The Borexino detector was designed to perform the first real-time measurement of the monoenergetic neutrinos from the electron capture on the 7Be in the core of the Sun. The measurement with a precision of 10% has already been reported. The goal of 5% can be obtained in the future thanks to the intense calibration campaign performed in order to tune the Borexino reconstruction code. The 8B solar neutrino analysis with the lowest threshold achieved so far of 3MeV has also been reported. The unprecedentedly low intrinsic radioactivity achieved in Borexino offers a unique tool for the sensitive anti-neutrino study in the MeV

energy range and made possible the first observation at more than 3 σ CL of the geo-neutrino (geo-¯νe) signal

Suvorov, Y.

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DOI 10.1393/ncc/i2011-10707-3Colloquia: LaThuile10IL NUOVO CIMENTO Vol. 33 C, N. 5 Settembre-Ottobre 2010New results and strategy of BorexinoY. Suvorov on behalf of the Borexino CollaborationLaboratori Nazionali del Gran Sasso - 67100 Assergi (AQ), Italy(ricevuto il 14 Settembre 2010; pubblicato online il 10 Gennaio 2011)Summary. — The Borexino detector was designed to perform the first real-timemeasurement of the monoenergetic neutrinos from the electron capture on the 7Bein the core of the Sun. The measurement with a precision of 10% has alreadybeen reported. The goal of 5% can be obtained in the future thanks to the intensecalibration campaign performed in order to tune the Borexino reconstruction code.The 8B solar neutrino analysis with the lowest threshold achieved so far of 3MeVhas also been reported. The unprecedentedly low intrinsic radioactivity achievedin Borexino offers a unique tool for the sensitive anti-neutrino study in the MeVenergy range and made possible the first observation at more than 3 σ CL of thegeo-neutrino (geo-ν¯e) signal.PACS 95.55.Vj – Neutrino, muon, pion, and other elementary particle detectors;cosmic ray detectors.PACS 29.40.Mc – Scintillation detectors.1. – IntroductionThe first real-time measurement of the solar neutrino fluxes in the sub-MeV energyrange became possible with the Borexino multiton liquid scintillator (LS) detector [1]which has been constantly taking data since May 15th of 2007. The main goal of theBorexino experiment is the 5% precision measurement of the monochromatic solar neu-trinos of the energy Eν = 0.862MeV that are emitted in the electron capture decay of7Be in the core of the Sun. The solar neutrinos in Borexino are detected via the neutrinoelastic scattering on the electrons of the LS target.The expected in Borexino rate of the 7Be solar neutrinos of about 50 counts perday (cpd) in 100 tons of LS imposes to achieve an extremely high level of purity of thetarget scintillator. The core of the Borexino detector should be 9-10 orders of magnitudeless radioactive than anything on Earth. Years of dedicated studies brought to thedevelopment of the successful purification strategy which allowed to obtain the desirablepurity level of the scintillator [2].c© Societa` Italiana di Fisica 8586 Y. SUVOROV on behalf of the BOREXINO COLLABORATIONFig. 1. – Schematic view of the Borexino detector.2. – Detector design and experiment locationThe Borexino detector is an unsegmented multiton liquid-scintillator detector, whichcan be subdivided into three concentric parts (see fig. 1).The first, external, part is a Water Tank (WT) with a cylindrical base (18m in thediameter) and a hemispherical top with a maximum height of 16.9m. The Water Tankis filled with 2400m3 of high-purity water which provides a 2m shielding against theexternal background (γ-rays and neutrons from the rock). Equipped by 208 PMTs thatcollect the Cerenkov light emitted by the muons in water, the WT also serves as amuon veto.Inside the WT the Stainless Steel Sphere (SSS), the scintillator container and theholding structure for the 2212 8′′PMTs mounted on its inner surface, is placed. TheSSS is divided into two parts: the most inner part, the core of the detector, is filledby the LS target, the