LXXXIV. Cosmic rays at60 m.w.e.Underground: I. The knock-on electrons and nuclear interactions produced by relativisticμ-mesons

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Abstract
A length of 126·6 m of minimum ionization track has been examined in Ilford G5 plates, coated, exposed, and processed at 60 m.w.e. underground, and a detailed study of knock-on electrons of energy greater than 50 mev has been made. After making suitable corrections for scanning efficiency, it is found that the theoretical cross section, as derived by Bhabha, fits the data well for knock-on energies greater than 100 mev. The mean free path for the production of these knock-on electrons is (4·5±1) m of emulsion corresponding to a cross section of (20±4) ×10-28 cm2/electron. The theoretical cross section for the μ-meson spectrum at 60 m.w.e. is 18·4×10-28 cm2/electron. A typical μ-meson star has been found. It is shown that the number of small stars with less than three dark tracks produced by μ-mesons is not very different from the number of larger stars with three or more dark tracks. No evidence for or against the existence of pairs of associated penetrating particles has been obtained.

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