Intermediate Structure and Particle-Hole States in Nuclear Reactions

The effects of nuclear two-particle-one-hole (2p-1h) states on the energy-averaged cross section are studied to see when they cause observable fluctuations. This is done by using the self-consistent perturbation methods of Brueckner to derive an optical potential for the elastic scattering of neutrons. It is then shown that for the case where the interaction between different 2p-1h states can be neglected and the coupling to more complicated configurations is small, the scattering matrix can be approximated to second order in the interaction by a slowly varying background term plus a resonant term. The width of this resonant term is shown to be equal to the escape width plus a spreading width caused by the coupling to the more complicated configurations. These widths are calculated for a number of 2p-1h configurations in various spherical nuclei in order to determine when the isolated resonance condition ($\Gamma <D$) is satisfied. It is found that for the light and doubly magic nuclei, individual resonances of this particular type should be observable only for incident neutron energies below 2-3 MeV. At higher energies, the density of 2p-1h states and the widths are too large for the occurrence of individual resonances. In the heavier nuclei ($A\gtrsim 60$) with partially filled shells, the 2p-1h states are broadened so much by the decay to the more complicated states that individual 2p-1h states cause no observable effects. The possibliity of other types of states causing observable fluctuations is not considered.