The Promises of Geoneutrinos

Neutrinos have been studied ever since they were predicted by Pauli with ever increasing vigor. The recent flurry of studies on neutrinos from the Earth is coming from its first detection in 2003 at Kamiokande which has already set some non trivial limits on the radiogenic heat flux. Geoneutrino detection in future experiments hold the promises of both improving our knowledge of neutrino's properties as well as of the geophysics of the Earth. It may even settle definitively the controversial issue of whether or not there could be a georeactor at the centre of our planet


The messengers of astronomy
Modern astronomy has opened up various windows to look at the Universe. The electromagnetic window has been for a long time confined to the optical part of the spectrum accessible through the use of reflecting telescopes. The invention of the radar led to the birth of radio-astronomy after WWII opening up the radio window. Then space age allowed us to place our telescopes beyond the atmosphere and thus free from atmospheric absorption and turbulence, opening up the infrared, ultraviolet, X rays and then gamma rays branches of astronomy. But astronomy, namely the study of the Universe, needs not stop at electromagnetic astronomy. Indeed other particle messengers beside the photon can be detected on Earth and inform us on the properties of matter whether stellar, galactic or even extragalactic. There is first cosmic ray astronomy where the messenger is mostly the proton. Being stable, the proton can propagate through huge distances but being a charged particle we lose directional information about the source save for ultra high protons beyond 10 18 eV. Graviton astronomy attempts to start with gravitational waves interferometers like LIGO and VIRGO. The idea is to detect the gravitational waves either primordial or coming from the asymmetric merging of massive compact objects like neutron stars or black holes.

Neutrino astronomy
Neutrino astronomy is a somewhat late comer in multi-messenger astronomy. It all started with the detection of a handful of neutrinos in 1987 when SN 1987A situated in the Large Magellanic Cloud galaxy next to our Milky Way exploded thus releasing most of its energy in neutrino form, some of which reached Earth on that fateful day of 23 February 1987. Solar neutrino astronomy started few years earlier when it was found a deficit on the number of solar neutrinos received in underground detectors. In addition, atmospheric neutrinos detection (Superkamiokande and Borexino detectors notably) coming from the decay of muons generated in cosmic rays air showers started in earnest. All this along with the various ongoing long baseline experiments has enabled us to determine most of the neutrino mixing matrix parameters. The first galactic neutrinos were detected in 2013 by the billion ton detector Ice Cube which is tracking neutrinos coming from below ground (and thus coming from the Northern hemisphere sky). At term, it should measure their flux and identify some of their sources which presumably are Active Galactic Nuclei (AGN) as well as Gamma-ray bursts and starburst galaxies. We can see displayed in  In addition to all these astrophysical neutrino sources, there exists a neutrinic cosmic background at slightly lower temperature and density than the photon CMB produced some 13.7 billion years ago a little bit prior to the matter photons decoupling. Due to their extremely low energy, there is no foreseeable experiment which could detect them. Although they are only the second most abundant particles in the Universe after the photon, yet due their non-zero mass, they most likely give the largest contribution to the mass of ordinary matter. Neutrino astronomy may also indirectly detect dark matter as dark matter candidates involves in many scenarios the weak interaction and thus very high energy neutrino pairs would be produced.

