TABLE OF CONTENTS The difference between direct and compound nuclear reactions has been discussed. In the direct reaction, the incident particle interacts with the target nucleus only several times at most. Therefore the change produced in the target must be a simple rearrangement of one or several nucleons or collective degrees of freedom. Because of the conservation laws, the change produced in the nucleus must be reflected in the energy and angular momentum carried by the outgoing particle.
The spectrum of protons emitted at a fixed angle was illustrated in Fig. 1.1. If the counter is moved successively to various angles, and the cross section to each energy group is recorded, then the angular distribution or differential cross section to specific states is thereby measured. The angular distribution of particles that have produced a specific nuclear level reflects the angular momentum and parity that was exchanged between the scattered particle and the nucleus in making the transition. This is because the particle must scatter in such a way as to obey the conservation laws, including angular momentum, and this imposes a restriction on the directions which it can scatter.
To illustrate how this comes about in a more quantitative way, let us consider the plane-wave Born approximation. The use of plane waves to describe low-energy reactions is not considered accurate enough to be reliable for most direct reactions, but the corrections introduce quantitative rather than qualitative changes.
Consider a direct reaction such as the (p, d) reaction, referred to as pickup, and...
