Unicorns, Bigfoot, and banshees have been scientifically debunked, but ghosts are real, sort of. Exceptionally elusive neutrinos and their antineutrino counterparts are known as ghost particles because they almost never interact with matter, due to their lack of electrical charge and almost nonexistent mass. However, these ghostly fundamental particles can help improve nuclear safeguards by providing a non-intrusive way to examine what happens inside nuclear reactors, including the spectral flux of antineutrinos after a power plant has been shut down. In a recent breakthrough, physicists measured this residual emission for the first time, detecting the faint, eerie glow of electron antineutrinos emanating from the darkness of the dormant cores of nuclear reactors in France. Led by physicists at the Max Planck Institute for Nuclear Physics (MPIK), researchers from the Double Chooz Collaboration have experimentally validated this predicted flow, which emanates from fuel burned inside two reactors, as well as from removed waste material placed in cooling pools. This flow has been theoretically restricted but never measured. The standard model that describes the fundamental composition of the Universe and its countless particles. (ScienceAlert) “Until now, reactor antineutrino experiments have focused primarily on operating reactors, where the antineutrino flux is much higher,” says Anthony Onillon, a physicist at MPIK and one of the study’s co-leaders. “Detection of the small residual signal after shutdown required exceptionally low backgrounds and careful analysis techniques developed by the Double Chooz collaboration over many years.” The Double Chooz experiment operated from 2011 to 2017, exploring the unimaginably immense amount of antineutrinos produced at the Chooz nuclear power plant in northern France. An illustration of the experiment site, detailing the reactors and detectors. (Double Chooz Collaboration)Double Chooz used two underground detectors at average distances of 400 meters (1,300 feet) and 1,050 meters (3,400 feet) from the power plant’s two operating reactors to study a strange property incomprehensible in the macroscopic world. A schematic illustration showing the Double Chooz setup. (Abrahão et al., Physical Review Letters, 2026) Neutrinos and antineutrinos change their flavor, or type, as they travel, through a process called oscillation. It’s like buying chocolate ice cream and transforming it into vanilla and then strawberry as it goes from the store to the house to the bowl, the Fermilab physicists explain, inventing Quantum Neapolitan in the process. But physicists can’t actually “see” antineutrinos; They can only discover their existence based on their interaction with other particles within delicately designed detectors. “Antineutrinos interact very rarely with matter,” explains Thierry Lasserre, a physicist at MPIK. “However, when one interacts within the Double Chooz detector, a characteristic double light signal is produced that can be distinguished from background events.” For another food-based analogy, imagine a collision between a truck full of Mentos and another full of cola, creating a telltale fountain of fizz. When a neutrino or antineutrino passes through the Double Chooz detector, it sometimes collides with a proton inside the “liquid scintillator” in the detector’s internal chamber. This collision generates a neutron and a positron, the antimatter counterpart of an electron. The positron annihilates instantly with a surrounding electron, generating a flash of light facilitated by the scintillator. The neutron is captured by gadolinium, a silvery rare earth metallic element within the scintillator, producing a second flash of light to provide “two-factor authentication” of the antineutrino event. An illustration of a Double Chooz detector. (Double Chooz Collaboration) This event is called reverse beta decay (IBD) and it occurs a lot in reactors during the fission of elements such as uranium and plutonium. It represents the largest source of man-made antineutrinos, and monitoring this flux may prove invaluable for reporting on nuclear safety and perhaps analyzing the changing composition of these combustible materials. In the study, the researchers observed an excess of IBD events across the energy range where the desired residual emission signal is likely to peak. Over 17.2 days of observation during which both Chooz reactor cores were shut down, the researchers found “very good agreement” between measured and expected antineutrino events; specifically, 106 ±18 measured events compared to 88 ±7 predicted events. This may represent less than 1% of the flow signal during reactor core operation. Related: All those decades-old nuclear tests have revealed something new about our planet’s core The accuracy of this technique can still be improved. It is sensitive to large changes in flow, but might not be able to tell if, for example, some spent fuel assemblies had disappeared, the researchers note. Overall, however, this work serves as an invaluable proof of concept for direct, non-intrusive nuclear testing, to potentially inform future nuclear safeguards standards. This research was published in Physical Review Letters. This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we take pride in our process, we are human. If you spot an error, please let us know.
