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The first year of Borexino data

The first year of Borexino data. LIP July 9, 2008 – Lisbon. Davide Franco Milano University & INFN. Outline. The role of neutrinos in particle physics and astro-particle The physics of Borexino The Borexino detector The “radio-purity” challenge The reached goals ( 7 Be and m n )

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The first year of Borexino data

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  1. The first year of Borexino data LIP July 9, 2008 – Lisbon Davide Franco Milano University & INFN

  2. Outline • The role of neutrinos in particle physics and astro-particle • The physics of Borexino • The Borexino detector • The “radio-purity” challenge • The reached goals (7Be and mn) • Near and far future goals Davide Franco – Università di Milano & INFN

  3. Neutrinos: cosmic messengers High energy protons 5·10Mpc Astrophysical source neutrinos High energy gammas 10 Mpc Low energy protons deflected • Ideal messenger: • neutral • stable • weakly interacting Davide Franco – Università di Milano & INFN

  4. Messengers from the Sun’s core • Core (0-0.25 Rs) • Nuclear reactions: T~1.5 107 °K • energy chains pp e CNO (neutrino production) • Radiative region (0.25-0.75 Rs) • Photons carry energy in ~ 105 y • Convective region (0.75-1 Rs) • Strong convection and turbulence • Complex surface phenomena • Corona (> 1 Rs) • Complex magneto-hydrodynamic phenomena • Gas at T~ 106 °K Davide Franco – Università di Milano & INFN

  5. The Standard Solar Model • It is an evolution model f the Sun, starting from the origin up to now: • Includes: • Hydrostatic equilibrium equations • Energetic and mass balance • Nuclear Reactions • Energy transport • Depends on several parameters and/or assumptions: • Initial chemical composition • Nuclear cross sections • Magnetic field, rotation • The model predicts the actual temperature and hence neutrino production • Main uncertainties due to chemical compound and nuclear cross sections Davide Franco – Università di Milano & INFN

  6. Solar nuclear reactions • pp chain • Fusion of 4 protons in a 4He nucleus with emission of 27 MeV energy • Main chain for “small-medium size” stars, like the Sun • CNO cycle • Carbon-Nitrogen-Oxygen or CNO cycle converts hydrogen to helium • It is dominant in population I “hot” stars • In the Sun, the CNO cycle contributes to 1-2 %, but there is no any experimental evidence Davide Franco – Università di Milano & INFN

  7. Neutrino Production In The Sun pp chain:pp, pep, 7Be, and 8Bn CNO cycle:13N, 15O, and 17Fn Davide Franco – Università di Milano & INFN

  8. Solar Neutrinos Davide Franco – Università di Milano & INFN

  9. Gallex GNO Homestake Sage SNO SuperK (real time) Borexino (real time) Solar Neutrino Spectra Davide Franco – Università di Milano & INFN

  10. The Standard Solar Model before 2004 One fundamental input of the Standard Solar Model is the metallicity of the Sun - abundance of all elements above Helium: The Standard Solar Model, based on the old metallicity derived by Grevesse and Sauval (Space Sci. Rev. 85, 161 (1998)), was in agreement within 0.5 in % with the solar sound speed measured by helioseismology. Davide Franco – Università di Milano & INFN

  11. The Sun Structure • The study of the Sun structure is supported by the “helium-seismology” • Pressure wave propagation in the Sun • Sun’s vibration modes depend on the internal structure • The Standard Solar Model can be tested by looking at the p-mode (acoustic waves), g-mode (density or gravity waves) and f-mode (surface density waves) Reconstructed speed of sound Surface waves Reconstruction Davide Franco – Università di Milano & INFN

  12. The Standard Solar Model after 2004 Latest work by Asplund, Grevesse and Sauval (Nucl. Phys. A 777, 1 (2006)) indicates a metallicity lower by a factor ~2. This result destroys the agreement with helioseismology Solar neutrino measurements can solve the problem! Davide Franco – Università di Milano & INFN

  13. Borexino goals: solar physics • First ever observations of sub-MeV neutrinos in real time • Check the balance between photon luminosity and neutrino luminosity of the Sun • CNO neutrinos (direct indication of metallicity in the Sun’s core) • pep neutrinos (indirect constraint on pp neutrino flux) • Low energy (3-5 MeV) 8B neutrinos • Tail end of pp neutrino spectrum? Davide Franco – Università di Milano & INFN

  14. pep 8B 7Be Borexino goals: neutrino physics Test of the matter-vacuum oscillation transition with 7Be, pep, and low energy 8B neutrinos Check of the mass varying neutrino model (Barger et al., PRL 95, 211802 (2005)) Limit on the neutrino magnetic moment by analyzing the 7Be energy spectrum and with Cr source Moreover: geoneutrinos and supernovae Davide Franco – Università di Milano & INFN

