Geoneutrino

A geoneutrino is a neutrino or antineutrino emitted in decay of radionuclide naturally occurring 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. A geoneutrino is a neutrino or antineutrino emitted in decay of radionuclide naturally occurring 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. Most geoneutrinos are electron antineutrinos originating in β− 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. As of 2016 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. JUNO experiment is under construction in Southern China. Another geoneutrino detecting experiment is planned at the China Jinping Underground Laboratory. 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. 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. 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 × 10 6 c m − 2 s − 1 {displaystyle scriptstyle mathrm { imes 10^{6}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.