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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.
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
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
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
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