Intensity of Heavily Ionizing Penetrating Particles at 4300-Meter Altitude

The counting efficiency of a low pressure Geiger-Mueller counter for the ionizing cosmic radiation penetrating various thicknesses of lead has been measured at 4300-meter altitude. From existing data it is possible to determine the counting efficiency expected for mesotrons alone at that altitude. For thicknesses greater than 100 g/${\mathrm{cm}}^2$ electrons are excluded, and the observed excess in counting efficiency over that expected for mesotrons can be interpreted as due to heavily ionizing particles. On the assumption that these are protons the data indicate, for example, that 17±5 percent of the radiation penetrating 167 g/${\mathrm{cm}}^2$ Pb consists of protons.