Two-Phonon Octupole Excitation inGd147

A half-life measurement gives $(57\mathrm{}\pm 13)B_W$ for the 1575-keV $E3\mathrm{}$ transition in ${}_{}{}^{147}{}_{}{}^{}\mathrm{Gd}$, which characterized the 2.572-MeV ${\left(\frac{19}{2}\right)}^{-}$ level as $\nu f_{\frac{7}{2}}\times 3^{-}\times 3^{-}$ two-phonon octupole state. This is the first identification of a nuclear two-phonon octupole excitation. The observed departures from harmonic vibration (twice the one-phonon energy and twice the transition rate) can be quantitatively understood in terms of the microscopic composition of the states, and can be derived from other observed features of the octupole vibrations in this region.