TABLE OF CONTENTS Many processes can occur when a neutron strikes an atom. Neutrons carry no charge and their interaction with electrons is negligible, but when an incident neutron comes close enough to the very small atomic nucleus to transfer some kinetic energy to the nucleus as a whole, the nucleus moves but has no internal excitation. This process is called nuclear elastic scattering and may be visualized as billiard balls colliding. The energy of nuclear motion eventually dissipates as heat.
More typically the neutron may be absorbed by the target nucleus that is then excited by the transfer of kinetic energy and the released binding energy of the incident particle, and a new heavier nucleus is formed, at least briefly. The highly excited intermediate nucleus can nearly instantaneously de-excite in several ways. It can emit electromagnetic radiation in the form of gamma rays. These gamma rays are called prompt to distinguish this mechanism from gamma emission following radioactive decay. This process is called radiative neutron capture. If the nuclear excitation energy is concentrated on a few nucleons (i.e., neutrons and protons), either by direct collision with the incident particle or by statistical processes, nucleons can be ejected. If the total excitation energy of the intermediate state is greater than the binding energy of nucleons within the nucleus, nucleon emission is favored. If not, simple prompt gamma emission returns the new isotope nucleus to a more stable state. This process of conversion...
