Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration

1. Getting the first 7Be n detection:scintillator purification, detector response and data analysis in Borexino Marco Pallavicini Università di Genova & INFN On behalf of the Borexino Collaboration

2. Contents • Physics goals, detector design, construction & filling • Design guidelines • Radiopurity issues • Plants and Filling • Detector response & Data analysis • Event selection • Detector response • Background content • Spectral fits M. Pallavicini - Università di Genova & INFN

3. 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) M. Pallavicini - Università di Genova & INFN

4. Abruzzo, Italy 120 Km from Rome Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e External Labs Borexino Detector and Plants M. Pallavicini - Università di Genova & INFN

5. 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) M. Pallavicini - Università di Genova & INFN

6. 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 M. Pallavicini - Università di Genova & INFN

7. 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 15 years of work in three slides (I) M. Pallavicini - Università di Genova & INFN

8. Picture gallery (I) Pmt sealing: PC & Water proof 2000 Nylon vessels installation (2004) PMT installation in SSS 2002 M. Pallavicini - Università di Genova & INFN

9. Water ( production rate 1.8 m3/h) RO, CDI, filters, N2 stripping U, Th: < 10-14 g/g 222Rn: ~ 1 mBq/m3 226Ra: <0.8 mBq/m3 18.2-18.3 MW/cm typical @ 20°C Scintillator IV: PC+PPO (1.5 g/l) OV & Buffer: PC+DMP (5 g/l) PC Distillation (all PC) 6 stages distillation 80 mbar, 90 °C Vacuum stripping with low Ar-Kr N2 Humidified with water vapor 60-70% PPO purification PPO is solid. A concentrated solution (120 g/l) in PC is done first (“master solution”) Master solution was purified with: Water extraction ( 4 cycles) Filtration Single step distillation N2 stripping with LAKN Filling operations Purging of the SSS volume with LAKN (early ‘06) Water filling (Aug. 06  Nov. 06) Replacement of water with PC+PPO or PC+DMP (Jan. 07  May. 07) Mixing online DATA TAKING from May 15, 2007 15 years of work in three slides (II) M. Pallavicini - Università di Genova & INFN

10. Picture gallery (II) Water Plant Storage area and Plants CTF and Plants M. Pallavicini - Università di Genova & INFN

11. LTA Low Argon Krypton Nitrogen Specification: 222Rn  7 µBq/m3 Ar  0.4 ppm Kr  0.2 ppt Expected signal from 39Ar, 85Kr and 222Rn in the Borexino FV  1 cpd (for each isotope) LAKN developed for: • IV/OV inflating/flushing • scintillator purification • blanketing and cleaning Production rate reaches 100 m3/h (STP) Achieved results: High Purity Nitrogen: 222Rn < 0.3 µBq/m3 222Rn: 8 Bq/m3 Ar: 0.01ppm Kr: 0.02 ppt Details discussed by G. Zuzel “Low-level techniques applied in the expe- riments looking for rare events”, Wed. 12.09, Solar & Low BG Techniques. 1 ppb Ar in N2 ~1.4 nBq/m3 for 39Ar; 0.1 ppt Kr in N2 ~0.1 µBq/m3 for 85K M. Pallavicini - Università di Genova & INFN

12. 15 years of work in three slides (III) M. Pallavicini - Università di Genova & INFN

13. What’s important of previous table… • 238U and 232Th content in the scintillator and in the nylon vessels meet specifications or sometimes are even below specs • GOAL: < 10-16 g/g (< 10 c/d/FV)ACHIEVED:< 10-17 g/g • 14C is ~ 10-18 g/g as expected (2.7 10-18 g/g measured) • Muon rejection is fine: < 10-4 • Two main backgrounds are still above specs, although are managable: • Off equilibrium 210Po as (no evidence of 210Pb or 210Bi at that level) • Some 85Kr contamination, probably due to a small air leak during filling M. Pallavicini - Università di Genova & INFN

14. Finally, May 15th, 2007 M. Pallavicini - Università di Genova & INFN

15. Our first result (astro-ph 0708.2251v2) • We have detected the scattering rate of 7Be solar ns on electrons 7Be n Rate:47 ± 7STAT ± 12SYS c/d/100 t How did we get here ? M. Pallavicini - Università di Genova & INFN

16. The starting point: no cut spectrum 14C dominates below 200 KeV 210Po NOT in eq. with 210Pb Arbitrary units Mainly external gs and ms Photoelectrons Statistics of this plot: ~ 1 day M. Pallavicini - Università di Genova & INFN

