Cloud Chamber Observations of the Cosmic Radiation Underground

In a study of the passage of fast charged particles through a cloud chamber containing two Pb plates at a depth below ground equivalent to 60 m water, examples of associated pairs of penetrating particles and of large angle scattering of single particles have been observed. If these events are interpreted as the production of secondary particles by μ-mesons, and as anomalous scattering of μ-mesons in Pb, the following cross sections per nucleon result: for penetrating secondary particles 4 × 10 -29 cm 2 , and for anomalous large angle scatter 2 × 10 -28 cm 2 .