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Lino Miramonti Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare

From the solar neutrino problem to the 7 Be neutrinos measurement: results and perspectives of the Borexino detector. Lino Miramonti Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare. The core of the Sun reaches temperatures of  15.5 million K .

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Lino Miramonti Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare

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  1. From the solar neutrino problem to the 7Be neutrinos measurement: results and perspectives of the Borexino detector Lino Miramonti Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare Baikal Summer School 20-27 July 2008

  2. The core of the Sun reaches temperatures of  15.5 million K. At these temperatures, nuclear fusion can occur which transforms 4 Hydrogen nuclei into 1 Helium nucleus How the Sun shines 1 Helium nucleus has a mass that is slightly (0.7%) smaller than the combined mass of the 4 Hydrogen nuclei. That “missing mass” is converted to energy to power the Sun. Baikal Summer School 20-27 July 2008

  3. Net reaction: 41H 14He + energy Mass of 4 H: 6.6943 x 10-27 kg Mass of 1He: 6.6466 x 10-27 kg Difference : 0.048 x 10-27 kg (0.7%) Using E=mc2 each fusion releases 4.3 · 10-12 J 26.7 MeV The current luminosity of the Sun is 4 · 1026 Watts, (600 million tons of Hydrogen per second is being converted to 596 million tons of Helium-4. The remaining 4 million tons is released as energy). Baikal Summer School 20-27 July 2008

  4. What about neutrinos? We start from 4 protons and we end with 1 He nucleus which is composed of 2 protons and 2 neutrons. This means that we have to transform 2 protons into 2 neutrons: (inverse -decay) In the inverse beta decay a proton becomes a neutron emitting a positron and an electron neutrinoe Since neutrinos only interact with matter via the weak force, neutrinos generated by solar fusion pass immediately out of the core and into space. The study of solar neutrinos was conceived as a way to test the nuclear fusion reactions at the core of the Sun. Baikal Summer School 20-27 July 2008

  5. Neutrino production in the Sun In our star  98% of the energy is created in this reaction The pp chain reaction There are different steps in which energy (and neutrinos) are produced  from: pp pep 7Be 8B hep Monocrhomatic ν’s (2 bodies in the final state) Baikal Summer School 20-27 July 2008

  6. Neutrino production in the Sun Beside pp chain reaction there is also the CNO cycle that become the dominant source of energy in stars heavier than the Sun (in the Sun the CNO cycle represents only 1-2 %)  from: 13N 15O 17F Baikal Summer School 20-27 July 2008

  7. Neutrino production in the Sun Neutrino energy spectrum as predicted by the Solar Standard Model (SSM) John Norris Bahcall (Dec. 30, 1934 – Aug. 17, 2005) 7Be: 384 keV (10%) 862 keV (90%) pep: 1.44 MeV Baikal Summer School 20-27 July 2008

  8. “…..to see into the interior of a star and thus verify directly the hypothesis of nuclear energy generation in stars.” Phys. Rev. Lett. 12, 300 (1964); Phys. Rev. Lett. 12, 303 (1964); Homestake: The first solar neutrino detector The first experiment built to detect solar neutrinos was performed by Raymond Davis, Jr. and John N. Bahcall in the late 1960's in the Homestake mine in South Dakota Davis and Bahcall Large tank of 615 tons of liquid perchloroethylene Neutrinos are detected via the reaction: ne+ 37Cl → 37Ar + e- Eth = 814 keV mostly 8B neutrinos Remove and detect 37Ar (1/2=35 days): 37Ar + e-37Cl* + e Homestake Solar Neutrino Detector Expected rate: Only 1 atom of 37Ar every six days in 615 tons C2Cl4! Baikal Summer School 20-27 July 2008

  9. ≈ 5 37Ar atoms were extracted per month bubbling helium through the tank. 1 SNU (Solar Neutrino Unit) = 1 capture/sec/1036 atoms Expected from SSM: 7.6 + 1.3 - 1.1 SNU Detected in Homestake: 2.56 ± 0.23 SNU The number of neutrino detected was about 1/3 lower than the number of neutrino expected →Solar Neutrino Problem (SNP) Baikal Summer School 20-27 July 2008

