Abstract
The penetrating showers observed with a large multiplate cloud chamber at 50 mwe and 250 mwe underground have been analyzed. The frequencies of the penetrating showers which are experimentally verified to be produced by mu-mesons have been interpreted with the Williams-Weiszäcker method and the constant photonuclear cross section of 1.4×10 -28 cm 2 /nucleon. It has been also shown that the angular distribution of secondary penetrating particles in multiple productions produced by mu-mesons is nearly isotropic in the center-of-mass system of the target nucleon and photon which is virtually associated with the incoming mu-mesons.