Fair Weather Neutron Bursts From Photonuclear Reactions By Extensive Air Shower Core Interactions In The Ground And Implications For Terrestrial Gamma-Ray Flash Signatures

We report on anomalously long duration (2 ms) count rate bursts following the impact of cosmic ray showers near a 7.62 cm x circle divide 7.62 cm LaBr3 scintillation detector at the High Altitude Water Cherenkov array in Mexico, previously described by Stenkin et al. (2001), and termed "neutron bursts." The largest burst produced 198 counts within 2 ms in our LaBr3 detector. We simulate the neutron burst albedo flux (that is, secondary emissions from an extensive air shower core impacting the ground), and show that (1) the characteristic spectra and count rates are well explained by neutron absorption in the ground and (2) any cosmic ray secondary that produces neutrons, either through hadron inelastic collisions, or photoneutron production by gamma-rays, produces the same characteristic spectra. This implies that other natural phenomena that produce downward beams of gamma-rays, like Terrestrial gamma ray flashes, should produce a similar "neutron burst" signature from the photoneutron reactions occurring in the soil.