Learning about the structure of giant resonances from their γ decay

The direct γ decays of the giant dipole resonance (GDR) and the giant quadrupole resonance (GQR) of Pb208 to low-lying states are investigated by means of a microscopic self-consistent model. The model considers effects beyond the linear response approximation. The experimental γ-decay widths from GQR to the first 3- state and the low-lying dipole states are well reproduced with the Skyrme interaction SkP. The strong sensitivity of γ decay to the isospin of the involved states is proven. By comparing the decay widths between GDR to 21+ and GQR to 31-, a much larger weight of the 31- component in the GQR wave function of Pb208 is deduced, with respect to the weight of the 21+ component in the GDR wave function. Thus, we have shown that γ decay is a unique probe of the resonance wave functions, and a test ground for nuclear structure models.