Antineutrinos from residual radioactive decay can reveal activity inside nuclear reactors even after they have been shut down.

A nuclear reactor does not become completely silent when it shuts down. Long after power production stops, radioactive, long-lived fission products continue decaying for months or years, releasing a faint stream of particles called antineutrinos. (Anti)neutrinos are the lightest and among the most elusive particles known in the Universe, passing through reactor structures and shielding with little obstruction.

The Double Chooz collaboration has now directly measured this lingering antineutrino emission for the first time. Led by Anthony Onillon and Thierry Lasserre of the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg, Germany, the research was recently published in Physical Review Letters.