Evolution of the giant dipole resonance in excited 120Sn and 208Pb 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 MeV, respectively, were studied by measuring high energy gamma rays from the decay of the resonance. The excited states were populated by inelastic 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 increase of the resonance width with increasing excitation energy was observed for both nuclei. The observed width evolution was compared to calculations employing a model that adiabatically couples the collective excitation 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 calculation could describe the width evolution only in Pb-208. Light-particle inelastic scattering populates low angular momentum states in the target nucleus. The observed width increase is therefore interpreted to be predominantly due to fluctuations in the nuclear shape induced by temperature. This interpretation is consistent with the adiabatic model calculations and with recent angular momentum-gated measurements of the GDR in excited Sn isotopes. (C) 1998 Published by Elsevier Science B.V.