Dissipative dynamics of the nuclear surface in the sd shell

The dynamics of the nuclear surfaces in the hot nuclei is a problem of great interest for heavy ion reactions at low and intermediate energies. One of the most important effects in this case is the dissipation which can be considered phenomenologically in a model with moving walls reservoir filled with gas at low pressure. Although, in the nuclear case this dissipation cannot be described realistically because of the short range interactions as for instance the pairing interaction which can play an important role. For the study of the temperature effects, pairing and dissipation in the dynamics of the nuclear surface, we have considered the case of the isoscalar quadrupole giant resonance in 28Si. The dynamical equations are of the 'Langevin' type, containing the phenomenological terms of noise and friction produced by the coupling with the thermal bath. If one uses a linear coupling with the quadrupole momentum, the numerical solution of the motion equation at t = 0 show that the friction term cannot lead to a complete energy dissipation. The friction may be related to the resonance width and it can have an important influence on the energy decay of the quadrupole vibration but a weak one on the pairing energy. Calculations at a temperature T = 2 MeV show that the quadrupolar motion is thermalized during a time interval of 1 h/MeV, and also that the pairing term may lead to important changes in the occupation numbers for the valence nucleons during the relaxation process