EVOLUTION OF THE GIANT-DIPOLE RESONANCE IN EXCITED SN-120 AND PB-208 NUCLEI POPULATED BY INELASTIC ALPHA-SCATTERING

The evolution of the giant dipole resonance (GDR) in Sn-120 and Pb-208 nuclei at excitation energies in the range of 30-130 MeV and 40-110 M eV, respectively, were studied by measuring high energy gamma rays fro m the decay of the resonance. The excited states were populated by ine lastic scattering of alpha particles at beam energies of 40 and 50 MeV /nucleon for Sn-120 and 40 MeV/nucleon for Pb-208. A systematic increa se of the resonance width with increasing excitation energy was observ ed for both nuclei. The observed width evolution was compared to calcu lations employing a model that adiabatically couples the collective ex citation to the nuclear shape, and to a model based on the collisional damping of nucleons. The adiabatic coupling model described the width evolution in both nuclei well, whereas the collisional damping calcul ation could describe the width evolution only in Pb-208. Light-particl e inelastic scattering populates low angular momentum states in the ta rget nucleus. The observed width increase is therefore interpreted to be predominantly due to fluctuations in the nuclear shape induced by t emperature. This interpretation is consistent with the adiabatic model calculations and with recent angular momentum-gated measurements of t he GDR in excited Sn isotopes. (C) 1998 Published by Elsevier Science B.V.