Introduction

The light elements (D to B, apart from 4He) have such fragile nuclei (see Table 9.1) that they tend to be destroyed, rather than created, in thermonuclear burning, although certain special processes can lead to stellar production of 3He, 7Li and 11B.

B2FH accordingly postulated for their creation an ‘x’-process involving spallation by fast particles at high energy, but low temperature and density. They considered stellar flares and supernova shells as possible sites, while also envisaging the possibility of 7Li creation in H-free helium zones in stars. Since then, it has been accepted that all D, some 3He and some 7Li come from the Big Bang (see Chapter 4), where rapid expansion and cooling allow traces of these elements to be preserved; and that a significant clue to the origin of Li, Be and B comes from their relative overabundance (by factors of 104 to 105) in Galactic cosmic rays (Fig. 9.1).

Sketch of cosmic-ray physics

Cosmic rays reaching the ground are secondary particles resulting from the impact of primary cosmic rays coming mainly from the Galaxy. The latter are mostly protons and α-particles with a sprinkling of heavier nuclei, coming in with a broad distribution of energies. The most energetic, with energies up to 1020 eV or so, are quite rare but are detected occasionally in the form of extensive air showers.