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Borexino and Solar Neutrinos

Borexino and Solar Neutrinos. Emanuela Meroni Università di Milano & INFN On behalf of the Borexino Collaboration. Physics and detection principles.

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Borexino and Solar Neutrinos

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  1. Borexino and Solar Neutrinos Emanuela Meroni Università di Milano & INFN On behalf of the Borexino Collaboration

  2. Physics and detection principles • Borexino aims to measure low energy solar neutrinos in real time by elastic neutrino-electron scattering in a volume of highly purified liquid scintillator • Mono-energetic 0.862 MeV 7Beνis the main target • Pep, CNO and possibly ppν • Geoneutrinos • Supernovaν • Detection via scintillation light • Very low energy threshold • Good position recostruction • Good energy resolution • Drawbacks: • No direction measurements • νinduced events can’t be distinguished fromβ-decay due to natural radioactivity Typical  rate (SSM+LMA+Borexino) Extreme radiopurity of the scintillator

  3. Detector design and layout Borexino detector at LNGS Stainless Steel Sphere: 2212 photomultipliers 1350 m3 Scintillator: 270 t PC+PPO in a 150 m thick nylon vessel Water Tank: gand n shield mwaterČdetector 208 PMTs in water 2100 m3 Nylon vessels: Inner: 4.25 m Outer: 5.50 m 20 legs Design based on the principle of graded shielding Carbon steel plates

  4. Background suppression strategies= 15 years of work • g’s from rocks, PMT, tank, nylon vessel • Detector design: concentric shells to shield the inner scintillator • Material selection and surface treatment • Clean construction and handling • Internal background (238U, 232Th, 40K, 39Ar, 85Kr, 222Rn) • Scintillator purification: • Distillation (6 stages distillation, 80 mbar, 90 °C) • Vacuum Stripping by LAK N2 (222Rn: 8 Bq/m3, Ar:0.01 ppm, Kr: 0.03 ppt) • Humidified with water vapor 30% • Master solution (PPO) purification: • Water extraction ( 5 cycles) • Filtration • Single step distillation • N2 stripping with LAKN • Leak requirements for all systems and plants < 10-8 atm/cc/s • Critical regions (pumps, valves, big flanges, small failures) were protected with additional nitrogen blanketing

  5. First result in August 2007 During the water filling Finally, May 15th, 2007 During the PC filling Liquid scintillator Low Ar and Kr N2 Hight purity water From Aug 2006 From Jan 2007 We have measured the scattering rate of 7Be solarnson electrons 7BenRate: 47 ± 7STAT ± 12SYSc/d/100 t August 16(2007): PLB 658, 101(2008)

  6. 47 ± 7statcpd/100tons for 862 keV 7Be solar n Syst. Error: 25% Using LMA with: dm122=7.92·10-5 eV2 sin2q12=0.314 and BPS07(GS98)

  7. t = 432.8 ns b a 212Bi 212Po 208Pb ~800 KeV eq. 2.25 MeV Specs: 232Th: 1. 10-16 g/g 0.035 cpd/ton Background: 232Th content Assuming secular equilibrium, 232Th is measured with the delayed coincidence: 212Bi-212Po =423±42 ns Time (ns) Events are mainly in the south vessel surface (probably particulate) z (m) Only few bulk candidates (m) (m) From 212Bi-212Po correlated eventsin the scintillator: 232Th: < 6 ×10-18 g(Th)/g (90% C.L.)

  8. t = 236 ms b a 214Bi 214Po 210Pb ~700 KeV eq. 3.2 MeV Specs: 238U: 1. 10-16 g/g Background: 238U content Assuming secular equilibrium, 238U is measured with the delayed coincidence: 214Bi-214Po =240±8s Time s Setp - Oct 2007 214Bi-214Po z (m) < 2 cpd/100 tons 238U: = 6.6 ± 1.7×10-18 g(U)/g

  9. NOTES • The bulk 238U and 232Th contamination is negligible • The 210Po background is NOT related neither to 238U contamination NOR to 210Pb contamination Background: 210Po • Not in equilibrium with 210Pb ! • 210Po decays as expected 210Po decay time: 204.6 days 60 cpd/1ton • 210Bi no direct evidence----> free parameter in the total fit cannot be disentangled, in the 7Be energy range, from the CNO

  10. b g 85Kr 85mRb 85Rb 514 keV 173 keV • = 1.46 ms - BR: 0.43% b 85Kr 85Rb 687 keV = 10.76 y - BR: 99.56% Background: 85Kr 85Kr  decay : (b decay has an energy spectrum similar to the 7Be recoil electron ) 85Kr is studied through : Only 4 events (are selected in the IV in ~120 d. 1.4 events were expected from 14C-210Po random coincidences the 85Kr contamination upper limits<35 counts/day/100 ton (at 90% C.L.) More statistics is needed---> Taken as free parameter in the total fit

  11.  are identified by the OD and by the ID OD eff: ~ 99% ID analysis based on pulse shape variables Pulse mean time, peak position in time Estimated overall rejection factor: > 104 (still preliminary) Cosmic m A muon in OD Muon angular distributions ID efficiency After cuts, m are not a relevant background for 7Be analysis • Residual background: < 1 c/d/100 t Muon flux:(1.21±0.05)h-1m-2

