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A geoneutrino is a neutrino or antineutrino emitted in decay of
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
naturally occurring in the Earth. Neutrinos, the lightest of the known
subatomic particles In physical sciences, a subatomic particle is a particle that composes an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a prot ...
, lack measurable electromagnetic properties and interact only via the
weak nuclear force In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interacti ...
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 Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier tu ...
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 chain In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directl ...
s 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 physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. ...
). In neutrino experiments, large underground liquid
scintillator A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbe ...
detectors record the flashes of light generated from this interaction. geoneutrino measurements at two sites, as reported by the
KamLAND The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an electron antineutrino detector at the Kamioka Observatory, an underground neutrino detection facility in Hida, Gifu, Japan. The device is situated in a drift mine shaft in th ...
and
Borexino Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier tu ...
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 Juno commonly refers to: *Juno (mythology), the Roman goddess of marriage and queen of the gods * ''Juno'' (film), 2007 Juno may also refer to: Arts, entertainment and media Fictional characters *Juno, in the film '' Jenny, Juno'' *Juno, in the ...
experiment is under construction in Southern China. Another geoneutrino detecting experiment is planned at the
China Jinping Underground Laboratory The China Jinping Underground Laboratory () is a deep underground laboratory in the Jinping Mountains of Sichuan, China. The cosmic ray rate in the laboratory is under 0.2 muons/m2/day, placing the lab at a depth of 6720  m.w.e. and maki ...
.


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 Krauss is a German surname. Notable people with the surname include: * Alison Krauss (born 1971), American bluegrass musician * Alexander Krauß (born 1975), German politician * Alexis Krauss (born 1985), musician of the noise pop duo Sleigh Bell ...
, 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 The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an electron antineutrino detector at the Kamioka Observatory, an underground neutrino detection facility in Hida, Gifu, Japan. The device is situated in a drift mine shaft in th ...
experiment at the Kamioka Observatory in Japan. In 2010 the
Borexino Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier tu ...
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

The Earth's interior radiates heat at a rate of about 47 TW (
terawatts The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Watt ...
), 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 Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
, the remaining power is distributed in the Earth mantle; the amount of U, Th, and K in the
Earth core The internal structure of Earth is the solid portion of the Earth, excluding its atmosphere and hydrosphere. The structure consists of an outer silicate solid crust, a highly viscous asthenosphere and solid mantle, a liquid outer core who ...
is probably negligible. Radioactivity in the Earth mantle provides internal heating to power
mantle convection Mantle convection is the very slow creeping motion of Earth's solid silicate mantle as convection currents carrying heat from the interior to the planet's surface. The Earth's surface lithosphere rides atop the asthenosphere and the two form ...
, 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: : \frac = 10\frac \frac \int\limits_V \mathrm^3\vec' \frac where d''φ''(''E''ν,''r'')/d''E''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 The Avogadro constant, commonly denoted or , is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. It is an SI defining co ...
(in mol−1), ''λ'' is decay constant (in s−1), d''n''(''E''ν)/d''E''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, the average survival probability. Integration over the energy yields the total antineutrino flux (in cm−2⋅s−1) from a given radionuclide: : \phi(\vec) = 10\frac \int\limits_V \mathrm^3\vec' \frac 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 nuclide In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the ...
s, because neutrinos do not carry off a constant fraction of the energy from the radiogenic
decay chain In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directl ...
s of these
primordial radionuclide In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the ...
s.


Geoneutrino detection


Detection mechanism

Instruments that measure geoneutrinos are large scintillation detectors. They use the
inverse beta decay Inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction involving an electron antineutrino scattering off a proton, creating a positron and a neutron. This process is commonly used in the detection of electron antineutrinos in neutr ...
reaction, a method proposed by
Bruno Pontecorvo Bruno Pontecorvo (; russian: Бру́но Макси́мович Понтеко́рво, ''Bruno Maksimovich Pontecorvo''; 22 August 1913 – 24 September 1993) was an Italian and Soviet nuclear physicist, an early assistant of Enrico Fermi and ...
that
Frederick Reines Frederick Reines ( ; March 16, 1918 – August 26, 1998) was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist ...
and
Clyde Cowan Clyde Lorrain Cowan Jr (December 6, 1919 – May 24, 1974) was an American physicist, the co-discoverer of the neutrino along with Frederick Reines. The discovery was made in 1956 in the neutrino experiment. Frederick Reines received the Nobel Pr ...
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 The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
and a neutron: :\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: :e^+ + e^- \rightarrow \gamma + \gamma With a delay of few tens to few hundred microseconds the neutron combines with a proton to form a
deuteron Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one n ...
: :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. 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.


