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 64Ni + 92Zr, 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 64Ni + 92Zr reaction with earlier results from the 12C + 144Sm asymmetric system.