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論文

Thundercloud project; Exploring high-energy phenomena in thundercloud and lightning

湯浅 孝行*; 和田 有希*; 榎戸 輝揚*; 古田 禄大; 土屋 晴文; 久富 章平*; 辻 結菜*; 奥田 和史*; 松元 崇弘*; 中澤 知洋*; et al.

Progress of Theoretical and Experimental Physics (Internet), 2020(10), p.103H01_1 - 103H01_27, 2020/10

We designed, developed, and deployed a distributed sensor network aiming at observing high-energy ionizing radiation, primarily gamma rays, from winter thunderclouds and lightning in coastal areas of Japan. Starting in 2015, we have installed, in total, more than 15 units of ground-based detector system in Ishikawa Prefecture and Niigata Prefecture, and accumulated 551 days of observation time in four winter seasons from late 2015 to early 2019. In this period, our system recorded 51 gamma-ray radiation events from thundercloud and lightning. Highlights of science results obtained from this unprecedented amount of data include the discovery of photonuclear reaction in lightning which produces neutrons and positrons along with gamma rays, and deeper insights into the life cycle of a particle-acceleration and gamma-ray-emitting region in a thunder-cloud. The present paper reviews objective, methodology, and results of our experiment, with a stress on its instrumentation.

論文

Photoneutron detection in lightning by gadolinium orthosilicate scintillators

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

Physical Review D, 101(10), p.102007_1 - 102007_6, 2020/05

 被引用回数:0 パーセンタイル:100(Astronomy & Astrophysics)

During a winter thunderstorm on November 24, 2017, a downward terrestrial gamma-ray flash took place and triggered photonuclear reactions with atmospheric nitrogen and oxygen nuclei, coincident with a lightning discharge at the Kashiwazaki-Kariwa nuclear power station in Japan. We directly detected neutrons produced by the photonuclear reactions with gadolinium orthosilicate scintillation crystals installed at sea level. Two gadolinium isotopes included in the scintillation crystals, $$^{155}$$Gd and $$^{157}$$Gd, have large cross sections of neutron captures to thermal neutrons such as $$^{155}$$Gd(n,$$gamma$$)$$^{156}$$Gd and $$^{157}$$Gd(n,$$gamma$$)$$^{158}$$Gd. De-excitation gamma rays from $$^{156}$$Gd and $$^{158}$$Gd are self-absorbed in the scintillation crystals, and make spectral-line features which can be distinguished from other non-neutron signals. The neutron burst lasted for $$sim$$100 ms, and neutron fluences are estimated to be $$>$$ 58 and $$>$$ 31 neutrons cm$$^{-2}$$ at two observation points at the power plant. Gadolinium orthosilicate scintillators work as valid detectors for thermal neutrons in lightning.

論文

Downward terrestrial gamma-ray flash observed in a winter thunderstorm

和田 有希*; 榎戸 輝揚*; 中澤 知洋*; 古田 禄大; 湯浅 孝行*; 中村 佳敬*; 森本 健志*; 松元 崇弘*; 牧島 一夫*; 土屋 晴文

Physical Review Letters, 123(6), p.061103_1 - 061103_6, 2019/08

 被引用回数:5 パーセンタイル:18.18(Physics, Multidisciplinary)

During a winter thunderstorm on 2017 November 24, a strong burst of gamma-rays with energies up to $$sim$$10 MeV was detected coincident with a lightning discharge, by scintillation detectors installed at Kashiwazaki-Kariwa Nuclear Power Plant at sea level in Japan. The burst had a sub-second duration, which is suggestive of photoneutron productions. The leading part of the burst was resolved into four intense gamma-ray bunches, each coincident with a low-frequency radio pulse. These bunches were separated by 0.7$$-$$1.5 ms, with a duration of $$<$$1 ms each. Thus, the present burst may be considered as a "downward" terrestrial gamma-ray flash (TGF), which is analogous to up-going TGFs observed from space. Although the scintillation detectors were heavily saturated by these bunches, the total dose associated with them was successfully measured by ionization chambers, employed by nine monitoring posts surrounding the power plant. From this information and Monte Carlo simulations, the present downward TGF is suggested to have taken place at an altitude of 2500$$pm$$500 m, involving $$8^{+8}_{-4} times10^{18}$$ avalanche electrons with energies above 1 MeV which is comparable to those in up-going TGFs.

論文

Gamma-ray glow preceding downward terrestrial gamma-ray flash

和田 有希*; 榎戸 輝揚*; 中村 佳敬*; 古田 禄大; 湯浅 孝行*; 中澤 知洋*; 森本 健志*; 佐藤 光輝*; 松元 崇弘*; 米徳 大輔*; et al.

