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The Last Secret Of The Sun's Nuclear Fusion Revealed

The Last Secret Of The Sun's Nuclear Fusion Revealed

Scientists from the Borexino collaboration, which also includes Russians, reported at the Neutrino-2020 conference on the first confirmation of the reactions of the carbon-nitrogen cycle in the Sun. One of the fundamental propositions of astrophysics predicted theoretically has received the experimental proof.

According to existing concepts, two main nuclear fusion reactions occur in The Sun's core. The first, known as the proton-proton chain, in which hydrogen merges with helium, gives about 99 percent of the star's energy.

The second is the cycle for carbon, nitrogen, and oxygen (CNO). It produces only about a percent of solar energy and is secondary to the Sun. However, for stars of one and a half times the mass of the Sun, this cycle is responsible for half of all energy. During this process, four protons merge to form a helium core, which releases two neutrinos (the lightest known elementary particles of matter), as well as other subatomic particles and a large amount of energy.

Scientists from the Borexino collaboration have previously detected neutrinos from the proton-proton chain and now have detected neutrinos from the carbon-nitrogen cycle. Physicists from the Laboratory of nuclear problems of the joint Institute for nuclear research named after V. p. Dzhelepov (JINR) took part in the study.

The measurement of the CNO-neutrino flux, the intensity of which is directly related to the abundance of elements in the star, should shed light on the mystery of the chemical composition of the Sun, about which there are various hypotheses. In particular, this will help to find out how many elements in the Sun are heavier than hydrogen and helium, that is, to determine its metallicity.

Even though millions of neutrinos pass through every square centimeter of the Earth's surface in a second, it is quite difficult to catch them, since they do not interact with matter.

The borexino underground particle detector, operating since 2007 at the Gran Sasso National Laboratory (Laboratori Nazionali del Gran Sasso) in Italy, consists of a giant nylon balloon submerged in water and filled with 278 tons of liquid hydrocarbons.

The vast majority of solar neutrinos pass freely through the earth and the detector, but very few of them bounce off the electrons in the hydrocarbons, producing flashes of light that are picked up by the photon sensors lining the water tank. These flashes are used to detect "Ghost particles."

Researchers have spent years discovering CNO neutrinos, which are relatively rare because they are formed in only a small portion of solar fusion reactions. Besides, the neutrino of the carbon-nitrogen cycle is easily confused with the neutrino formed as a result of the radioactive decay of bismuth-210, an isotope that seeps from the nylon balloon into the hydrocarbon mixture. Although the contamination is negligible — no more than a few dozen bismuth nuclei decompose per day-separating the solar signal from bismuth noise has required painstaking efforts that scientists have been putting in since 2014.

To eliminate the influence of bismuth, it was necessary to control any temperature imbalances in the tank that could cause fluid convection. To maintain a constant uniform temperature of hydrocarbons, the scientists wrapped the entire reservoir in an insulating coating and installed heat exchangers to automatically equalize the temperature.

Only in 2019, the noise of bismuth decreased so much that it allowed us to isolate the CNO-neutrino signal. By early 2020, researchers had collected enough particles to definitively declare the discovery of neutrinos from the carbon-nitrogen chain of nuclear fusion.

Thanks to this result, physicists are now reliably aware of two main processes that occur not only in the interior of the Sun but also in heavier stars, where the carbon-nitrogen cycle is the main one.

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