The Average Ionization and Energy Loss of Cosmic-Ray Mesotrons in Air

The average specific ionization of cosmic-ray mesotrons has been determined at sea level and at 100 feet underground from drop counts along diffuse cloud-chamber tracks. The cloud chamber was operated in such a way that the efficiency of condensation on ions was known. The average specific ionization, excluding energy transfers greater than 800 ev, is essentially 50 ion pairs per cm of air at N.T.P. for both locations. Since a 15 percent increase is expected for the measurements underground, it is suggested that the high energy cosmic rays may be protons. If all energy transfers less than ${10}^4$ ev are included, the average energy loss is 1.35×${10}^6$ ev per g/${\mathrm{cm}}^2$ of air, in reasonable agreement with the theory. The observed cross sections for energy transfers between 800 and 3200 ev agree with the Rutherford formula when the electron binding is taken into account.