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 (1.29±0.05)×1032 protons ×year represents an increase by a factor of two over a previous Borexino analysis reported in 2015. By observing 52.6−8.6+9.4(stat)−2.1+2.7(sys) geoneutrinos (68% interval) from U238 and Th232, a geoneutrino signal of 47.0−7.7+8.4(stat)−1.9+2.4(sys) TNU with −17.2+18.3% 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 U238 and Th232 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 21.2−9.0+9.5(stat)−0.9+1.1(sys) TNU corresponds to the production of a radiogenic heat of 24.6−10.4+11.1 TW (68% interval) from U238 and Th232 in the mantle. Assuming 18% contribution of K40 in the mantle and 8.1−1.4+1.9 TW of total radiogenic heat of the lithosphere, the Borexino estimate of the total radiogenic heat of the Earth is 38.2−12.7+13.6 TW, which corresponds to the convective Urey ratio of 0.78−0.28+0.41. 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.50 MoreReceived 5 September 2019Corrected 25 February 2020DOI:https://doi.org/10.1103/PhysRevD.101.012009Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNeutrino interactionsTechniquesNeutrino detectorsScintillatorsParticles & Fields

Highlights

  • Neglecting the small contribution (

  • The index i stands for the different components of nuclear fuel (235U, 238U, 239Pu, and 241Pu), pi is the power fraction of the component i, Qi is the energy released per fission of the component i taken from [99] with a 0.2% quoted uncertainty, φiðEνeÞ is the antineutrino spectrum originating from the fission of the ith component, σðEνeÞ is the inverse beta decay (IBD) cross section [84], and Pee is the survival probability use in Eq (11)

  • Borexino is 280-ton liquid scintillator neutrino detector located at Laboratori Nazionali del Gran Sasso (LNGS) in

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Summary

INTRODUCTION

The most abundant massive particles in the universe, are produced by a multitude of different. Large-volume liquid-scintillator neutrino experiments KamLAND [12,13,14,15] and Borexino [16,17,18] have demonstrated the capability to efficiently detect a geoneutrino signal These detectors are offering a unique insight into 200 years long discussion about the origin of the Earth’s internal heat sources. Both procedures are based on Borexino Monte Carlo (MC) [27], that was tuned on independent calibration data. The acronyms used within the text are listed in alphabetical order in the Appendix

WHY STUDY GEONEUTRINOS?
59 Æ 12 113 Æ 24 142 Æ 14 224 Æ 10
THE BOREXINO DETECTOR
Borexino data structure
The main DAQ
The FADC DAQ subsystem
Muon detection
External muon flag
Strict internal muon flag
Special muon flags
FADC muon identification
Internal large muon flag
Inner vessel shape reconstruction
Gatti optimal filter
Multilayer perceptron
ANTINEUTRINO DETECTION
EXPECTED ANTINEUTRINO SIGNAL
Neutrino oscillations
Geoneutrinos
Geoneutrino energy spectra
Geological inputs
Reactor antineutrinos
Atmospheric neutrinos
Georeactor
Summary of antineutrino signals
NONANTINEUTRINO BACKGROUNDS
Cosmogenic background
Hadronic background
Untagged muons
Fast cosmogenic neutrons
Accidental coincidences
Radon background
Muon vetoes
DATA SELECTION CUTS
Veto after internal muons
Time coincidence
Single cluster events
Double cluster events
Space correlation
Pulse shape discrimination
Energy cuts
Charge of prompt signal
Charge of delayed
Dynamical fiducial volume cut
Multiplicity cut
VIII. MONTE CARLO OF SIGNAL AND BACKGROUNDS
Monte Carlo spectral shapes
Detection efficiency
EVALUATION OF THE EXPECTED SIGNAL AND BACKGROUNDS WITH OPTIMIZED CUTS
Data set and exposure
TW Georeactor GR2
Fast neutrons
SENSITIVITY TO GEONEUTRINOS
Geoneutrino analysis in a nutshell
Sensitivity study
Expected sensitivity
RESULTS
Golden candidates
Analysis
Th and U as free fit parameters
Shape of the reactor spectrum
MC efficiency
Position reconstruction
Geoneutrino signal at LNGS
Extraction of mantle signal
Estimated radiogenic heat
Testing the georeactor hypothesis
CONCLUSION
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