Observation of a Low-Energy Octupole Resonance in Medium-Mass Nuclei

A giant-resonance-like structure at $E_{\mathrm{exc}}\sim \frac{32}{A^{\frac{1}{3}}}$ MeV is observed in ($\alpha ,{\alpha }^{\prime }$) spectra in nuclei from ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ to ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$. Analysis of the angular distributions leads to an assignment of $J^{\pi }=3^{-}$ and an energy-weighted sum-rule fraction of 16-22% for this structure. Comparison with random-phase-approximation calculations shows that $\sim \frac{2}{3}$ of the expected $1\hslash \omega$ octupole energy-weighted sum-rule strength is found at $\sim \frac{32}{A^{\frac{1}{3}}}$ MeV; the remainder occurs in low-lying $3^{-}$ states.