  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 $>$52 and
$>$31~neutrons~cm$^{-2}$ at two observation points inside the power plant.
Gadolinium orthosilicate scintillators work as valid detectors for thermal
neutrons in lightning.