  15. Detecting neutrinos • Solar neutrino flux on Earth F~ 1010 cm-2 s-1 with energy [0.1-10] MeV • Weakly interacting : s ~ 10-46 – 10-43 cm2 • At 1 MeV (F~ 108 cm-2 s-1) in water (s ~2 x10-46 cm2) in 30 days (T = 2.5 106 s) to see 1 event we need: Huge active masses and extremely low background are required Davide Franco – Università di Milano & INFN

  16. First requirement: underground Second requirement: ultra high purity materials and proper design to minimize contaminations Davide Franco – Università di Milano & INFN

  17. Milano Perugia Borexino Collaboration Genova Princeton University APC Paris Virginia Tech. University Munich (Germany) Dubna JINR (Russia) Kurchatov Institute (Russia) Jagiellonian U. Cracow (Poland) Heidelberg (Germany) Davide Franco – Università di Milano & INFN

  18. Abruzzo 120 Km da Roma Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e Laboratori esterni Borexino – Rivelatore e impianti Davide Franco – Università di Milano & INFN

  19. Detection principles and n signature • Borexino detects solar n via their elastic scattering off electrons in a volume of highly purifiedliquid scintillator • Mono-energetic 0.862 MeV 7Ben are the main target, and the only considered so far • Mono-energetic pep n , CNO n and possibly pp n will be studied in the future • Detection via scintillation light: • Very low energy threshold • Good position reconstruction • Good energy resolution BUT… • No direction measurement • The n induced events can’t be distinguished from other b events due to natural radioactivity • Extreme radiopurity of the scintillator is a must! Typical n rate (SSM+LMA+Borexino) Davide Franco – Università di Milano & INFN

  20. NW-1 NW-2 Events/3years/100 tons/0.05 MeV n-spectrum in Borexino NW-1 = 0.25 – 0.8 MeV (7Be) NW-2 = 0.8 – 1.4 MeV (pep & CNO) • LMA –BP2004 – LUNA • 3 years statistics • 100 tons Davide Franco – Università di Milano & INFN

  21. Borexino Background Expected solar neutrino rate in 100 tons of scintillator ~ 50 counts/day (~ 5 10-9 Bq/Kg) Just for comparison: - Low background nylon vessel fabricated in hermetically sealed low radon clean room (~1 yr) - Rapid transport of scintillator solvent (PC) from production plant to underground lab to avoid cosmogenic production of radioactivity (7Be) - Underground purification plant to distill scintillator components. - Gas stripping of scintlllator with special nitrogen free of radioactive 85Kr and 39Ar from air - All materials electropolishedSS or teflon, precision cleaned with a dedicated cleaning module BX scintillator must be 9/10 order of magnitude less radioactive than anything on earth! Davide Franco – Università di Milano & INFN

  22. Detector layout and main features Stainless Steel Sphere: 2212 PMTs 1350 m3 Scintillator: 270 t PC+PPO in a 150 mm thick nylon vessel Nylon vessels: Inner: 4.25 m Outer: 5.50 m Water Tank: g and n shield m water Č detector 208 PMTs in water 2100 m3 Carbon steel plates 20 legs Davide Franco – Università di Milano & INFN

  23. PMTs: PC & Water proof Nylon vessel installation Installation of PMTs on the sphere Davide Franco – Università di Milano & INFN

  24. Water Plant Storage area and Plants Davide Franco – Università di Milano & INFN

  25. Counting Test Facility • CTF è un prototipo su piccola scala di Borexino: • ~ 4 tons of scintillator • 100 PMTs • Buffer of water • Muon veto • Vessel radius: 1 m CTF ha dimostrato la fattibilità di Borexino Davide Franco – Università di Milano & INFN

  26. May 15, 2007 Davide Franco – Università di Milano & INFN

  27. Borexino background Davide Franco – Università di Milano & INFN

  28. Detector & Plants All materials carefully and painfully selected for: Low intrinsic radioactivity Low Rn emanation Good behaviour in contact with PC Pipes, vessels, plants: electropolished, cleaned with detergent(s), pickled and passivated with acids, rinsed with ultra-pure water down to class 20-50 The whole plant is vacuum tight Leak requirements < 10-8 atm/cc/s Critical regions (pumps, valves, big flanges, small failures) were protected with additional nitrogen blanketing PMTs (2212) Sealing: PC and water tolerant Low radioactivity glass Light cones (Al) for uniform light collection in fiducial volume Time jitter: 1.1 ns (for good spatial resolution, mu-metal shielding) 384 PMTs with no cones for m id Nylon vessels Material selection for chemical & mechanical strength Low radioactivity to get <1 c/d/100 t in FV Construction in low 222Rn clean room Never exposed to air “Few” details Davide Franco – Università di Milano & INFN

  29. Expected Spectrum Davide Franco – Università di Milano & INFN

  30. The starting point: no cut spectrum Davide Franco – Università di Milano & INFN

  31. m are identified by ID and OD OD eff: ~ 99% ID based on pulse shape analysis Rejection factor > 103 (conservative) m crossing the scintillator m crossing the buffer only Detecting (and rejecting) cosmic muons m pulses A muon in OD m track ID efficiency Davide Franco – Università di Milano & INFN

