Participants of the international experiment Borexino carried out in March 2010 at the underground laboratory of the National Institute of Nuclear Physics of Italy (INFN) located in the Gran Sasso mountains in the Central Apennines, registered geoneutrinos—particles coming out of the magma surrounding the Earth core. Specialists observed a stable antineutrino signal with an energy spectrum equal to the expected spectrum resulting from the beta-composition of radioactive elements of uranium-238 and thorium-232 chains. Thus, for the first time they proved a radiogenic contribution of neutrino to the heat produced in the interiors of the Earth. An article dedicated to this experiment has been published in the Europhysics Letters (EPL) magazine.

Researchers of the Joint Institute for Nuclear Research Yuri Gornushkin and Oleg Smirnov who participated in the experiment told about their results in the weekly newspaper Dubna: Science, Cooperation, Progress. Note: the experiment has been carried out in cooperation with research associates of the Joint Institute for Nuclear Research, Russian Scientific Center "Kurchatov Institute", Research Center for Nuclear Physics named after D. Skobeltsyn under Lomonosov Moscow State University (Moscow), and RAS St. Petersburg Institute for Nuclear Physics named after B. Konstantinov. As for foreign partners, Universities of Milan, Genoa, Perugia (Italy), Munich Technical University, Max Planck Institute (Germany), Laboratory of Astroparticles and Cosmology of the National Institute of Nuclear Physics and Particle Physics (France), Jagellonian University (Krakow, Poland), Princeton University and Virginia Polytechnic University (USA) took also part in the experiment.

Neutrinos are stable neutral particles weakly interacting with the substance. The Sun and cosmic rays are considered "typical" sources of neutrinos. But physicists assume that: electronic antineutrinos can also occur in the interiors of the Earth as a result of certain radioactive decomposition processes. These neutrinos can be registered only after thorough isolation of detectors from the background radiation of different nature. As a rule, detectors are installed under the earth: for example, Borexino neutron detector is located 1 km deep.

The detector itself represents a steel cylinder connected from above with a sphere of 16 m in diameter. Its internal structure has several layers and resembles a Russian matryoshka. The external layer is filled up with 2.4 thous. tons of super-pure water protecting the unit from natural radioactivity of rocks and structural materials. It can also register rare cosmic muons not absorbed in a thousand m thickness of surrounding the underground laboratory. The next layer is a steel sphere filled with 1,000 tons of super-pure pseudocumene— a hydrocarbon compound used to protect central part of the detector. From within, the sphere has 2,200 photo-electronic multipliers—sensitive sensors capable of registering very weak light flashes produced in the course of interaction of neutrinos. And, finally, the center of the structure is a transparent nylon ball, 4.25 m in radius, containing 300 tons of scintillating* liquid. When "colliding" with an electron of this organic substance, neutrino transfers a part of its energy, which results in a light flash registered by physicians.

According to the authors, registration of "elusive" particles is complicated by the presence of natural radioactive admixtures simulating neutrino interaction processes in any materials. Therefore, scientists focused much attention on selection of radiation-free substances to construct a detector and development of new technologies enabling to clean liquids and gases of natural admixtures to the earlier unattainable levels. Thus, in the course of the 10-year experiment, they have achieved meaningful results. At the moment, the central part of the Borexino underground unit, where neutrino interactions are registered, is constructed of the purest material on the planet, speaking in terms of inner radioactivity.

* Scintillators-special media where in the course of radiation there occur light flashes, so-called scintillations. -Ed.

Scientists believe that geoneutrinos formed as a result of decomposition of uranium and thorium elements, as well as potassium-40 and rubidium-87, are present in the crust and mantle of the Earth, greatly affect the process of heating of its depths. Released heat is supposed to trigger convective fluxes in the Earth's liquid mantle, which accelerates volcanic activity, movement of tectonic plates and, as a consequence, seismic activity of the crust. According to specialists, the Earth's magnetic field is preconditioned by a deep-laid heat isolation.

The mechanism of heat production in the depths of the Earth is a fundamental problem of geology, emphasize the authors. National and foreign scientists began to discuss the problem of geoneutrino as early as in the 1960s. For example, the well-known specialist in nuclear physics Academician Moisey Markov pointed out the possibility to register "elusive" particles in the reaction of reverse beta-decomposition of protons; this principle has been applied in the Borexino detector. But modern theoretical studies were initiated only in 1994 on the initiative of American physicians Lawrence Krauss, Sheldon Glashow and David Shramm. In 2004, excessive number of low-energy antineutrinos were registered by participants of KamLAND collaboration-Japanese and American scientists working with a similar detector in the Kamioka mine, Japan. But due to powerful internal sources of radioactivity and antineutrinos from nuclear power plants located nearby and practically similar to their "geological" counterparts, members of that group were very deliberate in their conclusions: excess of the registered events (flashes) just "shows" existence of geoneutrino.

In this sense, Gran Sasso mountains are more convenient to "catch" these particles as the laboratory is located in the center of Italy, which has no atomic stations and is far away from European nuclear power plans. It is necessary to take into account one more factor: purity of a liquid scintillator used in the Borexino unit is much higher than in any other detector. Consequently, the background radiation to measure geoneutrino signals is by 100 times lower than elsewhere.

Summarizing the above-said, the experiment carried out at the Borexino underground laboratory proved the hypothesis that radioactivity makes a significant, if not a decisive contribution to heating of the Earth. But other sources of thermal effect are also possible, for example, a process of gravitational differentiation of the terrestrial substance by density and tidal interaction with the Moon. The earlier accepted hypothesis on the existence of a natural nuclear georeactor in the center of the Earth, producing the major part of energy has been rejected based on the data obtained in Gran Sasso: the power of such natural "plant" in this place cannot exceed 3 TW, otherwise it would have made a significant contribution to the general signal of the detector.

According to scientists, detailed studies of radioactive particles and their effect on heating of the planet can be carried out by means of a net of neutrino sounding units located in different parts of the planet. Today such units are operating in Italy, Japan and Canadian province of Ontario (Sudbury Neutrino Observatory). Due to them specialists are expecting in near future to get valuable information that will enable them to make steps forward in understanding of the structure of our planet and sources of its internal heat.

Yu. GORNUSHKIN, O. SMIRNOV, Experiment "Borexino" Registers Neutrinos Appearing in the Depths of the Earth.— "Dubna: Science, Cooperation, Progress" weekly newspaper. No. 12-13, 2010

Prepared by Marina KHALIZEVA