Study of extensive air showers at Mount Norikura. III. Core structure and high-energy events

Abstract

An area of 3 × 4 m2 is covered by 48 plastic scintillators above and below a water tank 2 m in depth. From maps of the charged-particle density and the energy-flow density in the core region, properties of EAS cores and of the high-energy nucleon component have been studied. About 25% of observed cores show the complicated structure of a "multiple core". These can be understood as due to effects of high-energy nuclear particles having large transverse momenta of several GeV/c to a few tens of GeV/c. The frequency of occurrence of such events increases with the size of EAS only slowly, but it decreases rapidly with increasing distance between the main cores and subcores. There is no clear distinction in the average lateral density distribution of charged particles between these multiple-core EAS and ordinary EAS at points distant from the cores.Comparing the particle density in both layers of detectors (top and bottom), the activity of cores has been studied. This fluctuates more than would be expected from the lateral structure of the showers.