Two- to one-phonon E3 transition strength in ${}_{}{}^{148}{}_{}{}^{}\mathrm{Gd}$

In a plunger experiment the mean life of the (ν${\mathit{f}}_6^2$×$3^{\mathrm{-}}$×$3^{\mathrm{-}}$)${12}^{+}$ state at 3.981 MeV in ${}_{64}{}^{148}{}_{}{}^{}\mathrm{Gd}_{84}^{}{}_{}{}^{}$ was measured at τ=83(10) ps, giving 77(11)${\mathit{B}}_{\mathit{W}}$ for the 1286 keV ${12}^{+}$→$9^{\mathrm{-}}$ E3 transition rate, confirming the double-octupole character of the ${12}^{+}$ state. The observed deviations in energy and transition rate from harmonic vibration are shown to be caused by the exclusion principle acting between nucleons in the two phonons and are related to the dominant contributions to the ${}_{}{}^{148}{}_{}{}^{}\mathrm{Gd}$ octupole phonon of the low-lying Δl=Δj=3 proton and neutron in-shell $3^{\mathrm{-}}$ excitations which are of vital significance for the octupole mode in open-shell nuclei.