Lifetime of the31−state and octupole collectivity inZr96

The half-life of the 1897.2-keV, $3_1^{\mathrm{-}}$ state in ${}_{}{}^{96}{}_{}{}^{}\mathrm{Zr}$ has been measured as (67.8±4.3) ps using the recoil-distance technique following inelastic excitation by 105-MeV ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ ions. This is equivalent to a lifetime which is about 40% longer than those reported from two recent centroid-shift measurements. Our lifetime implies B(E3)↑=(0.180±0.018) ${\mathit{e}}^2$ ${\mathrm{b}}^3$, i.e., (47.1±4.7) W.u., which is the most enhanced one-phonon g.s.→$3_1^{\mathrm{-}}$ transition strength observed in nuclei. A recent quasiparticle random-phase approximation (QRPA) calculation reproduces our result. Serious doubts are raised about an earlier conclusion that the reduced octupole transition probabilities of ‘‘mirror’’ nuclei correspond to the harmonic low for small amplitude vibrations. The enhanced B(E1:$3_1^{\mathrm{-}}$→$2_1^{+}$)=(1.71±0.11) ${10}^{\mathrm{-}5}$ ${\mathit{e}}^2$b deduced from the present work is consistent with appreciable octupole deformation. The measured B(E3)↑ resolves problems in the interpretation of the isospin character of the E3 transition derived from inelastic scattering data.