UPDATED GEONEUTRINO MEASUREMENT WITH BOREXINO LIVIA LUDHOVA FOR BOREXINO COLLABORATION IKP-2, FORSCHUNGSZENTRUM JÜLICH AND RWTH AACHEN UNIVERSITY, GERMANY SEPTEMBER 10TH, 2019 TAUP 2019, TOYAMA, JAPAN
OUTLINE (OR WHERE IS THIS ENERGY COMING FROM?) • What are geoneutrinos and why to study them • Expected geoneutrino signal at LNGS (Italy) • Borexino and antineutrino detection • Borexino geoneutrino measurement: fresh new results • Geological interpretation
EARTH’S HEAT BUDGET Radiogenic heat & Geoneutrinos can help! Integrated surface heat flux: H tot = 47 + 2 TW Lithosphere Mantle Heat production in lithosphere “well” known Big uncertainty Mantle cooling 7 - 9 TW Heat production in mantle 1 – 27 TW 4 – 27 TW Core cooling 9 – 17 TW Core cooling Mantle cooling
Geoneutrinos: antineutrinos/neutrinos from the decays of long-lived radioactive isotopes naturally present in the Earth 238 U (99.2739% of natural U) à 206 Pb + 8 α + 8 e - + 6 anti-neutrinos + 51.7 MeV 232 Th à 208 Pb + 6 α + 4 e - + 4 anti-neutrinos + 42.8 MeV 235 U (0.7205% of natural U) à 207 Pb + 7 α + 4 e - + 4 anti-neutrinos + 46.4 MeV 40 K (0.012% of natural K) à 40 Ca + e - + 1 anti-neutrino + 1.32 MeV (BR=89.3 %) 40 K + e - à 40 Ar + 1 neutrino + 1.505 MeV (BR=10.7 %) q the only direct probe of the deep Earth q released heat and geoneutrino flux in a well fixed ratio q to measure geoneutrino flux = (in principle) = to get radiogenic heat q in practice (as always) more complicated … .. Earth shines in geoneutrinos: flux ~10 6 cm -2 s -1 leaving freely and instantaneously the Earth interior (to compare: solar neutrinos (NOT antineutrinos!) flux ~10 10 cm -2 s -1 )
GEONEUTRINOS AND WHY TO STUDY THEM • Main goal: contribution of the Nuclear physics Abundance of radiogenic heat (mainly of the Radiogenic mantle) to the total Earth’s radioactive heat surface heat flux , which is an elements (main goal) important margin, test, and input at the same time for many geophysical and geochemical models of the Earth; Distribution of radioactive elements • Further goals: U/Th bulk ratio, (geological models) tests and discrimination among geological models, Earth composition models, study of the mantle homogeneity or From geoneutrino stratification , insights to the To predict: Geoneutrino flux measurement: processes of Earth’formation, additional sources of heat?, idea of U-based georeactor Neutrino geoscience: truly inter-disciplinary field!
BOREXINO DETECTOR Laboratori Nazionali del Gran Sasso, Italy 3800 m.w.e 4300 muons/day crossing the inner detector 278 ton liquid scintillator (LS) • the world’s radio-purest LS detector < 9 × 10 -19 g(Th)/g LS , < 8 × 10 -20 g(U)/g LS • ~500 hit PMTs / MeV • energy reconstruction: 5 keV (5%) @ 1 MeV NIM A600 (2009) 568 • position reconstruction: 10 cm @ 1 MeV • pulse shape identification ( α / β , e + /e - ) Operating since 2007
ANTINEUTRINO DETECTION WITH LIQUID SCINTILLATORS Electron antineutrino detection: delayed coincidence Energy threshold = 1.8 MeV • Inverse Beta Decay (IBD) σ @ few MeV: ~10 -42 cm 2 • Charge current, electron flavour only (~100 x more than scattering) E prompt = E visible = T e+ + 2 x 511 keV ~ E antinu – 0.784 MeV e + ν e W n p
EXPECTED GEONEUTRINO SIGNAL AT GRAN SASSO LOCAL AND GLOBAL GEOLOGICAL INFORMATION GEONEUTRINO ENERGY SPECTRA • σ ( IBD) ~10 -42 cm 2 • <P ee > ~0.55 U, Th abundances & distribution + density profiles GEONEUTRINO SIGNAL AT LNGS 1 TNU (Terrestrial Neutrino Unit) = 1 event / 10 32 target protons (~1kton LS) / year with 100% detection efficiency S (U + Th) S(Th)/S(U) H (U + Th [TNU] +K) [TW] Local Crust (~500 km around LNGS) 9.2 ± 1.2 0.24 - Bulk Lithosphere (observed) 25.9 +4.9 0.29 8.1 +1.9 -4.1 -1.4 2.5 – 19.6 3.2 – 25.4 Mantle = Bulk Silicate Earth model 0.26 – lithosphere (assuming for BSE chondritic value of 0.27 ) 28.5 – 45.5 11.3 – 33.5 Total 0.27 (chondritic)
OPTIMIZED IBD SELECTION CUTS Efficiency: (86.98 ± 1.50)% Charge of prompt Time correlation Charge of delayed Space correlation Q d > 700 (860) – 3000 pe Q p > 408 pe dt = (2.5-12.5) µ s + (20-1280) µ s dR < 1.3 m Neutron capture τ = ( 254.5 ± 1.8) µ s • Neutron captures on proton • Prompt spectrum (2.2 MeV) and in about 1% of starts at 1 MeV 2 cluster event in 16 µ s DAQ gate cases on 12 C (4.95 MeV) • 5% energy resolution • Spill out effect at the nylon @ 1 MeV prompt delayed inner vessel border • Radon correlated 214 Po ( α + γ ) decays from 214 Bi and 214 Po fast coincidences Muon veto Multiplicity α / β discrimination Dynamic Fiducial Volume 2s || 1.6 s : 9 Li( β + n) > 10 cm from IV (prompt) No event with Q >400 pe MLP delayed > 0.8 ± 2 ms around promt/delayed 2 ms: neutrons • Exposure vs accidental bgr • Radon correlated 214 Po ( α + γ ) • IV has a leak: shape reco from • Several veto categories the data weekly • Strict and special muon tags • Suppressing undetected cosmogenic background, mostly Whole detector o multiple neutrons Cylinder o • Negligible exposure loss Only 2.2% exposure loss