  12. The time and the total charge are measured, and the position is reconstructed for each event . Absolute time is also provided (GPS) Position reconstruction • Position reconstruction algorithms • Base on time of flight fit to hit time distribution • developed with MC, tested and validated in CTF • cross checked and tuned in Borexino on selected events (14C, 214Bi-214Po, 11C) 14C 214Bi-214Po Radius (m)  distance(m) The fit is compatible with the expected r2-like shape with R=4.25m. Spatial resolution: 35 cm at 200 keV 16 cm at 500 keV (scaling as )

  13. Fiducial volume • the nominal Inner Vessel radius: 4.25m (278 tons of scintillator) • the effective I.V. radius has been reconstructed using: • # 14C events # Thoron (=80s) on the I.V. surface (emitted by the nylon) • # External background gamma # Teflon diffusers on the IV surface • maximum uncertainty: ~ +-12% z < 1.8 m, was done to remove gammas from IV endcaps Radial distribution z vs Rc scatter plot FV R2 gauss g from PMTs that penetrate the buffer FM: by rescaling background components known to be uniformly distributed within the LS and using the known LS mass (278.3 t)

  14. The light yield has been evaluated also by taking it as free parameter in a global fit on the total spectrum (14C,210Po,s210Po ,7Ben Compton edge) Light Yield 14C spectrum (b decay-156 keV, end point) The Light Yield has been evaluated fitting the 14C spectrum, (Borex. Coll. NIM A440, 2000) and the 11C spectrum The 11C sample is selected through the triple coincidence with muon and neutron. We limited the sample to the first 30 min of 11C time profile, which reduces the random coincidence to a factor 1/14. 11C spectrum(+ decay-960 keV) Light Yield = 500 +- 12 p.e./MeV The energy equivalent to the sum of the two quenched 511 keV gammas: E2511) = 0.83 +- 0.03 MeV. Energy resolution: 10% at 200 keV 8% at 400 keV 6% at 1 MeV

  15. Final spectrum after all cuts Understanding the final spectrum: main components 210Po (only, not in eq. with 210Pb!) 14C No cuts 85Kr+7Be n 11C After m cut 10C+ ext. bkg After FV cuts

  16. Spectral fit to determine the  signal 7BenRate: 47 ± 7STAT ± 12SYS c/d/100 t 47 days of live time (August 2007) • Strategy: • Fit the shoulder region only • Use between 14C end point and 210Po peak to limit 85Kr content • pep and 8B neutrinos fixed at SSM-LMA value • Other backgrounds (U, Th) negligible with the present radiopurity • 210Po peak not included in this fit • Fit components • 7Be, 85Kr • CNO+210Bi combined • very similar in this limited energy region • Light yield left free • Stat. error at present includes lack of knowledge of 85Kr • Syst. uncertainty comes from Fiducial Mass estimation (max error) These bins used to limit 85Kr content in fit

  17. Spectral fit in ~200 days • Improvements: • Better definition of FV (use internal source and diffuser balls deployed on the IV surface) • PMT charge equalization • LY (but still free parameter in a global fit on the total spectrum) • Better background measurements • Detector stability • Fit in the range 150-2000 keV Preliminary • Background issue: • 85Kr • 210Bi - 40K no signature • 11C : reduction by tagging -induced neutrons identification is in progress

  18. 7BenRate: 47 ± 7STAT ± 12SYSc/d/100 t August (2007) Comments on errors 6% • Statistical: • Right now, it includes combined the effect of statistics itself, the lack of knowledge of 85Kr content, and the lack of a precise energy calibration • These components are left free in the final fit, and contribute to the statistical error • Systematic (the evaluation is still in progress): • Fiducial volume determination: it is improved due to a better understanding of the detector response. • Max. range of 7Be  flux due to poor knowledge of the background 210Bi/40K which are in competition with CNO ’s: • Events selection (background subtraction: muons, Rn.. ), energy scale 15%

  19. Spherical cut around2.2 gamma to reject 11C event m+12C-->11C+n+m 11B+e++ne Cylindrical cut Around muon-track n capture g (2.2 MeV) Muon track Neutron production pep and CNO fluxes Simulated CNO Main problem: 11C 11C pep • ms may produce 11C by spallation on 12C • n are also produced ~ 90% of the times • Untill now, only the first neutron after a muon can be currently detected • Events that occur within 2 ms after a m are rejected

  20. 11C and neutrons after muons • electronics improvement to detect all the neutrons produced by a muon • Implementation of the main electronics • FADC in parallel to the main electronics

  21. What next @ possibly p-p neutrinos @ seasonal variations of the solar n flux due to the eccentricity of the Earth orbit 250-800 keV En. window

  22. What next (cont.) @ search for antineutrinos (from Sun, Earth,reactors) Borex. Coll. Eur. Phys.J. C47,2006 good tagging: +p n+e+ signal > 1 MeV ≈200ms neutron capture: signal 2.2 MeV --->> geoneutrinos Main bckg: from reactors In 300 tons: 7- 17 ev/y (BSE)- S/N=1 Antineutrinos from Reactors;long base line: ≈1000 km Rate: ≈ 20 ev/y

  23. CONCLUSIONS >> Borexino just started the study of the various solarneutrino sources below 2 MeV, with a real time detection ( pp,7Be, pep, CNO) >>Future goal (in a few months): • try to tag 11C • CNO study >> The program includes also the study of the antineutrinos (from Sun, Earth, Reactors) >> Borexino in also a useful observatory for the Supernova

  24. 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)

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