The general problem of the propagation of cosmic rays from their source until they are observed in the vicinity of the earth is reviewed. The effect of matter on the cosmic-ray composition is considered in detail, and the effect of diffusion is treated in the depth necessary to study some models of the cosmic radiation not previously considered in this manner. The results of these calculations show that there are some aspects of the experimental observations, such as the ratio of light to medium nuclei as a function of energy, which are not consistent with any equilibrium model of the cosmic radiation which does not include a rigidity dependence in the mean free path. This conclusion is independent of the source spectrum assumed and is valid for a large class of source distributions. A series of possible explanations for the discrepancy between the observed experimental data and the theoretical predictions of the equilibrium picture are considered, and shown to be unlikely. Faced with this dilemma, attention is turned to a simple nonequilibrium model, consisting of one fairly close single source superimposed on a general background. This picture was chosen because it seemed the simplest first step from equilibrium. The calculations show that this picture leads to considerably better agreement with the experimental data, especially on the ratio of light to medium nuclei as a function of energy/nucleon. At least one irreconcilable experimental datum remains, namely, the one measurement of the fluorine-to-oxygen ratio.

• Received 1 July 1968

DOI:https://doi.org/10.1103/PhysRev.175.1564