  10. Possible Explanations to the SNP • Standard Solar Model is not right • Homestake is wrong • Something happens to the  ..but Solar models have been tested independently by helioseismology(studies of the interior of the Sun by looking at its vibration modes), and the standard solar model has so far passed all the tests. Non-standard solar models seem very unlikely. bisede • New experiments (since about 1980) are of three types: • Neutrino scattering in water (Kamiokande, SuperKamiokande) • Radiochemical experiments (like Homestake, but probing different energies) (SAGE, GALLEX) • Heavy water experiment (SNO) Baikal Summer School 20-27 July 2008

  11. Kamiokande  SuperKamiokande:Real time detection • Kamiokandelarge water Cherenkov Detector • 3000 tons of pure water • 1000 PMTs • SuperKamiokandelarge water Cherenkov Detector • 50000 tons of pure water • 11200 PMTs SuperKamiokande Electrons are accelerated to speeds v > c/n “faster than light”. Reaction: Elastic Scattering on e- Eth = 7.5 MeV (for Kamiokande) Eth = 5.5 MeV (for SKamiokande) only 8B neutrinos (and hep) Results: Inferred flux  2 times lower than the prediction Neutrinos come from the Sun! (Point directly to the source) Baikal Summer School 20-27 July 2008

  12. …looking for pp neutrinos … Until the year 1990 there was no observation of the initial reaction in the nuclear fusion chain (i.e. pp neutrinos). This changed with the installation of the gallium experiments. Gallium as target allows neutrino interaction via 2 radiochemical experiment were built in order to detect solar pp neutrinos. ne+ 71Ga → 71Ge + e- Eth = 233 keV Less model-depended GALLEX (and then GNO) Located in the Gran Sasso laboratory (LNGS) in Italy. The tank contained 30 tonnes of natural gallium in a 100 tonnes aqueous gallium chloride solution Baikal Summer School 20-27 July 2008

  13. SAGE Located at Baksan underground laboratory in Russia Neutrino Observatory with 50 tons of metallic gallium running since 1990-present Results of Gallex/GNO and SAGE The measured neutrino signal were smaller than predicted by the solar model ( 60%). Calibration tests with an artificial neutrino source (51Cr) confirmed the proper performance of the detector. Baikal Summer School 20-27 July 2008

  14. All experiments detect less neutrino than expected from the SSM ! Rate measurement Reaction Obs / Theory Homestake ne + 37Cl  37Ar + e- 0.34  0.03 SAGE ne + 71Ga  71Ge + e- 0.59  0.06 Gallex+GNO ne + 71Ga  71Ge + e- 0.58  0.05 Super-K nx + e-nx + e-0.46  0.02 Baikal Summer School 20-27 July 2008

  15. nm nm ne …… something happens to the  ! Standard Model assumes that neutrinos are massless • Let us assume that neutrinos have (different) masses - Δm2 • Let us assume that the mass eigenstates(in which neutrinos are created and detected)  flavor eigenstates: ne, nm, nt are not mass eigenstates Mass eigenstates are n1, n2,n3 We can write: θ analogous to the Cabibbo angle in case of quarks In the simple case of 2  Being: Consider θ = 45° Baikal Summer School 20-27 July 2008

  16. In general this leads to the disappearance of the original neutrino flavour with the corresponding appearance of the “wrong” neutrino flavour Baikal Summer School 20-27 July 2008

  17. Three-flavor mixing νe , νμ , ν- flavor eigenstates ν1 , ν2 , ν3 - mass eigenstates with masses m1, m2, m3 U is thePontecorvo-Maki-Nakagawa-Sakata matrix (the analog of the CKM matrix in the hadronic sector of the Standard Model). 3 angles: θ12 ,θ13 , θ23 1 CP-violating Dirac phase: δ 2 CP-violating Majorana phases: α1 , α2(physical only if ν’s are Majorana fermions) Baikal Summer School 20-27 July 2008