  32. t~ 250 ms d + g n + p Detecting (and rejecting) cosmogenic neutrons t = 255 ± 5 ms A dedicated trigger starts after each muon opening a gate for 1.6 ms. An offline clustering algorithm identifies neutron in high multiplicity events A muon in OD Davide Franco – Università di Milano & INFN

  33. Muon and neutron cuts Residual background : << 1 c/d/100 t No cut m cut Davide Franco – Università di Milano & INFN

  34. Position reconstruction • Position reconstruction algorythms (we have 4 codes right now) • time of flight fit to hit time distribution • developed with MC, tested and validated in CTF • cross checked and tuned in Borexino with 214Bi-214Po events and 14C events z vs Rc scatter plot Resolution 214Bi-214Po (~800 KeV) 14±2 cm 14C (~100 KeV): 41±4 cm Davide Franco – Università di Milano & INFN

  35. Vessel Radius and Shape Vessel radius and origin calibrated exploiting fast coincidences of 222Rn and 220Rn chain segments emanated from vessel nylon 212Bi-212Po 214Bi-214Po t = 432.8 ns t = 236 ms The two plots confirms the CCD camera calibration results: vessel origin is shifted along positive z-axis by about 5 cm. Davide Franco – Università di Milano & INFN

  36. FV Spatial resolution 22 cm @ ~400 keV 220Rn-216Po 7Be energy region (mainly 210Po) 214Bi-214Po (~800 KeV) 14±2 cm 14C (~100 KeV): 41±4 cm m-induced neutrons FV 15 cm @ ~2200 keV Davide Franco – Università di Milano & INFN

  37. Spectrum after FV cut (100 tons) Davide Franco – Università di Milano & INFN

  38. 214Bi 214Po t = 236 ms b a 214Bi 214Po 210Pb ~700 keV eq. 3.2 MeV t = 236 ± 8 ms 238U content Assuming secular equilibrium and looking in the FV only: 0.02 cpd/tons corresponding to 238U = (1.9 ± 0.3)×10-17 g(U)/g Davide Franco – Università di Milano & INFN

  39. t = 442 ± 57 ns t = 432.8 ns b a 212Bi 208Pb 212Po ~1000 keV eq. 2.25 MeV 232Th content Events are mainly on the south vessel surface (probably particulate) Assuming secular equilibrium and looking in the FV only: 0.00256 cpd/toncorresponding to 232Th = (6.8±1.5)×10-18 g(Th)/g Davide Franco – Università di Milano & INFN

  40. a/b discrimination Full separation at high energy Small deformation due to average SSS light reflectivity a particles b particles ns 250-260 pe; near the 210Po peak 200-210 pe; low energy side of the 210Po peak 2 gaussians fit 2 gaussians fit a/b Gatti parameter a/b Gatti parameter Davide Franco – Università di Milano & INFN

  41. No radial cut R < 3.8 m FV (R < 3 m) Not from 210Pb 210Po contamination Davide Franco – Università di Milano & INFN

  42. a/b statistical subtraction Davide Franco – Università di Milano & INFN

  43. Energy calibration and stability • We have not calibrated with inserted sources (yet) • Planned for the near future • So far, energy calibration determined from 14C end point spectrum • Energy stability and resolution monitored with 210Po a peak • Difficult to obtain a very precise calibration because: • 14C intrinsic spectrum and electron quenching factor poorly known Light yield determined from 14C fit Light yield monitored with 210Po peak position Davide Franco – Università di Milano & INFN

  44. 14C 11C m-induced neutrons Energy scale MC vs data comparison of photoelectron time distributions from 14C MC-G4Bx Data LY = 500 (1%) p.e./MeV kB = 0.017 (15%) cm/MeV Ph.Y. ~ 9000 photons/MeV Davide Franco – Università di Milano & INFN

  45. New results with 192 days of statistics Davide Franco – Università di Milano & INFN

  46. New results with 192 days of statistics Davide Franco – Università di Milano & INFN

  47. Estimated 1σ Systematic Uncertainties* [%] 7Be Rate: 49±3stat±4syst cpd/100 tons , which means *Prior to Calibration Systematic and Final Result Expected interaction rate in absence of oscillations: 75±4 cpd/100 tons for LMA-MSW oscillations: 48±4 cpd/100 tons, which means: Davide Franco – Università di Milano & INFN

  48. Before Borexino Davide Franco – Università di Milano & INFN

  49. After Borexino Davide Franco – Università di Milano & INFN

  50. measured over predicted flux ratio Survival Probability Constraints on pp and CNO fluxes Combining Borexino 7Be results with other experiments,the expected rate in Clorine and Gallium experiments is • where • Ri,k and Pi,k are calculated in the hypothesis of high-Z SSM and MSW LMA • Rk are the rates actually measured by Clorine and Gallium experiments • f8B is measured by SNO and SuperK to be 0.87 ±0.07 • f7Be =1.02 ±0.10 is given by Borexino results Plus luminosity constraint: best determination of pp flux! Davide Franco – Università di Milano & INFN

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