mixture of the solvent pseudocumene and the fluor PPO at aconcentration of 1.5 g/l, enclosed in a thin 125μm nylon Inner Vessel (IV). Outside theIV the SSS is filled with a not scintillating mixture of pseudocumene and DMP (bufferliquid) at a concentration of 5.0 g/l. This buffer liquid represents a second layer (1.5m)of shielding mainly against the internal radiation from the PMTs and the constructionmaterials. In order to prevent a Rn diffusion toward the central part of the detector asecond nylon vessel (shroud) is placed in the SSS, close to the PMTs.The Borexino experiment is located in the Hall C of the Gran Sasso National under-ground Laboratory (LNGS, Italy). The surrounding mountain provides 3800 m.w.e. ofshielding against the cosmic radiation, the residual muon flux is 1.16±0.03m−2 h−1 withan average energy of 320± 4(stat)± 11(syst)GeV [3].3. – Solar neutrinosSince the first observation of the solar neutrinos in 1968 [4] a clear discrepancybetween the experimental results and the theoretical predictions was noticed, namedlater as the solar neutrino problem. Among various explanations the most favouriteappeared to be the solar neutrino flavour oscillations due to the MSW effect [5] thatNEW RESULTS AND STRATEGY OF BOREXINO 87was experimentally confirmed by the SNO experiment in 2001 [6]. Further experimentalinvestigation performed by SNO and KamLAND experiments allowed to constrain theMSW parameters to the so-called Large Mixing Angle (LMA) region in the θ12 : Δm2plane [7].The MSW-LMA scenario predicts the correlation of the neutrino oscillations (andtherefore the survival probability) with the neutrino energy [8]. In the energy rangeabove ∼ 3MeV the oscillations are dominated by matter effects, while below ∼ 0.5MeVa more important role is played by the vacuum effects. Before Borexino started to takedata, in real time only the matter-dominated region was studied by observation of 8Bneutrinos starting from 5MeV. The direct measurement of the survival probability (Pee)in the region between ∼ 0.5MeV and ∼ 3MeV, the so-called transition zone, and belowrepresents an important test of the MSW-LMA solution.3.1. 7Be solar neutrinos. – The detection of the 7Be solar monoenergetic electronneutrinos with energy of 0.862MeV produced in the process 7Be(e−, νe)7Li allowed forthe first time to test the MSW-LMA prediction in the transition zone.The present Borexino result based on 192 days of statistics is 49 ± 3(stat) ±4(syst) cpd/100 tons of LS [9]. Considering the high-metallicity Standard Solar Model(BS07(GS98) SSM), found rate corresponds to a flux Φ(7Be) = 5.08±0.25×109 cm−2 s−1.The expected signal in case of non-oscillating neutrinos is 74±4 cpd/100 tons. The MSW-LMA scenario predicts the rate of 48± 4 cpd/100 tons that is in a good agreement withobservation. The non-oscillation hypothesis (Pee = 1) is rejected at 4σ CL.Currently reached accuracy of 10% on the 7Be flux does not allow to resolve the re-cently arisen problem of the chemical composition of the Sun, the so-called Solar metal-licity problem [10].The originally expected precision of 5% is feasible thanks to the recent improvementsperformed in the Borexino reconstruction code basing on the new calibration data fromthe 2008-2009 calibration campaign.3.2. 8B solar neutrinos. – The electron neutrinos produced in the decay of 8B in thecore of the Sun were the most accessible for the experimentalists due to the high visibleend point energy of 16.3MeV. Boron solar neutrino spectroscopy, however, has been per-formed so far only by the water Cerenkov detectors KamiokaNDE, SuperKamiokaNDEand SNO.The main detection process is the elastic ν-e scattering, SNO experiment used alsothe nuclear reaction channels on deuterium contained in the heavy water target. Thehigh level of intrinsic contamination and low photon yield impose a high threshold in thewater Cerenkov experiments of 5MeV. The lowest threshold achieved so far is 3.5MeVafter the application of the new analysis technique performed by the SNO Collabora-tion [11].The Borexino liquid-scintillator