Communications Physics (Internet), 2(1), p.67_1 - 67_9, 2019/06

 被引用回数:4 パーセンタイル:23.32(Physics, Multidisciplinary)

Two types of high-energy events have been detected from thunderstorms. One is "terrestrial gamma-ray flashes" (TGFs), sub-millisecond emissions coinciding with lightning discharges. The other is minute-lasting "gamma-ray glows". Although both phenomena are thought to originate from relativistic runaway electron avalanches in strong electric fields, the connection between them is not well understood. Here we report unequivocal simultaneous detection of a gamma-ray glow termination and a downward TGF, observed from the ground. During a winter thunderstorm in Japan on 9 January 2018, our detectors caught a gamma-ray glow, which moved for $$/sim$$ 100 s with ambient wind, and then abruptly ceased with a lightning discharge. Simultaneously, the detectors observed photonuclear reactions triggered by a downward TGF, whose radio pulse was located within $$sim$$ 1 km from where the glow ceased. It is suggested that the highly-electrified region producing the glow was related to the initiation of the downward TGF.

論文

Termination of electron acceleration in thundercloud by intracloud/intercloud discharge

和田 有希*; Bowers, G. S.*; 榎戸 輝揚*; 鴨川 仁*; 中村 佳敬*; 森本 健志*; Smith, D.*; 古田 禄大*; 中澤 知洋*; 湯浅 孝行*; et al.

Geophysical Research Letters, 45(11), p.5700 - 5707, 2018/06

 被引用回数:10 パーセンタイル:16.78(Geosciences, Multidisciplinary)

An on-ground observation program for high energy atmospheric phenomena in winter thunderstorms along Japan Sea has been performed via lightning measurements of $$gamma$$-ray radiation, atmospheric electric field and low-frequency radio band. On February 11, 2017, the radiation detectors recorded $$gamma$$-ray emission lasting for 75 sec. The $$gamma$$-ray spectrum extended up to 20 MeV and was reproduced by a cutoff power-law model with a photon index of 1.36$$^{+0.03}_{-0.04}$$, being consistent with a Bremsstrahlung radiation from a thundercloud (as known as a $$gamma$$-ray glow). Then the $$gamma$$-ray glow was abruptly terminated with a nearby lightning discharge. The low-frequency radio monitors, installed $$sim$$50 km away from Noto School, recorded intra/inter-cloud discharges spreading over $$sim$$60km area with a $$sim$$300 ms duration. The timing of the $$gamma$$-ray termination coincided with the moment when a sequence of intra/inter-cloud discharges passed 0.7 km horizontally away from the radiation monitors. The atmospheric electric-field measurement presented that negative charge was located in the cloud base and not neutralized by the lightning discharge. This indicates that the $$gamma$$-ray source was located at an higher region than the cloud base.

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:51 パーセンタイル:2.74(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.

論文

Applications and imaging techniques of a Si/CdTe Compton $$gamma$$-ray camera

武田 伸一郎*; 一戸 悠人*; 萩野 浩一*; 小高 裕和*; 湯浅 孝行*; 石川 真之介*; 福山 太郎*; 齋藤 新也*; 佐藤 有*; 佐藤 悟朗*; et al.

Physics Procedia, 37, p.859 - 866, 2012/10

 被引用回数:11 パーセンタイル:3.24

ASTRO-Hミッションのために開発されたSi/CdTe半導体両面ストリップ検出器(DSD)を利用したコンプトンカメラを用い、放射線ホットスポットのモニタリングの実行可能性チェックを目的とした複数放射線源の画像化実験を行った。本装置は半導体検出器によって与えられた良好なエネルギー分解能により、既に商業的な画像処理システムが提供するホットスポットの画像可能力に加え、複数の放射性同位元素を同定する能力を有する。今回の実験では、$$^{133}$$Ba(356keV), $$^{22}$$Na(511keV)及び$$^{137}$$Cs(662keV)の三放射性同位元素を同時に測定し、これらの画像化に成功した。5つの検出器モジュール(有効面積: 1.7$$times$$10$$^{-3}$$cm$$^2$$)を積み重ねることによって、662keVの$$gamma$$線に対し、検出効率1.68$$times$$10$$^{-4}$$、及び、3.8度の角度分解能を確認した。本装置は、より多くの検出器モジュールをスタックすることにより、さらに大きな検出効率を達成することが可能である。

論文

Observation of an energetic radiation burst from mountain-top thunderclouds

土屋 晴文*; 榎戸 輝揚*; 鳥居 建男; 中澤 知洋*; 湯浅 孝行*; 鳥井 俊輔*; 福山 太郎*; 山口 貴弘*; 加藤 博*; 岡野 眞治*; et al.

Physical Review Letters, 102(25), p.255003_1 - 255003_4, 2009/06

 被引用回数:57 パーセンタイル:10.01(Physics, Multidisciplinary)

2008年9月20日に標高2770mの乗鞍観測所において$$gamma$$線と電子が同時に検出された。これらの放出は、雷雲活動によるもの考えられ90秒続いた。$$gamma$$線のエネルギーは10MeVに達し、距離60$$sim$$130mにある線源からの制動放射線によるものと推定された。電子は、雷雲中での加速によって生成されたものと考えられる。

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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