Time structure of individual extensive air showers

Extensive air showers detected by the GREX array have been sampled by means of highly segmented 8 m² bakelite RPC in the GREX/COVER_PLASTEX experiment. Delay distributions of particles with respect to the first arriving particle in the EAS front at PeV energies have been analysed for individual events in the core distance range of 0–100 m. It is shown that both mean arrival time and EAS front thickness in individual showers fluctuate strongly and cannot be a good measure of the distance from the EAS axis in a 0–100 m core distance interval.

Individual distributions have been compared with integrated inclusive distributions measured in the same experiment. Results indicate that the width of the individual distribution is systematically less than that of the inclusive distribution. It means that the bulk of particles in individual showers arrive as a relatively compact group delayed by different time intervals from the first arriving particle. Such fluctuations of the arrival time for most of the shower particles may be the consequence of large fluctuations in the shower longitudinal development.

Comparison with CORSIKA Monte Carlo simulations confirmed the difference between the mean width of inclusive and individual arrival time distribution. It revealed also the presence in the experiment of the excessive train of delayed particles near the shower core. This train is obviously due to the non-relativistic low energy hadrons most abundant in the shower core region.