detector has better energy resolution and higher levelof radiopurity with respect to the water Cerenkov detectors. This provides an advantagein search of the 8B neutrinos. At present the Borexino experiment performed the mea-surement of boron electron neutrinos based on 488 live days of statistics with a thresholdof 3MeV. The current threshold is limited by the 2.6 MeV gamma of the 208Tl decayfrom the 232Th chain. The observed rate is 0.217± 0.038(stat)± 0.01(syst) cpd/100 tonsof LS which corresponds to a flux of Φ(8B) = 2.4±0.04±0.01×106 cm−2 s−1 (consideringhigh metallicity SSM) [12]. The obtained result is in a good agreement with previousmeasurements.88 Y. SUVOROV on behalf of the BOREXINO COLLABORATIONFig. 2. – (Colour on-line) The mean electron neutrino survival probability (Pee) as a functionof neutrino energy. In red dots the Pee(7Be) and Pee(8B) measured by Borexino are shown.The mean electron neutrino survival probability for the 8B neutrinos at the effectiveenergy of 8.6MeV assuming BS07(GS98) SSM is found to be 0.35 ± 0.10. For the 7Bethis value is 0.56 ± 0.01 (see fig. 2). Removing the common sources of uncertainty, theratio between 7Be and 8B neutrinos survival probabilities becomes 1.60 ± 0.33 which is1.8 σ away from unity. This is the first measurement of the survival probability in boththe transition zone and in the matter-dominated oscillation region obtained by the samedetector.4. – CalibrationsDuring a period of 2008-2009 three intense calibration campaigns were performed inBorexino in order to study in detail the detector response function. Three types of theradioactive sources (α, β and γ) were used for the precise definition of the energy scalein the wide range starting from 0.12MeV up to 10MeV.The main source of uncertainty, as was reported in [9], is caused by the uncertaintyon the fiducial volume definition (6%). Performed calibration allowed us to reduce thisnumber down to 3.8%.5. – First observation of the geo-neutrino signalThe geo-neutrinos, the electron anti-neutrinos (ν¯e) generated in β decays of the long-lived radioactive isotopes mainly 40K, 238U and 232Th which are naturally present in theEarth interior, were introduced in 1960s by Marx [13], Markov [14] and Eders [15] andthe subject was reviewed in 2004 by Mantovani et al. [16].The first experimental investigation of geologically produced ν¯e’s was done by theKamLAND experiment in 2005 [17].NEW RESULTS AND STRATEGY OF BOREXINO 89Extreme cleanliness of the scintillator achieved in Borexino together with high photonyield and the large number of free target protons (∼ 1.7 × 1031) offer a unique tool forthe anti-neutrino (ν¯e) study in the MeV energy range.Borexino detects ν¯e by means of the well-established reaction of the inverse β decayν¯e+p→ e++n with a threshold of 1.806MeV. This process offers a strong signature givenby two signals correlated in space and time. The positron in the LS comes promptly torest and annihilates emitting two 0.511MeV γ-rays, giving a prompt signal, with a visibleenergy of Eprompt = Eν¯e− 0.782MeV. The emitted neutron, after the thermalization(τ ∼ 256μs), is typically captured on proton with a resulting emission of a 2.22MeV γwhich provide a coincident delayed signal. Only ν¯e from the 238U and 232Th series satisfythe threshold (Eν¯emax(238U) = 3.26MeV, Eν¯emax(232Th) = 2.25MeV), while the 40K ν¯eare below the threshold (Eν¯emax(40K) = 1.3MeV) and therefore remain undetected.The present ν¯e analysis is based on the 537.2 days of live-time collected betweenDecember 2007 and December 2009 that correspond, after all cuts, to an exposure of252.6 ton y [18].Two main sources of ν¯e’s were considered in the analysis: the reactor ν¯e’s whose energyspectrum extends up to ∼ 8MeV, and ν¯e’s from the Earth (1.8MeV < Eν¯e < 3.26MeV).The predicted geo-neutrino signal in Borexino with the visible energy up to ∼ 2.6MeVconsidering the Bulk Silicate Earth (BSE) model [16] is 2.5 +0.3−0.5 events in 100 ton y.The advantageous location of Borexino detector, far from nuclear plants, guaran-tees the