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

TAUP 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

TAUP 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

TAUP 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

TAUP 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 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)

TAUP 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

TAUP 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 The whole plant is vacuum tight Leak requirements < 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

TAUP 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)

TAUP 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: < g/g 222 Rn: ~ 1 mBq/m Ra: <0.8 mBq/m M  /cm 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

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

TAUP 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 , Solar & Low BG Techniques. 222 Rn: 8  Bq/m 3 Ar: 0.01 ppm Kr: 0.02 ppt

TAUP 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 ~ UndergroundCherenkov signal < 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~ ~ Old Oil, check in CTFThreshold cut ~ UDust~ g/g < g/g Distillation, Water Extraction < ThOrganometallic (?)(dust)(in scintillator)Filtration, cleanliness < BeCosmogenic ( 12 C)~ Bq/t< Bq/tonFast procurement, distillationNot yet measurable? 40 KDust,~ g/g< g/g scin.Water ExtractionNot yet measurable? PPO(dust)< 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

TAUP 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: < g/g (< 10 c/d/FV)ACHIEVED: < g/g 14 C is ~ g/g as expected ( g/g measured) Muon rejection is fine: < 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

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

TAUP Sendai, September 11-15, 2007M. Pallavicini - Università di Genova & INFN Our first result (astro-ph v2) 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 ?

TAUP 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

TAUP 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

TAUP 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

TAUP 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

TAUP 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 < 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

TAUP 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

TAUP 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

TAUP 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

TAUP 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

TAUP 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   = 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: < g/g 232 Th: < g/g

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

TAUP 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 Bi 210 Po peak not included in this fit These bins used to limit 85 Kr content in fit

TAUP 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

TAUP 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

TAUP 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