A determination of the specific ionization of cosmic-ray particles, first, by a count of the number of drops per cm along cosmic-ray tracks on cloud-chamber photographs and, second, by measurements of the energy loss in lead has shown that the great bulk of the cosmic-ray particles of positive charge are positive electrons. The primary ionization was found to be about 31 ion-pairs per cm in air at S.T.P., but the total energy loss represents about 120 ion pairs per cm in air. Approximately the same values of specific ionization were found for the positives as for the negatives. Positive and negative electrons were found to occur in nearly equal numbers and to have similar distributions in energy. Energy distribution curves are given for the positives and negatives. A brief description of the experimental procedure is given, and several track photographs are shown. In view of the discovery that hard gamma-rays of Th $C^{\prime \prime }$ give rise to positives and negatives in pairs, similar to the effects found in cosmic-ray studies, it is concluded that the absorption of the Th $C^{\prime \prime }$ rays is due in part to that by free negative electrons and in part to a nuclear effect which results in the production of pairs of positive and negative electrons, the former effect accounting for the greater part of the absorption of the Th $C^{\prime \prime }$ rays. The symmetry in occurrence of the positive and negative electrons found in the cosmic-ray studies shows that the nuclear effect which results in the production of positive and negative electrons in pairs represents the predominant part of the absorption for the range of energies as high as those of the cosmic rays and that the absorption by free negative electrons is relatively small.

• Received 19 June 1933

DOI:https://doi.org/10.1103/PhysRev.44.406