Measurement of geo-neutrinos from 1353 days of Borexino

We present a measurement of the geo--neutrino signal obtained from 1353 days of data with the Borexino detector at Laboratori Nazionali del Gran Sasso in Italy. With a fiducial exposure of (3.69 ± 0.16) × 1031 proton × year after all selection cuts and background subtraction, we detected (14.3 ± 4.4) geo-neutrino events assuming a fixed chondritic mass Th/U ratio of 3.9. This corresponds to a geo-neutrino signal Sgeo = (38.8 ± 12.0) TNU with just a 6 × 10−6 probability for a null geo-neutrino measurement. With U and Th left as free parameters in the fit, the relative signals are STh = (10.6 ± 12.7) TNU and SU = (26.5 ± 19.5) TNU. Borexino data alone are compatible with a mantle geo--neutrino signal of (15.4 ± 12.3) TNU, while a combined analysis with the KamLAND data allows to extract a mantle signal of (14.1 ± 8.1) TNU. Our measurement of a reactor anti--neutrino signal Sreact = 84.5+19.3−18.9 TNU is in agreement with expectations in the presence of neutrino oscillations.