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Karl-Heinz Schmidt
Dissipation phenomena in nuclear fission In the hybrid nuclear model one distinguishes between intrinsic and collective degrees of freedom. While intrinsic motion is essentially based on states of individual nucleons in the nuclear potential, collective motion is a coordinated motion of great part or all nucleons. Intrinsic excitations can be understood as heating. Examples of collective motion are rotations and vibrations. Also the elongation and the necking in of a fissioning nucleus is a collective motion. One of the basic properties of nuclear matter is the viscosity. It describes the coupling between intrinsic and collective degrees of freedom. This coupling can be realised in quite different ways. In a thin gas, collisions between molecules are seldom. The mean free path of the molecules is large. The viscosity is generated by collisions of individual molecules with the walls of the container. In a viscous liquid, collisions between molecules are very frequent. The mean free path of the molecules is small. The viscosity is generated by friction inside the liquid. Honey is an example of a very viscous liquid. In nucleus, both processes, collisions with the "container" (the nuclear potential) and collisions with other nucleons (friction) play a role. In the nucleus, the situation is complicated by quantum-mechanical phenomena. The nucleons as Fermions fill the nuclear potential up to a certain level, the Fermi energy, because each nucleon can only occupy one state. In an intrinsic excitation of moderate energy, only nucleons near the Fermi surface are involved; nucleons in deep-lying states cannot be excited. Such collisions are "Pauli blocked". This tends to reduce the nuclear viscosity. With increasing temperature, more and more nucleons are involved in intrinsic excitations and consequently the viscosity is expected to increase. At very low temperatures, the nucleus becomes superfluid due to pairing correlations, and the viscosity is expected to decrease considerably. Since fission corresponds to a typical large-scale motion process, it has been recognised as one of the most promising tools to investigate the nuclear viscosity. To this goal, a new experimental approach has been developed at GSI to investigate nuclear viscosity at small deformations based on measuring the nuclide distributions of fission products emerging from peripheral fragmentation reactions in inverse kinematics, and two new signatures of that phenomenon have been introduced. The systematic analysis of the large amount of data available at GSI, ranging from low to large fissilities, and from low to high excitation energies will lead to an improvement in the present knowledge on the viscosity of nuclear matter.
Publications: Jurado et al. (2003/2004) |
