Second proven fusion path in the Sun – Neutrinos confirm CNO fusion cycle recommended 82 years ago

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Milestone in astronomy: Researchers have for the first time discovered neutrinos from the so-called CNO cycle of the Sun – a fusion reaction predicted 82 years ago. Protons do not melt directly, but bind to helium by the catalysis of heavy elements such as carbon, nitrogen and oxygen. The first direct evidence of this CNO cycle has now been achieved by neutrinos – an important turning point for stellar research.

Our sun receives its energy Nuclear fusion – Hydrogen is converted to helium. About 99 percent of these fusion reactions occur in our star through the direct fusion of protons. But in early 1938 the physicists Hans Bethe and Carl Friedrich von Weissecker suggested that there should be a second connection. These so-called CNO cycles, in which carbon, nitrogen, and oxygen react with the fusion reaction.

Plan of the CNO fusion cycle. © Civit / CC-PI-SA 3.0

Neutrinos as fusion boats

In the sun, this CNO fusion accounts for only one percent of all fusion processes. But in the case of the biggest stars it is the dominant reaction pattern. “The CNO cycle is therefore the primary means of converting hydrogen into helium in the universe,” explains Gioachino Ranucci and his colleagues from the Borexino collaboration. But despite its importance, this fusion path has yet to be proven directly in a star.

That has now changed: 82 years after the theoretical prediction of the CNO cycle, researchers at the Borexino collaboration have now proven it experimentally. They did this with help Solar neutrinos – Almost massless particles released as a by-product of fusion reactions. Hundreds of billions of such neutrinos run unnoticed through our body every second. The fusion reaction of such a particle can be read from its energy with others.

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The glow of light in the underground laboratory

The problem, however, is that neutrinos from CNO fusion are relatively rare and have a low energy of up to a maximum of 1.700 kV. This makes them easier to use Radiation decay reactions To confuse released neutrinos. To detect CNO neutrinos, one must have diagnoses that are maximally protected against such mutations.

One such discovery is the borexino in the Gran Sasso laboratory. The underground neutrino detector is protected from the outside world by a thick rock, steel shell and several liquid tanks. At the center of the structure is a detector tank filled with 278 tons of organic liquid. When a neutrino collides with one of the liquid atoms in it, a small incandescent light is generated, which is recorded by photosynthesizers.

In order to search for solar CNO neutrinos, the researchers evaluated the detection data from July 2016 to February 2020 and filtered it in detail and statistically.

720 million CNO neutrinos per second and square centimeters

They discovered what they were actually looking for: among the many neutrinos from other sources, they were the first to identify particles from CNO fusion. Their discovery recorded an average of 7.2 CNO neutrinos and 100 tons of fluid per day. Considering that most of these CNO neutrinos go unnoticed, their actual size is enormous: “It could turn into 720 million CNO neutrinos, which flow into the earth every second and every centimeter,” Boraxino scientists explain.

This is the first time that the CNO cycle in the Sun has been detected directly by neutrinos formed during this fusion path. They confirm that such a CNO fusion takes place in the sun – along with the 82-year-old theory of Bethe and Van Weiss. At the same time, the measured values ​​match the samples, so that this fusion path is one percent of the total solar fusion reactions.

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“Milestone in Neutrino Physics”

Gianfolo Bellini, co-author of the University of Milan, said: “This gives us the first, amazing test evidence of how stars produce luminosity heavier than the Sun. At the same time, these measurements open up a way to more closely determine the content of heavier elements in the Sun and other stars. Affects the size of the CNO connection.

Gabriel Orebi Gunn, a physicist at the University of California, Berkeley, writes: , 2020; Thoi: doi: 10.1038 / s41586-020-2934-0)

Quell: Borexino Collaboration

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