  18. The MikheyevSmirnov Wolfenstein Effect (MSW)… or Matter Effect Neutrino oscillations can be enhanced by traveling through matter • The neutrino “index of refraction” depends on its scattering amplitude with matter: • The Sun is made of up/down quarks and electrons • e, , . All neutrinos can interact through NC equally. • e, Only electron neutrino can interact through CC scattering: • The “index of refraction” seen by e is different than the one seen by  and . The MSW effect gives for the probability of an electron neutrino produced at t=0 to be detected as a muon neutrino: Ne being the electron density. Baikal Summer School 20-27 July 2008

  19. Sudbury Neutrino Observatory 1000 tonnes D2O (Heavy Water) 12 m diameter Acrylic Vessel 18 m diameter PMT support structure 9500 PMTs 1700 tonnes inner shielding H2O 5300 tonnes outer shielding H2O Urylon liner radon seal depth: 6010 m.w.e. // 70 muons/day Baikal Summer School 20-27 July 2008

  20. n +  + + CC d p p e− e n +  + + n NC d p n x x +  + n e− n e− ES x x Neutrino Reactions in SNO • measures total 8B n flux from the Sun • equal cross section for all n flavors Baikal Summer School 20-27 July 2008

  21. CC, NC FLUXES MEASURED INDEPENDENTLY Best fit to data gives: The Total Flux of Active Neutrinos is measured independently (NC) and agrees well with solar model Calculations: 4.7 ± 0.5 (BPS07) Baikal Summer School 20-27 July 2008

  22. Summary of all Solar neutrino experiments before Borexino All experiments “see” less neutrinos than expected by SSM …….. (but SNO in case of NC) Baikal Summer School 20-27 July 2008

  23. electron neutrinos oscillate into non-electron neutrino with these parameters: Large mixing Angle (LMA) Region: MSW from S.Abe et al., KamLAND Collab. arXiv:0803.4312v1 SOLAR only KamLAND is a detector built to measure electron antineutrinos coming from 53 commercial power reactors (average distance of ~180 km ). The experiment is sensitive to the neutrino mixing associated with the (LMA) solution. SOLAR plus KamLAND Baikal Summer School 20-27 July 2008

  24. The Borexino detector at Laboratori Nazionali del Gran Sasso ….. a detector “to see” in real time solar neutrinos below 1 MeV Borexino Detector and Plants CTF Borexino Baikal Summer School 20-27 July 2008

  25. The Borexino solar physics goals Radiochemical Real time measurement (only 0.01 %!) Gallex/GNO SAGE Homestake SNO & SuperKamiokande Borexino is able to measure for the first time neutrino coming from the Sun in real_time with low_energy ( 200 keV) and high_statistic. Eth200 keV Borexino (real time) Baikal Summer School 20-27 July 2008

  26. 7Be pep 8B pp neutrinos • The main goal of Borexino is the measurement of 7Be neutrinos, thank to that it will be possible: • To test the Standard Solar Model and the MSW-LMA solution of the SNP • To provide a strong constraint on the 7Be rate, at or below 5%, such as to provide an essential input to check the balance between photon luminosity and neutrino luminosity of the Sun • To confirm the solar origin of 7Be neutrinos, by checking the expected 7% seasonal variation of the signal due to the Earth’s orbital eccentricity • To explore possible hints of non-standard neutrino-matter interactions or presence of mass varying neutrinos. • Additional Possibilities: • pep neutrinos (indirect constraint on pp neutrino flux) • 8B neutrinos with a low energy threshold • Tail end of pp neutrinos spectrum? Baikal Summer School 20-27 July 2008

  27. Solar Model Chemical Controversy • One fundamental input of the Standard Solar Model is the metallicity (abundance of all elements above Helium) of the Sun • The Standard Solar Model, based on the old metallicity [GS98]is in agreement within 0.5% with helioseismology (measured solar sound speed). • Recent work[AGS05]indicates a lower metallicity. → This result destroys the agreement with helioseismology • A lower metallicity implies a variation in the neutrino flux (reduction of  40% for CNO neutrino flux) → A direct measurement of the CNO neutrinos rate could help to solve this controversy A direct measurement of the CNO neutrinos rate (never measured up to now) could give a direct indication of metallicity in the core of the Sun Baikal Summer School 20-27 July 2008

