A Missing Piece of the Puzzle for the Solar Engine
First experimental proof of fusion process in massive stars
Scientists involved in the Borexino Collaboration have for the first time proven the existence of the CNO fusion cycle in nature. They detected solar neutrinos originating in this process, subordinate in our sun. However, CNO fusion is expected to play a crucial role in the universe, being the predominant energy generating process in more massive stars. With the aid of the observatory for almost undetectable “ghost particles”, located 1,400 metres below the Earth’s surface at the Gran Sasso massif near Rome, the researchers have found one of the missing pieces of the puzzle in how solar nuclear fusion works. They have now published their findings in the renowned journal Nature.
At its heart, the sun is a gigantic fusion reactor. Hydrogen nuclei continuously fuse with each other at a temperature of around 15 million degrees in the sun’s core, thus forming the element helium via a chain of different reactions. As this happens, different types of radiation and particles are constantly emitted, some of which are neutrinos. Billions of these neutrinos pass through every square centimetre on Earth every second, completely unnoticed and unhindered. This property of being able to pass through materials undisturbed makes them ideal probes for gaining a closer glimpse inside the solar furnace. They provide direct, uncompromised information about the conditions in the core of the sun.
Heavy elements in stellar plasma
"The sun generates its energy in complicated chains of nuclear reactions," explains Livia Ludhova, member of JARA-FAME, one of the two current scientific coordinators of the Borexino collaboration and head of the Neutrino Group at Jülich’s Nuclear Physics Institute, which made crucial contributions to the results of the experiment. "The key process is the fusion of hydrogen into helium, called 'hydrogen burning'. According to the current model of the sun, this takes place through two theoretically well understood mechanisms."
In less massive stars like our sun, the dominant one is the PP chain, which is started by the direct fusion of two protons. The other mechanism is the CNO cycle, sometimes known as the Bethe–Weizsäcker cycle. "It is enabled by the presence of elements heavier than helium in the stellar plasma, which are directly involved in the fusion reactions, as carbon, nitrogen, and oxygen" says Ludhova. "This is why it is called the CNO cycle. The name is derived from the symbols of these elements." Both mechanisms, the PP chain and the CNO cycle, generate energy and emit a whole spectrum of neutrinos.
"Because of the large number of massive stars it is presumed that the CNO cycle is the primary mechanism in the universe for stars to convert hydrogen into helium," says Ludhova. However, in our sun – a star with a relatively low mass – the process only plays a very minor role. "We expect that it is responsible for about one percent of the sun’s energy production," the physicist explains. "In theory, the process is well founded. But until now, no-one has ever managed to observe it in experiments."
First experimental proof
The same property that makes neutrinos so ideal for getting information from the core of the sun to Earth also makes them particularly difficult to capture. They pass almost unhindered through materials – including measuring instruments. They can only be detected in the Borexino detector if a neutrino happens to collide with an electron, to which it transfers some of its energy, that is then released in the form of a tiny flash of light when the electron moves. Measuring neutrinos requires large detectors, in which a few of the trillions passing through per day interact with matter and can therefore be detected.
Now the scientists working on the Borexino experiment have managed to find the first experimental proof of the existence of the CNO cycle – by directly observing neutrinos generated in this fusion process. "We have proven the existence of CNO neutrino interactions with high statistical significance," explains Ludhova. "This allowed us to calculate the total flux of CNO neutrinos on Earth at around 700 million per second through one square centimetre – around one hundredth of the total number of solar neutrinos."
Tiny light flashes occur in the detector when the rare reactions with neutrinos happen. Individual light particles, photons, are detected by around 2000 sensors mounted on the walls of the stainless steel sphere enclosing the scintillator, converting light into electrical impulses. ©: BOREXINO Collaboration
Thousands of light detectors 1,400 m below the Earth
The instrument that enabled this measurement is the Borexino observatory, where data on neutrinos from the sun has been gathered since 2007. The facility 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 containing 280 tonnes of a special scintillator fluid. Tiny light flashes occur in it when the rare reactions with neutrinos happen. Individual light particles, photons, are detected by around 2000 sensors converting light into electrical impulses. "The energy deposited by the neutrinos, a crucial parameter of the analysis, is correlated with the amount of light detected," explains Ludhova. "In order to perform this delicate measurement in the first place, the natural radioactivity in the Borexino detector had to be reduced by several orders of magnitude to an unprecedented level. In addition, the whole detector was thermally stabilized in the latest years, what characterizes the Borexino Phase III data set, used in the CNO discovery."
To protect against cosmic radiation, the tank is located below a 1,400-metre-thick layer of dolomitic rock at the Gran Sasso mountain massif near Rome, Italy. Nevertheless, some of these cosmic particles are still able to reach the detector – and overshadow the neutrino signals that are being searched for. To reduce this additional background source, the scientists have developed sophisticated analyses techniques. Such algorithms help to suppress events that are hindering identification of rare neutrino signals.
"Proof of the CNO neutrinos represents a significant milestone," Livia Ludhova explains. "It paves the way for us to better understand the composition of the sun’s core, particularly its metallicity." This is the standard term in astrophysics for the abundance of "heavy" chemical elements in stars and is one of the most important unanswered questions in solar physics today. "The Borexino detector has once again played a pioneering role in research into the sun and unlocked the deepest secrets of the processes that keep our star alive."
Original publication: Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun, Nature: https://www.nature.com/articles/s41586-020-2934-0