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We propose a unified theory of nuclear level densities, encompassing both spherical and deformed nuclei, using the static-path approximation.The theory appears numerically tractable for a pairing-plusquadrupole Hamiltonian.

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We show that the damping of nuclear collective excitations in the regime of chaotic intrinsic dynamics is well described by the coupling to doorway states. Subsequent collision processes which eventually lead the system into the compound nucleus eigenstates do not increase the damping width. We also argue that the damping width of collective excitations in nuclei does not increase without bounds as the excitation of the nucleus is increased, but acquires a limiting value. PACS numbers: 24.30.Cz, 21.60.Jz, 24.60.LzThe question concerning the variety of mechanisms at work in the damping of collective motion of many-body systems is a fascinating one, and central in a variety of fields ranging from condensed matter to atomic nuclei. An example is provided by the nuclear isovector giant dipole resonance (GDR), that is, the collective vibration of protons against neutrons. Once excited, the mode will relax its energy and angular momentum either by particle emission or by coupling to the complicated intrinsic motion of the nucleons, the compound nucleus. The relaxation of the resonance is recognized by the width of the associated bell-shaped strength function P(E), which is the probability distribution for finding the resonance at the excitation energy E.