  28. The Borexino neutrino physics goals Resonant Oscillations in Matter: the MSW effect For high energy neutrinos flavor change is dominated by matter oscillations For low energy neutrinos flavor change is dominated by vacuum oscillations Regime transition expected between 1-2 MeV • Test the fundamental prediction of MSW-LMA theory • Exploring the vacuum-matter transition. • Check the mass varying neutrino model • pep and 7Be neutrinos good sources to study the transition! before Borexino Limit on the neutrino magnetic momentby analyzing the 7Be energy spectrum and with artificial neutrino 51Cr source (MCi) Geoneutrinos and Neutrinos from Supernovae Baikal Summer School 20-27 July 2008

  29. Detection principles and  signature elastic scattering (ES) on electrons in very high purity liquid scintillator Detection via scintillation light: Very low energy threshold Good position reconstruction Good energy resolution But… No direction measurement The  induced events can’t be distinguished from other γ/β events due to natural radioactivity Extreme radiopurity of the scintillator is a must! pp 7Be pep+CNO 8B Baikal Summer School 20-27 July 2008

  30. Borexino design Core of the detector: 300 tons of liquid scintillator (PC+PPO)contained in a nylon vessel of 8.5 m diameter. The thickness of nylon is 125 µm. 1st shield: 1000 tons of ultra-pure buffer liquid (PC+DMP) contained in a stainless steel sphere of 13.7 m diameter (SSS). 2200 photomultiplier tubes pointing towards the center to view the light emitted by the scintillator. 2nd shield: 2400 tons of ultra-pure water contained in a cylindrical dome. 200 photomultiplier tubes mounted on the SSS pointing outwards to detect Cerenkov light emitted in the water by muons. Baikal Summer School 20-27 July 2008

  31. How many 7Be neutrinos we expect? 7Be flux on earth:  5·109 cm-2 s-1 Cross-section: Neutrino signal:  45 events/day/100 tons above threshold (between 250-800 keV) Including oscillations:  30 events/day/100 tons! (between 250-800 keV) Recoil nuclear energy of the e-  = 3.310-45 cm2 Baikal Summer School 20-27 July 2008

  32. How much background we can tolerate? Use 238U and 232Th intrinsic contamination at 10-16 g/g For Th: 4.06 x 10-4Bq/kg 0.035 cpd/ton! For U: 12.35 x 10-4Bq/kg 0.107 cpd/ton! In [250-800] keV expected about 20 events/day/100tons with offline analysis  S/N  30/20 • In 1998 through the Borexino Counting Test Facility (CTF) it was proved the feasibility to reach such a low level of contamination by purification methods • distillation (6 stages distillation, 80 mbar, 90 °C) • water extraction(5 cycles) • N2 stripping(by LAK N2222Rn: 8 Bq/m3, Ar:0.01 ppm, Kr: 0.03 ppt) • [Borexino coll. Astrop. Phys. 8 1998] Internal view of CTF Baikal Summer School 20-27 July 2008

