Scientists from the international collaboration Borexino announced the first observation of neutrinos from reactions of the carbon-nitrogen cycle in the Sun. This experimentally confirms the theoretical concepts of the secondary cycle of thermonuclear fusion in massive stars. The research results are published in the journal Nature.
The stars feed on the energy of thermonuclear reactions of the transformation of hydrogen into helium, taking place in their bowels. Such synthesis is possible in two ways: in the proton-proton (pp) chain, which includes only isotopes of hydrogen and helium, and during the secondary cycle, which is also called carbon-nitrogen, or CNO-cycle according to the symbols of carbon, nitrogen and oxygen – elements that act reaction catalysts. Nuclear reactions of both the primary and the secondary cycle are accompanied by the emission of characteristic neutrinos.
Proton-proton chains produce about 99 percent of the energy of the Sun and stars similar in size to it, so earlier scientists were able to observe only neutrinos from the pp-cycle. But it is believed that in heavy stars, with a mass of one and a half times more massive than the Sun, the carbon-nitrogen cycle prevails, and it was important to experimentally prove its existence.
Due to the extremely low probability of interaction with ordinary matter, neutrinos easily pass through the thickness of the Sun, preserving information about nuclear processes in the depths of the star and the conditions for their occurrence. It was a very difficult task to fix among solar neutrinos those that belong to the secondary cycle, since their signal was not much higher than the background one. But the scientists of the Borexino collaboration succeeded.
“Until recently, the question remained open whether it would be possible to register neutrinos from the CNO cycle. The registration of CNO neutrinos, in addition to the smallness of the flux itself, is complicated by the presence of the spectral component of the natural background, indistinguishable from their spectrum,” the press release of the Joint Institute for Nuclear Research says. in Dubna, the words of one of the participants in the experiment, a senior researcher at the Laboratory of Nuclear Problems. V.P. Dzhelepov JINR Oleg Smirnov.
The property of unimpeded penetration through matter allows neutrinos to store information about internal processes in the Sun, but this property makes them elusive for conventional particle detectors. Therefore, to register neutrinos, special detectors of very large mass are used with careful control of all processes that can reflect the interactions of neutrinos with electrons.
In those rare cases when a neutrino interacts with an electron, it transfers part of its energy to it. This process resembles the elastic collision of billiard balls. An electron, having received a certain initial velocity, gradually loses it in the course of interaction with the molecules of the medium. Part of the energy is emitted in the form of photons. Thus, the interaction of a neutrino with an electron leads to a flash of light, and several thousand photons scatter from the point of interaction in all directions.
These photons register thousands of light detectors, and special devices – photomultiplier tubes – allow you to estimate the energy transferred to the electron, as well as determine the point where the interaction took place.
The Borexino ultrasensitive detector, located in the largest underground laboratory in the world in Gran Sasso in Central Italy, uses about 100 tons of liquid scintillator as an active medium for neutrino detection.
“Despite the huge number of solar neutrinos passing through the detector (more than a sextillion per day), only fifty neutrinos leave a noticeable ‘trace’ in the detector during the same time. Scientists working on the data analysis were able to isolate a signal that can only be explained by the presence of neutrinos from CNO-cycle. Thus, the course of nuclear reactions of the CNO-cycle in the Sun has been proved. The total flux of neutrinos from the CNO-cycle is about one percent of the total flux of solar neutrinos “, – explains Oleg Smirnov.
The discovery is of paramount importance for astrophysics, since in stars more massive than the Sun, energy is released mainly due to the carbon-nitrogen cycle. Its mechanism has now been experimentally verified.