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Underground Laboratories and Low Background Experiments Pia Loaiza Laboratoire Souterrain de Modane Bordeaux, March 16 th, 2006
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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
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Evidence for dark matter From rotational velocity of galaxies : From clusters and superclusters From numerical simulations of LSS From Big Bang nucleosynthesis, the amount of baryons represents only 4% of the total matter/energy.
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The cosmological model after the year 2000
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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
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Dark matter candidate : WIMPs Present limits: < 1 event/kg per week (above 10 keV r 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.
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Background sources Experimental site : - Cosmic radiation - Environmental radioactivity Experimental set-up - Contamination in shielding and detector components Background sources How to reduce it Go deep Shielding Material selection
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Background sources Cosmic radiation Hadrons Easily stopped with few m.w.e
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Background sources Muons Cosmic radiation Stopped with some km.w.e, high energy component can be vetoed Modane
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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
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Deep underground laboratories in ILIAS Modane Canfranc
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Laboratoire Souterrain de Modane Characteristics: Muon flux : 4 / m 2 /day Neutron flux : 1.6 10 -6 cm 2 /s Radon concentration : 5 to 15 Bq/m 3 Edelweiss: direct dark matter search Nemo : double beta decay
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Background sources Environmental radioactivity Gammas from U, Th chains, 40 K in the rock Shielding with high Z material Neutrons From spontaneus fission and ( ,n) reactions Can be moderated with low Z materials. Archeological lead from an ancient boat (400 a.J.C) found in the coast of Bretagne, used in the Edelweiss shielding
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210 Pb deposition on surface Background sources Environmental radioactivity Radon, 222 Rn
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Background sources Environmental radioactivity Radon, 222 Rn Radon purification facility 15 mBq/m 3
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Contamination in shielding and detector components Background sources 238 U decay chain : Gamma emitters Mass spectrometry Need material selection Typical shielding materials: Pb, Cu, PE, Steel, Al, water Typical dangerous backgrounds: 238 U (in Pb, Al) 210 Pb (in Pb) 232 Th (in Al) 60 Co(steel, Cu) Cosmogenic activation: 60 Co, 57 Co, 56 Ni (in Cu)
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Background sources Contamination in shielding and detector components Need material selection Detector components : PMTs Electronic components Cabling Detector housing Target container Handling: 40 K(dust) 232 Th decay chain : Gamma emitters 228 Radiopurity Database
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Background discrimination Photons and electrons scatter from electrons WIMPs and neutrons scatter from nuclei and e- WIMPs and neutrons E phonons E charge Measure: - Ionization - Heat
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Towards a background free experiment Goal of next generation experiment: 10 -10 pb for WIMP-nucleon cross section 10 -10 pb Almost background free for 1 ton/year
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