2 00 3 Octupole response and stability of spherical shape in heavy nuclei

The isoscalar octupole response of a heavy spherical nucleus is analyzed in a semiclassical model based on the linearized Vlasov equation. The octupole strength function is evaluated with different degrees of approximation. The zero-order fixed-surface response displays a remarkable concentration of strength in the 1h̄ω and 3h̄ω regions, in excellent agreement with the quantum single-particle response. The collective fixed-surface response reproduces both the highand low-energy octupole rsonances, but not the low-lying 3− collective states, while the moving-surface response function gives a good qualitative description of all the main features of the octupole response in heavy nuclei. The role of triangular nucleon orbits, that have been related to a possible instability of the spherical shape with respect to octupole-type deformations, is discussed within this model. It is found that, rather than creating instability, the triangular trajectories are the only classical orbits contributing to the damping of low-energy octupole excitations. PACS: 24.10.Cn, 24.30.Cz