LIMITING TEMPERATURES FOR COLLECTIVE MOTION IN NUCLEI

The stability of nuclei is a balance between the kinetic energy of the nucleons, the Coulomb repulsion and the nuclear attraction, resulting in the fact that no compound nucleus seems to exist at temperatures above 6-8 MeV. Even at lower temperatures, the competition between single-particle and collective channels through which the compound nucleus cools down is such that the variety of collective modes may not exist at all temperatures. Furthermore, even it the coupling of a collective mode to the compound nucleus is adequate to secure its excitation at a given temperature, the width associated from mechanisms different from compound-nucleus decay may be so large that the mode is overdamped and consequently, not directly observable as a independent mode. In the present paper, information about these temperatures for the case of the giant dipole resonance will be discussed. It will be suggested that the first temperature is controlled by the times it takes the vibration and the neutron motion to couple to the compound nucleus, while the second temperature reflects the relaxation time associated with quadrupole surface deformations.