27.11.2020

Borexino detector. courtesy Borexino Collaboration.

AMHERST, Mass. – An international team of about 100 scientists of the Borexino Collaboration, including particle physicist Andrea Pocar at the University of Massachusetts Amherst, report in Nature this week detection of neutrinos from the sun, directly revealing for the first time that the carbon-nitrogen-oxygen (CNO) fusion-cycle is at work in our sun.

The CNO cycle is the dominant energy source powering stars heavier than the sun, but it had so far never been directly detected in any star, Pocar explains.

For much of their life, stars get energy by fusing hydrogen into helium, he adds. In stars like our sun or lighter, this mostly happens 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 since the 1930’s is that the CNO-cycle will be dominant in heavy stars.

Neutrinos emitted as part of these processes provide a spectral signature allowing scientists to distinguish those from the ‘proton-proton chain’ from those from the ‘CNO-cycle.’ Pocar points out, “Confirmation of CNO burning in our sun, where it operates at only one percent, reinforces our confidence that we understand how stars work.”

Beyond this, CNO neutrinos can help resolve an important open question in stellar physics, he adds. That is, how the sun’s central metallicity, as can only be determined by the CNO neutrino rate from the core, is related to metallicity elsewhere in a star. Traditional models have run into a difficulty – surface metallicity measures by spectroscopy do not agree with the sub-surface metallicity measurements inferred from a different method, helioseismology observations. 

Pocar says neutrinos are really the only direct probe science has for the core of stars, including the sun, but they are exceedingly difficult to measure. As many as 420 billion of them hit every square inch of the earth’s surface per second, yet virtually all pass through without interacting. Scientists can only detect them using very large detectors with exceptionally low background radiation levels. 

The Borexino detector lies deep under the Apennine Mountains in central Italy at the INFN’s Laboratori Nazionali del Gran Sasso. 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, alone in its class for low-background radiation, Pocar says. The project was initiated in the early 1990s by a group of physicists led by Gianpaolo Bellini at the University of Milan, Frank Calaprice at Princeton and the late Raju Raghavan at Bell Labs.

Until its latest detections, the Borexino collaboration had successfully measured components of the ‘proton-proton’ solar neutrino fluxes, helped refine neutrino flavor-oscillation parameters, and most impressively, even measured the first step in the cycle: the very low-energy ‘pp’ neutrinos, Pocar recalls.

Its researchers dreamed of expanding the science scope to also look for the CNO neutrinos – in a narrow spectral region with particularly low background – but that prize seemed out of reach. However, research groups at Princeton, Virginia Tech and UMass Amherst believed CNO neutrinos might yet be revealed using the additional purification steps and methods they had developed to realize the exquisite detector stability required.

Over the years and thanks to a sequence of moves to identify and stabilize the backgrounds, the U.S. scientists and the entire collaboration were successful. “Beyond revealing the CNO neutrinos which is the subject of this week’s Nature article, there is now even a potential to help resolve the metallicity problem as well,” Pocar says.

Before the CNO neutrino discovery, the lab had scheduled Borexino to end operations at the close of 2020. But because the data used in the analysis for the Nature paper was frozen, scientists have continued collecting data, as the central purity has continued to improve, making a new result focused on the metallicity a real possibility, Pocar says. Data collection could extend into 2021 since the logistics and permitting required, while underway, are non-trivial and time-consuming. “Every extra day helps,” he remarks.

Pocar has been with the project since his graduate school days at Princeton in the group led by Frank Calaprice, where he worked on the design, construction of the nylon vessel and the commissioning of the fluid handling system. He later worked with his students at UMass Amherst on data analysis and, most recently, on techniques to characterize the backgrounds for the CNO neutrino measurement.

This work was supported in the U.S. by the National Science Foundation. Borexino is an international collaboration also funded by the Italian National Institute for Nuclear Physics (INFN), and funding agencies in Germany, Russia and Poland.

