Although scientists have widely believed for a long time, now, that high-energy electrons in lightning have the potential to generate enough gamma rays that can trigger nuclear reactions in thunderclouds, no evidence to back the theory had been found. Well, not until recently!
In a study published Wednesday, in the journal NATURE, a team of Japanese researchers from Kyoto University claims to have found conclusive evidence to the century-old theory.
“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, explains. “Several groups have accumulated signatures of this phenomena, such as signals of either neutrons or positrons, which are the products of this reaction.”
During a February 6 study of a thunderstorm off the coast of the Sea of Japan this year, substantial amounts of radiation were recorded by the research team’s y-ray detectors at monitoring sites that were 0.5 to 1.7 kilometers away from the lightning.
Analysis of the less-than-one-millisecond y-ray flash and the ensuing half-a-second y-ray afterglow followed by a minute-long emission of γ-rays concentrated at 511 kiloelectronvolts of energy, were conclusive enough for the researchers to confirm that lightning did cause nuclear reactions in the sky.
Here’s the scientific explanation of the phenomenon published in the journal NATURE.
“During a thunderstorm on 6 February 2017 in Japan, a γ-ray flash with a duration of less than one millisecond was detected at our monitoring sites 0.5–1.7 kilometers away from the lightning. The subsequent γ-ray afterglow subsided quickly, with an exponential decay constant of 40–60 milliseconds, and was followed by prolonged line emission at about 0.511 megaelectronvolts, which lasted for a minute. The observed decay timescale and spectral cutoff at about 10 megaelectronvolts of the γ-ray afterglow are well explained by de-excitation γ-rays from nuclei excited by neutron capture. The center energy of the prolonged line emission corresponds to electron-positron annihilation, providing conclusive evidence of positrons being produced after the lightning.”
Experimental physicist Leonid Babich at the Russian Federal Nuclear Centre in Sarov accepts the study findings as “unequivocal evidence” of thunderstorms causing photonuclear reactions.
“This line is a conclusive indication of electron-positron annihilation, and represents unequivocal evidence that photonuclear reactions can be triggered by thunderstorms,” Babich explains.
The initial y-ray flash, the y-ray afterglow, and the prolonged y-ray emission together indicate a photonuclear reaction first theorized by Babich about a decade ago. Electrons accelerated by lightning to almost the speed of light have the potential to generate y-rays.
Collision of one of these y-rays with the nucleus of a nitrogen atom in the atmosphere causes a neutron to dislodge. Another nitrogen nucleus in the atmosphere then absorbs most of these dislodged neutrons, causing it get into an excited state due to the added energy. As the recipient nucleus settles down to its original state, it gives off the telltale y-ray glow.
Now, the nitrogen atom nucleus, which lost its neutron because of the colliding y-ray, becomes very unstable causing its radioactive decay and the resultant emission of a positron. There is almost an immediate annihilation of the positron along with an electron.
The positron-electron annihilation produces two 511-kiloelectronvolt photons, which, in effect, was the third afterglow.
Enoto and his team were fortunate that the short-lived radioactive cloud was low and moving in the direction of their detectors which were able to record the phenomenon. This is probably why photonuclear signature has been so elusive to scientists in the past.
While Enoto claims to have witnessed similar events in the past, none were as conclusive as the one explained in the journal NATURE.
In Enoto’s opinion, health risks from these aerial photonuclear reactions are unlikely.
“Since the radioactive isotopes are short-lived, spatially restricted, and [comprise a] relatively small amount compared to usual background radioactive environments, I think there is no health risk from this phenomena,” he says.
The obvious question that arises out of all this is:
Do all thunderstorms cause photonuclear reactions over our heads?
According to Enoto, further studies would be required to arrive at any conclusive answer to that question.
“This is still an open question,” he said.”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.”
— WeatherTalk (@WeatherTalk) November 23, 2017
— Phys.org (@physorg_com) November 22, 2017