Geoneutrinos: How Antineutrinos Reveal the Earth's Radioactive Heart

Inside the Earth, at temperatures reaching 5,700 Kelvin at the inner core, atoms of uranium, thorium, and potassium are undergoing slow radioactive decay. Each decay releases energy in the form of gamma rays, alpha particles, beta electrons — and antineutrinos. The antineutrinos, unlike every other decay product, pass through the rest of the planet unimpeded and emerge at the surface in an almost undiminished flux.

About 20 million of these geoneutrinos cross each square centimetre of the Earth’s surface every second, carrying in their spectrum a direct signature of the radiogenic heat flowing through our planet. Two experiments — KamLAND in Japan and Borexino in Italy — have detected them, answering a question that had been unanswerable for a century: what fraction of the heat that keeps the Earth’s interior molten comes from radioactive decay, and what fraction is left over from the planet’s formation?

Why geoneutrinos exist

Every beta-minus decay produces an antineutrino: $n\to p+e^{-}+{\bar{\nu }}_e$ This reaction, or variants of it, appears in the decay chains of several long-lived radioactive isotopes that are significant contributors to the Earth’s heat budget. The dominant ones are:

• Uranium-238 (half-life 4.47 billion years) — decays through a 14-step chain to lead-206, emitting 6 antineutrinos along the way with energies up to 3.27 MeV
• Thorium-232 (half-life 14.05 billion years) — decays through a 10-step chain to lead-208, with 4 antineutrinos and energies up to 2.25 MeV
• Potassium-40 (half-life 1.27 billion years) — decays directly to ⁴⁰Ca emitting a single antineutrino at up to 1.31 MeV

Uranium and thorium each decay through a long chain, with each intermediate isotope also radioactive. The antineutrinos emitted in each branch have distinct energy distributions, but experiments cannot resolve them individually; they see only the total flux above the detector’s energy threshold.

Potassium-40 presents a specific problem: because its maximum antineutrino energy (1.31 MeV) falls below the 1.806-MeV threshold of the inverse-beta-decay reaction used by most antineutrino detectors, its contribution cannot be measured directly. It must be inferred from geochemical models. This is one of the largest systematic uncertainties in geoneutrino interpretation.

The heat budget question

The total heat flowing out through the Earth’s surface — measured from borehole temperature gradients and seafloor heat-flow probes — is approximately 47 terawatts (TW). This is a power output comparable to 150 large nuclear reactors, continuously, spread over the planet’s 510 million km² surface.

Where does the 47 TW come from? Two sources compete:

Primordial heat. Energy stored during planetary formation: gravitational potential energy released during accretion, energy from core-mantle differentiation, heat from the giant impact that formed the Moon, and the kinetic energy of infalling planetesimals. All this thermal energy was initially stored in the Earth’s interior and is now slowly leaking out.

Radiogenic heat. Energy released by ongoing radioactive decay of U, Th, and K (and small contributions from other isotopes). This heat is continuously generated, not depleting on a timescale shorter than the isotope half-lives (~10⁹ years).

The ratio of these two sources — the Urey ratio — is a fundamental number in geophysics. A high Urey ratio (radiogenic heat dominant) implies an Earth whose thermal evolution has been steady over geological time. A low ratio implies that primordial heat dominates, and that the Earth is steadily cooling from initial conditions. The distinction has profound implications for when plate tectonics began, how quickly the core formed, and how long the geodynamo will continue to generate the magnetic field.

Geochemical models, based on measurements of U/Th/K in meteorites and upper-mantle samples, predict a radiogenic contribution of about 20 TW — roughly 40% of the total. But these models require extrapolation from a few accessible samples to the whole Earth, and their uncertainties are substantial.

A direct measurement of the radiogenic heat — from geoneutrinos — would settle the question.

KamLAND’s first detection

KamLAND (Kamioka Liquid Scintillator Antineutrino Detector) is a 1,000-ton liquid scintillator detector in the Kamioka mine of Japan, 1,000 metres underground. It was built in the early 2000s primarily to observe reactor antineutrinos from 55 Japanese reactors at baselines of 130 to 210 km — a measurement that definitively confirmed the solar-sector oscillation parameters and won the joint 2015 Nobel Prize.

As a by-product of its reactor programme, KamLAND was also sensitive to geoneutrinos. The reactor signal at 1-10 MeV dominates at most energies, but the lowest-energy end of the spectrum — below about 3 MeV — is a region where reactor contributions are small and geoneutrino events can be identified.

