In this chapter we consider the passage of energetic particles through matter. Nuclear reactions usually result in the production of such particles: α-particles, electrons, photons, nucleons, fission fragments, or whatever. In passing through matter, an energetic particle loses its energy, ultimately largely into ionisation. The instruments of nuclear physics are designed to detect and measure this deposited energy, and so it is upon these processes that our knowledge of nuclear physics rests.

The subject is also basic to an understanding of the biological effects of energetic particles, since a living cell can be damaged by the ionisation. This can be of positive benefit, as in the destruction of malignant tissue in cancer treatment, or a danger from which, for example, workers in the nuclear power industry must be shielded. Shielding calculations also depend on the physical principles set out in this chapter.

We limit the discussion to particles with kinetic energies up to around 10 MeV, in line with the nuclear physics described in Chapters 4–12. It is intended to give the reader a qualitative comprehension, rather than a compendium of the most accurate formulae and data available for quantitative work.

Charged particles

We consider first the passage of charged particles, such as protons and α-particles, through gases. For charged particles of energy < 10 MeV, the dominant mechanism for energy loss is the excitation or ionisation of the atoms (or molecules) of the gas: electrons being excited to higher bound energy levels in the atom, or detached completely.