Probing the giant dipole resonance in the compound nucleus Er-156

The giant dipole resonance (GDR) from the decay of excited Er-156 nuclei, populated in the reaction 241 MeV Ni-64 + Zr-92 has been studied employing a high-resolution 4 pi gamma-ray spectrometer. High-energy gamma-ray spectra have been measured in coincidence with both the spectrometer inner ball and the Ge detectors selecting discrete gamma-transitions in the residual nuclei Er-155,Er-154. The analysis of the high-energy gamma-ray spectra shows that the GDR can be reproduced by a single-lorentzian fit using standard parameters for strength, energy and width, only when a fold k > 10 is required in the inner ball. On the contrary, with lower-fold conditions (k less than or equal to 10) unrealistic GDR parameters are obtained, mainly due to the presence of light target contaminants. The GDR width determined for k > 10 compares well with systematics and with predictions of adiabatic calculations. The line-shape analysis of the gamma-ray spectrum associated with a single evaporation residue (Er-154) yields GDR parameters similar to those obtained for the high-fold selected spectra, with the exception of a slightly larger width. No signature is therefore observed for a pre-equilibrium component in the decay of Er-156 populated in a nearly symmetric reaction. Nevertheless, differences are evidenced in the comparison of the present spectrum from the Ni-64 + Zr-92 reaction with earlier results from the C-12 + Sm-144 asymmetric system.