Neutrinose Yield First Experimental Evidence of Catalyzed Fusion Dominant in Many Stars
Reported in an international team of about 100 scientists of the Boraxino collaboration, including particle physicist Andrea Pocar at the University of Massachusetts Amherst Nature This week the detection of neutrinos from the sun, revealed for the first time directly that the carbon-nitrogen-oxygen (CNO) fusion-cycle is working in our sun.
The CNO cycle is the major energy source that makes stars heavier than the sun, but it was never directly detected in any star until now, explains Pokar.
For their lives, stars get energy by fusing hydrogen into helium, he said. In stars like our sun or lighter, this is mostly through ‘proton-proton’ chains. However, many stars are heavier and warmer than our sun, and their composition includes elements heavier than helium, a quality known as metal. It is predicted since 1930 that the CNO-cycle will be prominent in heavy stars.
The neutrinos emitted as a part of these processes provide a spectral signature that allows scientists to distinguish the ‘proton-proton chain’ from those in the ‘CNO-cycle’. “The confirmation of our sun-burning CNO, where it operates at only one percent, strengthens our confidence that we understand how stars work,” explains Poker.
In addition, CNO neutrinos can help solve an important open question in stellar physics, he said. That is, the central metallicity of the Sun, as can only be determined by the CNO neutrino rate from the core, is related to metallicity elsewhere in a star. Traditional models have gone into a difficult one – surface metal measurement measures by spectroscopy do not agree with a different method, the subsurface metal measurements predicted from heliosomasology observations.
Pocar states that neutrinos are in fact the only direct investigative science that has the cores of stars including the Sun, but they are extremely difficult to measure. As 420 billion of them hit every square inch of the Earth per second, yet almost all pass without dialogue. Scientists can only detect them using very large detectors with exceptionally low background radiation levels.
The Borexino detector is located deep under the Appenine Mountains in central Italy at the Nazarelli Naznei del Gran Sasso of INFN. It detects neutrinos when flashes of light are produced when neutrinos collide with electrons in a 300-ton ultra-pure organic scintillator. Its great depth, size, and purity make Borexino a unique detector for this type of science, which is in its class for low-background radiation alone. The project was initiated in the 1990s by a group of physicists led by Gianpolo Bellini at the University of Milan, Frank Calaprice of Princeton and the late Raju Raghavan at Bell Labs.
Until its latest findings, the Borexino collaboration had successfully measured the components of the ‘proton-proton’ solar neutrino flux, helped refine neutrino flavor-oscillation parameters, and most effectively, even measured the first step in the cycle. : Very low energy ‘PP’ neutrinos, Pokar recalls.
Its researchers dreamed of expanding the scope of science for CNO neutrinos – especially in a narrow spectral region with low background – but the award seemed out of reach. However, research groups at Princeton, Virginia Tech and UMus Amherst believe that CNO neutrinos can be revealed using additional purification steps and methods that were developed to realize the minimal detector stability they require.
Thanks to a sequence of tricks to identify and stabilize the background over the years, the American scientist and the entire collaboration were successful. “Beyond disclosing CNO neutrinos which is the subject of this week Nature The article states that metal has the potential to help solve the problem.
Prior to the CNO neutrino discovery, the lab scheduled Borexino to cease operations in late 2020. But because the data used in the analysis Nature As the paper was frozen, scientists have continued to collect data, as central purity has continued to improve, creating a new result to focus on the true possibility of metallicity. Data collection may expand in 2021 because logistics and permissions are required, while ongoing, are non-trivial and time-consuming. “Every extra day helps,” he comments.
Pokar has been in the group led by Frank Calaprice with the project since his graduate school days at Princeton, where he worked on the design, construction of a nylon vessel, and fluid handling operations. He later worked with his students at UMass Amherst on data analysis and most recently, techniques to characterize the background of CNO neutrino measurements.
Reference: Experimental evidence of neutrinos formed in the CNO fusion cycle in the Sun by the “Boroxino Collaboration, 25 November 2020”. Nature.
DOI: 10.1038 / s41586-020-2934-0
This work was supported by the National Science Foundation in the US. Borexino is an international collaboration, funded by the Italian National Institute for Nuclear Physics (INFN), and funding agencies in Germany, Russia and Poland.