In nuclear and particle physics, a geoneutrino is a neutrino or antineutrino emitted during the decay of naturally occurring radionuclides in the Earth. Neutrinos, the lightest of the known subatomic particles, lack measurable electromagnetic properties and interact only via the weak nuclear force (when ignoring gravity). Matter is virtually transparent to neutrinos and consequently they travel, unimpeded, at near light speed through the Earth from their point of emission. Collectively, geoneutrinos carry integrated information about the abundances of their radioactive sources inside the Earth. A major objective of the emerging field of neutrino geophysics involves extracting geologically useful information (e.g., abundances of individual geoneutrino-producing elements and their spatial distribution in Earth's interior) from geoneutrino measurements. Analysts from the Borexino collaboration have been able to get to 53 events of neutrinos originating from the interior of the Earth.
Most geoneutrinos are electron antineutrinos originating in decay branches of 40K, 232Th and 238U. Together these decay chains account for more than 99% of the present-day radiogenic heat generated inside the Earth. Only geoneutrinos from 232Th and 238U decay chains are detectable by the inverse beta-decay mechanism on the free proton because these have energies above the corresponding threshold (1.8 MeV). In neutrino experiments, large underground liquid scintillator detectors record the flashes of light generated from this interaction. geoneutrino measurements at two sites, as reported by the KamLAND and Borexino collaborations, have begun to place constraints on the amount of radiogenic heating in the Earth's interior. A third detector (SNO+) is expected to start collecting data in 2017. The JUNO experiment has begun taking data in Southern China as of late 2025 and the DUNE experiment is now under construction in Lead, South Dakota. Another geoneutrino detecting experiment is planned at the China Jinping Underground Laboratory.
History
Neutrinos were hypothesized in 1930 by Wolfgang Pauli. The first detection of antineutrinos generated in a nuclear reactor was confirmed in 1956. The idea of studying geologically produced neutrinos to infer Earth's composition has been around since at least mid-1960s. In a 1984 landmark paper Krauss, Glashow & Schramm presented calculations of the predicted geoneutrino flux and discussed the possibilities for detection. First detection of geoneutrinos was reported in 2005 by the KamLAND experiment at the Kamioka Observatory in Japan. In 2010 the Borexino experiment at the Gran Sasso National Laboratory in Italy released their geoneutrino measurement. Updated results from KamLAND were published in 2011 and 2013, and Borexino in 2013 and 2015.
Geological motivation
+ Geologically significant antineutrino- and heat-producing radioactive decays and decay chains The Earth's interior radiates heat at a rate of about 47 TW (terawatts), which is less than 0.1% of the incoming solar energy. Part of this heat loss is accounted for by the heat generated upon decay of radioactive isotopes in the Earth interior. The remaining heat loss is due to the secular cooling of the Earth, growth of the Earth's inner core (gravitational energy and latent heat contributions), and other processes. The most important heat-producing elements are uranium (U), thorium (Th), and potassium (K). The debate about their abundances in the Earth has not concluded. Various compositional estimates exist where the total Earth's internal radiogenic heating rate ranges from as low as ~10 TW to as high as ~30 TW. About 7 TW worth of heat-producing elements reside in the Earth's crust, the remaining power is distributed in the Earth mantle; the amount of U, Th, and K in the Earth core is probably negligible. Radioactivity in the Earth mantle provides internal heating to power mantle convection, which is the driver of plate tectonics. The amount of mantle radioactivity and its spatial distribution—is the mantle compositionally uniform at large scale or composed of distinct reservoirs?—is of importance to geophysics.
The existing range of compositional estimates of the Earth reflects our lack of understanding of what were the processes and building blocks (chondritic meteorites) that contributed to its formation. More accurate knowledge of U, Th, and K abundances in the Earth interior would improve our understanding of present-day Earth dynamics and of Earth formation in early Solar System. Counting antineutrinos produced in the Earth can constrain the geological abundance models. The weakly interacting geoneutrinos carry information about their emitters' abundances and location in the entire Earth volume, including the deep Earth. Extracting compositional information about the Earth mantle from geoneutrino measurements is difficult but possible. It requires a synthesis of geoneutrino experimental data with geochemical and geophysical models of the Earth. Existing geoneutrino data are a byproduct of antineutrino measurements with detectors designed primarily for fundamental neutrino physics research. Future experiments devised with a geophysical agenda in mind would benefit geoscience. Proposals for such detectors have been put forward.
Geoneutrino prediction
Calculations of the expected geoneutrino signal predicted for various Earth reference models are an essential aspect of neutrino geophysics. In this context, "Earth reference model" means the estimate of heat producing element (U, Th, K) abundances and assumptions about their spatial distribution in the Earth, and a model of Earth's internal density structure. By far the largest variance exists in the abundance models where several estimates have been put forward. They predict a total radiogenic heat production as low as ~10 TW and as high as ~30 TW, the commonly employed value being around 20 TW. A density structure dependent only on the radius (such as the Preliminary Reference Earth Model or PREM) with a 3-D refinement for the emission from the Earth's crust is generally sufficient for geoneutrino predictions.
