Experiments on the Nature of Penetrating Events in Extensive Air Showers

The discharges, in coincidence with extensive showers, of individual counters of a group placed in a cavity under 15 cm. lead have been studied with particular reference to the number of these counters simultaneously discharged in a single event. It is shown that the observed multiplicity of discharge is not consistent with operation by particles, each discharging one counter only, distributed at random in the extensive shower falling on the apparatus. The multiplicity is also inconsistent with that expected for low energy photons at the extreme range of normal cascades, and with that arising from the secondary electrons of a reasonable μ-meson spectrum. Entry of energy into the shielded cavity is shown to take place over an area much smaller than the total area of the cavity, but the mechanism then leading to the observed multiplicity has not been identified.