Underground Laboratories and Low Background Experiments

1. Underground Laboratories and Low Background Experiments Pia Loaiza Laboratoire Souterrain de Modane Bordeaux, March 16th, 2006

2. Outline • 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

3. 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.

4. The cosmological model after the year 2000

5. 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

6. Dark matter candidate : WIMPs • Under certain SUSY models, the neutralino is the LSP: • stable • 10-1000 GeV • weakly interacting • no electric interaction. 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

7. Background sources Background sources How to reduce it • Experimental site : • - Cosmic radiation • - Environmental radioactivity Go deep Shielding • Experimental set-up • - Contamination in shielding • and detector components Material selection

8. Background sources Cosmic radiation Hadrons Easily stopped with few m.w.e

9. Modane Background sources Cosmic radiation Muons Stopped with some km.w.e, high energy component can be vetoed

10. Background sources Cosmic radiation Muon-induced neutrons • In the shielding materials • Can be tagged by a muon veto • In the rock • Highly energetic, up to GeV energies • Need deep underground sites

11. Boulby Mine Deep underground laboratories in ILIAS Canfranc Modane Modane Canfranc Gran Sasso

12. Laboratoire Souterrain de Modane Characteristics: Muon flux : 4  / m2/day Neutron flux : 1.6 10-6 cm2/s Radon concentration : 5 to 15 Bq/m3 Edelweiss: direct dark matter search Nemo : double beta decay

13. Background sources Environmental radioactivity • Gammas from U, Th chains, 40K 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

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

15. 15 mBq/m3 Background sources Environmental radioactivity Radon, 222Rn Radon purification facility

16. Background sources Contamination in shielding and detector components 238U decay chain : 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) Gamma emitters Mass spectrometry

17. Background sources Contamination in shielding and detector components 232Th decay chain : 228 Gamma emitters Need material selection Detector components : PMTs Electronic components Cabling Detector housing Target container Handling: 40K(dust) Radiopurity Database

18.  and e- Echarge WIMPs and neutrons Ephonons Background discrimination Photons and electrons scatter from electrons WIMPs and neutrons scatter from nuclei • Measure: • Ionization • - Heat

19. Towards a background free experiment 10-10 pb Goal of next generation experiment: 10-10 pb for WIMP-nucleon cross section Almost background free for 1 ton/year