“It is intellectually beautiful to confirm one of the fundamental predictions of star structure theory,” said astrophysicist Marc Pinsonneault of Ohio State University in Columbus. This is the general feeling of researchers who managed to capture neutrinos emanating from the Sun’s core – the first direct detection of these particles and the proof for decades-long predictions about how nuclear fusion happens in stars.
“With this result, we unveiled the two processes that feed the Sun,” said the physicist at Università di Milano Gioacchino Ranucci, one of the study’s authors, during the Neutrino 2020 virtual conference.
In the Sun’s nucleus, the CN reaction fuses four protons to form a helium nucleus, releasing some subatomic particles – including two neutrinos, the lightest elementary particles known to matter – and abundant amounts of energy. This fusion is responsible for less than 1% of the Sun’s energy, but it is the process that sustains larger stars.
The experiment was carried out by Borexino, a neutrino detector installed at the Laboratori Nazionali del Gran Sasso (named after the Gran Sasso mountain massif, which rises 1.4 km above the laboratory, in the interior of Italy), the largest underground research center in the world.
A few months before being deactivated, after 1 decade of operation, Borexino was able to confirm that part of the Sun’s energy is produced by a chain of reactions involving carbon and nitrogen nuclei (CN).
The process to capture the neutrinos is, in theory, simple: inside the Borexino, there is a giant nylon balloon, which is inflated with 278 tons of water with hydrocarbons. The neutrinos produced by the Sun go in a straight line across the Earth, but a small part, in its path through the Borexino, bounces off when it finds the electrons of the hydrocarbons, producing flashes of light that are, in turn, captured by photon sensors that coat the tank with water.
Almost absolute immobility
It seems easy and fast, but it is not: these neutrinos are relatively rare and easily confused with the particles produced by the radioactive decay of bismuth-210, an isotope that leaks from the balloon’s nylon into the hydrocarbon mixture.
Even though this contamination is minimal (a few dozen bismuth cores per day), separating neutrinos by their origin has occupied researchers since 2014. The work then consisted of ignoring what was occurring at the edges of the balloon, an area where the occurrence of bismuth-210 it was almost certain.
To avoid mixing, the tank was wrapped in an insulating blanket, keeping the hydrocarbon soup always at the same temperature to avoid convection currents. “The liquid must be extraordinarily still, moving at most a few tenths of centimeters per month”, explained the astrophysicist Aldo Serenelli, from the Instituto de Ciencias Espaciales (CSIC-IEEC).
Before the planets appear
The results only appeared in 2019, when the bismuth was finally removed from the equation and the neutrinos from the Sun stood out. In January of this year, there was already a sufficient record of particles for researchers to claim that neutrinos from the CN nuclear fusion chain had been detected.
“It is the first really direct evidence that the burning of hydrogen through the CN reaction operates in stars,” celebrated Serenelli.
Mainz physicist Michael Wurm of Johannes Gutenberg-Universität said, “These results confirm our understanding of the fusion processes within the Sun and deepen our knowledge of the early stages of the star’s life, before the appearance of the planets.”