The Specific Ionization and Mass of Cosmic-Ray Particles

The specific ionization of 120 cosmic-ray tracks has been measured by counting droplets in photographs of delayed expansion cloud chamber tracks. For values of $H\rho$ between ${10}^3$ and 2×${10}^5$ the specific ionization as a function of $H\rho$ is found to be in good agreement with theory. The predicted minimum ionization for electrons of $H\rho \sim {10}^4$ (∼2 Mev) is verified. These results are applied to the calculation of the mass of heavily ionizing cosmic-ray particles as observed by us and by others. The masses of all heavy cosmic-ray particles thus far reported, with one exception, do not seem to be in serious disagreement with a unique mass which lies in the limits $(200\mathrm{}\pm 50)m_0$.