Detectors and results


Existing detectors

KamLAND The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an electron antineutrino detector at the Kamioka Observatory, an underground neutrino detection facility in Hida, Gifu, Japan. The device is situated in a drift mine shaft in th ...
(Kamioka Liquid Scintillator Antineutrino Detector) is a 1.0 kiloton detector located at the Kamioka Observatory in Japan. Results based on a live-time of 749 days and presented in 2005 mark the first detection of geoneutrinos. The total number of antineutrino events was 152, of which 4.5 to 54.2 were geoneutrinos. This analysis put a 60 TW upper limit on the Earth's radiogenic power from 232Th and 238U. 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. Of 841 candidate antineutrino events, 106 were identified as geoneutrinos using unbinned maximum likelihood analysis. It was found that 232Th and 238U together generate 20.0 TW of radiogenic power.
Borexino Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier tu ...
is a 0.3 kiloton detector at
Laboratori Nazionali del Gran Sasso Laboratori Nazionali del Gran Sasso (LNGS) is the largest underground research center in the world. Situated below Gran Sasso mountain in Italy, it is well known for particle physics research by the INFN. In addition to a surface portion of t ...
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. 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. 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. SNO+ is a 0.8 kiloton detector located at
SNOLAB SNOLAB is a Canadian underground science laboratory specializing in neutrino and dark matter physics. Located 2 km below the surface in Vale's Creighton nickel mine near Sudbury, Ontario, SNOLAB is an expansion of the existing facilities c ...
near
Sudbury Sudbury may refer to: Places Australia * Sudbury Reef, Queensland Canada * Greater Sudbury, Ontario (official name; the city continues to be known simply as Sudbury for most purposes) ** Sudbury (electoral district), one of the city's federal el ...
, Ontario, Canada. SNO+ uses the original
SNO Tin(II) oxide (stannous oxide) is a compound with the formula SnO. It is composed of tin and oxygen where tin has the oxidation state of +2. There are two forms, a stable blue-black form and a metastable red form. Preparation and reactions Blue ...
experiment chamber. The detector is being refurbished and is expected to operate in late 2016 or 2017.


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. * Jinping Neutrino Experiment is a 4 kiloton liquid scintillation detector currently under construction in the
China JinPing Underground Laboratory The China Jinping Underground Laboratory () is a deep underground laboratory in the Jinping Mountains of Sichuan, China. The cosmic ray rate in the laboratory is under 0.2 muons/m2/day, placing the lab at a depth of 6720  m.w.e. and maki ...
(CJPL) scheduled for completion in 2022. *
LENA Lena or LENA may refer to: Places * Léna Department, a department of Houet Province in Burkina Faso * Lena, Manitoba, an unincorporated community located in Killarney-Turtle Mountain municipality in Manitoba, Canada * Lena, Norway, a village in ...
(Low Energy Neutrino Astronomy
website
is a proposed 50 kiloton liquid scintillation detector of the LAGUNA project. Proposed sites include Centre for Underground Physics in Pyhäsalmi (CUPP), Finland (preferred) and Laboratoire Souterrain de Modane (LSM) in Fréjus, France. This project seems to be cancelled. * at DUSEL (Deep Underground Science and Engineering Laboratory) at Homestake in Lead, South Dakota, USA * at BNO (Baksan Neutrino Observatory) in Russia
EARTH
(Earth AntineutRino TomograpHy)

(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.


Desired future technologies

* ''Directional antineutrino detection.'' Resolving the direction from which an antineutrino arrived would help discriminate between the crustal geoneutrino and reactor antineutrino signal (most antineutrinos arriving near horizontally) from mantle geoneutrinos (much wider range of incident dip angles). * ''Detection of antineutrinos from 40K decay.'' Since the energy spectrum of antineutrinos from 40K decay falls entirely below the threshold energy of inverse beta decay reaction (1.8 MeV), a different detection mechanism must be exploited, such as antineutrino scattering on electrons. Measurement of the abundance of 40K within the Earth would constrain Earth's volatile element budget.


References


Further reading

* * {{cite journal, last=McDonough, first=W. F., author2=Learned, J. G. , author3=Dye, S. T. , title=The many uses of electron antineutrinos, journal=Phys. Today, year=2012, volume=65, issue=3, pages=46–51, doi=10.1063/PT.3.1477, bibcode = 2012PhT....65c..46M


External links



describes deep ocean geo-neutrino detection projects with references and links to workshops.
Neutrino Geoscience 2015 Conference
provides presentations by experts covering almost all areas of geoneutrino science. Site also contains links to previous "Neutrino Geoscience" meetings.
Geoneutrinos.org
is an interactive website allowing you to view the geoneutrino spectra anywhere on Earth (see "Reactors" tab) and manipulate global geoneutrino models (see "Model" tab) Geophysics Neutrinos