Neutrinos, sometimes called ghost particles, rarely interact with matter even though trillions pass through every person each second, and they are produced in nuclear reactions such as those in the Sun's core. Detecting them is difficult because most traverse detectors without leaving any signal, and until now solar neutrinos had only been observed interacting with a small number of other target materials.
The new result comes from the SNO+ detector situated two kilometres underground in SNOLAB, a laboratory built in an active mine near Sudbury, Canada, where the rock overburden shields the experiment from cosmic rays and other background radiation that could obscure neutrino signals. The detector uses a 12-metre acrylic vessel containing about 800 tonnes of liquid scintillator surrounded by roughly 9,000 photomultiplier tubes to capture tiny flashes of light from particle interactions.
The team focused on events in which a solar neutrino strikes a carbon-13 nucleus in the liquid scintillator, converting it into radioactive nitrogen-13 that subsequently decays. To identify these rare reactions, the researchers applied a delayed-coincidence technique, searching for an initial flash from the neutrino interaction followed several minutes later by a second flash from the nitrogen-13 decay, a characteristic pattern that distinguishes true signals from background noise.
Over a 231-day run between 4 May 2022 and 29 June 2023, the analysis identified 5.6 such carbon-13 interaction events, consistent with an expectation of 4.7 events from solar neutrinos over the same period. The team reports that this agreement supports the interpretation that the flashes originate from solar neutrinos interacting with carbon-13 nuclei rather than from random background processes.
Neutrinos play a key role in understanding stellar physics, nuclear fusion, and the evolution of the universe, and the researchers say this result opens a new way to study low-energy neutrino interactions and rare nuclear processes. Lead author Gulliver Milton, a PhD student at 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 travelled vast distances to reach our detector."
SNO+ reuses the infrastructure of the original Sudbury Neutrino Observatory (SNO), which previously showed that solar neutrinos oscillate between electron, muon, and tau flavours on their way from the Sun to Earth, a discovery that contributed to the 2015 Nobel Prize in Physics. Co-author Professor Steven Biller of Oxford noted that decades of measurements of solar neutrinos now allow physicists to treat them as a test beam for probing other rare atomic reactions.
SNOLAB scientist Dr Christine Kraus explained that the measurement exploits the natural abundance of carbon-13 in the scintillator to study a specific reaction channel. "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."
Research Report:First Evidence of Solar Neutrino Interactions on 13C
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