PROBING THE GIANT-DIPOLE RESONANCE IN THE COMPOUND NUCLEUS ER-156

The giant dipole resonance (GDR) from the decay of excited Er-156 nucl ei, populated in the reaction 241 MeV Ni-64 + Zr-92 has been studied e mploying a high-resolution 4 pi gamma-ray spectrometer. High-energy ga mma-ray spectra have been measured in coincidence with both the spectr ometer inner ball and the Ge detectors selecting discrete gamma-transi tions in the residual nuclei Er-155,Er-154. The analysis of the high-e nergy gamma-ray spectra shows that the GDR can be reproduced by a sing le-lorentzian fit using standard parameters for strength, energy and w idth, only when a fold k > 10 is required in the inner ball. On the co ntrary, with lower-fold conditions (k less than or equal to 10) unreal istic GDR parameters are obtained, mainly due to the presence of light target contaminants. The GDR width determined for k > 10 compares wel l with systematics and with predictions of adiabatic calculations. The line-shape analysis of the gamma-ray spectrum associated with a singl e evaporation residue (Er-154) yields GDR parameters similar to those obtained for the high-fold selected spectra, with the exception of a s lightly larger width. No signature is therefore observed for a pre-equ ilibrium component in the decay of Er-156 populated in a nearly symmet ric reaction. Nevertheless, differences are evidenced in the compariso n of the present spectrum from the Ni-64 + Zr-92 reaction with earlier results from the C-12 + Sm-144 asymmetric system.