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Nature
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Photonuclear reactions triggered by lightning discharge

Naturevolume 551, pages481–484 (2017)Cite this article

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Abstract

Lightning and thunderclouds are natural particle accelerators1. Avalanches of relativistic runaway electrons, which develop in electric fields within thunderclouds2,3, emit bremsstrahlung γ-rays. These γ-rays have been detected by ground-based observatories4,5,6,7,8,9, by airborne detectors10 and as terrestrial γ-ray flashes from space10,11,12,13,14. The energy of the γ-rays is sufficiently high that they can trigger atmospheric photonuclear reactions10,15,16,17,18,19 that produce neutrons and eventually positrons viaβ+ decay of the unstable radioactive isotopes, most notably13N, which is generated via14N + γ → 13N + n, whereγ denotes a photon andn a neutron. However, this reaction has hitherto not been observed conclusively, despite increasing observational evidence of neutrons7,20,21 and positrons10,22 that are presumably derived from such reactions. Here we report ground-based observations of neutron and positron signals after lightning. 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 kilometres 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 centre energy of the prolonged line emission corresponds to electron–positron annihilation, providing conclusive evidence of positrons being produced after the lightning.

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Figure 1: Lightning discharges and subsecond decaying high-energy radiation.
Figure 2: De-excitation γ-ray spectra of the subsecond afterglow.
Figure 3: Count-rate histories of the annihilation signal.
Figure 4: γ-ray spectra during the prolonged annihilation signal.

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Acknowledgements

We thank the members of the radiation safety group of the Kashiwazaki-Kariwa nuclear power station, TEPCO Inc., for providing observation sites, H. Miyahara, N. Kawanaka and H. Ohgaki for discussions, H. Sakurai, M. Niikura and the Sakurai group members at RIKEN Nishina Center for providing Bi4Ge3O12 scintillation crystals, T. Tamagawa for project support, G. Bowers, M. Kamogawa and D. Smith for suggestions on our interpretation, S. Otsuka and H. Kato for supporting the detector developments, and the RIKEN Advanced Center for Computing and Communication for use of the HOKUSAI GreatWave supercomputing system for Monte Carlo simulations. This research is supported by JSPS/MEXT KAKENHI grant numbers 15K05115, 15H03653 and 16H06006, by SPIRITS 2017 and Hakubi projects of Kyoto University, and by the joint research programme of the Institute for Cosmic Ray Research (ICRR), The University of Tokyo. Our project is also supported by crowdfunding (‘Thundercloud Project’, using the academic crowdfunding platform ‘academist’), and we are grateful to Y. Shikano, Y. Araki, M. T. Hayashi, N. Matsumoto, T. Enoto, K. Hayashi, S. Koga, T. Hamaji, Y. Torisawa, S. Sawamura, J. Purser, S. Suehiro, S. Nakane, M. Konishi, H. Takami, T. Sawara and all of the backers of Thundercloud Project. We are grateful to M. Sakano of Wise Babel Ltd for linguistic help and to the ‘adachi design laboratory’ for supporting the crowdfunding acvitity. The background image inFig. 1 was provided by the Geospatial Information Authority of Japan.

Author information

Authors and Affiliations

  1. The Hakubi Center for Advanced Research and Department of Astronomy, Kyoto University, Kyoto, 606-8302, Japan

    Teruaki Enoto

  2. Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo, 113-0033, Japan

    Yuuki Wada, Yoshihiro Furuta, Kazuhiro Nakazawa & Kazufumi Okuda

  3. High Energy Astrophysics Laboratory, RIKEN Nishina Center, Saitama, 351-0198, Japan

    Yuuki Wada & Toshio Nakano

  4. Research Center for the Early Universe, The University of Tokyo, Tokyo, 113-0033, Japan

    Kazuhiro Nakazawa

  5. 55 Devonshire Road, Singapore, 239855, Singapore

    Takayuki Yuasa

  6. MAXI Team, RIKEN, Saitama, 351-0198, Japan

    Kazuo Makishima

  7. Graduate School of Science, Hokkaido University, Sapporo, 060-0808, Japan

    Mitsuteru Sato

  8. Department of Applied Energy, Graduate School of Engineering, Nagoya University, Aichi, 464-8603, Japan

    Yousuke Sato

  9. Advanced Institute for Computational Science, RIKEN, Hyogo, 650-0047, Japan

    Daigo Umemoto

  10. Nuclear Science and Engineering Center, Japan Atomic Energy Agency, Ibaraki, 319-1195, Japan

    Harufumi Tsuchiya

Authors
  1. Teruaki Enoto
  2. Yuuki Wada
  3. Yoshihiro Furuta
  4. Kazuhiro Nakazawa
  5. Takayuki Yuasa
  6. Kazufumi Okuda
  7. Kazuo Makishima
  8. Mitsuteru Sato
  9. Yousuke Sato
  10. Toshio Nakano
  11. Daigo Umemoto
  12. Harufumi Tsuchiya

