Expected geoneutrino signal at JUNO

Constraints on the Earth’s composition and on its radiogenic energy budget come from the detection of geoneutrinos. The KamLAND and Borexino experiments recently reported the geoneutrino flux, which reflects the amount and distribution of U and Th inside the Earth. The KamLAND and Borexino experiments recently reported the geoneutrino flux, which reflects the amount and distribution of U and Th inside the Earth. The JUNO neutrino experiment, designed as a 20 kton liquid scintillator detector, will be built in an underground laboratory in South China about 53 km from the Yangjiang and Taishan nuclear power plants. Given the large detector mass and the intense reactor antineutrino flux, JUNO aims to collect high statistics antineutrino signals from reactors but also to address the challenge of discriminating the geoneutrino signal from the reactor background.The predicted geoneutrino signal at JUNO is 39.7 $^{+6.5}_{-5.2}$ TNU, based on the existing reference Earth model, with the dominant source of uncertainty coming from the modeling of the compositional variability in the local upper crust that surrounds (out to $\sim$ 500 km) the detector. A special focus is dedicated to the 6{\deg} x 4{\deg} Local Crust surrounding the detector which is estimated to contribute for the 44% of the signal. On the base of a worldwide reference model for reactor antineutrinos, the ratio between reactor antineutrino and geoneutrino signals in the geoneutrino energy window is estimated to be 0.7 considering reactors operating in year 2013 and reaches a value of 8.9 by adding the contribution of the future nuclear power plants. In order to extract useful information about the mantle’s composition, a refinement of the abundance and distribution of U and Th in the Local Crust is required, with particular attention to the geochemical characterization of the accessible upper crust.

💡 Research Summary

The paper presents a comprehensive forecast of the geoneutrino signal that will be observed by the Jiangmen Underground Neutrino Observatory (JUNO), a 20‑kiloton liquid‑scintillator detector to be built in South China, roughly 53 km from the Yangjiang and Taishan nuclear power plants. Geoneutrinos—electron antineutrinos emitted in the β‑decays of ^238U and ^232Th—provide a unique probe of the Earth’s radiogenic heat budget and the distribution of these heat‑producing elements in the crust and mantle. The authors first review the status of geoneutrino measurements by KamLAND and Borexino, noting that while both experiments have detected the flux, limited statistics and substantial reactor‑antineutrino backgrounds have prevented a precise separation of crustal and mantle contributions.

JUNO’s large target mass and excellent energy resolution (≈3 %/√E) will yield several thousand inverse‑beta‑decay (IBD) events per year from reactor antineutrinos, offering unprecedented statistical power. However, the same proximity to powerful reactors also creates a formidable background in the 1.8–3.3 MeV energy window where geoneutrinos are observed. Using a worldwide reference model of reactor antineutrino emissions, the authors calculate that, for the 2013 reactor configuration, the reactor‑to‑geoneutrino signal ratio in this window is 0.7. When the planned expansion of the Yangjiang and Taishan plants is included, the ratio climbs to 8.9, underscoring the necessity of precise background modeling and spectral discrimination techniques.

The predicted geoneutrino signal at JUNO, based on the standard Reference Earth Model, is 39.7 TNU (Terrestrial Neutrino Units) with an asymmetric uncertainty of +6.5/‑5.2 TNU. The dominant source of this uncertainty is the compositional variability of the local upper crust within roughly 500 km of the detector. The authors devote special attention to a 6° × 4° “Local Crust” region surrounding JUNO, which alone contributes about 44 % of the total signal (≈17.5 TNU). By integrating detailed geological, geophysical, and geochemical data—rock type maps, crustal thickness, density models, and measured U/Th abundances—they quantify the contribution of this region but also reveal that the spread in possible U and Th concentrations leads to the bulk of the error budget.

In contrast, the mantle accounts for roughly 20 % of the total geoneutrino flux, but its contribution is currently obscured by the larger uncertainty in the crustal component. The paper argues that a refined characterization of the Local Crust would dramatically improve the ability to isolate the mantle signal, enabling a measurement of the mantle’s radiogenic heat production with an accuracy of order 10–15 %. To achieve this, the authors propose an integrated program of field sampling, high‑precision isotope analysis, and high‑resolution gravity and seismic tomography to constrain the upper‑crustal U/Th distribution to within ~10 %.

Methodologically, the study employs Bayesian inference combined with Markov‑Chain Monte‑Carlo (MCMC) simulations to simultaneously fit the reactor and geoneutrino spectra. The simulations indicate that, assuming the current Earth model, JUNO could determine the mantle U/Th ratio to within ~15 % after several years of data taking, provided the crustal uncertainties are reduced as suggested.

In summary, JUNO promises a leap forward in geoneutrino science thanks to its massive target and superb detector performance. Yet the key limitation is not statistical but systematic: the compositional heterogeneity of the local upper crust. By focusing future research on detailed geochemical mapping of the 6° × 4° area around the detector and by maintaining a real‑time monitoring system for reactor antineutrino fluxes, the experiment can extract a clean mantle signal. This will furnish a direct, model‑independent estimate of the Earth’s radiogenic heat budget, offering critical constraints for geophysical models of mantle convection, plate tectonics, and the thermal evolution of our planet.