RELATION BETWEEN DAMPING OF THE HOT GIANT-DIPOLE RESONANCE AND THE COMPLEX ADMITTANCE OF AN IRREVERSIBLE PROCESS

It is shown that the complex admittance, which describes the dissipati on of the giant dipole resonance (GDR) in hot nuclei, can be derived f rom the microscopic double-time Green function for the propagation of the GDR. The damping width of the GDR is calculated directly from the complex admittance without explicitly solving the equation for the pol es of the Green function. Using this method, a systematic study of the width of the GDR as a function of temperature T is carried out in Sn- 120 and Pb-208. Th, quantal width, caused by the coupling to ph config urations decreases slowly with increasing T. The thermal width, caused by the coupling to pp and hh configurations at T not equal 0, increas es sharply at low temperatures up to T similar to 3 MeV, and slowly at high temperatures, where it reaches a saturation in the region of T> 3-4 MeV. The calculated values of the total damping width of the GDR a re found in reasonable agreement with the experimental data in heavy-i on fusion reactions and inelastic alpha scattering. The mechanism of t he ''disappearance'' of the GDR at high temperatures is analyzed. The evidence of motional narrowing in the hot GDR is investigated.