Kinetic-theory description of isoscalar dipole modes

A semiclassical model, based on a solution of the Vlasov equation for finite systems with moving-surface, is employed to study the isoscalar dipole modes in nuclei. It is shown that, by taking into account the surface degree of freedom, it is possible to obtain an exact treatment of the centre of mass motion. It is also shown that a method often used to subtract the spurious strength in RPA calculations does not always give the correct result. An alternative analytical formula for the intrinsic strength function is derived in a simple confined-Fermi-gas model. In this model the intrinsic isoscalar dipole strength displays essentially a two-resonance structure, hence there are two relevant modes. The interaction between nucleons couples these two modes and changes the compressibility of the system. The evolution of the strength profile is then studied as a function of the compressibility of the nuclear fluid. Comparison with available data favours values of the incompressibility parameter of nuclear matter smaller than those suggested by the analysis of the monopole ``breathing'' mode.Comment: 17 pages, 4 figures, revised version to be published in Nucl. Phys.

Unified semiclassical approach to isoscalar collective modes in heavy nuclei

A semiclassical model based on the solution of the Vlasov equation for finite systems with a sharp moving surface has been used to study the isoscalar quadrupole and octupole collective modes in heavy spherical nuclei. Within this model, a unified description of both low-energy surface modes and higher-energy giant resonances has been achieved by introducing a coupling between surface vibrations and the motion of single nucleons. Analytical expressions for the collective response functions of different multipolarity can be derived by using a separable approximation for the residual interaction between nucleons. The response functions obtained in this way give a good qualitative description of the quadrupole and octupole response in heavy nuclei. Although shell effects are not explicitly included in the theory, our semiclassical response functions are very similar to the quantum ones. This happens because of the well known close relation between classical trajectories and shell structure. The role played by particular nucleon trajectories and their connection with various features of the nuclear response is displayed most clearly in the present approach, we discuss in some detail the damping of low-energy octupole vibrations and give an explicit expression showing that only nucleons moving on triangular orbits can contribute to this damping.Comment: 9 pages, 2 figures, Talk presented at the 8th International Spring Seminar on Nuclear Physics on Key Topics in Nuclear Structure, Paestum, Italy, May 23-27, 200

Dynamic effects of nuclear surface in isoscalar dipole modes

Dynamic surface effects in the isoscalar dipole modes of heavy nuclei are studied within a semiclassical model based on the solution of the Vlasov kinetic equation for finite Fermi systems with a moving surface. In order to clarify the role of dynamic surface effects we have considered an approximate solution, which takes into account only the part of the variation of the phase-space distribution function caused by the dynamic surface (the dynamic-surface approximation). It is shown that the dynamic surface effects have an essential influence on the features of the isoscalar dipole modes. The isoscalar dipole strength function has a two-resonance structure already in the dynamic-surface approximation, and the centroid energies of both the low-energy resonance and the high-energy resonance are close to corresponding centroid energies of exact strength function. Calculations of the velocity field in the dynamic-surface approximation show the vortex character of the low-energy isoscalar dipole resonance and the compression character of the high-energy one.Comment: 15 pages, 4 figures, typos correc

Experimental study on grooved Si and Ge crystals for Laue lens application

An experimental study on the method of indentations for bent crystals to realize a hard X-ray Laue lens has been done. We tested the diffraction properties of indented Si and Ge crystalline plates at European Synchrotron Radiation Facility (Grenoble, France). The samples were analyzed by diffraction of their (111) planes with hard X-rays from 150 to 600 keV. Crystals have shown significantly high diffraction efficiency, i.e. a Si crystal has exhibited up to 80% at 300 keV. A Ge crystal has confirmed the observation for a Si one, though the diffraction efficiency was about 60%. In both cases rocking curves showed flat-toped rectangular shapes, which demonstrates that the method of indentations evenly bends the crystals. Moreover, measured angular spread was always very close to the morphological curvature of the sample under investigation, showing that this method offers high reproducibility and, thus, easy control of diffraction properties of the crystals