Underground Laboratories and Low Background Experiments Pia Loaiza Laboratoire Souterrain de Modane Bordeaux, March 16 th, 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 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.

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 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 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 deep Shielding 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 Characteristics: Muon flux : 4  / m 2 /day Neutron flux : cm 2 /s Radon concentration : 5 to 15 Bq/m 3 Edelweiss: direct dark matter search Nemo : double beta decay

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

210 Pb deposition on surface Background sources Environmental radioactivity Radon, 222 Rn

Background sources Environmental radioactivity Radon, 222 Rn Radon purification facility 15 mBq/m 3

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)

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

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

Towards a background free experiment Goal of next generation experiment: pb for WIMP-nucleon cross section pb Almost background free for 1 ton/year