Natural neutrinos of terrestrial origin have been registered
Participants in the Borexino experiment announced that they were able to register electron antineutrinos, created as a result of the decay of radioactive isotopes in the Earth’s crust and mantle.
Neutrino – stable neutral particles, weakly interacting with matter, have been studied for a long time. Most of the experiments carried out were aimed at registering neutrinos emitted by the Sun or obtained as a result of the interaction of cosmic radiation with the nuclei of atoms in the Earth’s atmosphere. Having processed the data of these observations, scientists were able to clarify some questions of the physics of the Sun.
In addition, neutrinos (more precisely, electron antineutrinos) are born in the interior of the Earth as a result of the beta decay of the 40K isotope and some nuclides from the decay chains of the long-lived isotopes 238U and 232Th. Measuring the flow of such geoneutrino can give an answer to the question of how radioactive elements are distributed in the volume of the Earth and how important the processes of their decay play.
Register geoneutrino extremely difficult: the detector must have a huge volume and be located in a laboratory well protected from cosmic and other background radiation. It should also be borne in mind that geoneutrino have less energy than solar and atmospheric; this further complicates the experiment and forces specialists to use hydrocarbons for particle detection. In the volume of the detector, an electron antineutrino can interact with a proton to form a positron and a neutron. These particles, in turn, participate in the formation of gamma particles, which serve as evidence of the registration of geoneutrinos.
First detection message geoneutrino came in 2005. The data presented by the KamLAND collaboration were, however, not very reliable, since scientists had to highlight useful events against the background of an antineutrino flow from nearby Japanese and South Korean nuclear reactors (the KamLAND project is aimed specifically at studying reactor antineutrinos).
The Borexino experiment was initially focused on detecting low-energy solar neutrinos. The equipment, located several hundred kilometers away from the reactors, is installed in the underground Gran Sasso National Laboratory, protected by a layer of rocks one and a half kilometers thick. The detector has a complex structure; in its center is a nylon sphere 4.25 m in diameter, holding 278 tons of liquid scintillator – pseudocumene with additives diphenyloxazole… This volume is shielded from external radiation by 890 tons of solution of the same pseudocumene and dimethyl phthalate, enclosed in a second stainless steel sphere with a diameter of 13.7 m. The steel structure is housed in a huge tank filled with 2,400 tons of ultrapure water. A total of 2,212 photomultiplier tubes are used to collect the photons emitted by scintillation.
The authors processed the data obtained over two years (537.2 days of continuous operation of the installation). During this period, it was reported that about 10 – 9.9 (+4.1, -3.4) – events were recorded corresponding to geoneutrino… The taken spectra also make it possible to assess the prospects for the theory of a georeactor – a natural nuclear reactor in the Earth’s core. Counting the number of registered electron antineutrinos with suitable energy showed that the power of such georeactor cannot exceed 3 TW.
Gianpaolo Bellini, a spokesman for the Borexino research group, considers this result to be the first confirmed case of geoneutrino detection. The participant of the previously mentioned experiment KamLAND Atsuto Suzuki, of course, does not agree with him. “In 2008, KamLAND had 73 ± 27 registered events, while Borexino now has only 9.9 (+4.1, -3.4),” reminds the scientist. “I think everything is clear here even without my explanations.”
Both parties, however, agree that they require more data. At present, several large-scale experiments are planned at once, including the ambitious project of installing a detector weighing 10 thousand tons at the bottom of the Pacific Ocean.
The inner surface of a steel sphere, on which photomultiplier tubes are installed (photo of the Borexino project participants).