Ionization of High Energy Cosmic-Ray Electrons

Abstract

The ionization produced by high energy cosmic-ray electrons has been measured in a cloud chamber in order to determine whether the probable ionization produced by a high velocity particle continues to rise logarithmically with the energy, or whether it takes on a constant value as has been predicted theoretically. A lateral clearing field was arranged so that the ionization could be measured in the upper half of the cloud chamber; lead plates were placed in the lower half so that the electron would produce a shower from which its energy could be estimated. The ionization produced by electrons was measured and compared to the minimum ionization as determined from the average ionization produced by mesotrons. According to the theoretical formula of Halpern and Hall, the maximum ionization should be $1.4 I_{min}$. In most of the pictures taken the ionization was consistent with this value. There were several pictures, however, where the ionization was extremely high. These may either indicate that the theoretical formula is invalid, or that they may result from the coincidence of two or more particles. Such an event does not seem unlikely when the production of pairs is considered. When a high energy gamma-ray produces an electron-positron pair, the probability that the two particles come off in practically the same direction is very high; in the cloud chamber they would produce one track with doubly dense ionization.