Neutrinos are among the most puzzling particles known to science and are often called ‘ghost particles’ because they so rarely interact with matter. Trillions pass through each person every second without leaving any mark. These particles are created during nuclear reactions, including those inside the Sun’s core. Their extremely weak interactions make them exceptionally challenging to study. Only a few materials have ever been shown to respond to solar neutrinos. Scientists have now added another to that short list by observing neutrinos convert carbon atoms into nitrogen inside a massive underground detector.
This achievement came from a project led by Oxford researchers using the SNO+ detector, which sits two kilometers underground at SNOLAB in Sudbury, Canada. SNOLAB operates within an active mine and provides the shielding needed to block cosmic rays and background radiation that would otherwise overwhelm the delicate neutrino measurements.
Capturing a Rare Two-Part Flash From Carbon-13
The research team focused on detecting moments when a high-energy neutrino hits a carbon-13 nucleus and converts it into nitrogen-13, a radioactive form of nitrogen that decays roughly ten minutes later. To spot these events, they relied on a ‘delayed coincidence’ technique that searches for two related bursts of light: the first from the neutrino striking the carbon-13 nucleus and the second from the decay of nitrogen-13 several minutes afterward. This paired signal makes it possible to confidently distinguish true neutrino events from background noise.
Over a span of 231 days, from May 4, 2022, to June 29, 2023, the detector recorded 5.6 such events. This matches expectations, which predicted that 4.7 events would occur due to solar neutrinos during this period.
A New Window Into How the Universe Works
Neutrinos behave in unusual ways and are key to understanding how stars operate, how nuclear fusion unfolds, and how the universe evolves. The researchers say this new measurement opens opportunities for future studies of other low-energy neutrino interactions.
Lead author Gulliver Milton, a PhD student in the University of Oxford’s Department of Physics, said: “Capturing this interaction is an extraordinary achievement. 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 traveled vast distances to reach our detector.”
Co-author Professor Steven Biller (Department of Physics, University of Oxford) added: “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. 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!”
Building on the SNO Legacy and Advancing Neutrino Research
SNO+ is a successor to the earlier SNO experiment, which demonstrated that neutrinos switch between three forms known as electron, muon, and tau neutrinos as they travel from the Sun to Earth. According to SNOLAB staff scientist Dr. Christine Kraus, SNO’s original findings, led by Arthur B. McDonald, resolved the long-standing solar neutrino problem and contributed to the 2015 Nobel Prize in Physics. These results paved the way for deeper investigations into how neutrinos behave and their significance in the universe.
“This discovery uses the natural abundance of carbon-13 within the experiment’s liquid scintillator to measure a specific, rare interaction,” Kraus said. “To our knowledge, these results represent the lowest energy observation of neutrino interactions on carbon-13 nuclei to date and provides the first direct cross-section measurement for this specific nuclear reaction to the ground state of the resulting nitrogen-13 nucleus.”