In 2005, KamLAND published the first geoneutrino detection at $4.5\sigma$ significance: 28 events above 9 background events, consistent with a geoneutrino flux prediction from geochemical models. The result was limited by statistics but was the first direct experimental constraint on Earth’s radiogenic heat.

Subsequent KamLAND data releases (2011, 2013, 2022) improved the measurement with better background subtraction and longer exposure. The 2022 analysis, with 3,260 days of exposure, measured a total geoneutrino flux of $32_{-3}^{+4}$ events per year per kiloton — corresponding to a radiogenic heat of $17.7_{-6.0}^{+6.7}$ TW.

Borexino’s deep-Italian complement

Borexino, at the Gran Sasso National Laboratory 1,400 metres beneath Italy’s Apennine Mountains, is a 300-ton ultrahigh-purity liquid scintillator detector. Its scintillator radiopurity — achieved through multi-stage distillation and water extraction — is about a factor of 1,000 lower in background than any comparable detector.

Because Borexino is in a continental location far from any nuclear reactor (only 87 reactor-related events per year from distant European reactors), its geoneutrino signal-to-background ratio is excellent. In 2010 Borexino announced its first geoneutrino detection; by 2019 the accumulated analysis included 53 geoneutrino events against fewer than 20 background events, at $6\sigma$ significance, giving a measured flux of $(50.4\pm 8.1)\times 10^5$ cm⁻² s⁻¹.

The two experiments are complementary. KamLAND sees a flux dominated by crust-and-mantle contributions under thin Japanese crust. Borexino sees a flux dominated by thick continental crust beneath the Apennines, plus the mantle underneath. The difference between the two, once corrected for crust contributions using geological surveys, gives a constraint on the mantle’s uranium and thorium content.

What the measurements tell us

Combined fits of the KamLAND and Borexino data, interpreted using reference Earth models (BSE — Bulk Silicate Earth — models), give a radiogenic heat contribution from U and Th of about: $H_{\text{rad}}(\text{U+Th})=14.7_{-4.2}^{+4.6}\text{ TW}$ Adding the inferred ⁴⁰K contribution of about 4 TW gives a total radiogenic heat of roughly 19 ± 6 TW. The total surface heat flow is 47 TW. The implied Urey ratio is therefore $\text{Ur}=\frac{H_{\text{rad}}}{H_{\text{total}}}\approx 0.4\pm 0.15$ consistent with standard BSE models but excluding the most extreme “high-radiogenic” (Ur ~ 0.8) and “low-radiogenic” (Ur ~ 0.2) scenarios that had been proposed before direct measurement became possible. The Earth is a hybrid: substantial primordial heat remains, but ongoing radioactive decay contributes significantly.

The next generation

Three future programmes will sharpen the picture:

JUNO at Jiangmen, China, is primarily a reactor neutrino experiment for mass-ordering determination, but its 20-kiloton scintillator mass will also record hundreds of geoneutrinos per year — roughly 500–600 events annually, depending on location and geology. With a few years of data, JUNO will refine the radiogenic heat measurement to ~10% uncertainty, sharp enough to distinguish competing BSE models.

Hanohano, a proposed deep-ocean liquid scintillator detector, would be deployable far from continental crust and close to the thinner, Th-and-U-poor oceanic crust. Its signal would be dominated by the Earth’s mantle, providing the first direct measurement of mantle U/Th content.

Theia, a proposed water-based liquid scintillator detector, combines the scale of water Cherenkov detectors with the low-energy sensitivity of liquid scintillator. At the ~50-kt mass proposed, its geoneutrino sensitivity would rival JUNO’s.

Ocean-floor geoneutrino detection in particular would transform the field. A detector at 4 km depth in the mid-Pacific, far from reactors and continental crust, would see the mantle contribution essentially cleanly — something terrestrial detectors cannot do even in principle.

Why this matters

Geoneutrinos connect two disciplines that rarely speak to each other: particle physics and geophysics. The same apparatus built to measure oscillation parameters and confirm the Standard Model also directly probes the thermal engine that keeps plate tectonics operating, drives the geodynamo that protects the surface from the solar wind, and ultimately determines whether a terrestrial planet remains habitable over billion-year timescales.

Every beta decay inside the Earth produces an antineutrino that carries away its tiny share of energy unimpeded to space. Twenty million per square centimetre per second, arriving continuously. Until 2005, no one had seen one. Now we see them, and from them we read a message the Earth has been transmitting for 4.5 billion years — a message about its own heat, its own chemistry, and how long it will keep the surface warm enough for life.