From From INFN Gran Sasso (IT)
25 November 2020
The Borexino scientific collaboration, an experiment at the Gran Sasso National Laboratories of the Italian National Institute for Nuclear Physics (INFN) [below], publishes today, November 25th on Nature the announcement of the first ever detection of neutrinos produced in the Sun by the CNO cycle (carbon-nitrogen-oxygen [CNO]). It is an experimental result of historical value, which completes a chapter of physics that started in the 1930 decade of the last century. The implication of this new measure for understanding stellar mechanisms is enormous: in fact, since the CNO cycle is predominant in the most massive stars than the Sun, with this observation Borexino has reached the experimental evidence of what is in fact the dominant channel in the universe for hydrogen burning.
Previously Borexino had already studied in detail the main mechanism of energy production in the Sun, the proton-proton chain, through the individual detection of all neutrino fluxes that originate from it. Now, by measuring the neutrinos produced by the CNO cycle, which is present in the Sun at 1% level, Borexino provides the first experimental evidence of the existence of this additional energy generation mechanism.
“Now we finally have the first groundbreaking, experimental confirmation of how the stars, heavier than the Sun, shine” points out Gianpaolo Bellini, professor at the University of Milan and INFN researcher, one of the founding fathers of the experiment and spokesperson of Borexino for 22 years. Bellini led the group of researchers and technicians of the University of Milan (La Statale) and of the INFN division of Milan that exactly 30 years ago started the conception of the experiment. “This is the culmination of a thirty years long effort, which began in 1990, and of more than ten years of Borexino’s discoveries in the physics of the Sun, neutrinos and finally stars,” Bellini concludes.
Since 1990 the Milan group played a key role in the design and construction of the detector, with a contribution from the University of Princeton, and afterwards with the INFN groups of Genoa, Gran Sasso, Perugia, within the international collaboration Solar neutrinos can only be observed with highly sensitive detectors, which can exclude most sources of background signals. To achieve the required sensitivity, the Borexino experiment was built with an onion-like design, characterized by layers of increasing radio purity, making it a unique detector in the world for the ultra-low background level achieved, never obtained by any other experiment. In addition, the depth of the experimental Hall of the Gran Sasso Underground Labs protects it from cosmic radiation, with the exception of neutrinos that pass through Earth matter undisturbed. Measuring the neutrinos of the CNO cycle was a complicated task that required a great deal of effort in both hardware and software.
“Despite the exceptional successes previously achieved and an already ultra-pure detector, – explains Gioacchino Ranucci, researcher of the INFN section of Milan, current co-spokesperson of Borexino – we had to work hard to further improve the suppression and understanding of the very low residual backgrounds, so that we could identify the neutrinos of the CNO cycle.”
“The detection of neutrinos produced in the CNO cycle announced by Borexino is the crowning of a relentless, years-long effort that has led us to push the liquid-scintillator technology beyond any previously reached limit, and to make Borexino’s core the least radioactive place in the world,” comments Marco Pallavicini, professor at the University of Genoa and member of the INFN Executive Board, currently co-spokesperson for the experiment.
An 80 year long history
The existence of the CNO cycle was first theorized in 1938, when scientists Hans Bethe and Carl Friedrich von Weizsacker independently proposed that the fusion of hydrogen in stars could also be catalyzed by the heavy nuclei carbon, nitrogen and oxygen, in a cyclical series of nuclear reactions, in addition to proceeding according to the sequence of the proton-proton chain.
Despite indirect evidence from astronomical and astrophysical observations, direct experimental confirmation of the hypothesized stellar energy generation mechanism could not be easily obtained. The attempts to unveil it focused on neutrinos, particles produced in abundance in these reactions, leading to the start-up, in the 1960s, of the Solar Neutrino scientific program, which originated results of great importance for particle physics.
With this measure, Borexino, who is nearing the conclusion of his scientific activity, after demonstrating how the Sun shines, also leaves the neutrino field with the enduring legacy of the first observation of CNO neutrinos, a revolutionary achievement obtained through an impressive experimental effort, which will remain for the future as one of the fundamental successes of astrophysics and astroparticle physics.
In the course of this fascinating enterprise of unravelling the mysteries of the Sun and stars, which lasted almost a century, solar neutrinos were also instrumental in identifying the phenomenon of neutrino oscillation, one of the greatest discoveries of particle physics of the new millennium.
See the full article here .
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INFN Gran Sasso (IT) is the largest underground laboratory in the world devoted to neutrino and astroparticle physics, a worldwide research facility for scientists working in this field of research, where particle physics, cosmology and astrophysics meet. It is unequalled anywhere else, as it offers the most advanced underground infrastructures in terms of dimensions, complexity and completeness.
LNGS is funded by the National Institute for Nuclear Physics (INFN), the Italian Institution in charge to coordinate and support research in elementary particles physics, nuclear and sub nuclear physics
Located between L’Aquila and Teramo, at about 120 kilometres from Rome, the underground structures are on one side of the 10-kilometre long highway tunnel which crosses the Gran Sasso massif (towards Rome); the underground complex consists of three huge experimental halls (each 100-metre long, 20-metre large and 18-metre high) and bypass tunnels, for a total volume of about 180.000 m3.
Access to experimental halls is horizontal and it is made easier by the highway tunnel. Halls are equipped with all technical and safety equipment and plants necessary for the experimental activities and to ensure proper working conditions for people involved.
The 1400 metre-rock thickness above the Laboratory represents a natural coverage that provides a cosmic ray flux reduction by one million times; moreover, the flux of neutrons in the underground halls is about thousand times less than on the surface due to the very small amount of uranium and thorium of the Dolomite calcareous rock of the mountain.
The permeability of cosmic radiation provided by the rock coverage together with the huge dimensions and the impressive basic infrastructure, make the Laboratory unmatched in the detection of weak or rare signals, which are relevant for astroparticle, sub nuclear and nuclear physics.
Outside, immersed in a National Park of exceptional environmental and naturalistic interest on the slopes of the Gran Sasso mountain chain, an area of more than 23 acres hosts laboratories and workshops, the Computing Centre, the Directorate and several other Offices.
Currently 1100 scientists from 29 different Countries are taking part in the experimental activities of LNGS.
LNGS research activities range from neutrino physics to dark matter search, to nuclear astrophysics, and also to earth physics, biology and fundamental physics.
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