In writing this text we were concerned to assert the continuing importance of nuclear physics in an undergraduate physics course. We set the subject in the context of current notions of particle physics. Our treatment of these ideas, in Chapters 1 to 3, is descriptive, but it provides a unifying foundation for the rest of the book. Chapter 12, on β-decay, returns to the basic theory. It also seems to us important that a core course should include some account of the applications of nuclear physics in controlled fission and fusion, and should exemplify the role of nuclear physics in astrophysics. Three chapters are devoted to these subjects.

Experimental techniques are not described in detail. It is impossible in a short text to do justice to the ingenuity of the experimental scientist, from the early discoveries in radioactivity to the sophisticated experiments of today. However, experimental data are stressed throughout: we hope that the interdependence of advances in experiment and theory is apparent to the reader.

We have by and large restricted the discussion of processes involving nuclear excitation and nuclear reactions to energies less than about 10 MeV. Even with this restriction there is such a richness and diversity of phenomena that it can be difficult for a beginner to grasp the underlying principles. We have therefore placed great emphasis on a few simple theoretical models that provide a successful description and understanding of the properties of nuclei at low energies.

Nuclear potential wells

In the last chapter, we set out a semi-empirical theory for the binding energy of an atomic nucleus, and quantum-mechanical considerations came in only rather indirectly. Experimental atomic masses show deviations from the smooth curve given by the semi-empirical mass formula, deviations which we said were of quantum-mechanical origin. Since a nucleus in its ground state is a quantum system of finite size, it has angular momentum J, with quantum number j which is some integral multiple of ½. If j ≠ 0 the nucleus will have a magnetic dipole moment, and it may have an electric quadrupole moment as well.

The nuclear angular momentum and magnetic moment manifest themselves most immediately in atomic spectroscopy, where the interaction between the nuclear magnetic moment and the electron magnetic moments gives rise to the hyperfine structures of the electronic energy levels. In favourable cases both j and the magnetic moment may be deduced from this hyperfine splitting.

The observed values of nuclear angular momenta give strong support to the validity of a simple quantum-mechanical model of the nucleus: the nuclear shell model. In this model, each neutron moves independently in a common potential well that is the spherical average of the nuclear potential produced by all the other nucleons, and each proton moves independently in a common potential well that is the spherical average of the nuclear potential of all the other nucleons, together with the Coulomb potential of the other protons.

Life on Earth has evolved and is sustained by the light and heat of the Sun. In addition to this essential and almost entirely benign flux of electromagnetic energy, living organisms have always been subject to the hazards of natural ionising radiation. In the twentieth century man's activities added somewhat to these hazards. On the other hand, ionising radiation is used to great advantage in industry and for diagnostic and therapeutic purposes in medicine, and nuclear power is not without its benefits. The interaction between ionising radiation and living tissue is therefore a matter of great interest and importance.

Ionising radiation and biological damage

The basic unit of living tissue is the cell. Cells are complex structures enclosed by a surface membrane. A cell has a central nucleus. This contains DNA (deoxyribonucleic acid) molecules, which code the structure, function, and replication of the cell. The famous ‘double helix’ of the DNA molecule has a diameter of about 2 nm. About 80% of a cell consists of water.

The induction of cancer or of hereditary disease by low levels of ionising radiation is believed to be related to damage to the DNA molecules. This can happen by direct ionisation of the molecule, or indirectly through ionisation of the water molecules in the cell. The break-up of a water molecule may produce a hydroxyl (OH)- ion that is highly reactive chemically and may attack the DNA molecule.

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.

Accelerators

All accelerators employ electric fields to accelerate stable charged particles (electrons, protons, or heavier ions) to high energies. The simplest machine would be a d.c. high-voltage source (called a Van der Graaff accelerator), which can, however, only achieve beam energies of about 20 MeV. To do better, one has to employ a high frequency a.c. voltage and carefully time a bunch of particles to obtain a succession of accelerating kicks. This is done in the linear accelerator, with a succession of accelerating elements (called drift tubes) in line, or by arranging for the particles to traverse a single (radio-frequency) voltage source repeatedly, as in the cyclic accelerator.

Linear accelerators (linacs)

Figure 11.1 shows a sketch of a proton linac. It consists of an evacuated pipe containing a set of metal drift tubes, with alternate tubes attached to either side of a radio-frequency voltage. The proton (hydrogen ion) source is continuous, but only those protons inside a certain time bunch will be accelerated. Such protons traverse the gap between successive tubes when the field is from left to right, and are inside a tube (therefore in a field-free region) when the voltage changes sign. If the increase in length of each tube along the accelerator is correctly chosen then as the proton velocity increases under acceleration the protons in a bunch receive a continuous acceleration. Typical fields are a few MeV per metre of length. Such proton linacs, reaching energies of 50 MeV or so, are used as injectors for the later stages of cyclic accelerators.

Introduction

As indicated in Chapters 1 and 2, we are faced in nature with several types of fundamental interaction or field between particles. Each field has its distinct characteristics, such as space–time transformation properties (vector, tensor, scalar etc.), a particular set of conservation rules that are obeyed by the interaction and a characteristic coupling constant that determines the magnitude of the collision cross-sections or decay rates mediated by the interaction.

The fact that the strength of the gravitational interaction between two protons, for example, is only 10−38 of their electrical interaction has always been a puzzle and a challenge, and many attempts have been made to understand the interrelation between the different fields. In the last decades, the belief has grown that the strong, weak, electromagnetic and gravitational interactions are but different aspects of a single universal interaction, which would be manifested at some colossally high energy. At the everyday energies met with in laboratory studies in particle physics, it is necessary to assume that this symmetry is badly broken, at these mass or energy scales which are puny relative to the unification energy.

The first successful attempt to unify two apparently different interactions was achieved by Clerk Maxwell in 1865. He showed that electricity and magnetism could be unified into a single theory involving a vector field (the electromagnetic field) interacting between electric charges and currents.