Photonuclear reactions triggered by lightning discharge
Photonuclear reactions triggered by lightning discharge
复制标题
DOI：
10.1038/nature24630
复制
发表时间：
2017-11-23
期刊：
影响因子：
64.8
通讯作者：
Tsuchiya, Harufumi
中科院分区：
文献类型：
--
作者：
Enoto, Teruaki;Wada, Yuuki;Tsuchiya, Harufumi
闪电和雷暴云是天然的粒子加速器（1）。相对论逃逸电子的雪崩，在雷暴云内的电场中发展（2，3），发射出韧致辐射伽马射线。这些伽马射线已被地面观测站（4-9）、机载探测器（10）和来自太空的地面伽马射线闪光（10-14）探测到。伽马射线的能量足够高，它们可以触发大气光核反应（10，15 -19），通过不稳定的放射性同位素的β（+）衰变产生中子和最终的正电子，最值得注意的是N-13，其通过N-14 +伽马-> N-13 + n产生，其中伽马表示光子，n表示中子。然而，这种反应迄今为止还没有被最终观察到，尽管越来越多的观测证据表明，中子（7，20，21）和正电子（10，22）可能来自这种反应。在这里，我们报告了闪电后的中子和正电子信号的地面观测。在2017年2月6日日本的雷暴期间，我们在距离闪电0.5-1.7公里的监测点检测到持续时间不到1毫秒的伽马射线闪光。随后的伽马射线余辉迅速消退，指数衰减常数为40-60毫秒，随后是大约0.511兆电子伏的延长线发射，持续了一分钟。观察到的衰变时间尺度和光谱截止在约10兆电子伏的γ射线余辉很好地解释了去激发γ射线从中子俘获激发的核。延长线发射的中心能量对应于电子-正电子湮灭，提供了闪电后正电子产生的确凿证据。
Lightning and thunderclouds are natural particle accelerators(1). Avalanches of relativistic runaway electrons, which develop in electric fields within thunderclouds(2,3), emit bremsstrahlung gamma-rays. These gamma-rays have been detected by ground-based observatories(4-9), by airborne detectors(10) and as terrestrial gamma-ray flashes from space(10-14). The energy of the gamma-rays is sufficiently high that they can trigger atmospheric photonuclear reactions(10,15-19) that produce neutrons and eventually positrons via beta(+) decay of the unstable radioactive isotopes, most notably N-13, which is generated via N-14 + gamma -> N-13 + n, where gamma denotes a photon and n a neutron. However, this reaction has hitherto not been observed conclusively, despite increasing observational evidence of neutrons(7,20,21) and positrons(10,22) 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 gamma-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 gamma-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 time-scale and spectral cutoff at about 10 megaelectronvolts of the gamma-ray afterglow are well explained by de-excitation gamma-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.