17. m are identified by the OD and by the ID OD eff: ~ 99% ID analysis based on pulse shape variables Deutsch variable: ratio between light in the concentrator and total light Pulse mean time, peak position in time Estimated overall rejection factor: > 104 (still preliminary) m cuts Outer detector efficiency Preliminary m with OD tag No OD tag < 1% A muon in OD m track ID efficiency M. Pallavicini - Università di Genova & INFN

18. Spectrum after m cut (above 14C) • After cuts, m are not a relevant background for 7Be analysis • Residual background: < 1 c/d/100 t No cuts After m cut M. Pallavicini - Università di Genova & INFN

19. 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 M. Pallavicini - Università di Genova & INFN

20. Fiducial volume cut • External background is large at the periphery of the IV • g from materials that penetrate the buffer • They are removed by a fiducial volume cut • R < 3.276 m (100 t nominal mass) • Another volumetric cut, z < 1.8 m, was done to remove some Rn events caused by initial scintillator termal stabilization Preliminary Radial distribution z vs Rc scatter plot R2 gauss FV M. Pallavicini - Università di Genova & INFN

21. Spectrum after FV cut • External background is the dominant background component in NW, except in the 210Po peak region Clear 7Be shoulder No cuts After FV cuts 11C No ms M. Pallavicini - Università di Genova & INFN

22. 11C and neutrons after muons • ms may produce 11C by spallation on 12C • n are also produced ~ 90% of the times • Only the first neutron after a muon can be currently detected • Work in progress to try to improve this • Events that occur within 2 ms after a m are rejected Preliminary Neutron Capture Time Neutron spatial distribution t ~ 210 ms M. Pallavicini - Università di Genova & INFN

23. Final spectrum after all cuts Understanding the final spectrum: main components 210Po (only, not in eq. with 210Pb!) 14C 85Kr+7Be n 11C Last cut: 214Bi-214Po and Rn daughters removal M. Pallavicini - Università di Genova & INFN

24. 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 M. Pallavicini - Università di Genova & INFN

25. t = 432.8 ns t = 236 ms b b a a 214Bi 212Bi 212Po 214Po 210Pb 208Pb ~800 KeV eq. ~700 KeV eq. 3.2 MeV 2.25 MeV 238U and 232Th content 212Bi-212Po Assuming secular equilibrium, 232Th and 238U are measured with the delayed concidences: 232Th Events are mainly in the south vessel surface (probably particulate) 212Bi-212Po 214Bi-214Po Only 3 bulk candidates 238U: < 2. 10-17 g/g 232Th: < 1. 10-17 g/g M. Pallavicini - Università di Genova & INFN

26. 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 M. Pallavicini - Università di Genova & INFN

27. 7Be signal: fit without a/b subtraction • Strategy: • Fit the shoulder region only • Use between 14C end point and 210Po peak to limit 85Kr content • pep neutrinos fixed at SSM-LMA value • Fit components: • 7Be n • 85Kr • CNO+210Bi combined • very similar in this limited energy region • Light yield left free 210Po peak not included in this fit 7Be n CNO + 210Bi 85Kr These bins used to limit 85Kr content in fit M. Pallavicini - Università di Genova & INFN

28. The large 210Po background is subtracted in the following way: For each energy bin, a fit to the a/b Gatti variable is done with two gaussians From the fit result, the number of a particles in that bin is determined This number is subtracted The resulting spectrum is fitted in the energy range between 270 and 800 KeV A small 210Po residual background is allowed in the fit Results are totally consistent with those obtained without the subtraction 7Be signal: fit a/b subtraction of 210Po peak 2 gaussians fit a b The two analysis yield fully compatible results M. Pallavicini - Università di Genova & INFN

29. Comments on errors • 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: • Mostly due to fiducial volume determination • With 45 days of data taking, and without an internal source calibration, we estimate an upper limit of 25% for this error • Can be much improved even without internal calibration with more statistics and better understanding of the detector response M. Pallavicini - Università di Genova & INFN

30. Conclusions • Borexino has performed the first real time detection of sub/MeV solar neutrinos • Quite surprising even for us, after just two months of data • A clear 7Be neutrino signal is visible after a few cuts • We made no attempt to under-estimate the errors. • Better results to come in the near future • The central value is well in agreement with MSW/LMA. • Significant improvements are expected shortly In memory of: Cristina Arpesella, Martin Deutsch, Burkhard Freudiger, Andrei Martemianov and Sandro Vitale M. Pallavicini - Università di Genova & INFN