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Physicists Confirmed The Registration Of The Solar Neutrino CNO-Cycle For The First Time

Physicists Confirmed The Registration Of The Solar Neutrino CNO-Cycle For The First Time

Analysis of the Borexino detector data for the first time allowed physicists to confirm the detection of neutrinos that are formed inside the Sun during the CNO cycle. The registration of these particles is experimental evidence of nuclear reactions of this type in the bowels of stars, and further research will clarify the chemical composition of the Sun. The results were reported by scientists at the virtual conference "Neutrino 2020", their publication in a peer-reviewed journal is currently absent.

The CNO cycle is a set of nuclear reactions in which hydrogen is converted to helium with the participation of carbon (C), nitrogen (N), and oxygen (O). Scientists believe that this cycle is one of the main processes of thermonuclear fusion for massive main-sequence stars, but it also occurs in the bowels of lighter stars — including inside the Sun.

Since it is not possible to observe the interior of a star directly, physicists obtain information about thermonuclear processes using indirect measurements. The data source here are neutrinos — extremely light (rest energy is estimated at less than 0.12 electron volts) elementary particles, which, in particular, become products of CNO-cycle reactions. The peculiarity of neutrinos is that they interact very weakly with matter.

On the one hand, this simplifies the work of scientists — particles emitted in the bowels of the Sun almost freely reach the observer and at the same time almost do not lose energy — it is estimated that tens of billions of neutrinos per second pass through every square centimeter near The earth's surface (hundreds of millions of them are born in the CNO cycle). On the other hand, the registration of particles becomes much more complicated — only a small part of the colossal number of neutrinos interacts with matter. To detect a sufficient number of events, you need to build massive detectors, carefully isolate them from noise, and continue the experiment for up to several years.

Participants in the Borexino project, led by Gioacchino Ranucci from the National Institute of nuclear physics in Italy, processed neutrino detector data between July 2016 and February 2020. The installation is equipped with several layers of protection, each of which filters out background particles of cosmic radiation and terrestrial radioactivity. In the Central part of the detector is a ball of 280 tons of liquid scintillator in a nylon shell, which is surrounded by 2200 photoelectronic multipliers (FEU).

When neutrinos are scattered on the scintillator's electrons, radiation is generated, which is detected by the FEU and converted into an output signal — its amplitude can be used to determine the energy of the detected particle. The device allows you to register about a hundred events every day, but the difficulty is that the detector captures neutrinos from all possible processes at once-from the proton-proton cycle, which is the basis of thermonuclear reactions in the Sun, to radioactive decays in the earth's interior. In order to isolate the CNO-cycle neutrino in the energy distribution of events, it is necessary to use theoretical models and calculate the contributions of background reactions to the total particle flow with high accuracy.

By performing these calculations and using them to process long-term detector statistics, physicists were able to reliably confirm the registration of the CNO-cycle neutrino for the first time. According to the authors, the statistical significance of the hypothesis reached the level of 5σ. Thus, scientists have obtained the first reliable experimental evidence of thermonuclear reactions of this type in the bowels of a star. Future research will use this data to Refine the carbon, nitrogen, and oxygen content of the Sun and probably provide more information about the physics of stars.