Arrival-time distribution of air-shower electrons near the core

Abstract
The longitudinal structure of air-shower discs has been studied by measuring the arrival-time distribution of electrons in air showers with sizes 105.5 to 107.5, at distances less than 75 m from the core, in the Akeno air-shower array (900 m above sea level). The average T10-80, the time between arrival of 10% and 80% of the integrated full signal, which corresponds to the thickness of the air-shower disc, is found to increase with core distance from about 2 ns at less than 10 m to 13 ns at 70 m. Any dependence on shower size, age parameter or zenith angle cannot be seen at the small core distance observed. An empirical formula for the arrival-time distribution as a function of core distances is given.