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MADRID, Nov. 26 (EUROPA PRESS) –
Particle physicists have detected neutrinos from the Sun, directly revealing this for the first time the CNO (carbon-nitrogen-oxygen) fusion cycle is at work in our star.
The CNO cycle is the dominant energy source that powers stars heavier than the Sun, but until now it has never been detected directly in any star, he explains. it’s a statement co-author Andrea Pocar of the University of Massachusetts Amherst. The results are published in Nature.
For much of their life, stars obtain energy by fusing hydrogen into helium. In stars like our sun or lighter, this happens mainly through the “proton-proton” chains. However, many stars are heavier and hotter than our sun and include elements heavier than helium in their composition, a quality known as metallicity. The prediction from the 1930s is that the CNO cycle will be dominant in heavy stars.
Neutrinos emitted as part of these processes provide a spectral signature that allows scientists to distinguish those in the “proton-proton chain” from those in the “CNO cycle”. Pocar Notes: “The confirmation that the CNO burns in our sun, where it operates at only 1%, strengthens our confidence that we understand how the stars work.”
In addition to this, CNO neutrinos can help solve an important open question in stellar physics, he adds. That is, how the central metallicity of the sun, which can only be determined by the velocity of the CNO neutrinos of the nucleus, is related to the metallicity in other parts of a star. Traditional models have encountered a difficulty: the spectroscopic measurements of the surface metallicity do not agree with the measurements of the underground metallicity deduced from a different method, the helioseismological observations.
Pocar says it neutrinos are actually the only direct probe science has for the core of stars, including the sun, but they are extremely difficult to measure. Up to 420 billion of them hit every square inch of the earth’s surface per second, but virtually all of them pass by without interacting. Scientists can only detect them using very large detectors with exceptionally low background radiation levels.
The Borexino detector is located in the Apennines of central Italy at the INFN Gran Sasso National Laboratories. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300 tons of ultra-pure organic scintillator. Its great depth, size and purity make Borexino a unique detector for this type of science, one of a kind for low background radiation, says Pocar. The project was started in the early 1990s.
Until its latest findings, Borexino collaboration successfully measured the components of “proton-proton” solar neutrino fluxes, helping to refine the oscillation parameters of the neutrino “flavor” and, more impressive, even measured the first step of the cycle: the very low-energy “pp” neutrinos, Pocar recalls.
Its researchers dreamed of expanding the scope of science to also search for CNO neutrinos, in a narrow spectral region with a particularly low background, but that prize seemed out of reach. However, research groups at Princeton, Virginia Tech, and UMass Amherst believed that CNO neutrinos they could still be developed using the additional purification steps and methods they had developed to achieve the exquisite detector stability required.
Over the years and thanks to a sequence of movements to identify and stabilize the antecedents, the American scientists and all the collaboration have been successful. “In addition to revealing CNO neutrinos, the subject of this week’s Nature paper, now there is also the potential to help solve the metallicity problem“says Pocar.
Prior to the discovery of the CNO neutrino, the lab had scheduled Borexino to finish its operations in late 2020. But because the data used in the analysis for the Nature paper was frozen, the scientists continued to collect data, as the purity of the nucleus it continued to improve, making a new metallic-focused result possible, says Pocar. Data collection could be extended until 2021 as the necessary logistics and permits, although ongoing, are not trivial and time-consuming. “Each additional day helps”, comments.
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