The energy spectrum of heavy primary cosmic-ray nuclei was measured and found to be in agreement with the assumption that no primaries with energy below the geomagnetic cutoff enter the earth. The mean free path of the primary nuclei in lead was measured. If the transparency of nuclear matter is taken into account, the present results for lead and the measurements by others for lighter elements are in good agreement. If the expression $R_A=R_0\times A^{\frac{1}{3}}$ is used for the radius of the nucleus, the data indicate for $R_0$ the value 1.3×${10}^{-13\mathrm{}}$ cm. It is also shown that the ratio of the abundances of the charge groups $Z=12\mathrm{} \mathrm{to} 17$ and $Z=18\mathrm{} \mathrm{to} 28$ is ∼2.1.

• Received 28 July 1954

DOI:https://doi.org/10.1103/PhysRev.96.1378