Contributions

T.E., Y.W., Y.F., K.O., K.N., T.Y., T.N. and H.T. were responsible for the detector developments, data analyses and interpretation; T.E. is the project leader and wrote the draft of the manuscript; Y.W. made a major contribution to the detector development, installation and, in particular, analysis; Y.F. led the Monte Carlo simulations using Geant4; K.N. led the installation of the instruments at Kashiwazaki-Kariwa in 2016 and the laboratory experiment outlined in Methods section ‘Initial flash’; T.Y. led the development of the new data acquisition system after 2015; D.U. provided the data from 2012; and M.S., Y.S., K.M. and H.T. contributed to the data interpretation.

Corresponding author

Correspondence toTeruaki Enoto.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Additional information

Reviewer InformationNature thanks L. Babich and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Figure 1 Location of the observation sites.

a, Visible image of the geostationary satellite Himawari 8 at 06:00utc on 6 February 2017. The square and circle indicate Kashiwazaki-Kariwa and Kuju, respectively.b–d, Precipitation intensity map between 08:20 and 08:40utc on the same day, retrieved from the radar system of the Japan Meteorological Agency. Orange squares indicate Kashiwazaki-Kariwa nuclear power station.

Extended Data Figure 2 Detector response to the initial radiation flash.

a–c, Time histories of the maximum (black) and minimum (red) ADC values in the ADC-sampled waveforms of the photons detected with detectors A (a), B (b) and C (c). Normally, the minimum value is equal to the baseline (about 0 V at ADC = 2,050 ch), but undershoot was observed in our experiments (see Methods section ‘Initial flash’). An energy of 10 MeV corresponds to ADC increases of 1,395 ch, 1,218 ch and 404 ch for detectors A, B and C, respectively. The data gap for detector A is due to overflow of memory buffer in the ADC board.

Extended Data Figure 3 Illustration of lightning-triggered physical processes.

a, Physical processes during a chain of radiation events induced by the photonuclear reactions.b, Diffusion of neutrons produced in lightning and drift of the positron-emitting cloud.

Extended Data Figure 4 Neutron cross-section on nitrogen and time profile of scattered neutrons.

a, Neutron cross-section on14N (black) as a function of neutron kinetic energy40,41, including elastic (green) and inelastic (blue) scattering, charged-particle production (yellow) and neutron capture (red).b, Kinetic energy (black) and relative number of neutrons (red) as a function of time. The initial energy of neutrons is assumed to be 10 MeV and the initial number of neutrons is normalized to 1. Dashed lines indicate the times of thenth scatterings.

Extended Data Figure 5 De-excitation γ-ray spectra compared with simulations.

a–c, Background-subtracted γ-ray spectra of the subsecond γ-ray afterglow, with black crosses indicating ±1σ errors, for detectors A (a), B (b) and C (c). The source events are extracted for the periodt = 40–100 ms for detector A andt = 20–200 ms for detectors B and C. The curves show the Monte Carlo simulations of de-excitation γ-rays from atmospheric nitrogen (green dashed line), surrounding materials (blue dashed line), the detector itself (magenta dashed line) and their total (red solid line). The simulated spectra are normalized by the total counts above 1 MeV.

Extended Data Figure 6 Observed annihilation spectrum and simulated models.

The background-subtracted spectrum in the delayed phase for detector A, accumulated overt = 11.1–62.8 s, is plotted, with black crosses indicating ±1σ errors. The simulated model curves are overlaid, for assumed distances to the base of the positron-emitting cloud of 0 m (that is, the detector is within the cloud; red), 40 m (green), 80 m (blue) and 160 m (magenta). The models are normalized by the total counts in the 0.4–0.6-MeV band.

Extended Data Table 1 Specifications of our detectors and values obtained

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Enoto, T., Wada, Y., Furuta, Y.et al. Photonuclear reactions triggered by lightning discharge.Nature551, 481–484 (2017). https://doi.org/10.1038/nature24630

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Editorial Summary

Generating radioactive isotopes in thunderstorms

Lightning, particularly the very energetic γ-ray flashes, can theoretically generate radioactive isotopes through the interaction of relativistic electrons with atoms and molecules in the air. Some weak observational evidence for this was recently claimed. Teruaki Enoto and collaborators report observations of a coastal thunderstorm in Japan on 6 February 2017, in which they see a clear signature of positron annihilation associated with γ-ray flashes, combined with γ-rays arising in the de-excitation of nuclei excited by neutron capture. They conclude that the positrons arise from the decay of neutrons after the lightning.

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