Probing the giant dipole resonance in the compound nucleus 156Er

The giant dipole resonance (GDR) from the decay of excited 156Er nuclei, populated in the reaction 241 MeV View the MathML source, has been studied employing a high-resolution 4π γ-ray spectrometer. High-energy γ-ray spectra have been measured in coincidence with both the spectrometer inner ball and the Ge detectors selecting discrete γ-transitions in the residual nuclei 155,154Er. The analysis of the high-energy γ-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 ≤ 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 γ-ray spectrum associated with a single evaporation residue (154Er) 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 156Er populated in a nearly symmetric reaction. Nevertheless, differences are evidenced in the comparison of the present spectrum from the View the MathML source reaction with earlier results from the View the MathML source asymmetric system.