For the first time, scientists have witnessed lightning triggering
nuclear reactions in the atmosphere, confirming a hypothesis dating back
almost a century.
It's long been predicted that high-energy electrons in lightning can
produce gamma rays that induce nuclear reactions in thunderclouds, but
before now, the phenomenon has never been conclusively observed.
"The photonuclear reaction in the atmosphere has been theoretically
expected [to be] triggered by such high energy radiation," one of the
researchers, astrophysicist Teruaki Enoto from Kyoto University in
Japan, explained to ScienceAlert.
"Several groups have accumulated signatures of this phenomena, such
as signals of either neutrons or positrons, which are the products of
this reaction."
(Leonid Babich/Nature)
Since the 1980s, scientists have detected these kinds of signals
using ground-based observatories, aircraft, and satellites, but before
now it was difficult to confirm experimentally that nuclear reactions
were yielding the neutrons, positrons, or particles observed.
Here, Enoto and fellow researchers made use of radiation detectors
installed at the Kashiwazaki-Kariwa nuclear power station in Niigata,
along the coast of the Sea of Japan.
During a thunderstorm in February this year, the team detected
"intense radiation" from lightning strikes just off the coast, including
a brief gamma ray flash.
This was followed by a prolonged gamma ray line
at an energy of 0.511 megaelectronvolts (MeV) – the energy signature
you'd expect to see from positrons and electrons after a nuclear
reaction.
"This line is a conclusive indication of electron–positron
annihilation, and represents unequivocal evidence that photonuclear
reactions can be triggered by thunderstorms," explains experimental
physicist Leonid Babich from the Russian Federal Nuclear Centre, in a
commentary on the research in Nature.
According to Enoto, the findings show us there's more going on in thunderstorms than we may tend to imagine.
"Usually people think lightning can interact with electrons in atoms," he told ScienceAlert.
"The photonuclear reactions indicate that lightning also interacts
even with nuclei if gamma rays have sufficiently high energy to knock
out neutrons from the nuclei."
In addition to generating neutrons and positrons, the process
observed is also important because it's only the second time we've seen
radioactive isotopes being naturally produced in the atmosphere – with
the other example being particles produced by cosmic rays.
But just because there's now firm evidence that lightning is
generating radioactive particles in the sky, there's no reason to be
concerned.
"Since the radioactive isotopes are short-lived, spatially
restricted, and [comprise a] relatively small amount compared to usual
background radiative environments, I think there is no health risk from
this phenomena," Enoto says.
As to whether this means all thunderstorms are capable of kickstarting these reactions, the researchers aren't entirely sure.
"This is still an open question," Enoto says.
"We have proved the existence of photonuclear reactions at least from
one lightning discharge… in order to answer your question, we need more
statistical and quantitative studies. I am personally suspecting that
the photonuclear reaction would happen at powerful events."
The findings are reported in Nature.
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