Astroparticle Physics

78 6 Primary Cosmic Rays6.1 Charged Componentof Primary Cosmic Rays“Coming out of space and incident onthe high atmosphere, there is a thin rainof charged particles known as primarycosmic rays.”C. F. PowellFig. 6.1Elemental abundance of primarycosmic rays for 1 ≤ Z ≤ 28The elemental abundance of primary cosmic rays is shownin Figs. 6.1 and 6.2 in comparison to the chemical compositionof the solar system. Protons are the dominant particlespecies (≈ 85%) followed by α particles (≈ 12%). Elementswith a nuclear charge Z ≥ 3 represent only a 3%fraction of charged primary cosmic rays. The chemical compositionof the solar system, shown in Figs. 6.1 and 6.2, hasmany features in common with that of cosmic rays. However,remarkable differences are observed for lithium, beryllium,and boron (Z = 3–5), and for the elements below theiron group (Z