On collective dynamics in low-energy nucleus-nucleus collisions: Nuclear elastoplasticity

Collective nuclear dynamics at velocities small compared to the Fermi velocity is briefly reviewed. For finite collective velocities where the adiabatic approximation is no longer valid, a qualitatively new feature of collective nuclear motion is predicted: elastoplasticity. For finite Fermi systems this dynamical behaviour results from a coherent coupling between collective and intrinsic degrees of freedom and subsequent equilibration by residual two-body collisions. Within a non-markovian transport-theoretical approach, the elastic response is described by scaling of the diabatic single-particle wave functions according to the collective deformation, while two-body dissipation is accounted for by a relaxation equation. This dissipative diabatic dynamics ascribes elastoplasticity to nuclei and establishes a link between time-dependent Hartree-Fock and markovian transport theories. Isoscalar giant quadrupole vibrations of nuclei and mass diffusion in nucleus-nucleus collisions are considered as well-established examples for the elastic and plastic limits, respectively, of elastoplasticity. The transition region may be explored in central nucleus-nucleus collisions. Observable effects from the approach phase, like the hindrance of fusion reactions and the diabatic emission of nucleons are discussed. Further signatures of nuclear elastoplasticity are pointed out. (orig.)