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論文

Gamma-ray observations at the coastal area of Japan Sea in winter seasons

土屋 晴文; 榎戸 輝揚*; 和田 有希*; 古田 禄大; 中澤 知洋*; 湯浅 孝行*; 楳本 大悟*; 牧島 一夫*; GROWTH collaboration*

Proceedings of Science (Internet), 358, 6 Pages, 2021/07

Since 2006, the GROWTH experiment has been successfully operating at the coastal area of Japan Sea. The GROWTH experiment aims at elucidating how particles in lightning and thunderclouds are accelerated to relativistic energies to produce gamma rays and occasionally neutrons. According to observations done by the GROWTH experiment, it is found that there are two types of radiation bursts associated with winter thunderstorms. One is long bursts lasting for a few tens of seconds to a few minutes, being not clearly related to lightning. The other is short bursts in association with lightning. To better understand the production mechanism of these radiation bursts, we have developed a small-type of radiation detectors and increased observational points with the new detectors. In this presentation, we show an overview of observations done by the GROWTH experiment. Then we focus on recent several findings observed by the new detectors. One, which is categorized into long bursts, implies a relationship between a long burst and an intra/inter-cloud discharge. Another is a combination of short bursts and long ones, showing simultaneous detections of prompt gamma rays extending up to 10 MeV and the 511-keV annihilation ones. These gamma-ray signals demonstrate the occurrence of photonuclear reactions in lightning. Based on these results, we discuss the production mechanism of gamma rays related to thunderstorms.

論文

Photonuclear reactions triggered by lightning discharge

榎戸 輝揚*; 和田 有希*; 古田 禄大*; 中澤 知洋*; 湯浅 孝行*; 奥田 和史*; 牧島 一夫*; 佐藤 光輝*; 佐藤 陽祐*; 中野 俊男*; et al.

Nature, 551(7681), p.481 - 484, 2017/11

 被引用回数:71 パーセンタイル:97.24(Multidisciplinary Sciences)

Relativistic electrons accelerated by electric fields of lightnings and thunderclouds emit bremsstrahlung $$gamma$$ rays, which have been detected at ground observations. The energy of the $$gamma$$ rays is sufficiently high to potentially invoke atmospheric photonuclear reactions $$^{14}$$N($$gamma$$, n)$$^{13}$$N, which would produce neutrons and eventually positrons via $$beta^{+}$$ decay of generated unstable radioactive isotopes, especially $$^{13}$$N. However, no clear observational evidence for the reaction has been reported to date. Here we report the first detection of neutron and positron signals from lightning with a ground observation. During a thunderstorm on 6 February 2017 in Japan, an intense $$gamma$$-ray flash ($$<$$1 ms) was detected at our monitoring sites. The subsequent initial burst quickly subsided with an exponential decay constant of 40-60 ms, followed by a prolonged line emission at $$sim$$0.511MeV, lasting for a minute. The observed decay timescale and spectral cutoff at $$sim$$10 MeV of the initial emission are well explained with de-excitation $$gamma$$ rays from the nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to the electron-positron annihilation, and hence is the conclusive indication of positrons produced after the lightning. Our detection of neutrons and positrons is unequivocal evidence that natural lightning triggers photonuclear reactions.

口頭

日本海沿岸における冬の雷や雷雲に伴う放射線観測

土屋 晴文; 榎戸 輝揚*; 和田 有希*; 古田 禄大*; 中澤 知洋*; 湯浅 孝行*; 奥田 和史*; 牧島 一夫*; 佐藤 光輝*; 佐藤 陽祐*; et al.

no journal, , 

日本海沿岸地域にある原子力発電所や自治体が持つモニタリングポストにより、冬の雷の発生や雷雲の通過に伴った3MeV以上の高エネルギー放射線量の増大が観測されていた。こうした放射線は、雷や雷雲がもつ電場によって加速された電子の制動放射$$gamma$$線であると考えられている。また、近年、雷や雷雲に付随して中性子や陽電子の信号を捉えたという報告もあった。これは、制動放射だけではなく様々な高エネルギー反応が雷や雷雲の中で起こっている可能性を示唆しており、活発な議論がなされている。本発表では、現在までに原子力センシング研究グループが観測したロングバーストと呼ばれる雷雲の通過に伴って放射線量が数分ほど増大する事象と、雷に同期して発生する継続時間が1秒以下のショートバーストの観測結果について報告する。特に、2017年2月の落雷に伴って発生したショートバーストでは、雷が光核反応を誘発し、中性子, 陽電子および酸素や炭素の放射性同位体を生成した実験的な証拠を初めて確認することに成功した。本発表では、それらの生成メカニズムについても議論する。

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