Gamma decay of the ¹⁵⁴Sm Isovector Giant Dipole Resonance: Smekal-Raman Scattering as a Novel Probe of Nuclear Ground-State Deformation

The γ decays of the Isovector Giant Dipole Resonance (GDR) of the deformed nucleus ¹⁵⁴Sm from 2⁺₁-Smekal-Raman and elastic scattering were measured using linearly polarized, quasi-monochromatic photon beams. The two scattering processes were disentangled through their distinct angular distributions. Their branching ratio and cross sections were determined at six excitation energies covering the ¹⁵⁴Sm GDR. Both agree with the predictions of the geometrical model for the GDR and establish γ decay as an observable sensitive to the structure of the resonance. Consequently, the data place strong constraints on the nuclear shape, including the degree of triaxiality. The derived ¹⁵⁴Sm shape parameters β=0.2926(26) and γ=5.0(14) agree well with other measurements and recent Monte Carlo Shell-Model calculations.