Geo-neutrinos, electron anti-neutrinos produced in beta decays of naturally occurring radioactive isotopes in the Earth, are a unique direct probe of our planet's interior. We report the first observation at more than 3 sigma C.L. of geo-neutrinos, performed with the Borexino detector at Laboratori Nazionali del Gran Sasso. Anti-neutrinos are detected through the neutron inverse beta decay reaction. With a 252.6 ton yr fiducial exposure after all selection cuts, we detected 9.9(-3.4)(+4.1)((+14.6)(-8.2))geo-neutrino events, wills errors corresponding to a 68.3% (99.73%) C.L. From the In C.. profile, the statistical significance of the Borexino geo-neutrino observation corresponds to a 99.997% C.L.
Our measurement of the geo-neutrinos rate is 3.9(-1.3)(+1.6)((+5.8)(-3.2)) events/(100 ton yr).
The observed prompt positron spectrum above 2.6 MeV is compatible with that expected from European nuclear reactors (mean base line of approximately 1000 km). Our measurement of reactor anti-neutrinos excludes the non-oscillation hypothesis at 99.60% C.L. This measurement rejects the hypothesis of an active geo-reactor in the Earth's core with a power above 3 TW at 95% C.L. (C) 2010 Elsevier B.V. All rights reserved.