Progress in the study of the gamma decay of the Giant Dipole Resonance in very hot rotating nuclei

The γ decay of the Giant Dipole Resonance in 132 Ce nuclei has been measured in the compound temperature range 2.5 - 4.1 MeV using a symmetric reaction 64 Ni (300, 400, 500 MeV) + 68 Zn and an asymmetric one 16 O (130, 250 MeV) + 116 Sn. Both cases produce 132Ce compound nuclei at the same excitation energy and angular momentum. Light charged particles and evaporation residues have been measured in coincidence with high energy γ-rays. The data show that pre-equilibrium emission is present only in the asymmetric reaction while only statistical emission from a fully equilibrated 132 Ce compound is present in the symmetric one. The measured Giant Dipole Resonance widths show an almost linear increase with the effective temperature. Such a measured trend is reproduced by the thermal fluctuation model which includes the lifetime of the compound nucleus. The comparison between the high energy γ-ray spectra in the symmetric and asymmetric reactions shows a dynamic dipole emission in the 16 O-induced reaction. The yield is consistent with what has been previously measured in the same mass and energy range