Damping of nuclear excitations

TLDR

Nuclear excitations of particle and vibrational character exhibit damping through fragmentation of single‑particle levels, neutron strength function systematics, and elastic scattering absorption, with a close link between single‑particle and collective vibrational damping. The authors review the theory and empirical evidence for damping of simple nuclear excitations. They summarize collective vibrational states and compare them with theory, presenting a successful model in which simple excitations mix with surface vibrations to produce damping. The model shows surface damping dominates below ~15 MeV, vibrational damping is suppressed by particle–hole coherence, underpredicts spreading widths by up to a factor of two, and suggests additional, poorly understood degrees of freedom contribute to damping.

Abstract

The authors review the theory and the empirical evidence of damping of simple nuclear excitations. The excitations considered are the particle states and vibrational states. The particle damping phenomena include the fragmentation of single-particle levels, the systematics of neutron strength functions, and the optical absorption of elastic scattering. Information on the known collective vibrational states is summarized and compared with theory. A theoretical model that has found considerable success is based on a damping mechanism in which the simple excitations mix with the surface vibrations. This implies that the surface damping dominates for excitation energies below about 15 MeV. There is a close relation between the single-particle damping and the damping of collective vibrations. However, the vibrational damping is strongly suppressed by the coherence between the particle and the hole. While the model reproduces many of the observed features of the data rather well, it tends to underpredict the spreading width by as much as a factor of 2. Thus other degrees of freedom, not well understood at present, may play a role in the damping.