Comprehensive geoneutrino analysis with Borexino

Abstract

This paper presents a comprehensive geoneutrino measurement using the Borexino detector, located at Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The analysis is the result of 3262.74 days of data between December 2007 and April 2019. The paper describes improved analysis techniques and optimized data selection, which includes enlarged fiducial volume and sophisticated cosmogenic veto. The reported exposure of ð1.29 0.05Þ × 1032 protons × year represents an increase by a factor of two over a previous Borexino analysis reported in 2015. By observing 52.6þ9.4 −8.6 ðstatÞ þ2.7 −2.1 ðsysÞ geoneutrinos (68% interval) from 238U and 232Th, a geoneutrino signal of 47.0þ8.4 −7.7 ðstatÞþ2.4 −1.9 ðsysÞ TNU with þ18.3 −17.2 % total precision was obtained. This result assumes the same Th/U mass ratio as found in chondritic CI meteorites but compatible results were found when contributions from 238U and 232Th were both fit as free parameters. Antineutrino background from reactors is fit unconstrained and found compatible with the expectations. The null-hypothesis of observing a geoneutrino signal from the mantle is excluded at a 99.0% C.L. when exploiting detailed knowledge of the local crust near the experimental site. Measured mantle signal of 21.2þ9.5 −9.0 ðstatÞ þ1.1 −0.9 ðsysÞ TNU corresponds to the production of a radiogenic heat of 24.6þ11.1 −10.4 TW (68% interval) from 238U and 232Th in the mantle. Assuming 18% contribution of 40K in the mantle and 8.1þ1.9 −1.4 TW of total radiogenic heat of the lithosphere, the Borexino estimate of the total radiogenic heat of the Earth is 38.2þ13.6 −12.7 TW, which corresponds to the convective Urey ratio of 0.78þ0.41 −0.28 . These values are compatible with different geological predictions, however there is a ∼2.4σ tension with those Earth models which predict the lowest concentration of heat-producing elements in the mantle. In addition, by constraining the number of expected reactor antineutrino events, the existence of a hypothetical georeactor at the center of the Earth having power greater than 2.4 TW is excluded at 95% C.L. Particular attention is given to the description of all analysis details which should be of interest for the next generation of geoneutrino measurements using liquid scintillator detectors.