A Hodoscope Study of Penetrating Cosmic-Ray Showers. I. Local Showers

Showers produced locally in a lead absorber were studied with a hodoscope arrangement. It is shown that, in order to record "local penetrating showers," a rigid selection demanding high penetrating power of the secondaries as well as of the primary must be used. If this is done, some of the discrepancies of recent investigations are removed. For showers capable of penetrating at least 200 g/cm² Pb one finds a collision mean free path of (162±{}10) g/cm², and for showers of at least 100 g/cm² penetration a mean free path of (196±{}13) g/cm². Besides, "local soft showers" due to electronic or photonic secondaries of ordinary μ-mesons were studied. Their frequency at 3260-m elevation is about 1 in 10⁵ traversals of the lead absorber.