Cosmic-Ray Particles

Abstract

Specific Ionization. In a previous experiment, a number of thin straight tracks formed in a Wilson cloud-apparatus were identified as cosmic-ray particle tracks, by means of a Geiger-counter. The specific ionization along these has now been determined and is found not to exceed 36 ion-pairs per cm, in air at 1 atmosphere. This is less than one third the ionization calculated by Kolh\"orster and Tuwim, whose value is believed to be erroneous, at least for individual tracks, for reasons set forth in this paper."Group Phenomenon". On 148 stereopictures of tracks attributed to cosmic-ray particles, 20 groups of 2 or more such tracks were found. The tracks of a group usually converge to a point near at hand, which suggests that they have a common source. Each group is believed to be made by secondary electrons ejected by one photon. Possible explanations of the origin of the groups have been considered; the one that seems most favorable supposes that a cosmic-ray photon may interact with an atomic nucleus and eject one or more fast $\ensuremath{\beta}$-particles from it; the convergent groups are, then, formed when 2 or more particles are ejected from one nucleus. The production of groups may affect the interpretation of various ionization and Geiger-counter experiments with cosmic-rays. Some energy calculations have been made on the basis of the measured specific ionization of the cosmic-ray particles; for example, a minimum of 5\ifmmode\times\else\texttimes\fi{}${10}^{8}$ $e$-volts energy are required to send an electron through the earth's atmosphere. An ionization experiment is proposed, by which it should be possible to tell whether or not cosmic-rays produce groups of tracks by nuclear disruption.