Onset of quenching of the giant dipole resonance at high excitation energies

The evolution of the giant dipole resonance (GDR) properties in nuclei of mass A = 120 to 132 has been investigated in an excitation energy range between 150 and 270 MeV through the study of complete and nearly complete fusion reactions using (11)6Sn beams at 17A and 23A MeV from the cyclotron of the Laboratorio Nazionale del Sud impinging on C-12 and Mg-24 targets. gamma rays and light charged particles were detected using the multi-element detector array MEDEA in coincidence with evaporation residues detected by using mass and charge identification spectrometry with telescope (MACISTE). Light-charged-particle energy spectra were analyzed within the framework of a multiple-source-emission scenario by using a fitting procedure to determine the amount of pre-equilibrium emission and deduce the excitation energies reached in the compound nuclei. A detailed analysis of the gamma-ray spectra and their comparison with statistical model calculations is presented. Evidence of a quenching of the GDR gamma yield was found at 270 MeV excitation energy. The quenching effect becomes progressively more important with increasing excitation energy, as observed when the comparison is extended to data from the reaction Ar-36 + Mo-96 at 37A MeV where hot nuclei were populated up to 430 MeV excitation energy. A coherent scenario emerges indicating the existence of a limiting excitation energy for the collective motion of about E*/A = 2.1 MeV for systems of mass A = 105 to 111 while a slightly lower value was observed for nuclei of mass A similar to 132. The existence of a possible link between GDR disappearance and the liquid-gas phase transition is discussed.