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

Gamma decays of the isovector giant dipole resonance (GDR) of the deformed nucleus ${}^{154}$Sm from $2_1^{+}$-Smekal-Raman and elastic scattering were measured using linearly polarized, quasimonochromatic 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 ${}^{154}$Sm GDR. Both agree with the predictions of the geometrical model for the GDR and establish $\gamma$ 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 ${}^{154}$Sm shape parameters $\beta =0.2926(26)$ and $\gamma =5.0(14)$ agree well with other measurements and recent Monte Carlo Shell-Model calculations.