  During the 2010 rainy season in Yangbajing (4300 m above sea level) in Tibet,
China, a long-duration count enhancement associated with thunderclouds was
detected by a solar neutron telescope and neutron monitors installed at the
Yangbajing Comic Ray Observatory. The event, lasting for $\sim$40 min, was
observed on July 22, 2010. The solar neutron telescope detected significant
$\gamma$-ray signals with energies $>$40 MeV in the event. Such a prolonged
high-energy event has never been observed in association with thunderclouds,
clearly suggesting that electron acceleration lasts for 40 min in
thunderclouds. In addition, Monte Carlo simulations showed that $>$10-MeV
$\gamma$ rays largely contribute to the neutron monitor signals, while $>$1-keV
neutrons produced via a photonuclear reaction contribute relatively less to the
signals. This result suggests that enhancements of neutron monitors during
thunderstorms are not necessarily a clear evidence for neutron production, as
previously thought.
