Further Observations of Cosmic-Ray Events in Nuclear Emulsions Exposed Below Ground

Earlier work with underground emulsion has been extended by making observations at greater depths, and by obtaining more data at depths previously employed. The final result for the interaction cross section of fast μ-mesons with nucleons is (4.6±0.5) × 10 -30 cm 2 . The flux of fast π-mesons is certainly less than 0.17% of the flux of fast μ-mesons, and a more probable figure for this ratio is 0.04%.