GOLDEN CANDIDATES: 154 Prompt charge spectrum Delayed charge spectrum • December 9, 2007 to April 28, 2019 • 3262.74 days of data taking • Average FV = (245.8 ± 8.7) ton n+ 1 H n+ 12 C • Exposure = (1.29 ± 0.05) x 10 32 proton x year • Including systematics on position reconstruction and muon veto loss, for 100% detection eff. Distribution in time Radial distribution Distance to the Inner Vessel
NEUTRINO BACKGROUNDS Reactor antineutrinos Atmospheric neutrinos Mueller et al 2011 With “5 MeV bump” Energy Geoneutrino Reactor > 1 MeV window antineutrino Signal [TNU] 84.5 +1.5 -1.4 79.6 +1.4 -1.3 Events 2.2 ± 1.1 3.3 ± 1.6 9.2 ± 4.6 # Events 97.6 +1.7 -1.6 91.9 +1.6 -1.5 • For all ~440 world reactors (1.2 TW total power) ü their nominal thermal powers (PRIS database of IAEA) • Estimated 50% uncertainty on the prediction ü monthly load factors (PRIS database) • Indications of overestimation ü distance to LNGS (no reactors in Italy) • Included in the systematic error • 235 U, 238 U, 239 Pu , and 241 Pu fuel • Atmospheric neutrino fluxes ü power fractions for different reactor types from HKKM2014 (>100 MeV) and FLUKA (<100 MeV) ü energy released per fission ü energy spectra (Mueller at al. 2011 and Daya Bay) • Matter effects included • P ee electron neutrino survival probability • IBD cross section • Detection efficiency = 0.8955 ± 0.0150 Charge spectrum after IBD selection cuts
Accidentals NON-ANTINEUTRINO BACKGROUNDS R acc = (3029.0 ± 12.7) s -1 including scaling factor exp(-R muon x 2s) = 0.896 9 Li ( β +n) events < 2s after muons 12 C( 210 Po( α ) , n) 16 O due to the 2 s muon veto before delayed IBD-like events in dt = 2 -20 s Y n = (1.45 ± 0.22) x 10 -7 τ measured = (0.260 ± 0.021) s ε IBD-like = 0.56 for 210 Po in LS Charge of prompt (0.260 $\pm$ 0.021)\, Distance from muon track < 210 Po rate> = (12.75 ± 0.08) cpd/ton
SPECTRAL FIT with chondritic Th/U ratio Reactor expectations with and without 5 MeV bump # Geoneutrino events 8 σ 5 σ 3 σ 1 σ # Reactor events Prompt charge [photoelectrons]: 1 MeV ~500 photoelectrons Resulting number of geoneutrinos (median value) • Unbinned likelihood fit of charge spectrum of 154 prompts + 9.4 ( stat ) − 2.1 + 2.7 ( sys ) events • S(Th)/S(U) = 2.7 (corresponds to chondritic Th/U mass ratio of 3.9) 52.6 − 8.6 • Reactor signal unconstrained and result compatible with expectations + 18.3 % • 9 Li, accidentals, and ( α , n) bgr constrained according to expectations total precision − 17.2 • Systematics includes atmospheric neutrinos, shape of reactor spectrum, vessel shape and position reconstructions, detection efficiency
GEONEUTRINO SIGNAL AT LNGS + 8,4 ( stat ) − 1.9 + 2,4 ( sys ) TNU 47.0 − 7.7 LOC = local crust = (9.2 ± 1.2) TNU FFL = far-field lithosphere = (4.0 +1.4 _1.0 ) TNU MANTLE (U + Th abundances) = BSE model – LITHOSPHERE Intermediate scenario 2 layer distribution of U and Th in the mantle J: Javoy at al., 2010 L&K: Lyubetskaya and Korenaga, 2007 T: Taylor, 1980 M&S: Mc Donough and Sun, 1995 A : Anderson, 2007 In agreement with expectations W : Wang, 2018 P&O: Palme and O’Neil, 2003 T&S : Turcotte and Schubert, 2002
SPECTRAL FIT with Th and U fit independently # 232 Th events 3 σ 2 σ 1 σ Chondritic ratio 232 Th / 238 U ratio Prompt charge [photoelectrons]: 1 MeV ~500 photoelectrons # 238 U events Resulting number of geoneutrinos (median value) no sensitivity to measure Th/U ratio 50.4 events +46.8 -44.05 % total precision • In agreement with the fit with Th/U fixed • Larger error
MANTLE GEONEUTRINO SIGNAL Likelihood q obs = 5.4479 p value = 9.796 x 10 -3 # Mantle events Prompt charge [photoelectrons]: 1 MeV ~500 photoelectrons • Fit performed with signal from lithosphere constrained to Mantle signal (median value) (28.8 ± 5,6) events with S(Th)/S(U) = 0.29 + 10.7 events 23.7 − 10.1 • Mantle PDF constructed with S(Th)/S(U) = 0.26, maintaining the global Th/U ratio as in CI chondrites + 9.6 TNU 21.2 − 9.1 • Sensitivity study using log-likelihood ratio method: null hypothesis rejected with 99.0% C.L.
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