Abstract
Borexino is a liquid scintillator detector primary designed to observe solar neutrinos. Due to its low background level as well as its position in a nuclear free country, Italy, Borexino is also sensitive to geo-neutrinos. Borexino is leading this interdisciplinary field of neutrino geoscience by studying electron antineutrinos which are emitted from the decay of radioactive isotopes present in the crust and the mantle of the Earth. With 2056 days of data taken between December 2007 and March 2015, Borexino observed 77 antineutrino candidates. If we assume a chondritic Th/U mass ratio of 3.9, the number of geo-neutrino events is found to be 23.7+6.5 -5.7(stat) +0.9-0.6 (syst). With this measurement, Borexino alone is able to reject the null geo-neutrino signal at 5.9σ, to claim a geo-neutrino signal from the mantle at 98% C.L. and to restrict the radiogenic heat production for U and Th between 23 and 36 TW.
Highlights
Geo-neutrinos are electron antineutrinos which are produced by the decay of radioactive isotopes present in the crust and the mantle of our planet
In order to measure the number of geo-neutrinos and antineutrinos from nuclear reactors, we implement an unbinned maximum likelihood fit of the prompt energy spectrum of our antineutrino candidates
Nbg − (Nbg)estest with: fνe (Ei, Ngeo, Nreact) = fgeo(Ei, Ngeo) + freact(Ei, Nreact) fbg(Ei, Nacc, NLiHe, Nαn) = facc(Ei, Nacc) + fLiHe(Ei, NLiHe) + fαn(Ei, Nαn) where Nexp corresponds to the expected total number of events and i runs over the N = 77 antineutrino candidates. fgeo, freact, facc, fLiHe and fαn are the individual spectra of the geo-neutrinos, the antineutrinos from nuclear reactors, the accidental coincidences, the 9Li-8He events and the (α, n) events
Summary
Geo-neutrinos are electron antineutrinos which are produced by the decay of radioactive isotopes present in the crust and the mantle of our planet. In order to measure the number of geo-neutrinos and antineutrinos from nuclear reactors, we implement an unbinned maximum likelihood fit of the prompt energy spectrum of our antineutrino candidates. Nbg − (Nbg)est (δbg)est with: fνe (Ei, Ngeo, Nreact) = fgeo(Ei, Ngeo) + freact(Ei, Nreact) fbg(Ei, Nacc, NLiHe, Nαn) = facc(Ei, Nacc) + fLiHe(Ei, NLiHe) + fαn(Ei, Nαn) where Nexp corresponds to the expected total number of events and i runs over the N = 77 antineutrino candidates.
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