Observation of a downward terrestrial gamma-ray flash and the flash-induced neutrons

Tsuchiya, Harufumi ; Wada, Yuki*; Enoto, Teruaki*; Nakazawa, Kazuhiro*

Gamma Ray Observation of Winter Thundercloud (GROWTH) has been operating at the coastal area of the Japan sea since 2006. The area is famous for its frequent thunderstorms in winter seasons. GROWTH aims at elucidating how electrons are accelerated to relativistic energies inside lightning and thunderclouds. To this end, we installed newly developed radiation systems at the Kashiwazaki Kariwa nuclear power plant to observe bremsstrahlung photons and neutrons that originate from the accelerated electrons. The radiation systems have bismuth germanate (BGO) scintillation detectors and gadolinium orthosilicate (GSO) scintillation detectors. During a winter thunderstorm on 24 November 2017, a lightning discharge occurred near the power plant and caused a downward terrestrial gamma-ray flash (TGF) emitted from electrons through the bremsstrahlung process. Observations by the radiation systems showed that the TGF lasts for 10 ms and extends to around 10 MeV. The TGF was so intense that the BGO detectors were saturated during the main part of the flash. However, using the BGO data and data obtained from ionization chambers of monitoring posts at the power plant, we evaluated the total dose of the TGF and the number of 1 MeV electrons responsible for the TGF. The evaluated number of 1 MeV electrons, was consistent with that for a TGF frequently observed with space detectors. This implies that the production mechanism of TGFs detected on the ground is the same as or similar to that observed in space. In addition, the GSO detectors caught neutrons for ms after the lightning. The subsequent neutrons are thought to be generated via photonuclear reactions with atmospheric nitrogen and oxygen nuclei. These observations indicate that electrons are accelerated to at least 10 MeV in lightning. In this presentation, we discuss the production processes of the intense downward TGF and the subsequent neutrons.