GIANT-DIPOLE RESONANCE GAMMA-RAY DECAY IN HOT NUCLEI

Properties of the giant dipole resonances (GDR) in hot nuclei have bee n discussed based on neutron and gamma-ray data measured in coincidenc e with fusion residues produced in the Ar-40 + Ni, Mo-92 and Sn-122 re actions at E = 26 MeV/nucleon. Obtained neutron spectra show that the thermally equilibrated nuclei are produced up to about 500 MeV in exci tation energy and can be reproduced with statistical calculations. On the other hand, the gamma-ray spectra cannot be explained by statistic al gamma-ray emission through GDR; yields of the decay gamma rays are much less than those predicted by standard statistical calculations. T his clearly indicates that there exists the limiting temperature of th e nucleus for the GDR gamma-ray decay, which was deduced for mass appr oximately 70, 105 and 130. The spectra were analyzed with the model in cluding the equilibration time for the GDR. The analysis requires that the GDR disappears for E(x) > 300 MeV, although the equilibration tim e attributes very much to the disappearance of the GDR gamma-ray decay in hot nuclei.