A joint Fermilab/SLAC publication

Unraveling the processes that power the sun

06/24/20

The Borexino experiment announces the first detection of neutrinos from a secondary cycle that fuels our closest star.

The sun is powered by nuclear fusion, a process through which hydrogen is converted into helium, emitting large amounts of energy.

A side effect of this process is the generation of neutrinos, ghostly fundamental particles that rarely interact with their surroundings. The sun is the source of the majority of the Earth’s neutrinos, sending trillions of these particles raining down on our planet each day.

Solar neutrinos have provided physicists with an invaluable tool to study how the sun works, and what it is made of. Using these elusive particles, scientists have been able to confirm that more than 99% of fusion reactions in the sun are proton-proton chain reactions, which use hydrogen as the main source of fuel.

At this week’s Neutrino 2020 meeting, physicists on the Borexino neutrino experiment, located at Gran Sasso Laboratory in Italy, have announced the first-ever detection of neutrinos from another, less common fusion process: the carbon-nitrogen-oxygen (CNO) cycle, which uses carbon, nitrogen and oxygen as catalysts to fuel the conversion of hydrogen to helium.

“With these results, Borexino marks the first detection ever of CNO solar neutrinos,” Borexino spokesperson Gioacchino Ranucci of INFN Milano said during the Neutrino 2020 presentation. “We have completely unraveled the two processes powering the sun.”

Theorists have long suspected the existence of the CNO cycle. If this process didn’t occur, that would mean that the amount of these three elements in the sun is much lower than expected, says André de Gouvêa, a theoretical physicist and professor at Northwestern University. “We know that they’re supposed to be there unless we’re missing something big.”

In order to detect CNO neutrinos, physicists needed a highly sensitive detector capable of blocking out most sources of background noise.

To achieve the sensitivity, the Borexino detector was built with an onion-like design. It has multiple layers: The transparent, spherical core is filled with 280 tons of liquid scintillator (a material that emits light when a neutrino interacts with the electrons within it), which is encased in a large, stainless-steel sphere filled with a buffer liquid and studded with 2200 light sensors. The outer sphere is held within an even larger stainless-steel tank filled with 2400 tons of ultrapure water. The entire detector is buried 1.4 kilometers (about 0.9 miles) underground.

Borexino has been taking data since 2007. The collaboration made headlines around the world in 2014 when it announced the first real-time detection of neutrinos from proton-proton chain reactions.

Identifying the CNO cycle neutrinos was no simple feat. Even with a highly sensitive detector, Borexino physicists needed to remove a key source of background: the decay of the isotope Bismuth-210. This was achieved by studying the decay rate of Polonium-210, a process that is in equilibrium with Bismuth-210 decay but easier to measure.

As Ranucci described during the Neutrino 2020 presentation, the quest for CNO neutrinos “turned into the quest for Bismuth-210 through Polonium-210.”

However, since the decay rate of Polonium-210 is highly sensitive to fluctuations in temperature, the team needed to carefully monitor, understand and suppress the temperature variation within the hall that houses the detector. “This is the outcome of the relentless, years-long effort to stabilize the detector and understand the [Polonium-210] behavior in the inner vessel,” Ranucci said.

CNO neutrinos are particularly interesting to solar physicists because they provide one of the most direct measures of the sun’s metallicity—the content of “metals,” which are what astrophysicists call elements other than hydrogen and helium. Studying the properties of CNO neutrinos can help physicists disentangle whether the sun skews toward heavier metallicity, meaning higher metal content, or lighter metallicity, meaning lower metal content.

Although physicists cannot draw firm conclusions about solar metallicity with the latest Borexino result, the collaboration plans to further constrain their measurements in hopes of providing insights in the future.

Finding out the composition of the sun—and how it works—can help scientists understand how other, similar stars work as well. “There’s a whole range of what the stars can look like, depending on how heavy they are, how old they are, and the mechanism through which they were born,” de Gouvêa says. “By learning as much as we can about the sun, we can take that information and also apply it to other stars for which we have more limited knowledge.”

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