), hep-ph/9912394)

but: new detectors -> Eth= 0.5 keV!will significantly improve the sensitivity to the annualmodulation signature

good chance to see an annual modulation

Status of Genius-TF

Infrastructure built at Gran Sasso

5 detectors are ready (crystals + signal/HV contacts)

Cold hardware is being built in Heidelberg

New electronics: MPIK, Heidelberg

Setup to be installed in GS during the next months

First results expected by end of 2002

Conclusions for GENIUS

technology: simple, well understoodproduction of 40 HPGe detectors: straightforwardLiN purification method proved by BOREXINO

1 event/100 kg yr keV can be reached if:

tank diameter: 12 mproduction of cosmogenics in Ge minimizedsurface contaminations under control

cover large part of MW- parameter spacedetect a WIMP signature

backup slides

Muon induced neutrons

Negative muon capture