Geoneutrinos
Detecting geoneutrinos, that is neutrinos produced in the Earth interior from the decay of radioactive nuclides has become a reality in the past decade. The birth of what can be called neutrino geophysics took place at first with the KamLAND detector in Japan in 2005 when a non zero anti-neutrino signal was laboriously separated from the nuclear reactors background. It was then confirmed and consolidated at the Borexino detector at the Gran Sasso facility in Italy in 2010. Since then, more detections took place at those facilities, and several other dedicated ones are about to enter the fray. The idea of detecting geoneutrinos was put forward in the early sixties by Eden [1], Marx [2] and Markov [3], and argued further about its possibility in the eighties notably by Krauss, Schramm and Glashow [4]. The basic idea which makes this new tool for geophysics so exciting is its ability to determine quantitatively the geothermal heat that flows from the Earth occurring from radioactive elements like 235 U, 232 Th and 40 K. An average value of this internal heat is 60mW/m 2 which gives an overall value of 30-44 TW for the whole Earth. Indeed, a crucial issue facing geophysics is that unlike the density profile of the Earth which is rather well known from seismologic data, the elemental composition is not known save from geophysical arguments and hints from composition from chondritic meteorites as we have not been able to probe directly the Earth interior beyond a 12km depth. The Bulk Silicate Earth [5] model (BSE ) has been built describing the primitive mantle, that is after core separation, and which notably predicts the global distribution of the radiogenic elements. Although compatible with most observations, it has been gotten by a chain of arguments and can't be taken with total confidence. Furthermore it has to be in practise supplemented with estimates from the fractions of Uranium in the crust and in continental mass as this play a crucial role in evaluating the expected antineutrinos rates at the specific detector location. We see that the antineutrinos from those facilities could be reduced dramatically if we choose adequately the location of our detector. Furthermore, it has little to say about the possible existence of Uranium in the Earth core as it has been speculated by some dissident geophysicists. Arguments are based on the lithophile character of Uranium and Thorium which, according to the BSE model, prefer despite their high density, to be in the silicate or the molten rocks surrounding the core rather than in the core itself made of iron and other metals having some chemical affinity with it. Additionally, the Urey's ratio which measures the total energy radiated by Earth with respect to the radiogenic heat and which is currently estimated to be 0.5 to 0.6, strongly suggests that the radiogenic energy is the dominant source of the Earth energy outflow without being the only one, although a rate of 1 is not excluded. Now a beautiful way to probe the issue of the Earth energy source is to detect anti neutrinos from the decay of the Uranium and Thorium nuclides as they are directly related to the radiogenic heat. Let us recall, to underscore the importance of the issue, that the Earth internal heat flow is a crucial parameter to understand Earth as it drives every process inside it, most notably plate tectonic, volcanoes, the Earth magnetic field and the like.
Since the antineutrinos are coming from the natural beta decay of the radiogenic nuclides, it comes mainly through the three chains:  Adopting a specific mass distribution model (Like the BSE), we can find the differential luminosity for geoneutrinos which enables us to make predictions on the radiogenic energy output based on the signal at each detector. Luckily, solar neutrinos do not constitute a background since the reaction above is not occurring. The real background is from the some 450 nuclear reactors disseminated over much of the lands and which make the detector's location a crucial factor in the analysis as the geoneutrinos count will be deduced after subtracting out those reactors antineutrinos from the overall detection rates. The results using the BSE model is summarized in this table [7]. Thus the total radiogenic heat is the sum of the three contributions and give 18.4TW, which constitutes a substantial fraction of the total internal Earth outflow. Notice that according to the BSE paradigm, no contribution has been assumed from the core since it is supposed to be Uranium and Thorium depleted. Since all of the estimates, whether the overall energy outflow or the radiogenic heat, are uncertain by a factor of possibly two, the ranges of the various estimates in the literature for the radiogenic heat being between 19TW to 31TW, while the overall Earth output ranges between 30TW to 44TW, thus uncertain by a factor of possibly two. It is clear thust hat the question of the origin of Earth heat is not settled. Yet the necessity of including the radiogenic output is strong while at the same time the other energy sources which might have been important in the past like gravitational energy release during chemical differentiation, the tidal fiction, meteoritic impact are majored. This leaves some place for other models, called sometimes "heretical" geophysical models, which ranges from challenging the standard BSE abundances or the cosmochemical estimates on which it is partially based (The CI chondrites might not be the original material from which the Earth was formed), to more severe modifications like assuming that a large amount of Potassium might be locked inside the Earth's core [8], thus driving up the Earth's energy budget. An even more revolutionary modification to the standard BSE model is the core geo-reactor hypothesis by Herndon [9] whereby a large amount of Uranium at the Earth centre would have formed a breeder reactor. Unambiguous determination of the radiogenic heat production will be needed before neutrino geophysics could say to have contributed decisively to geophysical issues.

The geoneutrinos detectors
The first detector to have caught low energy geoneutrinos was KamLAND (Kamioka Liquid Scintillator Antineutrino Detector) located at the Kamioka Observatory in Japan, thus heralding the birth of neutrinos geophysics. It consists of a 1000t scintillator detector surrounded by 1845 PMTs and located at some 1000m underground to avoid the cosmic ray background. Yet the nuclear reactors background is formidable and has to be extracted from the detected events to isolate the signal. Borexino is the second geoneutrinos detector in use. It consists of a real time 0.3 kiloton scintillator calorimeter originally used to detect the 7 Be from the Sun, and is situated as some 1500 m underground at the Laboratori Nazionali del Gran Sasso in Italy. After a first run in 2011 which first demonstrated the capability of geoneutrino detection with a 3σ signal, a second run (1352 days) in 2013 identified 14 geoneutrinos out of 46 detected, the remaining ones being accounted for by the nuclear reactors and only one of them from the detector tank thanks to the great radio purity of the liquid scintillator. It thus beneficed with respect to KamLAND from a much lesser nuclear reactor background. The data seems to reject the Earth's core georeactor hypothesis, limiting in any case its power to no more than 3 TW (95% confidence level).

