In the next three chapters we discuss propagation of cosmic rays in space and production of secondary particles that trace the propagation. We begin in this chapter with a description of the Milky Way galaxy and propagation of primary and secondary nuclei. We also discuss production of secondary positrons and antiprotons. In Chapter 10 we discuss propagation of cosmic rays in intergalactic space, which is essential for understanding the highest energy portion of the cosmic ray spectrum. Then in Chapter 11 we discuss production of gamma rays and neutrinos and how they reflect the propagation of cosmic rays in space.

Two of the most important facts with implications for origin of cosmic rays were already mentioned in Chapter 1:

1. From the ratio of primary to secondary nuclei it can be inferred that, on average, cosmic rays in the GeV range traverse 5–10 g/cm2 equivalent of hydrogen between injection and observation.

2. This effective grammage decreases as energy increases, at least as far as observations extend, as illustrated by the decreasing intensity of the secondary nucleus boron in Figure 1.4 compared to primary nuclei such as carbon and oxygen.

Since the thickness of the disk of the Galaxy is about 10−3 g/cm2, (1) implies that cosmic rays travel distances thousands of times greater than the thickness of the disk during their lifetimes. This suggests diffusion in a containment volume that includes the disk of the Galaxy. The fact that the amount of matter traversed decreases with energy suggests that higher-energy cosmic rays spend less time in the Galaxy than lower-energy ones (although this may be only part of the explanation). It also suggests that cosmic rays are accelerated before most propagation occurs. If, on the contrary, acceleration and propagation occurred together, one would expect a constant ratio of secondary/primary cosmic rays – or even an increasing ratio for some stochastic mechanisms in which it takes longer to accelerate particles to higher energy.

Acceleration and transport of cosmic rays are nevertheless very closely related, particularly in the theory of shock acceleration by supernova blast waves. In that case, diffusive scattering of particles by irregularities in the magnetic field plays a crucial role for acceleration as well as for propagation.