  33. Primary sources of radioimpurities Baikal Summer School 20-27 July 2008

  34. Experimental Hall C Baikal Summer School 20-27 July 2008

  35. External dome 18 m Baikal Summer School 20-27 July 2008

  36. Stainless Steel Sphere (SSS) Baikal Summer School 20-27 July 2008

  37. PMTs ready to be mounted Clean Room Baikal Summer School 20-27 July 2008

  38. Borexino inner detector Baikal Summer School 20-27 July 2008

  39. Optical fiber istallation Baikal Summer School 20-27 July 2008

  40. Borexino inner detector Baikal Summer School 20-27 July 2008

  41. Nylon vessels (Princeton Univ.) Baikal Summer School 20-27 July 2008

  42. Nylon vessels installation Baikal Summer School 20-27 July 2008

  43. Nylon vessels installed and inflated Baikal Summer School 20-27 July 2008

  44. 2002-2004 The project is stopped for local problems 2005 Re-commissioning of all the set ups 2006 Restart of all operations - detector filled with purified water 2007 Detector filled with purified scintillator (PC+1.5 g/l PPO), PC plus quencher (5.0 g/l),purified water May 15th 2007 Borexino starts the data taking with the detector completely filled. water filling May 15th, 2007 Scintillator filling Liquid scintillator Low Ar and Kr N2 Hight purity water From Aug 2006 From Jan 2007 Baikal Summer School 20-27 July 2008

  45. t = 432.8 ns b a 212Bi 212Po 208Pb ~800 keV eq. 2.25 MeV Background: 232Th content Specs: 232Th: 10-16 g/g Assuming secular equilibrium, 232Th is measured with the delayed coincidence: 212Pb 212Bi 212Po α=6.04 MeV α=8.79 MeV  = 432.8 ns 208Tl 208Pb Events are mainly in the south vessel surface (probably particulate) z (m) Only few bulk candidates (m) (m) From 212Bi-212Po correlated events in the scintillator: 232Th: = 6.8 ± 1.5 ×10-18 g(Th)/g 0.25 cpd/100tons Baikal Summer School 20-27 July 2008

  46. t = 236 ms b a 214Bi 214Po 210Pb ~700 keV eq. 3.2 MeV Specs: 238U: 10-16 g/g Background: 238U content 214Pb 214Bi 214Po Assuming secular equilibrium, 238U is measured with the delayed coincidence: α=7.69 MeV  = 236 s 210Pb 210Bi 210Po 214Bi-214Po 210Pb z (m) 214Bi-214Po  = 240±8s Time s (m) From 214Bi-214Po correlated events in the scintillator: 238U: = 1.6 ± 0.1 ×10-17 g(U)/g 1.9 cpd/100tons Baikal Summer School 20-27 July 2008

  47. Background: 210Po NOTES • The bulk238U and 232Th contamination is negligible • The 210Po background is NOT related neither to 238U contamination NOR to 210Pb contamination 210Po decay time: 204.6 days 214Pb 214Bi 214Po α=7.7 MeV 210Pb 210Bi 210Po α=5.4 MeV 210Po decays α: Q=5.4 MeV light yield quenched by  13 206Pb 210Bi no direct evidence  free parameter in the total fit (cannot be disentangled, in the 7Be energy range, from the CNO) • Not in equilibrium with 210Pb ! • 210Po decays as expected Baikal Summer School 20-27 July 2008

  48. b 85Kr 85Rb End point 687 keV = 10.76 y - BR: 99.56% Background: 85Kr 85Kr has an energy spectrum similar to the 7Be recoil electron The 85Kr content in the scintillator was probed through the rare decay sequence: that offers a delayed coincidence tag. Our best estimate for the activity of 85Kr is 29±14 cpd/100 tons Baikal Summer School 20-27 July 2008

  49. Cosmic m rejection SSS Inner Detector  flux: 1.21±0.05 m-2h-1  are detected in Outer Detector and Inner Detector; Outer Detector Muon angular distributions • Outer Detector efficiency > 99% • Inner Detector analysis is based on time pulse shape variables Estimated overall rejection factor: > 104 After cuts,  residual background: < 1 cpd/100 ton Baikal Summer School 20-27 July 2008

  50. For each event the time and the total charge are measured. Absolute time is also provided (GPS) Position reconstruction &α/β discrimination The position of each event is reconstructed with an algorithms based on time of flight fit to hit time distribution of detected photoelectrons α particles (developed with MC, tested and validated in CTF - cross checked and tuned in Borexino on selected events 14C, 214Bi-214Po, 11C) β particles Spatial resolution of reconstructed events: 16 cm at 500 keV (scaling as ) 14C Good separation at high energy Radius (m) The fit is compatible with the expected r2-like shape with R=4.25m. Baikal Summer School 20-27 July 2008

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