Cloud chamber photographs at 4300 meters elevation show positive and negative electron tracks similar to those observed at sea-level, but positive-negative electron showers occur more frequently and, in general, consist of more numerous tracks. Showers of 2-4 tracks, 5-10 tracks, and 11-100 tracks occur respectively, 8.6, 21 and 29 times as frequently per unit time at 4300 meters as they do near sea-level. Further measurements on the energy loss in lead of electrons up to 400 MEV, are given. They show that in this range of energies the energy loss in lead is roughly proportional to the incident energy. About one percent of the exposures on Pike's Peak reveal the presence of strongly ionizing particles which in most cases seem to be protons. The proportion of such tracks is considerably greater than at Pasadena. These heavy tracks in general bear only little relation in direction to that of the incoming beam, and usually arise from a type of nuclear disintegration not heretofore observed. The energies of these heavily ionizing particles may rise to values so high as 150 MEV, thus indicating that the source of the particle energies is in the cosmic rays.

• Received 9 June 1936

DOI:https://doi.org/10.1103/PhysRev.50.263