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

2. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 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

3. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 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

4. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Abruzzo, Italy 120 Km from Rome Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e Borexino Detector and Plants External Labs

5. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Detection principles and signature Borexino detects solar via their elastic scattering off electrons in a volume of highly purified liquid scintillator Mono-energetic 0.862 MeV 7 Be are the main target, and the only considered so far Mono-energetic pep, CNO and possibly pp 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 induced events can’t be distinguished from other  events due to natural radioactivity Extreme radiopurity of the scintillator is a must! Typical rate (SSM+LMA+Borexino)

6. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Detector layout and main features 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 PMTs 1350 m 3 Nylon vessels: Inner: 4.25 m Outer: 5.50 m

7. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 15 years of work in three slides (I) 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  id Nylon vessels Material selection for chemical & mechanical strength Low radioactivity to get <1 c/d/100 t in FV Construction in low 222 Rn clean room Never exposed to air

8. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Picture gallery (I) 2000 Pmt sealing: PC & Water proof 2002 PMT installation in SSS Nylon vessels installation (2004)

9. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 15 years of work in three slides (II) Water ( production rate 1.8 m 3 /h) RO, CDI, filters, N 2 stripping U, Th: < 10 -14 g/g 222 Rn: ~ 1 mBq/m 3 226 Ra: <0.8 mBq/m 3 18.2-18.3 M  /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 N 2 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 N 2 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

10. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Picture gallery (II) CTF and Plants Water Plant Storage area and Plants

11. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Low Argon Krypton Nitrogen High Purity Nitrogen: 222 Rn < 0.3 µBq/m 3 LTA 1 ppb Ar in N 2 ~1.4 nBq/m 3 for 39 Ar; 0.1 ppt Kr in N 2 ~0.1 µBq/m 3 for 85 K LAKN developed for: - IV/OV inflating/flushing - scintillator purification - blanketing and cleaning Production rate reaches 100 m 3 /h (STP) Specification: 222 Rn  7 µBq/m 3 Ar  0.4 ppm Kr  0.2 ppt Expected signal from 39 Ar, 85 Kr and 222 Rn in the Borexino FV  1 cpd (for each isotope) Achieved results: Details discussed by G. Zuzel “Low-level techniques applied in the expe- riments looking for rare events”, Wed. 12.09, Solar & Low BG Techniques. 222 Rn: 8  Bq/m 3 Ar: 0.01 ppm Kr: 0.02 ppt

12. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 15 years of work in three slides (III) RadioIsotopeConcentration or FluxStrategy for Reduction NameSourceTypicalRequiredHardwareSoftwareAchieved  cosmic~200 s -1 m -2 ~ 10 -10 UndergroundCherenkov signal <10 -10 at sea level Cherenkov detectorPS analysis(overall) Ext.  rock Water Tank shieldingFiducial Volumenegligible Int.  PMTs, SSS Material SelectionFiducial Volumenegligible Water, Vessels Clean constr. and handling 14 CIntrinsic PC/PPO~ 10 -12 ~ 10 -18 Old Oil, check in CTFThreshold cut ~ 10 -18 238 UDust~ 10 -5 -10 -6 g/g < 10 -16 g/g Distillation, Water Extraction < 10 -17 232 ThOrganometallic (?)(dust)(in scintillator)Filtration, cleanliness < 10 -17 7 BeCosmogenic ( 12 C)~ 3 10 -2 Bq/t< 10 -6 Bq/tonFast procurement, distillationNot yet measurable? 40 KDust,~ 2 10 -6 g/g< 10 -14 g/g scin.Water ExtractionNot yet measurable? PPO(dust)< 10 -11 g/g PPODistillation 210 PbSurface contam. Cleanliness, distillationNot yet measurable? from 222 Rn decay (NOT in eq. with 210 Po) 210 Po Surface contam. Cleanliness, distillationSpectral analysis ~ 60 from 222 Rn decay  stat. subtraction ~ 0.01 c/d/t 222 Rnair, emanation from~ 10 Bq/l (air)< 1 c/d/100 tWater and PC N 2 stripping,Delayed coincidence< 0.02 c/d/t materials, vessels~100 Bq/l (water)(scintillator)cleanliness, material selection 39 ArAir (nitrogen)~17 mBq/m 3 (air)< 1 c/d/100 tSelect vendor, leak tightnessNot yet measurable? 85 Kr Air (nitrogen)~ 1 Bq/m 3 in air< 1 c/d/100 tSelect vendor, leak tightnessSpectral fit ~ 0.2 <0.01 ppt (learn how to measure it)fast coincidence <0.35

13. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN What’s important of previous table… 238 U and 232 Th 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 14 C 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 210 Po  s (no evidence of 210 Pb or 210 Bi at that level) Some 85 Kr contamination, probably due to a small air leak during filling

14. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Finally, May 15 th, 2007

15. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Our first result (astro-ph 0708.2251v2) We have detected the scattering rate of 7 Be solar s on electrons 7 Be Rate: 47 ± 7 STAT ± 12 SYS c/d/100 t How did we get here ?

16. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN The starting point: no cut spectrum 14 C dominates below 200 KeV 210 Po NOT in eq. with 210 Pb Mainly external  s and  s Photoelectrons Statistics of this plot: ~ 1 day Arbitrary units

17. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN  cuts  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: > 10 4 (still preliminary)  with OD tag No OD tag < 1% Outer detector efficiency ID efficiency A muon in OD  track

18. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Spectrum after  cut (above 14 C) After cuts,  are not a relevant background for 7 Be analysis Residual background: < 1 c/d/100 t No cuts After  cut

19. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 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 214 Bi- 214 Po events and 14 C events 214 Bi- 214 Po (~800 KeV) 14±2 cm 14 C (~100 KeV): 41±4 cm z vs R c scatter plot Resolution

20. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Fiducial volume cut External background is large at the periphery of the IV  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 Radial distribution R2R2 gauss z vs R c scatter plot FV

21. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Spectrum after FV cut External background is the dominant background component in NW, except in the 210 Po peak region No cuts No  s Clear 7 Be shoulder After FV cuts 11 C

22. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 11 C and neutrons after muons  s may produce 11 C by spallation on 12 C 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  are rejected Neutron Capture Time  ~ 210  s Neutron spatial distribution

23. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Final spectrum after all cuts  Kr+  Be 14 C 210 Po (only, not in eq. with 210 Pb!) 11 C Understanding the final spectrum: main components Last cut: 214 Bi- 214 Po and Rn daughters removal

24. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Energy calibration and stability We have not calibrated with inserted sources (yet) Planned for the near future So far, energy calibration determined from 14 C end point spectrum Energy stability and resolution monitored with 210 Po  peak Difficult to obtain a very precise calibration because: 14 C intrinsic spectrum and electron quenching factor poorly known Light yield monitored with 210 Po peak position Light yield determined from 14 C fit

25. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 238 U and 232 Th content Assuming secular equilibrium, 232 Th and 238 U are measured with the delayed concidences: 212 Bi 212 Po 208 Pb   = 432.8 ns 2.25 MeV ~800 KeV eq. Only 3 bulk candidates 232 Th Events are mainly in the south vessel surface (probably particulate) 214 Bi- 214 Po 212 Bi- 212 Po 214 Bi 214 Po 210 Pb   = 236  s 3.2 MeV ~700 KeV eq. 238 U: < 2. 10 -17 g/g 232 Th: < 1. 10 -17 g/g

26. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN  /  discrimination  particles Small deformation due to average SSS light reflectivity  particles 250-260 pe; near the 210 Po peak 200-210 pe; low energy side of the 210 Po peak 2 gaussians fit Full separation at high energy ns  Gatti parameter

27. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 7 Be signal: fit without  subtraction Strategy: Fit the shoulder region only Use between 14 C end point and 210 Po peak to limit 85 Kr content pep neutrinos fixed at SSM-LMA value Fit components: 7 Be 85 Kr CNO+ 210 Bi combined very similar in this limited energy region Light yield left free 7 Be 85 Kr CNO + 210 Bi 210 Po peak not included in this fit These bins used to limit 85 Kr content in fit

28. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 7 Be signal: fit  subtraction of 210 Po peak The large 210 Po background is subtracted in the following way: For each energy bin, a fit to the  Gatti variable is done with two gaussians From the fit result, the number of  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 210 Po residual background is allowed in the fit Results are totally consistent with those obtained without the subtraction 2 gaussians fit   The two analysis yield fully compatible results

29. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 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: 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

30. TAUP 2007 - Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN 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 7 Be 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