Simple Nuclear Excitations Distributed among Closely Spaced Levels

A review is given of the limited information available about the correct description of simple nuclear excitations when they are mixed with more complicated excitations to form the quasi-stationary highly excited states of nuclei. Examples are given which illustrate the energy region over which a simple excitation appears, the existence of local variations or intermediate structure with a characteristic energy of about 100 keV, and correlations which might exist between two different simple excitations. The types of simple excitations considered are single-particle states, analogue states, and electric dipole excitation. There is little quantitative information about the gross spreading of single-particle strength despite the successes of the optical model and the popularity of transfer reactions. There may be some evidence for intermediate structure in reported variations of neutron scattering cross-sections, but the interpretation is ambiguous. A convincing and educational example of a single-particle ''doorway'' is given by the Monahan and Elwyn analysis of s-wave neutron resonances in Fe. The fragmentation of an analogue state among the T< states is another interesting example of experimentally observable spreading of strength. Additional information on the spreading of strength in highly excited states (e.g. in 90Y) can be obtained by studying the more easily seen corresponding analogues (e.g. in 90Zr). Information on the spreading of electric dipole strength can be obtained directly from gamma-ray interaction cross-sections. The non-statistical neutrons emitted from the giant dipole resonance provide guidance for the theoretical description of the resonance. At energies well below the giant resonance there is evidence for substantial intermediate structure. In the same energy region comparisons between (γ, γ) and (γ, Po) cross-sections are particularly sensitive to correlations. Many more relevant experiments will be needed before the spreading of strength and the correlations between different simple excitations are understood. (author)