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The sun gets its energy from the fusion of hydrogen into helium. However, since 1938 theories have suggested that, in addition to a direct fusion of hydrogen nuclei, there must also be a second fusion pathway catalyzed by heavier elements: the so-called CNO cycle. Now, for the first time, researchers have managed to directly demonstrate this second cycle of solar fusion. With the help of a neutrino detector under the Italian Alps, they detected the particles that are released as a by-product of the CNO cycle. This is a fundamental advance for solar research and astrophysics.
Our sun is a gigantic melting furnace: the immense pressure of solar gravity and a heat of more than 15 million degrees cause hydrogen nuclei to fuse inside it. Around 600 million tons of hydrogen are converted to helium every second. In the sun, most of these fusion reactions occur through the direct fusion of protons. This so-called proton-proton fusion makes up about 99 percent of solar fusion. But as early as 1938, physicists Hans Bethe and Carl Friedrich von Weizäcker independently postulated from each other that there must be a second fusion path in stars. Here, heavy elements like carbon, oxygen and nitrogen act one after another as catalysts for the fusion of hydrogen into helium. According to the models, this reaction pathway, called the CNO cycle for the abbreviations of the elements, should account for about one percent of nuclear fusion in less massive stars like the sun, but represent the dominant form of fusion in more massive stars massive.
Neutrinos as messengers from inside the sun
But how can these processes be demonstrated inside the sun? This is where neutrinos come in – virtually massless particles that are released as a by-product of fusion reactions. Hundreds of billions of these solar neutrinos flow through our bodies every second without our noticing. Because these particles hardly interact with other matters. Therefore, they can only be detected in detectors that contain huge amounts of water, ice or other detector material. If a neutrino collides with an atom, this causes a small flash of light that is captured by photosensors. From the energy and spectrum of these light signals, scientists can draw conclusions about the properties of the trigger neutrinos and their origin. However, the detection of solar neutrinos is particularly difficult because, due to their low energy, they can easily be confused with the neutrinos released during radioactive decay reactions.
One system with which the detection of solar neutrinos is still possible is the Borexino underground detector at the Gran Sasso Laboratory. It is shielded in various ways against interfering radiation from decaying neutrinos. In addition to the rock cover and a layer of water, the 278 tonnes of organic detector liquid is surrounded by two more zones of protective liquids. Thanks to this configuration, researchers from the Borexino collaboration have already managed to detect solar neutrinos in the proton-proton collision in recent years. However, to detect the much rarer neutrinos of the CNO cycle, they had to develop additional purification steps and statistical filtering methods. “The biggest challenge was identifying the small excess; it only corresponds to a handful of events per day and 100 tons of detector liquid, ”the scientists explain.
720 million CNO neutrinos per second per square centimeter
But it succeeded: After evaluating detector data from July 2016 to February 2020, physicists have now clearly identified for the first time the neutrinos that were released during the sun’s CNO fusion cycle. Based on this, its neutrino detector captured an average of 7.2 CNO neutrinos per day and 100 tons of liquid. “That can be converted into 720 million CNO neutrinos that flow towards the earth every second and square centimeter,” explain the Borexino scientists. Their values confirm that fusion occurs in the sun according to the CNO cycle and also fit the models according to which this fusion path represents about one percent of the total solar fusion reactions. “With this we finally have the first groundbreaking experimental proof of how stars make their brightness heavier than the sun,” says co-author Gianpaolo Bellini of the University of Milan.
Physicist Gabriel Orebi Gann of the University of California at Berkeley, who is not involved in the study, sees it in a similar light. He writes in an accompanying comment: “The Borexino collaboration presents results that represent a milestone in neutrino physics. This tremendous achievement brings us closer to a complete understanding of our sun and massive star formation. “
Those: The Borexino Collaboration (INFN Gran Sasso Laboratories), Nature, doi: 10.1038 / s41586-020-2934-0
