Researchers using the SNO+ detector in Canada have observed solar neutrinos converting carbon-13 into nitrogen-13, marking one of the lowest-energy neutrino interactions detected. This achievement relied on tracking paired light bursts separated by minutes. The finding builds on prior neutrino research that earned a Nobel Prize.
Neutrinos, elusive particles produced in the Sun's core, rarely interact with matter, earning them the nickname 'ghost particles.' In a breakthrough, scientists at the SNO+ experiment captured these particles inducing a transformation in carbon atoms deep underground.
The SNO+ detector, located two kilometers below ground at SNOLAB in Sudbury, Canada, shields sensitive measurements from cosmic rays. Operating in an active mine, it uses a liquid scintillator containing carbon-13 to detect interactions. The team employed a 'delayed coincidence' method, identifying events through an initial light flash from a neutrino striking a carbon-13 nucleus, followed by a second flash from the decay of the resulting radioactive nitrogen-13 after about ten minutes.
Data collection spanned 231 days, from May 4, 2022, to June 29, 2023, yielding 5.6 such events—aligning closely with the predicted 4.7 from solar neutrinos. This observation provides the first direct measurement of the cross-section for this reaction to nitrogen-13's ground state.
Lead author Gulliver Milton, a PhD student at the University of Oxford's Department of Physics, stated: "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 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!"
SNO+ succeeds the SNO experiment, which resolved the solar neutrino problem and contributed to the 2015 Nobel Prize awarded to Arthur B. McDonald. SNOLAB staff scientist Dr. Christine Kraus noted: "This discovery uses the natural abundance of carbon-13 within the experiment's liquid scintillator to measure a specific, rare interaction... these results represent the lowest energy observation of neutrino interactions on carbon-13 nuclei to date."
These results, published in Physical Review Letters in 2025, open doors to studying other low-energy neutrino processes, enhancing insights into stellar nuclear fusion and cosmic evolution.