low contribution from the reactor-ν¯e’s in the geo-neutrino energy window(∼ 1–2.6MeV). Among all nuclear plants considered in the analysis 97.5% of the overallsignal is given by the 194 reactors in Europe while other 245 reactors around the worldcontribute to only 2.5%. Based on data about all European nuclear plants (nominalthermal power and monthly load factor) from IAEA and EDF the calculated expectedreactor-ν¯e signal in whole energy range (up to 8MeV) in case of 100% detection effi-ciency is 5.7± 0.3 events in 100 ton y (with neutrino oscillations) and 9.9± 0.5 events in100 ton y (without neutrino oscillations). Moreover, only ∼ 35% of the total reactor-ν¯esignal will contaminate the geo-neutrinos energy window, while remaining 65% will bein the reactor-ν¯e window (2.6–8MeV).The intense and detailed study of all possible sources of background, mainly thesources of events capable to mimic the ν¯e signature, such as radioactive spallation isotopes(8He, 9Li), fast neutrons and fake ν¯e from the (α-n) reaction including the study ofaccidentals, bring to the overall background rate of 0.14 ± 0.02 events in 100 ton y.See [18] for more details.A total of twenty-one (21) ν¯e candidates were selected after application of all cuts(see fig. 3). The expected background in the whole data set is 0.40± 0.05 events. There-fore, the resulting signal-to-background ratio is an unprecedented ∼ 50 : 1. The bestestimates of the geo-ν¯e and the rector-ν¯e rates obtained from the unbinned maximumlikelihood analysis are: Ngeo = 9.9+4.1−3.4 (+14.6−8.2 ) and Nreact = 10.7+4.3−3.4 (+15.8−8.0 ) at 68.3% CL(99.73% CL).6. – Future plansThe recent efforts of the Borexino Collaboration are focused on the achievement ofthe 5% precision measurement of the 7Be solar neutrino flux and on the study of the pep,CNO and possibly pp solar neutrinos. In order to extend the Borexino solar neutrinoprogram, further purification of the detector will be done in the near future in order toreduce the contamination which prevents this analysis, mainly 85Kr and 210Bi.90 Y. SUVOROV on behalf of the BOREXINO COLLABORATIONFig. 3. – The light yield spectrum for the positron signals of 21 selected ν¯e candidates and thebest fit. See [18] for details.For what concerns the geo-ν¯e study a greater statistics is needed (accumulation of1000 ton-y exposure is planned) in order to try to perform the spectral measurement of238U and 232Th components of the geo-ν¯e signal.REFERENCES[1] Alimonti G. et al. (Borexino Collaboration), Nucl. Instrum. Methods A, 600 (2009)568.[2] Alimonti G. et al. (Borexino Collaboration), Nucl. Instrum. Methods A, 609 (2009)58.[3] Ambrosio M. et al. (MACRO Collaboration), Astropart. Phys., 10 (1999) 11.[4] Davis R., Harmer D. and Hoffman K., Phys. Rev. Lett., 20 (1968) 1205.[5] Mikheev S. and Smirnov Y., Sov. J. Nucl. Phys., 42 (1985) 913.[6] Ahmad Q. et al. (SNO Collaboration), Phys. Rev. Lett., 87 (2001) 071301.[7] Abe S. et al. (KamLAND Collaboration), Phys. Rev. Lett., 100 (2008) 221803.[8] Barger et al., Phys. Rev. Lett., 95 (2005) 211802; Friedland A. et al., Phys. Lett. B,594 (2004) 347.[9] Alimonti G. et al. (Borexino Collaboration), Phys. Rev. Lett., 101 (2008) 091302.[10] Bahcall J., Serenelli A. and Basu S., Astrophys. J. Suppl., 165 (2006) 400; Pena-Garay C. and Serenelli A., arXiv:0811.2424v1 (2008); Serenelli A., arXiv:0910.3690(2009).[11] Ahmad Q. et al. (SNO Collaboration), arXiv:0910.2984v1 (2009).[12] Bellini G. et al. (Borexino Collaboration), arXiv:0808.2868v3 (2010).[13] Marx G. and Menyhard N., Mitt. Sternwarte, Budapest, No. 48 (1960).[14] Markov M. A., Neutrino (Nauka, Moscow) 1964.[15] Eders G., Nucl. Phys., 78 (1966) 657.[16] Mantovani F. et al., Phys. Rev. D, 69 (2004) 013001.[17] Araki T. et al. (KamLAND Collaboration), Nature, 436 (2005) 499.[18] Bellini G. et al. (Borexino Collaboration), Phys. Lett. B, 687 (2010) 299.

New results and strategy of Borexino

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New results and strategy of Borexino

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