Earth & Conservation

In ‘Shocking’ Discovery, Lightning Triggers Nuclear Reactions

For the first time ever, scientists proved that lightning strikes are powerful enough to knock out neutrons and form radioactive isotopes.

If stars are nature’s fusion reactors, then lightning is its particle accelerator. The powerful electrical and magnetic fields generated by a lightning strike emit a flash of gamma rays that burst out in all directions, colliding with atmospheric gases like a celestial game of billiards.

For years, scientists have wondered if these collisions were powerful enough to knock neutrons out of stable nuclei, creating radioactive isotopes of gases like nitrogen and oxygen. Thanks to a powerful winter thunderstorm and some well-placed radiation detectors, a team of Japanese researchers captured the first definitive proof that lightning can trigger a type of nuclear reaction.

As reported Nov. 22 in the journal Nature, a cluster of radiation detectors at the Kashiwazaki-Kariwa nuclear power station on the Sea of Japan recorded gamma ray and positron emissions from an offshore lightning strike on February 6, 2016. The lightning data was exactly what you’d expect to see following photonuclear reactions, the collision of high-energy photons with atmospheric nuclei.

The result of such reactions is a radioactive isotope. Nitrogen-14, for example, is the most abundant particle in the atmosphere, with seven protons and seven neutrons. If a neutron is knocked out of nitrogen-14, it becomes the radioactive isotope nitrogen-13, which quickly decays after just nine seconds into the stable isotope carbon-13.

Before this new lightning discovery, it was believed that all isotopes, both radioactive and stable, were formed in only two ways: from nucleosynthesis in stars, or from collisions in the upper atmosphere between cosmic rays and atmospheric nuclei. The isotope carbon-14, for example, famous for its slow and steady rate of decay, is formed when cosmic rays collide with nitrogen-14 nuclei in the atmosphere, adding a neutron and popping out a proton.

What the Japanese lightning researchers found was an entirely new channel for producing isotopes of nitrogen, carbon, and oxygen, some of which are abundant in nature.
Teruaki Enoto is a physicist and astronomer at Kyoto University in Japan. Back in 2006, he and some collaborators launched the Gamma-Ray Observation of Winter Thunderclouds project, or GROWTH, to try to capture definitive radiation data from lightning storms. After a stint at NASA Goddard Space Flight Center as an X-ray astronomer, Enoto returned to Japan in 2015 and installed several highly sensitive radiation detectors at the nuclear power plant.
Winter storms along the coast of Japan have the dual advantages of being both very powerful and very low to the ground, Enoto told Seeker, making it easier for ground-based detectors like his to capture gamma-rays and other particle emissions.
Enoto said the radiation data captured from a single lightning strike in 2016 points to several exciting new ways of understanding lightning and its role in the universe. First and foremost, the data proved that lightning is a source of at least a small fraction of all radioactive and stable isotopes on Earth, although much more research is needed to figure out how many different isotopes are generated by lightning storms and at what volume.
Second, said Enoto, the new data expands our understanding of lightning’s effects on atmospheric particles, which scientists used to believe was limited to electrons.
“This detection of nuclear reactions implies that nuclei can change in the lightning, which means that nuclear physics can be applied to lightning physics,” he said.  
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The lightning discovery also reveals the hidden nature of lightning storms as “positron generators.” Positrons are a byproduct of photonuclear reactions. They are positively charged subatomic particles that are paired with electrons before undergoing a decay process known as annihilation.
“After each lightning strike, there’s a positron-electron cloud quietly passing above our head,” said Enoto. “This is a new phase of lightning that we hadn’t known before.”
Don’t worry, though. There’s nothing dangerous about the radiation emitted by photonuclear reactions in lightning storms, explained Enoto, since the levels are almost too low to be detected by even his ultra-sensitive equipment.
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