The Specific Primary Ionization of Cosmic Rays in Helium

Values for the specific primary ionization of 46 cosmic rays in helium have been determined from cloud-chamber photographs. A magnetic field allowed separation into two groups: 21 electrons with energies near the energy for minimum ionization had a mean ionization of 7.33±{}0.12 per cm, and 26 mesotrons with energies greater than the energy for minimum ionization had a mean ionization of 7.23±{}0.12 per cm. The expected value for the latter figure, taking into account a logarithmic rise of ionization with energy, is 8.80 ions/cm. The experimental value for the minimum specific primary ionization in helium (corrected for the ionization in the vapor) is 6.5±{}0.1 at N.T.P. Three protons were observed in the H_ρ range in which they can easily be identified; in this H_ρ range three particles constituted approximately one percent of the radiation.