Observations of Cosmic-Ray Events in Nuclear Emulsions Exposed below Ground

Ilford Nuclear research plates were manufactured in a laboratory at an equivalent depth of 60 m. of water below ground. After having been stored at various depths, the plates were processed below ground. The observed frequencies of μ-mesons stopped in the plates are consistent with those expected from measurements on the energy-spectrum of cosmic rays. Several π-mesons stopping in the plates were observed, and reasons for believing these to have been locally produced in the matter near the plates are discussed. Forty-two nuclear disintegrations have been observed, the frequency at a depth of 60 m. water equivalent being of the order of 5 × 10 -3 stars/cm 3 /day. Approximately one third of these are attributed to the electromagnetic interaction of μ-mesons on their passage through nuclei. The remainder are attributed in part to neutrons from this first group of stars, and in part to the photons of the soft component underground. Four examples of stars accompanied by showers of particles at minimum ionization have been observed, and are discussed.