TABLE OF CONTENTS The discussion of the plane-wave Born matrix element for a direct reaction taught us that these reactions are sensitive to the spins and parities of the nuclear levels and to the radial distribution of particles in the nucleus. Although the point has not been stressed so far, the whole discussion also implies that a careful analysis of these reactions must yield information on the degree to which particular levels are simple shell-model states and the degree to which they have more complicated structure. In addition, we shall later discuss reactions in which several nucleons are transferred and see how they depend very sensitively on the correlation of nucleons in the nucleus. For these reasons, direct reactions have proven to be the principal means by which the detailed microscopic structure of nuclei can be discovered. This provides us with the motivation for improving and developing the theory in the many ways that will be discussed in later chapters. In this chapter we begin by considering some of the useful but more formal results of scattering theory.
The nucleus is a complicated A-particle system for which the exact wave function is unavailable. However, it has been shown (and known for an even longer time) that the properties of the nucleon-nucleon interaction, namely its short-range repulsion and relatively weak attraction, taken together with the Pauli exclusion principle, conspire in such a way as to make a nuclear shell model a fairly good approximation for some purposes.
