Geoneutrinos: what can particles tell us about the Earth’s heat?

Radioactive decay supplies one part of a larger budget

The Earth releases heat left from its formation and differentiation, as well as energy from ongoing radioactive decay. These contributions are physically different. A present surface heat flow is not, by itself, a measurement of how much uranium, thorium or potassium the planet contains.

Beta decays within uranium and thorium chains produce electron antineutrinos, alongside energy retained in matter. Their weak interactions let particles reach a detector from deep regions. The useful link is a common decay process: its nuclear bookkeeping relates emitted particles to radiogenic heat under specified chain assumptions.

Neutrinos carry away part of the decay energy rather than heating the rock with all of it. A heat conversion must therefore use the deposited-energy budget, not simply multiply a count by a total decay energy. It must also distinguish decay-chain activity from the abundance of a parent isotope.

Sources: [1] · [2] · [3]

The familiar pair selects only part of the emission

KamLAND in Japan and Borexino in Italy used inverse beta decay: a prompt positron is followed by neutron capture. The paired signals suppress many single-pulse backgrounds. The method retains its threshold and efficiency, so it cannot record every antineutrino generated inside the Earth.

Some uranium- and thorium-chain branches reach above the threshold. Potassium-40 beta-decay antineutrinos do not. This instrumental boundary is why potassium’s heat contribution needs other assumptions or a different measurement channel; absence from this sample is not absence from the planet.

The selected uranium and thorium contributions overlap in prompt energy, though their upper endpoints differ. Spectral fitting can help distinguish their yields when statistics and backgrounds permit. Neither a single low-energy pair nor a simple event count identifies which layer or parent chain produced every interaction.

Sources: [1] · [4] · [2]

A reactor source becomes a background to the Earth question

Reactor antineutrinos produce the same interaction channel and extend into the geoneutrino energy region. Their expected contribution depends on operating histories, spectra, distances and oscillations. A detector designed to study reactors must model that familiar source when it asks about the Earth instead.

KamLAND’s 2022 geoneutrino analysis benefited from a long period of greatly reduced Japanese reactor activity after 2011. This changed the signal-to-background balance. It did not switch off all backgrounds or make the Earth contribution identifiable without a fit.

Borexino’s comprehensive analysis, published in 2020 after a 2019 preprint, treated the reactor contribution as a fitted component and controlled accidental and cosmogenic backgrounds. Separate laboratories bring different reactor environments and local geology, giving valuable checks without forcing identical measured fluxes.

Sources: [2] · [3]

The nearby crust must be understood before the distant mantle

A detector measures a local flux integrated over many directions and distances. Nearby radioactive material can contribute strongly because flux from a small source decreases with distance squared. Continental crust is also enriched in heat-producing elements, making local composition particularly important.

Inferring a mantle contribution requires predicting the crustal and other lithospheric components with geological surveys, composition models and uncertainties. Subtracting one exact-looking number from a fitted total would conceal that uncertainty. The resulting mantle interval must carry both the particle and geological errors.

The same global heat can generate different local fluxes if the material is distributed differently. Material nearer the detector is weighted more heavily than material concentrated deep below. A conversion from flux to watts therefore needs a spatial model; this sample does not supply a resolved interior photograph.

Sources: [1] · [2] · [3]

A heat inference should keep its Earth model attached

The 2022 KamLAND paper reported strong disagreement with a fully radiogenic model under a homogeneous mantle distribution of heat-producing elements. The distribution assumption matters to the conclusion. Removing it while keeping the quoted strength would turn a conditional model test into a broader claim than the analysis supports.

Borexino’s 2020 analysis translated its mantle signal to heat with its own lithosphere model and added an assumed potassium contribution for total radiogenic heat. Comparing those results requires matching the reservoirs, composition assumptions and uncertainties. A shared unit of watts is not enough to make two inferences identical.

Geoneutrinos have nevertheless opened a direct particle probe of radioactive activity inside the Earth. More sites and better crust information can sharpen the separation of reservoirs. The gain is a new constraint on the planet’s energy sources, with primordial cooling and inaccessible decay channels kept visible in the remaining budget.

Sources: [2] · [3] · [1]

Try it in the Lab

Primary sources & revision

1. Fiorentini, Lissia & Mantovani · Geo-neutrinos and Earth’s interior (2007)
2. KamLAND Collaboration · Uranium and thorium abundances from geoneutrino spectroscopy (2022)
3. Borexino Collaboration · Comprehensive geoneutrino analysis (2019 preprint; 2020 publication)
4. Strumia & Vissani · Precise quasielastic neutrino/nucleon cross section (2003)

First published 2026-10-10; last revised 2026-10-10. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication.

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