Electric Dipole Decays of31- Octupole States

The $\gamma$-ray decay modes of the octupole $3_1$- state have been determined for ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$, ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$, and ${}_{}{}^{66}{}_{}{}^{}\mathrm{Zn}$ from measurements of ($p, p^{\prime }\gamma$) spectra using a coincidence-anticoincidence Ge(Li)-NaI(Tl) spectrometer. Available information for ${}_{}{}^{46}{}_{}{}^{}\mathrm{Ti}$, ${}_{}{}^{62}{}_{}{}^{}\mathrm{Ni}$, ${}_{}{}^{70}{}_{}{}^{}\mathrm{Ge}$, and ${}_{}{}^{72}{}_{}{}^{}\mathrm{Ge}$ is also summarized. A large variation in the ratio of reduced $E1\mathrm{}$ transition probabilities, $R=\frac{B(E1:{3_1}^{-}\to {2_2}^{+})}{B(E1:{3_1}^{-}\to {2_1}^{+})}$, has been observed. The experimental value $R\le 0.5$ for ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$ is much lower than the shell-model estimate ($R=\infty$) due to Soga, which assumed a pure ${\left(f_{\frac{7}{2}}\right)}^8$ configuration and characterized the $2_1^{+}$, $2_2^{+}$, and $3_1^{-}$ states according to their isospin properties. This result seems to require a substantial isovector component in the $3_1^{-}$ states of ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$. Comparisons are also made with Leper's estimate for the $B(E1:3\mathrm{}\to 2)$ value based on a vibrational model, as well as with the predictions of Soga, using perturbation theory to...