Terrestrial antineutrino (geoneutrino) measurement of the Earth's heat-producing elements: the never-observed $^{40}$K component and the mantle distribution

Radioactive decays of $^{238}$U, $^{232}$Th and $^{40}$K inside the Earth emit electron antineutrinos ("geoneutrinos") whose flux and spectrum at the surface encode the abundance and distribution of these heat-producing elements and hence the planet's radiogenic heat. In 1984, Krauss, Glashow and Schramm posed the problem of making this "antineutrino astronomy" a quantitative geophysical tool: developing detectors able to register the tiny terrestrial signal while discriminating reactor and cosmic (supernova) backgrounds, and using the measurement to constrain the deep-Earth radioactivity that no rock sample can reach. The first half of that program has been achieved, with a technology very different from the one envisioned: inverse-beta-decay spectroscopy in kiloton-scale underground liquid-scintillator detectors (KamLAND 2005, Borexino 2010) has measured the uranium- and thorium-chain geoneutrino flux and constrained the Earth's radiogenic heat, and neutrino oscillations are now independently characterized. The open core is the completion of the original objective: the $^{40}$K component — expected to carry roughly a fifth to a third of the radiogenic heat — has never been observed because its endpoint (1.311 MeV) lies below the inverse-beta-decay threshold (1.806 MeV); the mantle contribution is extracted only through model-dependent subtraction of the crustal flux; and mapping the spatial distribution of heat-producing elements requires multi-site or direction-sensitive detectors. The problem is to close these gaps and determine the abundance, distribution and radiogenic heat of U, Th and K in the Earth from direct antineutrino measurement.

Background

The Earth emits electron antineutrinos ($\bar{\nu}_e$) because its interior contains the long-lived radioactive isotopes $^{238}$U, $^{232}$Th and $^{40}$K, whose beta decays — and the beta decays in the uranium and thorium decay chains — produce antineutrinos in the MeV energy range. These "geoneutrinos" interact only through the weak nuclear force and escape from the deep interior essentially unattenuated, so their flux and energy spectrum at the surface are a direct messenger of the amount and distribution of the heat-producing elements (HPEs), the same decays that generate part of the Earth's internal heat. How large that radiogenic heat contribution is remains a central open quantity in Earth science: the total heat flowing out of the Earth's surface is about 44–47 terawatts (TW), but the split between residual primordial heat (secular cooling of the planet) and ongoing radioactive heating is known only through compositional models of the Bulk Silicate Earth (BSE — the composition of the Earth minus its core), for which several competing classes exist (cosmochemical, geochemical and geodynamical models, built from different meteorite analogues and geophysical constraints). Direct rock sampling reaches only the outer few hundred kilometers; the mantle's content of heat-producing elements cannot be measured chemically.

Krauss, Glashow and Schramm posed this agenda in their 1984 Nature review "Antineutrino astronomy and geophysics": radioactive decays inside the Earth produce antineutrinos that may be detectable at the surface; their flux and spectrum contain important geophysical information; and new detectors need to be developed that discriminate between antineutrino sources, including the cosmic background — the diffuse supernova neutrino background (DSNB), the accumulated antineutrino glow of all past core-collapse supernovae, whose magnitude is tied to the cosmic supernova frequency. In the early 1980s no practical detection scheme existed. The review analyzed radiochemical detection — resonant charged-current capture (absorption of an antineutrino by a nucleus, with emission of a positron) on low-threshold isotopes such as $^{79}$Br, producing long-lived or stable daughter atoms to be counted later by resonance ionization spectroscopy, a laser technique capable of counting individual atoms of a chosen element — and concluded that the required technology lay far in the future. It also stressed that the true HPE abundances of the deep Earth were unknown, so any flux prediction was model-dependent; that neutrino oscillations, then unconfirmed, could distort the measured spectrum; and that mapping lateral variations of radioactivity (for example between continents and ocean basins) would require detectors at several sites.

Neutrino oscillations — the quantum-mechanical flavor change of neutrinos during propagation, which suppresses the $\bar{\nu}_e$ survival probability at MeV energies to about 0.55 over terrestrial baselines — were subsequently established and precisely measured; KamLAND's reactor-antineutrino disappearance measurement (Eguchi et al. 2003) was the first direct evidence. This removed the degeneracy between oscillation effects and compositional effects that the 1984 review had worried about: modern geoneutrino analyses apply the independently measured oscillation parameters rather than treating them as unknowns.

