Lightning strikes leave behind a radioactive cloud

Gamma rays produced by lightning hit atomic nuclei, transforming them.

All of these phenomena are powered by the fact that the electric fields within thunderstorms are able to accelerate electrons to extremely high energies. Whenever these electrons move along a curved path, they emit radiation that’s proportional to their energy. As a result, a storm can be associated with bursts of gamma rays, extremely high-energy photons.

Gamma rays rays are primarily noted for their interaction with the electrons of any atoms they run into—it’s why they’re lumped in the category of ionizing radiation. But they can also interact with the nucleus of the atom. With sufficient energy, they can kick out a neutron from some atoms, transforming them into a different isotope. Some of the atoms this occurs with include the most abundant elements in our atmosphere, like nitrogen and oxygen. And, in fact, elevated neutron detections had been associated with thunderstorms in the past.

But a team in Japan managed to follow what happens with the transformed atomic nucleus. To do so, they set up a series of detectors on the site of a nuclear power plant and watched as thunderstorms rolled in from the Sea of Japan. As expected, these detectors picked up a flash of high-energy photons associated with a lightning strike, the product of accelerated electrons. These photons came in a variety of energies and faded back to background levels within a couple of hundred milliseconds.

But about 10 seconds later, the number of gamma rays started to go back up again, and this stayed elevated for about a minute. In contrast to the broad energy spectrum of the initial burst, these were primarily in the area of 500 kilo-electronVolts. That happens to be the value you’d get if you converted an electron’s mass into energy.

What’s going on here? The researchers suggest that this is a product of the atoms left behind when neutrons were ejected. In the case of the most common isotope of nitrogen, 14N, this leaves behind 13N, which is unstable and has a short half-life. The same is true for 15O, which is left behind when the most common form of oxygen kicks out a neutron. These unstable isotopes will typically decay in a matter of seconds, which is right about the time frame for the afterglow the researchers were detecting.

(The neutrons that are kicked out typically recombine with other atoms. For example, adding a neutron to 14N causes it to kick out a proton, which forms a hydrogen atom. The remaining nucleus is 14C, a relatively long-lived radioactive isotope of carbon.)

The 500keV photons the authors were seeing weren’t a direct product of the radioactive decay. Instead, 13N and 15O decay by releasing a neutrino and a positron, the antimatter equivalent of the electron. These positrons will then bump into an electron in the environment and annihilate it, converting each of the particles into a gamma ray with the energy equivalent of the electron’s mass. That’s exactly the energy the researchers were detecting.

The authors suggest that a lightning bolt creates an entire region in the atmosphere enriched with unstable nitrogen and oxygen, which they managed to detect as it drifted over their sensors: “A region, or ‘cloud,’ filled with these isotopes emits positrons for more than 10 minutes and moves by wind above our detectors without experiencing much diffusion, owing to a low mobility of the isotopes,” the team wrote.

This doesn’t mean that thunderstorms are major radiation risks. But over time, the storms collectively provide a small influence on the isotopes that comprise Earth’s atmosphere. We’re apparently not just limited to the carbon that was here when the Earth formed; instead, we’ve made a bit of our own since.

Nature, 2017. DOI: 10.1038/nature24630  (About DOIs).

Ars Technica sits down with Shawn Frayne, one of the creators of HoloPlayer One, the world’s first interactive lightfield development kit.