The Georeactor in the Earth's Core hypothesis
This radical hypothesis proposed by M. Herndon is certainly not the geophysicist's paradigm as it assumes the existence of a natural nuclear reactor with power up to 10 TW operating in the center of the Earth. Yet, as argued before and although it is not mainstream theory, it can't be ruled out by any experiment yet. A case for its plausibility can be made from the occurrence of natural reactors like the Oklo one in Gabon which has undergo in the past a spontaneous ignition of uranium in mineral deposits some 1.7 billion years ago. If the geo-reactor exists, its anti-neutrino will contribute to the radiogenic outflow. Now with KamLAND and Borexino data, we have for the first time better constraints on its existence, namely an upper limit on the power of the geo-reactor although it is not strong enough to exclude the hypothesis altogether. The best fit is: 0 6.4 5.9 5.9 H TW    As for the upper limit on the geo-reactor power, it is 19 TW at 90% C.L with some dependency from the choice of the oscillation parameters. Ideally, a way which could determine the geoneutrino's angular distribution [10] could help settle the issue as we could in particular discriminate the antineutrinos coming from the core from those coming from the mantle. It could also discriminate those from nuclear reactors and the crust coming close to horizontal from those from the mantle reaching the detector with a pronounced dip angle. That would imply other detection techniques as the one used till now and based on inverse beta decay on protons which is not directional [11]. Let us call I( the intensity of geoneutrinos at the Earth surface, so that F the flux distribution of neutrinos would be, following Fields, Brian D. et al: Dividing the Earth in concentric spherical layers, we can compute the differential neutrino flux at the surface or close enough to it according to various hypothesis on the distribution of the radiogenic elements according to the depth. Now assuming various global abundances for the uranium in the core, we can see the great discriminating power that could provide us a directional capability at the detector.

The future detectors
Quite few detectors are planned to study further the geoneutrinos. We will focus on two of them, -SNO+ which is about to take data, and the other one Hanohano quite revolutionary by all standards and which is in an advanced stage of planning. Two others also with great potential for geophysics will be briefly mentioned at the end of the section.
-The SNO+ (Sudbury Neutrino Observatory+) detector at SNOLAB near Sudbury in Canada is filled with 780 tons of liquid scintillator in a 12 m diameter acrylic sphere shielded by some ultra pure water along with 9000 photomultipliers. geology is extremely well studied. The antineutrino signal will be dominated at about the 80% level by the crustal component. It should provide data on the composition of the continents as well as place limits on the various models of the composition of the continental crust.
-The Hanohano deep-ocean transportable detector It is a proposed mobile sinkable 10 kton liquid scintillator detector to be carried by a ship and deployed in the deep ocean at some 3 to 5 km depth aiming at measuring geo-neutrinos in a quite innovative way. It is to be positioned at the oceanic crust near Hawaii since the oceanic crust is thin and assumedly depleted in U and Th with respect to continental crust which will furthermore reduce background from nuclear power reactors. Yet being portable it should be paced at various oceanic locations and be lowered there. Its expected detection rate is about 100 geo-neutrinos per year (For only around 12 reactor antineutrinos per year), and it should be able to also measure the Th/U ratio of the mantle at the 10% precision level in addition to potentially determine the neutrino mass hierarchy.
-The Daya Bay 2 experiment [12] in China is characterized by a very large mass of some 20 kton and is supposed to detect up to 400 geo-neutrinos per year. Unfortunately, being very close from a reactor, the e  background is huge. In the mean time, the facility made the news in 2012 when an antineutrino disappearance experiment improved much on the value of the  13 mixing angle, finding it yet surprisingly larger than expected. -The LENA (Low Energy Neutrino Astronomy) detector scientific consists of a huge 50 kton liquid scintillator multidisciplinary neutrino detector for which geoneutrinos measurement is one of its main scientific goals [12]. It has the potential of detecting some 1000 geoneutrinos per year. The diagram below shows the running and planned experiments.

Conclusion
Geoneutrinos have the potential to provide us with information on the Earth interior unavailable by other means and test in a unique way the standard model of geophysics. Most notably, it should enable us to settle the long standing controversy on the source of Earth's heat, whether geophysics or radiogenic, the ratio of U/Th, and their respective amount in the crust and the mantle, and even the core if any. And although it started as an offshoot of the new field of neutrino astrophysics it has acquired a life on its own. The coming to age of multi kilotons detectors promises to usher neutrino geophysics into a golden age.