Geoneutrino

A geoneutrino is a

Geologically significant antineutrino- and heat-producing radioactive decays and decay chains^[14]

${\displaystyle {\begin{array}{rclr}{\ce {^{238}_{92}U}}&{\ce {->}}&{\ce {^{206}_{82}Pb}}+8\alpha +6e^{-}+6{\bar {\nu }}_{e}&+\ 51.698\,{\ce {MeV}}\\{\ce {^{235}_{92}U}}&{\ce {->}}&{\ce {^{207}_{82}Pb}}+7\alpha +4e^{-}+4{\bar {\nu }}_{e}&+\ 46.402\,{\ce {MeV}}\\{\ce {^{232}_{90}Th}}&{\ce {->}}&{\ce {^{208}_{82}Pb}}+6\alpha +4e^{-}+4{\bar {\nu }}_{e}&+\ 42.652\,{\ce {MeV}}\\{\ce {^{40}_{19}K}}&{\ce {->[{89.3\,\%}]}}&{\ce {^{40}_{20}Ca}}+e^{-}+{\bar {\nu }}_{e}&+\ 1.311\,{\ce {MeV}}\\{\ce {^{40}_{19}K}}+e^{-}&{\ce {->[{10.7\,\%}]}}&{\ce {^{40}_{18}Ar}}+\nu _{e}&+\ 1.505\,{\ce {MeV}}\end{array}}}$

The

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.^[22]

Geoneutrino prediction

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.[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.^[25] 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:

${\displaystyle {\frac {\mathrm {d} \phi (E_{{\bar {\nu }}_{e}},{\vec {r}})}{\mathrm {d} E_{{\bar {\nu }}_{e}}}}=10{\frac {\lambda XN_{A}}{M}}{\frac {\mathrm {d} n(E_{{\bar {\nu }}_{e}})}{\mathrm {d} E_{{\bar {\nu }}_{e}}}}\int \limits _{V}\mathrm {d} ^{3}{\vec {r}}'{\frac {A({\vec {r}}')\rho ({\vec {r}}')P_{ee}(E_{{\bar {\nu }}_{e}},|{\vec {r}}-{\vec {r}}'|)}{4\pi |{\vec {r}}-{\vec {r}}'|^{2}}}}$

where dφ(E_ν,r)/dE_ν is the fully oscillated antineutrino flux energy spectrum (in cm⁻²⋅s⁻¹⋅MeV⁻¹) at position r (units of m) and E_ν 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), N_A is the Avogadro constant (in mol⁻¹), λ is decay constant (in s⁻¹), dn(E_ν)/dE_ν is the antineutrino intensity energy spectrum (in MeV⁻¹, normalized to the number of antineutrinos n_ν produced in a decay chain when integrated over energy), and P_ee(E_ν,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 P_ee can be replaced with a simple factor ⟨P_ee⟩ ≈ 0.55,^[14][26] the average survival probability. Integration over the energy yields the total antineutrino flux (in cm⁻²⋅s⁻¹) from a given radionuclide:

${\displaystyle \phi ({\vec {r}})=10{\frac {n_{{\bar {\nu }}_{e}}\langle P_{ee}\rangle \lambda XN_{A}}{M}}\int \limits _{V}\mathrm {d} ^{3}{\vec {r}}'{\frac {A({\vec {r}}')\rho ({\vec {r}}')}{4\pi |{\vec {r}}-{\vec {r}}'|^{2}}}}$

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

Geoneutrino detection

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:

${\displaystyle {\bar {\nu }}_{e}+p\rightarrow e^{+}+n}$

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:

${\displaystyle e^{+}+e^{-}\rightarrow \gamma +\gamma }$

With a delay of few tens to few hundred microseconds the neutron combines with a proton to form a

${\displaystyle n+p\rightarrow d+\gamma }$

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.^[25]

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.

Geoneutrino measurements are often reported in Terrestrial Neutrino Units (TNU; analogy with Solar Neutrino Units) rather than in units of flux (cm⁻² s⁻¹). TNU is specific to the inverse beta decay detection mechanism with protons. 1 TNU corresponds to 1 geoneutrino event recorded over a year-long fully efficient exposure of 10³² free protons, which is approximately the number of free protons in a 1 kiloton liquid scintillation detector. The conversion between flux units and TNU depends on the thorium to uranium abundance ratio (Th/U) of the emitter. For Th/U=4.0 (a typical value for the Earth), a flux of 1.0 × 10⁶ cm⁻² s⁻¹ corresponds to 8.9 TNU.^[14]

Detectors and results

Existing detectors

Borexino is a 0.3 kiloton detector at Laboratori Nazionali del Gran Sasso near L'Aquila, Italy. Results published in 2010 used data collected over live-time of 537 days. Of 15 candidate events, unbinned maximum likelihood analysis identified 9.9 as geoneutrinos. The geoneutrino null hypothesis was rejected at 99.997% confidence level (4.2σ). The data also rejected a hypothesis of an active georeactor in the Earth's core with power above 3 TW at 95% C.L.^[7]

A 2013 measurement of 1353 days, detected 46 'golden' anti-neutrino candidates with 14.3±4.4 identified geoneutrinos, indicating a 14.1±8.1 TNU mantle signal, setting a 95% C.L limit of 4.5 TW on geo-reactor power and found the expected reactor signals.^[27] In 2015, an updated spectral analysis of geoneutrinos was presented by Borexino based on 2056 days of measurement (from December 2007 to March 2015), with 77 candidate events; of them, only 24 are identified as geonetrinos, and the rest 53 events are originated from European nuclear reactors. 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.^[28]

SNO+ is a 0.8 kiloton detector located at SNOLAB near Sudbury, Ontario, Canada. SNO+ uses the original SNO experiment chamber. The detector is being refurbished and is expected to operate in late 2016 or 2017.^[29]

Planned and proposed detectors

• Ocean Bottom KamLAND-OBK OBK is a 50 kiloton liquid scintillation detector for deployment in the deep ocean.
• JUNO (Jiangmen Underground Neutrino Observatory, website) is a 20 kiloton liquid scintillation detector currently under construction in Southern China. The JUNO detector is scheduled to become operational in 2023.^[30]
• Jinping Neutrino Experiment is a 4 kiloton liquid scintillation detector currently under construction in the China JinPing Underground Laboratory (CJPL) scheduled for completion in 2022.^[31]
• Laboratoire Souterrain de Modane (LSM) in Fréjus, France.^[32]
  This project seems to be cancelled.
• at
  DUSEL (Deep Underground Science and Engineering Laboratory) at Homestake in Lead, South Dakota, USA^[33]
• at BNO (Baksan Neutrino Observatory) in Russia^[34]
• EARTH (Earth AntineutRino TomograpHy)
• Hanohano (Hawaii Anti-Neutrino Observatory) is a proposed deep-ocean transportable detector. It is the only detector designed to operate away from the Earth's continental crust and from nuclear reactors in order to increase the sensitivity to geoneutrinos from the Earth's mantle.^[22]