Scientists detect nuclear reactor’s ghostly afterglow for first time

Scientists have for the first time measured the faint antineutrino signal that lingers after a nuclear reactor is shut down, detecting it at the Chooz nuclear power plant in northern France. Antineutrinos are the lightest and most elusive known particles in the Universe and can pass through the reactor and its shielding with little interference.

The findings were published in Physical Review Letters on August 14, 2026.

Anthony Onillon and Thierry Lasserre of the Max-Planck-Institut für Kernphysik in Heidelberg, Germany, led the measurement.

The Double Chooz detector sits underground about 400 meters from the plant's two reactor cores and contains more than 30 cubic meters of liquid scintillator, which flashes when an antineutrino interacts.

Researchers analyzed 17.2 days of data collected while both reactor units were fully shut down and recorded around 100 antineutrino candidate events tied to residual radioactivity in the cores and nearby spent-fuel cooling pools.

The detected signal closely matched simulations of the remaining nuclear fuel and the decay of long-lived fission products. That provides the first direct experimental confirmation of predictions for shut-down reactors and spent fuel.

This capability suggests antineutrino detectors could provide independent information about reactors during maintenance and after shutdown, potentially aiding reactor monitoring, nuclear safety and safeguards.

Detecting the tiny residual signal required exceptionally low backgrounds and analysis techniques developed over many years, according to Onillon.

Until now, reactor antineutrino experiments focused mainly on operating reactors, where the flux is much larger.

Double Chooz was originally built to study neutrino oscillations and helped measure the mixing angle θ13, a key parameter for how neutrinos change type.

Initial results from JUNO-TAO, presented at Neutrino 2026, are using reactor-off data to isolate the weak antineutrino signal from spent nuclear fuel. The Double Chooz measurement provides the first published benchmark for studying residual signals from shut-down reactors and spent-fuel pools.