Photoneutron detection in lightning by gadolinium orthosilicate scintillators.

During a winter thunderstorm on 24 November 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, 155Gd and 157Gd, have large cross sections of neutron captures to thermal neutrons such as 155Gd(n,γ)156Gd and 157Gd(n,γ)158Gd. Deexcitation gamma rays from 156Gd and 158Gd 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 ~100 ms, and neutron fluences are estimated to be >52 and >31 neutrons cm-2 at two observation points inside the power station. Gadolinium orthosilicate scintillators work as valid detectors for thermal neutrons in lightning. [ABSTRACT FROM AUTHOR]