Giant dipole resonance γ-ray decay in hot nuclei

Properties of the giant dipole resonances (GDR) in hot nuclei have been discussed based on neutron and γ-ray data measured in coincidence with fusion residues produced in the 40Ar + Ni, 92Mo and 122Sn reactions at E = 26 MeV/nucleon. Obtained neutron spectra show that the thermally equilibrated nuclei are produced up to about 500 MeV in excitation energy and can be reproduced with statistical calculations. On the other hand, the γ-ray spectra cannot be explained by statistical γ-ray emission through GDR; yields of the decay γ rays are much less than those predicted by standard statistical calculations. This clearly indicates that there exists the limiting temperature of the nucleus for the GDR γ-ray decay, which was deduced for mass ∼ 70, 105 and 130. The spectra were analyzed with the model including the equilibration time for the GDR. The analysis requires that the GDR disappears for Ex > 300 MeV, although the equilibration time attributes very much to the disappearance of the GDR γ-ray decay in hot nuclei. (orig.)