Quelle: University of Massachusetts Amherst

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Solar CNO neutrinos observed for the first time

'Ghostly' particles are proof of the secondary fusion process that powers our Sun

25 November 2020

Scientists who are members of the Borexino Collaboration have provided the first experimental proof of the occurrence of the so-called CNO cycle in the Sun: They have managed to directly detect the distinctive neutrinos generated during this fusion process. This is an important milestone on the route to better understanding the fusion processes that occur in the Sun. At the same time, although the CNO cycle plays a minor role in our Sun, it is most likely the predominant way of producing energy in other more massive and hotter stars. The Borexino Collaboration's findings have been published in the latest issue of the journal Nature.

How does the Sun generate energy? As a gigantic fusion reactor, it continuously converts hydrogen into helium – a process also referred to as 'hydrogen burning'. Essentially, this involves two types of processes. On the one hand, there is the proton-proton reaction (pp reaction). This begins with the direct fusion of two hydrogen nuclei to create the intermediate hydrogen isotope deuterium from which helium is subsequently formed. On the other hand, the heavier elements carbon (C), nitrogen (N) and oxygen (O) are involved in the second type of reaction chain, known as the CNO cycle or Bethe-Weizsäcker cycle. While the pp reaction is predominant in smaller stars such as our Sun, the CNO cycle is the main process for generating energy in more massive and hotter stars.

As is the case with all fusion processes that occur within the Sun, countless neutrinos are produced in addition to helium and the enormous amounts of energy which cause the Sun and its sister stars to shine. The neutrinos reach the Earth in their billions and normally pass through it unhindered. "However, we are able to detect these neutrinos using the Borexino experiment's huge detector located 1400 meters underground," points out Prof. Michael Wurm, a neutrino physicist at the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) and a member of the Borexino Collaboration. "They provide us with clear insights into the processes in the Sun's core."

While the Borexino Collaboration has been able to detect neutrinos originating from several reactions along the pp chain in recent years, their current achievement has been to explicitly identify neutrinos released in the CNO cycle, which are significantly less abundant in comparison. "Although on the basis of model calculations we expected the CNO cycle also to occur in the Sun, direct evidence of this has never been obtained before. Only a characteristic neutrino signal can provide conclusive proof that this actually happens – now we have that conclusive proof without a shadow of a doubt."

In addition, the research team was also able to estimate the total flow of CNO neutrinos reaching the Earth. About 700 million of them fly through a square centimeter of our planet each second, but this accounts for only one hundredth of the total number of solar neutrinos. "This is consistent with the theoretical expectations that the CNO cycle in the Sun is responsible for about one percent of the energy it produces," adds Dr. Daniele Guffanti, a postdoc in Michael Wurm's team and also a member of the Borexino Collaboration.

The two neutrino physicists from Mainz consider the new results to be an important milestone along the route to obtaining a complete understanding of the fusion processes which not only drive our Sun but also massive stars, and make these latter light up our night sky. It also paves the way for a better insight into the elements that compose the solar core, particularly with regard to how frequently heavier elements such as carbon, nitrogen and oxygen can be found in the solar plasma in addition to hydrogen and helium – researchers call this metallicity. Neutrinos might once again be our only guides to help us discover this.

About the Borexino detector

The Borexino detector has been collecting data on solar neutrinos since 2007. It is located in the largest underground laboratory in the world, the Laboratori Nazionali del Gran Sasso in Italy. At the heart of the Borexino detector is an extremely thin-walled, spherical nylon balloon that contains 280 tons of special scintillator fluid. A neutrino interacts with the detector material just a few hundred times a day. This then generates tiny flashes of light which are detected by around 2.000 extremely sensitive sensors.

In order to make sure that the detected signals actually come from neutrinos, the scientists have to switch off other potential signal sources or filter them out during data analysis – this includes natural background radioactivity and interference caused by cosmic radiation, especially that associated with muons. But even though the tank is shielded under a 1.400-meter thick layer of rock in the Gran Sasso massif near Rome, some muons are still able to reach it, while radioactive decay can produce signals that at first glance cannot be distinguished from a real neutrino signal. The Mainz team is specialized in developing sophisticated analysis techniques that help suppress such background events, so that the rare neutrino signals can be reliably identified.

Quelle: Johannes Gutenberg-Universität Mainz