The geoneutrino signal predictions are crucial for two main reasons: 1) they are used to interpret geoneutrino measurements and test the various proposed Earth compositional models; 2) they can motivate the design of new geoneutrino detectors. The typical geoneutrino flux at Earth's surface is few × 106 cm−2⋅s−1. As a consequence of (i) high enrichment of continental crust in heat producing elements (~7 TW of radiogenic power) and (ii) the dependence of the flux on 1/(distance from point of emission)2, the predicted geoneutrino signal pattern correlates well with the distribution of continents. At continental sites, most geoneutrinos are produced locally in the crust. This calls for an accurate crustal model, both in terms of composition and density, a nontrivial task.
Antineutrino emission from a volume V is calculated for each radionuclide from the following equation:
where dφ(Eν,r)/dEν is the fully oscillated antineutrino flux energy spectrum (in cm−2⋅s−1⋅MeV−1) at position r (units of m) and Eν is the antineutrino energy (in MeV). On the right-hand side, ρ is rock density (in kg⋅m−3), A is elemental abundance (kg of element per kg of rock) and X is the natural isotopic fraction of the radionuclide (isotope/element), M is atomic mass (in g⋅mol−1), NA is the Avogadro constant (in mol−1), λ is decay constant (in s−1), dn(Eν)/dEν is the antineutrino intensity energy spectrum (in MeV−1, normalized to the number of antineutrinos nν produced in a decay chain when integrated over energy), and Pee(Eν,L) is the antineutrino survival probability after traveling a distance L. For an emission domain the size of the Earth, the fully oscillated energy-dependent survival probability Pee can be replaced with a simple factor ⟨Pee⟩ ≈ 0.55, the average survival probability. Integration over the energy yields the total antineutrino flux (in cm−2⋅s−1) from a given radionuclide:
The total geoneutrino flux is the sum of contributions from all antineutrino-producing radionuclides. The geological inputs—the density and particularly the elemental abundances—carry a large uncertainty. The uncertainty of the remaining nuclear and particle physics parameters is negligible compared to the geological inputs. At present it is presumed that uranium-238 and thorium-232 each produce about the same amount of heat in the Earth's mantle, and these are presently the main contributors to radiogenic heat. However, neutrino flux does not perfectly track heat from radioactive decay of primordial nuclides, because neutrinos do not carry off a constant fraction of the energy from the radiogenic decay chains of these primordial radionuclides.
Geoneutrino detection Antineutrino Global Map (AGM2015) depicts electron antineutrino emissions of natural origins (from earth's crust and mantle both) - combined with similar emissions worldwide from manmade reactors too.
However, geoneutrinos energy spectrum is known to be limited to 3.26 MeV maximum (for cases of uranium-238 decay, while in comparison Thorium-232 decay is only upto 2.25 MeV). Hence, geoneutrinos exceeding the detection threshold 1.8 MeV of inverse beta decay (IBD) detectors and upto the expected cutoff of 3.26 MeV are isolated statistically from the overall antineutrino emissions measurements which can go upto even 10 MeV. The shortlisted emissions in 1.8 to 3.26 MeV band which still contain an overlap of both natural and manmade emissions are further refined through spectral fits analysis.
The separated natural geoneutrinos data is useful for compositional models study of the Earth - with its mostly metallic iron inner and outer cores and the silicate-constituting mantle and crust layers - for understanding Earth’s internal dynamics of mantle convection, plate tectonics and deep-seated radioactivity.
Detection mechanism Instruments that measure geoneutrinos are large scintillation detectors. They use the inverse beta decay reaction, a method proposed by Bruno Pontecorvo that Frederick Reines and Clyde Cowan employed in their pioneering experiments in 1950s. Inverse beta decay is a charged current weak interaction, where an electron antineutrino interacts with a proton, producing a positron and a neutron:
Only antineutrinos with energies above the kinematic threshold of 1.806 MeV—the difference between rest mass energies of neutron plus positron and proton—can participate in this interaction. After depositing its kinetic energy, the positron promptly annihilates with an electron:
With a delay of few tens to few hundred microseconds the neutron combines with a proton to form a deuteron:
The two light flashes associated with the positron and the neutron are coincident in time and in space, which provides a powerful method to reject single-flash (non-antineutrino) background events in the liquid scintillator. Antineutrinos produced in man-made nuclear reactors overlap in energy range with geologically produced antineutrinos and are also counted by these detectors.
Because of the kinematic threshold of this antineutrino detection method, only the highest energy geoneutrinos from 232Th and 238U decay chains can be detected. Geoneutrinos from 40K decay have energies below the threshold and cannot be detected using inverse beta decay reaction. Experimental particle physicists are developing other detection methods, which are not limited by an energy threshold (e.g., antineutrino scattering on electrons) and thus would allow detection of geoneutrinos from potassium decay.