6 - Nuclear reactions

Summary

Introduction

The study of nuclear reactions provides considerable information on the structure of nuclei as well as on the nature of their interaction. In this chapter there is first a brief discussion of some experimental details, before the general features of nuclear reactions are described. Then methods for describing reactions are developed. First some general predictions are given followed by a discussion of the simplest reactions, elastic and inelastic scattering, for both light and heavy ions. After this the theory of compound nucleus reactions is presented and the occurrence of slow neutron resonances described. Isobaric analogue resonance and isospin forbidden reactions are then discussed.

At higher incident energies direct reactions become important and the use of first-order perturbation theory, the Born approximation, for the description of pick-up and stripping reactions by both light and heavy ions is described. For heavy-ion direct reactions new features are seen reflecting the more classical behaviour of the ions: selectivity arising through kinematic matching and characteristic bell-shaped angular distributions. At energies above the Coulomb barrier, as well as compound nucleus (fusion) reactions, deep-inelastic reactions are also observed, with substantial cross-sections for very heavy ions. In these reactions there is a considerable transfer of the incident energy to internal excitation of the product nuclei. For even higher energies, a new phase of nuclear matter, a quark-gluon plasma, is predicted to be found in reactions with relativistic heavy ions, and it is in this area that nuclear and particle physics overlap.