Geoneutrino

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.

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.

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.

As of 2016[update] 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.

[3] In a 1984 landmark paper Krauss, Glashow & Schramm presented calculations of the predicted geoneutrino flux and discussed the possibilities for detection.

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.

Existing geoneutrino data are a byproduct of antineutrino measurements with detectors designed primarily for fundamental neutrino physics research.

[22] Calculations of the expected geoneutrino signal predicted for various Earth reference models are an essential aspect of neutrino geophysics.

[24] 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.

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,[14][26] the average survival probability.

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.

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.

[5] A 2011 update of KamLAND's result used data from 2135 days of detector time and benefited from improved purity of the scintillator as well as a reduced reactor background from the 21-month-long shutdown of the Kashiwazaki-Kariwa plant after Fukushima.

The analysis shows that the Earth crust contains about the same amount of U and Th as the mantle, and that the total radiogenic heat flow from these elements and their daughters is 23–36 TW.