The detection route that actually succeeded was not the radiochemical one but inverse beta decay (IBD), $\bar{\nu}_e + p \to e^+ + n$, in large underground liquid-scintillator detectors. IBD has a threshold of 1.806 MeV: antineutrinos from the $^{238}$U and $^{232}$Th chains (spectra extending up to 3.27 and 2.25 MeV respectively) are visible, and the coincidence between the prompt positron signal and the delayed neutron-capture gamma ray provides powerful background rejection. Signal strengths are customarily quoted in TNU (terrestrial neutrino units), 1 TNU being one detected event per $10^{32}$ target protons per year. KamLAND (Japan; a kiloton-scale scintillator detector built primarily for reactor antineutrinos) reported the first experimental investigation of geoneutrinos in 2005, and Borexino (Italy; a few-hundred-ton ultrapure scintillator detector) made the first observation at more than 3$\sigma$ in 2010. By 2020–2022 the two experiments had accumulated of order two hundred geoneutrino events, measured the total U+Th signal (tens of TNU at both sites), constrained the uranium and thorium heat contributions separately (KamLAND, 2022), and extracted a mantle signal (Borexino, 2020) by subtracting a detailed model of the crustal flux (e.g., the reference Earth model of Huang et al. 2013). SNO+ (Canada; a kiloton-scale liquid-scintillator detector located deep in an underground mine) reported the first Western-Hemisphere measurement in 2026, and JUNO (China; a 20-kiloton liquid-scintillator detector), which completed construction in 2025, is expected to reach about 8% precision on the total flux within a decade (Li 2026).

Two essential gaps remain, and they define the open core of the 1984 program. First, $^{40}$K: its beta branch produces antineutrinos with an endpoint (the maximum antineutrino energy) of only 1.311 MeV, below the IBD threshold, so every existing detector is blind to it. Yet $^{40}$K is expected to contribute roughly a fifth to a third of the Earth's radiogenic heat (Wang & Chen 2020; LiquidO Collaboration 2026), and its abundance is inferred only from BSE models: the silicate Earth retains of order a tenth to a third of the potassium abundance predicted from chondritic meteorites (primitive meteorites that preserve the average composition of the solar system's rocky material), and whether the "missing potassium" was lost during planet formation or segregated into the core is debated. Detecting the $^{40}$K geoneutrino component requires a fundamentally different technique: antineutrino–electron elastic scattering with event-by-event directionality (to reject the overwhelming solar-neutrino background), or charged-current capture on low-threshold isotopes such as $^{63}$Cu, $^{79}$Br or $^{106}$Cd, with the event tagged in real time by detecting the emitted positron (Leyton et al. 2017; Wang & Chen 2020; LiquidO Collaboration 2026). Second, the mantle: at continental detector sites the crust supplies the majority of the U/Th geoneutrino signal, so the mantle abundance — the quantity that discriminates BSE models and fixes the mantle radiogenic heat — is currently obtained only through model-dependent crustal subtraction. Breaking this degeneracy requires measurements at oceanic sites (thin crust, large mantle fraction of the signal), a network of detectors at multiple sites, improved crust modeling, and/or direction-sensitive detection. The cosmic background (DSNB) is relevant here as a background to be separated: its spectrum lies at tens of MeV, well above the geoneutrino range below 3.3 MeV, so energy cuts discriminate it from the terrestrial signal, and its detection is pursued as an independent astrophysical objective (Abe et al. 2025; Mori et al. 2026).

Problem Statement

Determine the abundance and distribution of the heat-producing elements $^{238}$U, $^{232}$Th and $^{40}$K in the Earth — and hence the planet's radiogenic heat power — from direct measurement of the terrestrial antineutrino (geoneutrino) flux and spectrum at the surface, with discrimination of the non-terrestrial antineutrino backgrounds (nuclear-reactor antineutrinos and the diffuse supernova neutrino background). This is the completion of the program posed by Krauss, Glashow and Schramm in "Antineutrino astronomy and geophysics" (1984): to turn the antineutrinos emitted by terrestrial radioactivity into a quantitative, source-resolved probe of the Earth's interior. The objective is to determine, from direct geoneutrino measurement, the Earth's heat-producing-element inventory and radiogenic heat:

1. A measurement of the geoneutrino signal from each of the three heat-producing isotopes, in particular the never-observed $^{40}$K component, whose beta-decay spectrum (endpoint 1.311 MeV) lies below the 1.806 MeV threshold of the inverse-beta-decay channel used by all existing measurements and therefore requires a detection technique other than inverse beta decay.
2. A determination of the mantle contribution to the geoneutrino signal that is robust to the details of the crustal flux model, so that the deep-Earth abundance of heat-producing elements is not obtained solely by model-dependent subtraction of the crustal signal.
3. The resulting radiogenic heat power of the Earth and its partition between crust and mantle, with uncertainties small enough to discriminate among the competing bulk-silicate-Earth compositional models (cosmochemical, geochemical, geodynamical).

The common scientific objective uniting these three parts is the direct measurement of the Earth's internal radioactivity: each part measures one piece of the same flux (total, isotope-resolved, depth-resolved), and together they fix the radiogenic heat that enters mantle-convection and thermal-history models. The following parts of the 1984 agenda are already discharged by the literature and constitute the baseline rather than the open target: the feasibility and first execution of U/Th geoneutrino detection (KamLAND, Borexino, SNO+), the characterization of neutrino oscillations (now independently measured), and the control of cosmic-ray muon backgrounds at underground sites. The diffuse supernova neutrino background enters this problem as a background to be discriminated from the terrestrial signal, not as a discovery target; its detection is a separate objective pursued elsewhere. The potassium component and the mantle inventory are the open target; an upper bound on the $^{40}$K flux or a crust-model-robust mantle constraint addresses part of it.

References

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