Neutrinos are released during nuclear fusion reactions in the Sun’s core.Credit: Detlev Van Ravenswaay / Science Photo Library
Physicists have filled in the last missing detail of how nuclear fusion powers the Sun by trapping neutrinos emanating from the star’s core.
The detection confirms decades-long theoretical predictions that some of the Sun’s energy is made by a chain of reactions involving carbon and nitrogen nuclei. This process fuses four protons into a helium nucleus, releasing two neutrinos, the lightest known elementary matter particles, as well as other subatomic particles and large amounts of energy. This carbon-nitrogen (CN) reaction is not the Sun’s only fusion path: it produces less than 1% of the Sun’s energy, but it is believed to be the dominant energy source in larger stars.
“It is intellectually beautiful to really confirm one of the fundamental predictions of stellar structure theory,” says Marc Pinsonneault, an astrophysicist at Ohio State University in Columbus.
The findings, which have not yet been peer-reviewed, were reported on June 23 by the Borexino underground experiment in central Italy, at the Neutrino 2020 virtual conference.
The facility has previously made the first direct neutrino detections from three distinct steps in a separate reaction that explains most of the Sun’s fusion.one,two,3. “With this result, Borexino has completely unraveled the two processes that feed the Sun,” said Borexino co-spokesperson Gioacchino Ranucci, a physicist at the University of Milan, Italy, who presented the results.
The findings are a final milestone for Borexino, which is still taking data, but may now be set to close in a year. “We ended up with an explosion,” says Marco Pallavicini of the University of Genoa, Italy, the other spokesperson for the experiment.
Balloon detector
The Borexino solar neutrino experiment occupies a room under more than 1 kilometer of rock at the Gran Sasso National Laboratories, where it has been in operation since 2007. The detector consists of a giant nylon balloon filled with 278 tons of liquid hydrocarbons, and immersed in water. The vast majority of neutrinos from the Sun traverse Earth and Borexino in a straight line, but a small amount bounces off the hydrocarbon electrons, producing flashes of light that are picked up by the photon sensors that line the water tank.
Neutrinos in the Sun’s CN reaction chain are relatively rare, as they are responsible for only a small fraction of solar fusion. Furthermore, CN neutrinos are easy to confuse with those produced by the radioactive decay of bismuth-210, an isotope that escapes from the balloon’s nylon into the hydrocarbon mixture.
Although the contamination exists in extremely low concentrations, at most a few dozen bismuth nuclei decompose per day within Borexino, the separation of the solar signal from the bismuth noise required a painstaking effort that started in 2014. It couldn’t be helped For bismuth-210 to leak from the balloon, so the goal was to reduce the rate at which the element leaked into the fluid medium, while ignoring any signal from the outer edge. To do this, the team had to control any temperature imbalances in the tank, which would produce convection and mix its contents faster. “The liquid must be remarkably still, moving at most a few tenths of an inch per month,” says Pallavicini.
To keep the hydrocarbons at a constant and uniform temperature, they wrapped the entire tank in an insulating blanket and installed heat exchangers to automatically balance the temperature. Then they waited. It was only in 2019 that the bismuth noise became quiet enough for the neutrino signal to stand out. In early 2020, researchers had collected enough particles to claim the discovery to detect neutrinos from the CN nuclear fusion chain.
“It is the first really direct evidence that the hydrogen that burns through CN operates in stars,” says Aldo Serenelli, an astrophysicist at the Institute for Space Sciences in Barcelona, Spain. “So this is really incredible.”
Speculation of the sun’s surface
In addition to confirming theoretical predictions about the powers of the Sun, the detection of CN neutrinos could shed light on the structure of the nucleus, specifically the concentrations of elements that astrophysicists call metals (somewhat heavier than hydrogen and helium).
The amounts of neutrinos observed by Borexino appear consistent with standard models in which the Sun’s core has a “metallicity” similar to its surface. But more updated studies have begun to question that assumption, says Serenelli.
These studies suggest that metallicity is less. And because these elements regulate how quickly heat diffuses from the Sun’s core, it implies that the core is slightly cooler than previous estimates. Neutrino production is extremely sensitive to temperature, and overall, the various amounts of neutrinos observed by Borexino appear to be consistent with the older metallicity values, Serenelli says, not the new ones.
As a possible explanation, he and other astrophysicists have suggested that the core has a greater metallicity than the outer layers. Its composition could reveal more about the early stages of the Sun’s life, before the formation of the planets removed some of the metals that accumulated in the young star.
