The two primary types of radiation, galactic cosmic rays (Figures 2.1 and 2.2) and solar radiation (Figure 2.3) vary according to their source and the event producing them. Both are altered substantially in particle type and energy as the primary galactic cosmic and solar particles traverse Earth's atmosphere and the primaries and secondaries are trapped in its magnetosphere as shown in Figure 2.4. The radiation quality likewise changes as the primary and secondary radiation particles traverse the spacecraft or the crew members themselves.1 Figure 2.5 shows the estimated change in the dose equivalent at various depths in different shielding materials in comparison to its value at zero shield thickness. The large changes as a function of depth and material arise because the primary particles not only are attenuated but also produce secondary particles with a range of types of energy. As a result, the radiation quality is modified and so is the equivalent dose. It is the combination of primary particles, attenuated primaries, and secondary particles at the biologically relevant site that determines the biological effects, not the primary spectrum per se.
The major components of galactic cosmic radiation (GCR) are energetic protons and heavier ions with even atomic numbers, which are more abundant than those with odd numbers (see Figure 2.1). The relative abundance of each particle type generally decreases with increasing atomic number, although a significant increase occurs at iron-56 followed by a sharp decrease at higher numbers. There is a broad but consistent distribution in the energy per nucleon, with a peak in abundance in the vicinity of 1 GeV/nucleon (see Figure 2.2).