Solar Neutrinos Transform Carbon-13 Into Nitrogen-13 Deep Underground

Far beneath the surface, in the chthonic reaches of Earth's crust, scientists have finally observed solar neutrinos transforming carbon-13 into nitrogen-13.

This is the first observation of this uncommon neutrino-induced nuclear reaction. It shows that some of the Universe's most elusive, intangible particles can still subtly alter matter in the underground darkness, far below the surface.

"This discovery uses the natural abundance of carbon-13 within the experiment's liquid scintillator to measure a specific, rare interaction," says physicist Christine Kraus of SNOLAB, the Canadian neutrino observatory where the detection took place.

"To our knowledge, these results represent the lowest energy observation of neutrino interactions on carbon-13 nuclei to date and provide the first direct cross-section measurement for this specific nuclear reaction to the ground state of the resulting nitrogen-13 nucleus."

Solar neutrinos and underground detection at SNOLAB

Neutrinos rank among the most plentiful particles in the vast Universe. They are produced during high-energy events, including supernova explosions and the atomic fusion occurring in stellar cores, which means they are found virtually everywhere.

Yet they carry no electric charge, have an almost negligible mass, and interact only rarely with the particles they pass. Hundreds of billions of neutrinos are travelling through your body at this moment, passing straight through it like ghosts. For that reason, they are fondly called ghost particles.

Occasionally, however, a neutrino collides with another particle. Such an impact creates an extraordinarily faint burst of light and a cascade of other particles. At Earth's surface, these signals are difficult to identify because cosmic rays and background radiation overwhelm them.

This is why several of the most effective neutrino detectors operate deep underground, where the Earth's crust acts as a shield against radiation. Vast chambers there are equipped with photodetectors and filled with liquid scintillator, which boosts the minute signals from infrequent neutrino interactions as they flare in total, silent darkness.

Neutrinos created in the Sun's core continually pass through Earth. Their energies lie within a well-established range, allowing them to be readily separated from atmospheric and astrophysical neutrinos, which are much more energetic and much rarer. At the 2-kilometre depth of SNOLAB's SNO+ detector, almost every event within this energy range comes from the Sun.

How carbon-13 becomes nitrogen-13

The team, led by physicist Gulliver Milton of the University of Oxford in the UK, examined SNO+ data gathered from 4 May 2022 to 29 June 2023. They sought a particular sign of a neutrino interacting with carbon-13 in the scintillator liquid.

When a solar electron neutrino hits a carbon-13 nucleus, two things happen. First, as the nucleus absorbs the neutrino, it produces an electron, a negatively charged particle.

A carbon atom's nucleus contains 13 particles: six positively charged protons and seven uncharged neutrons. The neutrino-triggered weak interaction changes one of these neutrons into a proton and releases an electron.

As its proton number rises from six to seven, the atom ceases to be carbon and becomes nitrogen-13, containing seven protons and six neutrons.

Roughly 10 minutes afterwards, the newly formed nitrogen-13 - an unstable radioactive nitrogen isotope with, as you might expect, a 10-minute half-life - decays and emits a recognisable anti-electron, or positron.

Taken together, the interaction produces a distinctive two-stage flash called a delayed coincidence. In effect, the scientists can search for an electron followed 10 minutes later by a positron, providing the signature of a neutrino changing carbon-13 into nitrogen-13.

Rare carbon-13 neutrino events provide a new measurement

Across 231 days of observational data, the researchers found 60 candidate events. When they processed those candidate-event data using their statistical model, it estimated 5.6 neutrino-driven carbon-nitrogen transmutations. This was very close to the 4.7 events they had predicted.

"Capturing this interaction is an extraordinary achievement," Milton says. "Despite the rarity of the carbon isotope, we were able to observe its interaction with neutrinos, which were born in the Sun's core and travelled vast distances to reach our detector."

The finding is exciting. Verifying theoretical predictions is always satisfying, since it indicates that the science is proceeding in the right direction.

It also provides a fresh measurement of the likelihood of this particular low-energy neutrino-carbon reaction. As a result, it establishes a new nuclear-physics benchmark that should prove useful for future research.

"Solar neutrinos themselves have been an intriguing subject of study for many years, and the measurements of these by our predecessor experiment, SNO, led to the 2015 Nobel Prize in physics," says physicist Steven Biller of the University of Oxford.

"It is remarkable that our understanding of neutrinos from the Sun has advanced so much that we can now use them for the first time as a 'test beam' to study other kinds of rare atomic reactions!"

The research has been published in Physical Review Letters.