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About 99% of the solar energy emitted as neutrinos is produced by nuclear reaction sequences initiated by proton-proton (pp) fusion in which hydrogen is converted to helium, according to scientists, including physicist Andrea Pocar from the University of Massachusetts at Amherst. Today, they report the new results of Borexino, one of the most sensitive neutrino detectors on the planet, located at the bottom of the Apennine Mountains in Italy.
"The neutrinos emitted by this chain represent a unique tool for solar physics and neutrinos," they explain. Their new article in Nature reports "the first comprehensive study of all the components of the pp chain made by Borexino". These components include not only pp neutrinos, but others called 7-beryllium (7Be), pep and 8-boron (8B) neutrinos. The pp fusion reaction of two protons to produce deuteron, deuterium nuclei, is the first step of a reaction sequence responsible for about 99% of the Sun's energy production, says Pocar.
He adds: "Today's innovations are progressive, it's not a leap, but it's the culmination of over 10 years of data taking with the experiment to show immediately the entire spectrum energy of the Sun. Our results reduce uncertainty, which reduces uncertainty. may not be flashy, but it is a type of advancement that is often not sufficiently recognized by science. The advantage is that the measurements become more precise because with more data and thanks to the work of young dedicated physicists, we better understand the experimental apparatus. "
"Borexino offers the best measurement ever made for pp, 7Be and pep neutrinos," he adds. "Other experiments measure 8B neutrinos more precisely, but our measurement, with a lower threshold, is compatible with them."
In addition, "once you have more accurate data, you can integrate them into the model of behavior of the Sun, then the model can be further refined.This helps to better understand the Sun.The neutrinos have explained to us how the Sun burns and, in turn, the Sun has provided us with a unique source for studying the behavior of neutrinos, and Borexino, planned for a further two to three years, has greatly enhanced our understanding of the Sun. "
For previous studies on pp, 7B, pep and 8B neutrinos, the team focused on each separately in targeted analyzes of data collected in restricted energy windows, "like trying to characterize a forest. taking a picture of each type of tree, "notes Pocar. "Several images give you an idea of the forest, but it's not the same as the picture of the entire forest."
"What we have done now is to take a single photo that reflects the entire forest, the whole spectrum of all the neutrinos in one." Instead of zooming in to look at small pieces, we see all this at once We understand our detector, and now we are confident and confident that our one shot is valid for the whole spectrum of neutrino energies. "
Solar neutrinos spring from the star at the center of our system at a speed close to that of light, reaching up to $ 420 billion, reaching every inch of the earth's surface per second. But as they interact only through the weak nuclear force, they virtually do not cross the material, which makes them very difficult to detect and distinguish traces of nuclear disintegration of ordinary materials, says Pocar.
The Borexino instrument detects neutrinos when they interact with the electrons of an ultra pure organic liquid scintillator in the center of a large sphere surrounded by 1,000 tons of water. Its great depth and its many layers of protection resembling onions make the nucleus the most radiation-free support on the planet. It is the only detector on Earth able to simultaneously observe the entire spectrum of solar neutrinos, which is now accomplished, he notes.
The physicist UMass Amherst, one of the principal investigators of a team of more than 100 scientists, is particularly interested in the fact that he is now focusing on another type of solar neutrino, the CNO neutrinos that he hopes to usefully solve an important open question. in stellar physics, it is the metallicity or the metal content of the Sun.
"Two models predict different levels of elements heavier than helium.For astronomers, it is a metal in the Sun, a lighter metallicity and a heavier model" , he notes. CNO neutrinos are emitted in a cyclic fusion reaction sequence different from the pp and subdominant chain in the Sun, but considered as the main source of energy for heavier stars. CNO solar neutrino flux is strongly affected by solar metallicity.
Pocar said: "Our data may show a slight preference for heavy metallicity, so let's look at this because the Sun's neutrinos, particularly the NOCs, can help us unravel that."
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