1. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Borexino and Solar Neutrinos Emanuela Meroni Università di Milano & INFN On behalf of the Borexino Collaboration

2. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 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 7 Be ν is the main target Pep, CNO and possibly pp ν Geoneutrinos Supernova ν Detection via scintillation light Very low energy threshold Good position recostruction Good energy resolution Drawbacks: No direction measurements ν induced events can’t be distinguished from  -decay due to natural radioactivity Extreme radiopurity of the scintillator Typical rate (SSM+LMA+Borexino)

3. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Detector design and layout Water Tank:  and n shield  water Č detector 208 PMTs in water 2100 m 3 20 legs Carbon steel plates Scintillator: 270 t PC+PPO in a 150  m thick nylon vessel Stainless Steel Sphere: 2212 photomultipliers 1350 m 3 Nylon vessels: Inner: 4.25 m Outer: 5.50 m Design based on the principle of graded shielding Borexino detector at LNGS

4. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Background suppression strategies = 15 years of work  ’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 ( 238 U, 232 Th, 40 K, 39 Ar, 85 Kr, 222 Rn) –Scintillator purification: Distillation (6 stages distillation, 80 mbar, 90 °C) Vacuum Stripping by LAK N 2 ( 222 Rn: 8  Bq/m 3, 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 N 2 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. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 First result in August 2007 During the water filling During the PC filling Finally, May 15 th, 2007 We have measured the scattering rate of 7 Be solar s on electrons 7 Be Rate: 47 ± 7 STAT ± 12 SYS c/d/100 t August 16(2007): PLB 658, 101(2008) From Aug 2006From Jan 2007 Hight purity water Liquid scintillator Low Ar and Kr N 2

6. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 47 ± 7 stat cpd/100tons for 862 keV 7 Be solar Using LMA with:  m 12 2 =7.92·10 -5 eV 2 sin 2  12 =0.314 and BPS07(GS98) Syst. Error: 25% Expected rate (cpd/100 t) No oscillation 75 ± 4 BPS07(GS98) HighZ 49 ± 4 BPS07(AGS05) LowZ 44 ± 4

7. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Background: 232 Th content Assuming secular equilibrium, 232 Th is measured with the delayed coincidence: 212 Bi 212 Po 208 Pb   = 432.8 ns 2.25 MeV ~800 KeV eq. From 212 Bi- 212 Po correlated events in the scintillator : 232 Th: < 6 ×10 -18 g(Th)/g (90% C.L.) Specs: 232 Th: 1. 10 -16 g/g 0.035 cpd/ton Only few bulk candidates 212 Bi- 212 Po Time (ns)  =423±42 ns Events are mainly in the south vessel surface (probably particulate) z (m) (m)

8. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Background: 238 U content Assuming secular equilibrium, 238 U is measured with the delayed coincidence: 214 Bi 214 Po 210 Pb   = 236  s 3.2 MeV ~700 KeV eq. 214 Bi- 214 Po  =240±8  s Time  s 214 Bi- 214 Po z (m) Setp - Oct 2007 Specs: 238 U: 1. 10 -16 g/g < 2 cpd/100 tons 238 U: = 6.6 ± 1.7×10 -18 g(U)/g

9. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Background: 210 Po NOTES The bulk 238 U and 232 Th contamination is negligible The 210 Po background is NOT related neither to 238 U contamination NOR to 210 Pb contamination 210 Po decay time: 204.6 days 210 Bi no direct evidence----> free parameter in the total fit cannot be disentangled, in the 7 Be energy range, from the CNO 60 cpd/1ton Not in equilibrium with 210 Pb ! 210 Po decays as expected

10. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Only 4 events (  are selected in the IV in ~120 d. 1.4 events were expected from 14 C- 210 Po random coincidences the 85 Kr contamination upper limits <35 counts/day/100 ton (at 90% C.L.) More statistics is needed---> Taken as free parameter in the total fit Background: 85 Kr 85 Kr is studied through : 85 Kr  decay : (  decay has an energy spectrum similar to the 7 Be recoil electron ) 85 Kr  85 Rb 687 keV  = 10.76 y - BR: 99.56% 85 Rb 85 Kr 85m Rb  = 1.46  s - BR: 0.43% 514 keV  173 keV 

11. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Cosmic   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: –> 10 4 (still preliminary) ID efficiency A muon in OD Muon flux :(1.21±0.05)h -1 m -2 Muon angular distributions After cuts,  are not a relevant background for 7 Be analysis –Residual background: < 1 c/d/100 t

12. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 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 ( 14 C, 214 Bi- 214 Po, 11 C) The fit is compatible with the expected r 2 -like shape with R=4.25m. The time and the total charge are measured, and the position is reconstructed for each event. Absolute time is also provided (GPS) 14 C Radius (m) Spatial resolution: 35 cm at 200 keV 16 cm at 500 keV (scaling as ) 214 Bi- 214 Po  distance(m)

13. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Fiducial volume Radial distribution R2R2 gauss z vs R c scatter plot FV  the nominal Inner Vessel radius: 4.25m (278 tons of scintillator)  the effective I.V. radius has been reconstructed using: # 14 C events # Thoron (  =80s) on the I.V. surface (emitted by the nylon) # External background gamma # Teflon diffusers on the IV surface maximum uncertainty : ~ +-12% FM: by rescaling background components known to be uniformly distributed within the LS and using the known LS mass (278.3 t)  from PMTs that penetrate the buffer z < 1.8 m, was done to remove gammas from IV endcaps

14. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Light Yield The 11 C sample is selected through the triple coincidence with muon and neutron. We limited the sample to the first 30 min of 11 C time profile, which reduces the random coincidence to a factor 1/14. The Light Yield has been evaluated fitting the 14 C spectrum, (Borex. Coll. NIM A440, 2000) and the 11 C spectrum 14 C spectrum (   decay-156 keV, end point) 11 C spectrum(  + decay-960 keV) Light Yield = 500 +- 12 p.e./MeV The energy equivalent to the sum of the two quenched 511 keV gammas: E 2  511) = 0.83 +- 0.03 MeV. Energy resolution: 10% at 200 keV 8% at 400 keV 6% at 1 MeV The light yield has been evaluated also by taking it as free parameter in a global fit on the total spectrum ( 14 C, 210 Po,  210Po, 7 Be  Compton edge )

15. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Final spectrum after all cuts Understanding the final spectrum: main components 14 C No cuts  Kr+  Be 11 C After  cut After FV cuts 10 C+ ext. bkg 210 Po (only, not in eq. with 210 Pb!)

16. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Spectral fit to determine the signal Stat. error at present includes lack of knowledge of 85 Kr Syst. uncertainty comes from Fiducial Mass estimation (max error) 47 days of live time (August 2007) Strategy: –Fit the shoulder region only –Use between 14 C end point and 210 Po peak to limit 85 Kr content –pep and 8 B neutrinos fixed at SSM- LMA value –Other backgrounds (U, Th) negligible with the present radiopurity – 210 Po peak not included in this fit Fit components – 7 Be, 85 Kr –CNO+ 210 Bi combined very similar in this limited energy region –Light yield left free These bins used to limit 85 Kr content in fit 7 Be Rate: 47 ± 7 STAT ± 12 SYS c/d/100 t

17. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 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 Background issue: – 85 Kr – 210 Bi - 40 K no signature – 11 C : reduction by tagging  -induced neutrons identification is in progress

18. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Comments on errors Statistical: –Right now, it includes combined the effect of statistics itself, the lack of knowledge of 85 Kr 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 7 Be flux due to poor knowledge of the background 210 Bi/ 40 K which are in competition with CNO ’s: –Events selection (background subtraction: muons, Rn.. ), energy scale 7 Be Rate: 47 ± 7 STAT ± 12 SYS c/d/100 t August (2007)   15%   6%

19. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008  s may produce 11 C by spallation on 12 C –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  are rejected   C   C  n+     +e + + e n capture  MeV) Cylindrical cut Around muon-track Spherical cut around 2.2 gamma to reject 11 C event Neutron production Muon track Main problem: 11 C pep and CNO fluxes CNO 11 C pep Simulated

20. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 11 C 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. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 What next @ possibly p-p neutrinos @ seasonal variations of the solar  flux due to the eccentricity of the Earth orbit 250-800 keV En. window

22. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 @ search for antineutrinos (from Sun, Earth,reactors) Borex. Coll. Eur. Phys.J. C47,2006 good tagging: +p n+e + signal > 1 MeV ≈200  s 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 What next (cont.)

23. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 CONCLUSIONS >> Borexino just started the study of the various solar  neutrino sources below 2 MeV, with a real time detection ( pp, 7 Be, pep, CNO) >> Future goal (in a few months): –try to tag 11 C –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. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008 Borexino collaboration Kurchatov Institute (Russia) Dubna JINR (Russia) Heidelberg (Germany) Munich (Germany) Jagiellonian U. Cracow (Poland) Perugia Genova APC Paris Milano Princeton University Virginia Tech. University

25. Emanuela Meroni Univ. & INFN Milano NO-VE April 15-18, 2008