The Earth seen through Neutrinos

Geoneutrinos

Our great old planet has kept since its birth many long lived radioactive atomic nuclei (238U, 235U, 232Th, 40K). This is what we call “natural radioactivity”. This radioactivity is quite huge but badly known. The power coming from it is estimated to be about 20 Tera Watts (TW) (about 20,000 nuclear plants!) and the antineutrinos (ν) coming from this radioactivity are numerous: about 6 millions per second and per cm2.

The idea of studying Earth composition by detecting the antineutrinos from “natural radioactivity” has been around since at least mid-1960s [Ede66]. In 1984 Krauss, Glashow and Schramm [Kra84] presented detailed calculations of the geoneutrino flux and possibilities for their detection.

At the beginning the antineutrinos emitted by nuclear power plants seems an irreducible background for the detection of these low energy “natural radioactivity” antineutrinos and the most favorable place to detect geoneutrinos was identified as New Zealand, the farthest place from northern hemisphere numerous nuclear power plants.

In spite of that, the first detection of geoneutrinos was done in 2005 by the KamLAND [Ara05], an experiment primarily dedicated to the observation of antineutrinos emitted by nuclear power plants located at a typical distance of 200 km. In 749 days KamLAND detected 152 ν but less than 60 were attributed to the geoneutrino component. After Fukushima the reactor background was reduced and KamLAND estimated the quantity of heat produced by radioactive nuclei in the Earth to be 20 TW.

In 2010 Borexino [Bel10], in Italy, released its first geoneutrino measurement, complemented in 2013 [Bel13]. With 24 interactions attributed to geoneutrinos, Borexino estimated the heat released to be 23-26 TW.

Although still rather uncertain, the heat released by the radioactivity in the Earth represents an important fraction of the total heat released measured as 47 TW.

In 1993 J.M. Herdon [Her93] postulated the existence of a fast neutron breeder reactor at the center of the Earth. This proposition, very controversial in the geoscience community, could explain the measured abundance of 3He in oceanic basalt. It could offer also an explanation to the production of the Earth magnetic field.

Several projects dedicated to the measurements of geoneutrinos have been proposed like Hano-Hano close to Hawaii, Lena in Finland and SNO+ in Canada. The various locations allows to disentangle the contribution due to the crust and to the mantle of the Earth.

Since 2005 regular symposium gathers neutrino physicists and specialists of geosciences. The last one was held in Paris in 2015.

Studies of the Earth

There already exists some proposals to use high energy atmospheric neutrinos or man-made neutrino beams to make an Earth tomography, ie to provide a map of the matter density inside our planet. One of the most spectacular and somehow futuristic was described in  [Ruj83]. The idea consisted of an accelerator of particles producing an orientable beam of neutrinos. This device would be on a vessel of a size similar to an aircraft carrier. Neutrinos having crossed the Earth would be detected on a movable detector on the other side of the planet. Such a device could produce a full image of the interior of the Earth either by measuring the attenuation or by using the change of flavor induced by the matter effect.

The large neutrino telescopes infrastructures is also an opportunity to deploy oceanographic, geophysical and biological instrumentation in deep-sea environment, allowing long-term and real-time investigations.The KM3NeT deep-sea installations in France and in Italy already provide access to the Earth and Sea science community.

Further information

During the conference on the History of the Neutrino (Sept. 5-7, 2018 in Paris) the  history of Geoneutrinos was reviewed by L. Ludhova (Forschungszentum Julich, Germany) : here the slides and the video of her talk.

For recent developments on the topics : Wikipedia on Geoneutrinos

References

 Author(s)TitleReference
Ara05T. Araki et al. Experimental investigation of geologically produced antineutrinos with KamLANDNature 436 (2005) 499
Bel10G. Bellini et al. Observation of Geo-neutrinos Phys. Lett. B687 (2010) 299
Bel13G. Bellini, A. Ianni, L. Ludhova, F. Mantovani, W.F. McDonough Geo-neutrinos Prog. in Part. and Nucl. Phys. 73 (2013) 1
Ede66G. EderTerrestrial neutrinos Nucl. Phys. 78 (1966) 657
Fio07Gianni Fiorentini, Marcello Lissia, Fabio Mantovani Geo-neutrinos and earths’s interior Physics Reports 453 (2007) 117
Her06J.M. HerndonSolar System Processes Underlying Planetary Formation, Geodynamics and the GeoreactorNeutrino Geophysics Proceedings of Neutrino Sciences 2005, Earth, Moon and Planets 99 (2006) 53
Kra84L. Krauss, S.L. Glashow, D.N. Schramm Antineutrino Astronomy and Geophysics Nature 310 (1984) 191
Mar69G. MarxGeophysics by neutrinosCzech. J. Phys. B 19 (1969) 1471
Ruj83A. de Rujula, S.L. Glashow, R. Wilson, G. Charpak Neutrino Exploration of the Earth Physics Reports 99 (1983) 341

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