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This paper describes the Borexino detector and the high-radiopurity studies and tests that are integral part of the Borexino technology and development. The application of Borexino to the detection and studies of geoneutrinos is discussed.
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Borexino is a massive calorimetric liquid scintillation detector whose installation has been completed in the underground Gran Sasso Laboratory. The focus of the experiment is on the direct and real time measurement of the flux of neutrinos produced in the $^{7}Be$ electron capture reaction in the Sun. Furthermore, recent studies about the reduction of the $^{11}C$ background through suitable rejection techniques demonstrated the possibility to open an interesting additional observation window in the energy region of the pep and CNO solar neutrinos. Beyond the solar neutrino program, the detector will be also a powerful observatory for antineutrinos from Supernovae, as well as for geoneutrinos, profiting from a very low background from nuclear reactors.
Geo-neutrinos, electron antineutrinos from natural radioactive decays inside the Earth, bring to the surface unique information about our planet. The new techniques in neutrino detection opened a door into a completely new inter-disciplinary field of Neutrino Geoscience. We give here a broad geological introduction highlighting the points where the geo-neutrino measurements can give substantial new insights. The status-of-art of this field is overviewed, including a description of the latest experimental results from KamLAND and Borexino experiments and their first geological implications. We performed a new combined Borexino and KamLAND analysis in terms of the extraction of the mantle geo-neutrino signal and the limits on the Earths radiogenic heat power. The perspectives and the future projects having geo-neutrinos among their scientific goals are also discussed.
Geoneutrinos are electron antineutrinos ($bar u_e$) generated by the beta-decays of radionuclides naturally occurring inside the Earth, in particular $^{238}$U, $^{232}$Th, and $^{40}$K. Measurement of these neutrinos provides powerful constraints on the radiogenic heat of the Earth and tests on the Earth models. Since the prediction of $bar u_e$s in geoneutrino flux is subject to neutrino oscillation effects, we performed a calculation including detailed oscillation analysis in the propagation of geoneutrinos and reactor neutrinos generated around the Earth. The expected geoneutrino signal, the reactor neutrino background rates and the systematic error budget are provided for a proposed 3-kiloton neutrino detector at the Jinping underground lab in Sichuan, China. In addition, we evaluated sensitivities for the geoneutrino flux, Th/U ratio and power of a possible fission reactor in the interior of Earth.
This paper presents a geoneutrino measurement using 3262.74 days of data taken with the Borexino detector at LNGS in Italy. By observing $52.6 ^{+9.4}_{-8.6} ({rm stat}) ^{+2.7}_{-2.1}({rm sys})$ geoneutrinos (68% interval) from $^{238}$U and $^{232}$Th, a signal of $47.0^{+8.4}_{-7.7},({rm stat)}^{+2.4}_{-1.9},({rm sys})$ TNU with $^{+18.3}_{-17.2}$% total precision was obtained. This result assumes the same Th/U mass ratio found in chondritic CI meteorites but compatible results were found when contributions from $^{238}$U and $^{232}$Th were fit as free parameters. Antineutrino background from reactors is fit unconstrained and found compatible with the expectations. The null-hypothesis of observing a signal from the mantle is excluded at a 99.0% C.L. when exploiting the knowledge of the local crust. Measured mantle signal of $21.2 ^{+9.6}_{-9.0} ({rm stat})^{+1.1}_{-0.9} ({rm sys})$ TNU corresponds to the production of a radiogenic heat of $24.6 ^{+11.1}_{-10.4}$ TW (68% interval) from $^{238}$U and $^{232}$Th in the mantle. Assuming 18% contribution of $^{40}$K in the mantle and $8.1^{+1.9}_{-1.4}$ TW of radiogenic heat of the lithosphere, the Borexino estimate of the total Earth radiogenic heat is $38.2 ^{+13.6}_{-12.7}$ TW, corresponding to a convective Urey ratio of 0.78$^{+0.41}_{-0.28}$. These values are compatible with different geological models, however there is a 2.4$sigma$ tension with those which predict the lowest concentration of heat-producing elements. By fitting the data with a constraint on the reactor antineutrino background, the existence of a hypothetical georeactor at the center of the Earth having power greater than 2.4 TW at 95% C.L. is excluded. Particular attention is given to all analysis details, which should be of interest for the next generation geoneutrino measurements.
Borexino is an organic liquid scintillator detector located in the underground Gran Sasso National Laboratory (Italy). It is devoted mainly to the real time spectroscopy of low energy solar neutrinos via the elastic scattering on electrons in the target mass. The data taking campaign started in 2007 and led to key measurements of 7}Be and 8B solar neutrinos as well as antineutrinos from the earth (geo-neutrinos) and from nuclear power reactors. Borexino is also a powerful tool for the study of cosmic muons that penetrate the Gran Sasso rock coverage and thereby induced signals such as neutrons and radioactive isotopes which are today of critical importance for upcoming dark matter and neutrino physics experiments. Having reached 4y of continuous data taking we analyze here the muon signal and its possible modulation. The muon flux is measured to be (3.41+-0.01)E-4/m2/s. A modulation of this signal with a yearly period is observed with an amplitude of (1.29+-0.07)% and a phase of (179+-6) d, corresponding to June 28th. Muon rate fluctuations are compared to fluctuations in the atmospheric temperature on a daily base, exploiting the most complete atmospheric data and models available. The distributions are shown to be positively correlated and the effective temperature coefficient is measured to be alpha_T = 0.93 +- 0.04. This result is in good agreement with the expectations of the kaon-inclusive model at the laboratory site and represents an improvement over previous measurements performed at the same depth.
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