Arrival Times of Air Shower Particles at Large Distances from the Axis

A study has been made of the relative times of arrival of shower particles at large distances (200 to 1500 m) from the shower axis. Data were obtained at the MIT Volcano Ranch Station, using an array of 20 scintillation detectors, one of which was shielded part of the time. The shower size, direction, and core location were determined for each event. We describe the spatial distribution of shower particles at a given instant by means of three curved surfaces: the median surface for the penetrating particles (muons), the median surface for the electrons, and the extreme front. We find that the average median surface for the muons is approximately spherical, the center being located at an atmospheric depth of 320±70 g ${\mathrm{cm}}^{-2\mathrm{}}$, and that the average median surface for the electrons has a radius of curvature of about 1 km at a distance from the axis of 450 m. The electron radius of curvature increases at greater distances. Assuming that the extreme front is spherical, its average center must be located above 320 g ${\mathrm{cm}}^{-2\mathrm{}}$. We measured the radius for curvature of the extreme front for a small number of individual showers, but were not able to improve upon that limit.