Underground Laboratories and

Low Background Experiments

Pia LoaizaLaboratoire Souterrain de Modane

Bordeaux, March 16th, 2006

Outline

1) Introduction to dark matter, as an example of rare event searches

2) Background sources - Cosmic rays Underground Labs - Environmental radioactivity - Contamination in shielding and detector components

Evidence for dark matter

NGC 6503150

100

50

0 0 10 20 30Radius (kpc)

Vel

ocit

y (k

m/s

)

gas

halo

disk

observationsNGC 6503150

100

50

0 0 10 20 30Radius (kpc)

Vel

ocit

y (k

m/s

)

gas

halo

disk

observations

• From rotational velocity of galaxies :

• From clusters and superclusters

• From numerical simulations of LSS

From Big Bang nucleosynthesis, the amount of baryons representsonly 4% of the total matter/energy.

The cosmological model after the year 2000

Dark matter candidates

Nature of dark matter- Non-baryonic- Cold

Favoured scenario: dark matter is made of WIMPs (Weakly Interacting Massive Particle)

WIMPs, a generic new type of unknown subatomic particles

SUSY WIMPs :• Not invented to solve the dark matter problem• Relic from the Big Bang

Dark matter candidate : WIMPs

Present limits: < 1 event/kg per week (above 10 keVr in Ge)

Sensitivity for next generation :~ 1 event/100 kg per year

NEED EXTREMELY LOW BACKGROUNDS

Under certain SUSY models, the neutralino is the LSP:- stable- 10-1000 GeV- weakly interacting- no electric interaction.

Background sources

• Experimental site : - Cosmic radiation - Environmental radioactivity

• Experimental set-up - Contamination in shielding and detector components

Background sources How to reduce it

Go deepShielding

Material selection

Background sources

Cosmic radiation

Hadrons

Easily stopped with few m.w.e

Background sources

Muons

Cosmic radiation

Stopped with some km.w.e, high energy component can be vetoed

Modane

Background sources

Cosmic radiation

• In the shielding materials Can be tagged by a muon veto

• In the rock Highly energetic, up to GeV energies Need deep underground sites

Muon-induced neutrons

Deep underground laboratories in ILIAS

Modane

Canfranc

Laboratoire Souterrain de Modane

FRANCE ITALIE

AltitudesDistances

1228 m 1298 m1263 m0 m 6210 m 12 868 m

Characteristics:

Muon flux : 4 / m2/dayNeutron flux : 1.6 10-6 cm2/sRadon concentration : 5 to 15 Bq/m3

Edelweiss: direct dark matter searchNemo : double beta decay

Background sourcesEnvironmental radioactivity

• Gammas from U, Th chains, 40K in the rock

Shielding with high Z material

• NeutronsFrom spontaneus fission and (,n) reactions

Can be moderated with low Z materials.

Archeological lead from an ancientboat (400 a.J.C) found in the coast of Bretagne, used in the Edelweissshielding

210Pb deposition on surface

Background sources

Environmental radioactivity

Radon, 222Rn

Background sources

Environmental radioactivity

Radon, 222Rn

Radon purification facility

15 mBq/m3

Contamination in shielding and detector components

Background sources

238U decay chain :

Gamma emitters

Mass spectrometry

Need material selection

Typical shielding materials:Pb, Cu, PE, Steel, Al, water

Typical dangerous backgrounds:238U (in Pb, Al)210Pb (in Pb)232Th (in Al)60Co(steel, Cu)

Cosmogenic activation: 60Co, 57Co, 56Ni (in Cu)

Background sources

Contamination in shielding and detector components

Need material selection

Detector components : PMTsElectronic componentsCablingDetector housingTarget container

Handling:40K(dust)

232Th decay chain :

Gamma emitters

228

Radiopurity Database

Background discrimination

Photons and electrons scatter from electronsWIMPs and neutrons scatter from nuclei

and

e-

WIMPs and neutrons

Ephonons

Ech

arge

Measure:- Ionization- Heat

Exposure (kgd)

Sen

siti

vity

(p

b)

10-10

10-9

10-8

10-7

10-6

10 103102 104 105 106

Exposure (kgd)

Sen

siti

vity

(p

b)

10-10

10-9

10-8

10-7

10-6

10 103102 104 105 106

1 evts/kg/d1 evts/kg/d

Towards a background free experiment

Goal of next generation experiment: 10-10 pb for WIMP-nucleon cross section

10-10 pb

1 evts/t/week1 evts/t/week

5 evts/t/year5 evts/t/year

Almost background free for 1 ton/year

… BG subtraction… BG subtraction