The Average Specific Ionization of Cosmic-Ray Mesons in Hydrogen, Helium, and Argon

Abstract

Counter-controlled, cloud-chamber photographs have been taken of cosmic-ray meson tracks in hydrogen, helium, and argon gases. The chamber was operated in such a way that the average probable ionization could be determined from the counted number of drops that condensed on positive ions. Clusters of droplets that required energy transfers up to 25 \ifmmode\times\else\texttimes\fi{} (average energy spent per ion pair) were included. The observed results were corrected for the ionization in the alcohol and water vapors used in the chamber. The ionization correction amounted to less than 25 percent for hydrogen and helium gases and less than one percent for argon gas. The average probable ionization per cm at N. T. P. in hydrogen gas was 8.00, in helium gas 9.67, and in argon gas 65.5. The probable error was about seven percent. The results were used to check with theory. There was very good agreement between theory and experiment for hydrogen, argon, and air, but in the case of helium the discrepancy exceeded the estimated experimental error.