Lightning can trigger an atmospheric nuclear reaction that leads to the production of radioactive isotopes, according to a study in this week’s Nature.

It has been proposed that the energy from gamma-rays within lightning can cause photonuclear reactions in the atmosphere, which would produce neutrons and, eventually, positrons (the antimatter counterpart of electrons). However, the reaction had yet to be conclusively observed.

Using four radiation detectors, Teruaki Enoto and colleagues were able to detect neutron and positron signals during a thunderstorm on 6 February 2017 in Japan. From their data, the authors propose that a burst of lightning-triggered gamma-ray photons collided with atmospheric nuclei and initiated nuclear reactions. The atmospheric photonuclear reactions generated neutrons and unstable radioactive isotopes, which generated positrons during decay. The authors suggest that this discovery makes lightning only the second known natural channel on Earth, after the atmospheric cosmic-ray interaction, in which isotopes such as 13C, 14C and 15N are produced.