Direct proof of the two-phonon character of the dipole excitations inNd142andSm144around 3.5 MeV

Abstract

Resonant inelastic proton scattering experiments were carried out to investigate the $\gamma$ decay of the $1^{-}$ state at an excitation energy of around 3.5 MeV in the $N=82\mathrm{}$ nuclei ${}_{}{}^{142}{}_{}{}^{}\mathrm{Nd}$ and ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$. $\gamma$-ray branching ratios were determined from $\gamma$-proton coincidences in both nuclei. The newly observed $\gamma$ decay of the dipole excitation at 3225 keV in ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ to the one-phonon states, in particular the ($1^{-}$ → $3_1^{-}$) $E2\mathrm{}$ transition to the octupole phonon state, is the first direct proof of the two-phonon character of the 1${}_{}{}^{-}{}_{}{}^{}\mathrm{state}$ in any $N=82\mathrm{}$ nucleus.