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

Radioactive decay is only part of the Earth’s heat

The Earth gives off heat left from its formation and differentiation, plus energy from radioactive decay that is still going on. These are physically different sources. The heat flowing out of the surface today does not, by itself, tell us how much uranium, thorium or potassium the planet contains.

Beta decays in the uranium and thorium chains release electron antineutrinos along with energy that stays in the rock. Because antineutrinos interact only weakly, they reach a detector even from deep inside. The same decays make both the heat and the antineutrinos, so nuclear data link the particles to the radiogenic heat, given stated assumptions about the decay chains.

The antineutrinos carry away part of the decay energy, so not all of it heats the rock. Converting to heat therefore uses the energy actually deposited, not a count multiplied by the total decay energy. It must also keep the activity of a decay chain distinct from the abundance of its parent isotope.

Sources: [1] · [2] · [3]

The paired signal catches only some of the antineutrinos

KamLAND in Japan and Borexino in Italy used inverse beta decay, in which a prompt positron signal is followed by a neutron capture. Requiring the pair removes many single-pulse backgrounds. The method still has an energy threshold and limited efficiency, so it cannot record every antineutrino produced inside the Earth.

Some branches of the uranium and thorium chains produce antineutrinos above the threshold. Antineutrinos from potassium-40 beta decay fall below it. That is why potassium’s share of the heat needs other assumptions or a different detection channel. Missing from this sample does not mean missing from the planet.

The uranium and thorium signals overlap in prompt energy, though they end at different maximum energies. When statistics and backgrounds allow, fitting the spectrum can separate their yields. No single event, and no total count, tells you which layer of the Earth or which decay chain produced each interaction.

Sources: [1] · [4] · [2]

Reactor antineutrinos become the background

Reactor antineutrinos interact the same way and overlap the geoneutrino energy range. How many are expected depends on each reactor’s operating history, spectrum and distance, and on oscillations. A detector built to study reactors has to model that familiar source when it turns to the Earth.

KamLAND’s 2022 geoneutrino analysis benefited from the long period after 2011 when Japanese reactors ran far less. That shifted the balance between signal and background. It did not remove all backgrounds, and the Earth’s contribution still had to be extracted with a fit.

Borexino’s comprehensive analysis, posted as a preprint in 2019 and published in 2020, fitted the reactor contribution as one component and controlled accidental and cosmogenic backgrounds. Laboratories in different places have different nearby reactors and local geology, so they provide valuable cross-checks; their measured fluxes need not be identical.

Sources: [2] · [3]

The nearby crust must be understood before the distant mantle

A detector measures the flux at one site, summed over all directions and distances. Nearby radioactive material can contribute strongly, because the flux from a small source falls off as the square of the distance. Continental crust is also rich in heat-producing elements, so the local composition matters a great deal.

To infer the mantle’s share, the crust and the rest of the lithosphere must first be predicted from geological surveys and composition models, with their uncertainties. Subtracting one exact-looking number from the fitted total would hide that uncertainty. The mantle result must carry both the particle-measurement and the geological errors.

The same total heat can give different fluxes at a site if the material is arranged differently, because material near the detector counts for more than material deep below. Converting flux to watts therefore needs a model of where the material is; these data are not a resolved picture of the interior.

Sources: [1] · [2] · [3]

Each heat estimate depends on its Earth model

The 2022 KamLAND paper reported strong disagreement with a fully radiogenic model, assuming the heat-producing elements are spread evenly through the mantle. That assumption is part of the conclusion. Quoting the strength of the disagreement without it would claim more than the analysis supports.

Borexino’s 2020 analysis converted its mantle signal to heat with its own lithosphere model, and added an assumed potassium contribution to estimate the total radiogenic heat. Comparing the two results means matching the reservoirs, composition assumptions and uncertainties. Quoting both in watts does not make the two inferences the same.

Even so, geoneutrinos give a direct particle probe of radioactivity inside the Earth. More sites and better crust data can separate the reservoirs more sharply. The result is a new constraint on the planet’s energy sources, while heat from primordial cooling and from decays this method cannot see still has to be accounted for separately.

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.