Potential of Geo-neutrino Measurements at JUNO

The flux of geoneutrinos at any point on the Earth is a function of the abundance and distribution of radioactive elements within our planet. This flux has been successfully detected by the 1-kt KamLAND and 0.3-kt Borexino detectors with these measurements being limited by their low statistics. The planned 20-kt JUNO detector will provide an exciting opportunity to obtain a high statistics measurement, which will provide data to address several questions of geological importance. This paper presents the JUNO detector design concept, the expected geo-neutrino signal and corresponding backgrounds. The precision level of geo-neutrino measurements at JUNO is obtained with the standard least-squares method. The potential of the Th/U ratio and mantle measurements is also discussed.

💡 Research Summary

The paper “Potential of Geo‑neutrino Measurements at JUNO” evaluates how the upcoming Jiangmen Underground Neutrino Observatory (JUNO), a 20‑kiloton liquid‑scintillator detector, can dramatically improve geo‑neutrino science compared with the existing 1‑kt KamLAND and 0.3‑kt Borexino experiments. Geo‑neutrinos are electron antineutrinos produced in the β‑minus decays of the Earth’s heat‑producing elements (U, Th, and K). Their flux encodes the distribution of these radioactive elements and therefore the radiogenic contribution to the Earth’s surface heat flow (≈ 46 ± 3 TW). Current measurements are limited by low statistics, yielding uncertainties of order 20 % on the total flux and on the Th/U ratio, and they cannot separate the mantle contribution from the crustal one.

JUNO’s design is described in detail. The central detector consists of a 17‑m‑radius acrylic sphere filled with 20 kt of linear alkylbenzene (LAB) scintillator, instrumented with about 17 000 20‑inch photomultiplier tubes (PMTs) mounted on a stainless‑steel truss. An outer water pool equipped with PMTs acts as a Cherenkov muon veto, and a top tracker further improves muon tagging. The detector will be placed 700 m underground, reducing cosmic‑ray backgrounds. An IBD (inverse beta decay) detection efficiency of 80 % and a free‑proton target of 1.285 × 10³³ are assumed.

The expected geo‑neutrino signal is calculated using the Reference Earth Model (RM) of Huang et al., which divides the silicate Earth into eight lithospheric reservoirs (ice, water, soft and hard sediment, upper, middle and lower continental crust, lithospheric mantle) and two mantle reservoirs (depleted and enriched). The model assigns U and Th abundances on a 1° × 1° grid based on geochemical and geophysical constraints. The lithospheric U contribution is 23.2 + 5.9 − 4.8 TNU (Terrestrial Neutrino Units, 1 TNU = 10⁻³² events yr⁻¹ per proton). Assuming a chondritic Th/U mass ratio of 3.9, the Th contribution follows directly. Mantle contributions are parameterized by two extreme β factors (β_low = 12.15 TNU, β_high = 17.37 TNU) corresponding to a thin basal layer of U or a uniform distribution throughout the mantle. The resulting total signal S(U+Th) varies from ~0 to 70 TNU as the combined radiogenic heat H(U+Th) spans 0–45 TW, defining a band bounded by the two β scenarios.

Backgrounds are examined in depth. The dominant background arises from reactor antineutrinos, especially the two nearby nuclear power plants (Yangjiang and Taishan) with a combined thermal power of 36 GW. Using the standard IBD rate formula, the authors estimate 980 ± 27 TNU from all world reactors (2013 data) and 1510 ± 423 TNU from the two local plants alone. Systematic uncertainties (thermal power, fission fractions, IBD cross‑section, oscillation parameters) sum to about 2.8 %. Non‑antineutrino backgrounds include β‑n decays from ⁹Li and ⁸He produced by cosmic muons (≈ 84 day⁻¹ before veto, reduced to 1.8 ± 0.36 day⁻¹ after muon veto cuts), fast neutrons (≈ 0.01 day⁻¹, negligible), and (α,n) reactions on ¹³C (suppressed by achieving ultra‑low U/Th/K contamination levels of 10⁻¹⁵–10⁻¹⁶ g/g). Overall, after applying muon veto and IBD selection, the non‑reactor background is sub‑percent compared with the expected geo‑neutrino signal.

Statistical precision is evaluated with a standard least‑squares (χ²) method that incorporates Poisson counting statistics and systematic uncertainties on signal and background components. Simulations assuming 10 years of data (≈ 10⁴ geo‑neutrino events) indicate that JUNO can measure the total geo‑neutrino flux with ~3 % uncertainty, the Th/U ratio to better than 10 %, and the mantle contribution with ~15 % uncertainty. This represents a 5–10‑fold improvement over KamLAND and Borexino, sufficient to discriminate between competing bulk‑Earth compositional models (cosmochemical, geochemical, geodynamical) and to test hypotheses about mantle layering (e.g., a two‑layer mantle with distinct U/Th distributions).

Because JUNO is situated near a continental margin, its geo‑neutrino flux has a relatively large crustal component. By jointly fitting the measured spectrum with the detailed regional lithospheric model (the nearest 500 km) and the global mantle model, the experiment can extract the mantle signal despite the dominant crustal background. The authors argue that this capability will provide the first high‑precision mantle geo‑neutrino measurement from a continental site, complementing future measurements from oceanic sites (e.g., SNO+).

In conclusion, the paper demonstrates that JUNO’s large target mass, excellent energy resolution, deep underground location, and sophisticated muon veto system will enable a high‑statistics, low‑background geo‑neutrino dataset. The resulting precision on the total flux, Th/U ratio, and mantle contribution will substantially advance our understanding of Earth’s radiogenic heat budget, compositional structure, and mantle dynamics, and will open new interdisciplinary collaborations among particle physicists, geologists, and nuclear‑security experts.