Cloud Tracks of Cosmic Rays in the Substratosphere

A cloud chamber in a magnetic field of 708 oersteds was carried in an airplane to a height of 29,300 feet. The maximum measurable $H\rho$ was 5.6 × ${10}^5$ which, for an electron, corresponds to an energy of 1.7 × ${10}^8$ ev. Out of 155 pictures taken at altitudes greater than 15,000 feet, 51 slow mesotrons and 39 proton tracks were identified. The number of slow mesotron tracks at these altitudes is 9 percent of the number of electron and fast mesotron tracks. One photograph seems to show a slow mesotron shower. Some of the tracks with very high specific ionization may be due to the fragments of heavy nuclei.