Giant dipole resonances in very hot nuclei of mass A 115

Gamma rays, light charged particles, and evaporation residues emitted from hot nuclei formed in the Ar-36+Zr-90 reaction at 27 MeV/nucleon have been measured at the GANIL facility with the 4 pi barium fluoride multidetector MEDEA. The combination of the residue and particle measurements shows that nuclei with masses around 115 and excitation energies between 350 and 550 MeV are produced. The gamma spectra measured in coincidence with the evaporation residues exhibit three components: a low-energy statistical component, a high-energy contribution due to nucleon-nucleon bremsstrahlung during the initial stages of the collision, and a contribution from the decay of the giant dipole resonance built on highly excited states. The characteristics of the bremsstrahlung component are in agreement with previously published systematics. The gamma yield from the decay of the giant dipole resonance remains constant over the excitation energy range studied. A comparison with other experiments shows that the N/Z asymmetry in the entrance channel does not affect the gamma yield. Statistical calculations performed using the code CASCADE and supposing a fixed width and full sum rule strength for the dipole resonance strongly overpredict the data. The hypothesis of a continuously increasing width of the resonance with temperature gives a better agreement with experiment near the centroid of the resonance but overpredicts the gamma spectra at higher energies. The best account of the data is given by assuming a cutoff of gamma emission from the resonance above an excitation energy of approximately 250 MeV. This cutoff is discussed in terms of the rime necessary to equilibrate the dipole oscillations with the hot compound nucleus. Finally, some evidence is given for a possible new low-energy component of the dipole strength at very high temperatures.