Data analysis of Borexino detector for the first time, allowed physicists to confirm the detection of neutrinos generated inside the Sun during the CNO-cycle. Registration of these particles is an experimental evidence of the occurrence of nuclear reactions of this type in the interiors of stars, and further studies will allow to clarify the chemical composition of the Sun. The results, scientists reported on the virtual conference “Neutrino 2020”, published in the peer-reviewed journal at the moment.
The CNO-cycle is a set of nuclear reactions in which hydrogen is converted into helium, with the participation of carbon (C), nitrogen (N) and oxygen (O). Scientists believe that this cycle is one of the main processes of fusion for massive stars of the main sequence, but occurs in depths more than light luminaries — including inside the Sun.
Because to see the inside of a star directly, information about thermonuclear physics is obtained by indirect measurements. The data source here are neutrinos — extremely light (the energy of peace is estimated at less than 0.12 electron volts) of an elementary particle, which, in particular, be reaction products of the CNO-cycle. Feature of neutrinos is that they interact very weakly with matter.
On the one hand, this simplifies the work of scientists in the bowels of the emitted light of particles almost freely reach the observer with almost no loss of energy — it is estimated that near the Earth’s surface, flying tens of billions of neutrinos per second (of which hundreds of millions are born in the CNO-cycle). On the other hand, significantly complicated the registration of the particles from the huge number of neutrinos are only a small part interacts with matter. To detect sufficient number of events to build massive detectors, carefully isolate them from noise and to continue the experiment up to several years.
The participants of the project Borexino under the leadership of Jakino of Ranucci (Gioacchino Ranucci) from the National Institute of nuclear physics in Italy held data processing neutrino detector in the period from July 2016 to February 2020. The unit is equipped with multiple layers of protection, each of which filters out background particles of cosmic rays and terrestrial radioactivity. In the Central part of the detector is a ball of 280 tons of liquid scintillator in a nylon sheath, which is surrounded by 2200 photomultiplier tubes (PMT).
In the scattering of neutrinos on electrons of scintillator occurs radiation a photomultiplier to register and convert to the output signal, its amplitude can determine the energy of the detected particles. The unit allows a daily register of the order of a hundred events, but the difficulty lies in the fact that the detector detects neutrinos from all possible processes, from the proton-proton cycle, which is the basis of thermonuclear reactions in the Sun, and ending with the radioactive decays in the earth. To the distribution of events in energy to distinguish neutrinos of the CNO-cycle, it is necessary to draw the theoretical model and accurately calculate the contributions of background reactions in the total particle flux.
Having done these calculations and process with them using multi-year statistics of the detector, physicists have been able to reliably confirm the registration of neutrinos of the CNO-cycle, according to the authors, the statistical significance of the hypothesis has reached the level of 5σ. Thus, scientists have obtained the first reliable experimental evidence of thermonuclear reactions of this type in the interior of stars. Future research will allow based on the data, to clarify the content of carbon, nitrogen and oxygen in the composition of the Sun and likely to get more information about the physics of stars.
Earlier we talked about how to install Borexino has allowed to determine the energy of the proton-proton cycle in the Sun, and to assess heating of the Earth by nuclear decay in the mantle. Read more about the history of neutrino experiment can be found in the material “Laboratory under the mountain”.