Two- to one-phonon [ital E]3 transition strength in [sup 148]Gd

In a plunger experiment the mean life of the ([nu][ital f][sub 6][sup 2][times]3[sup [minus]][times]3[sup [minus]])12[sup +] state at 3.981 MeV in [sub 64][sup 148]Gd[sub 84] was measured at [tau]=83(10) ps, giving 77(11)[ital B][sub [ital W]] for the 1286 keV 12[sup +][r arrow]9[sup [minus]] [ital E]3 transition rate, confirming the double-octupole character of the 12[sup +] 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 [sup 148]Gd octupole phonon of the low-lying [Delta][ital l]=[Delta][ital j]=3 proton and neutron in-shell 3[sup [minus]] excitations which are of vital significance for the octupole mode in open-shell nuclei.