Electric Dipole Decays of31- Octupole States

Abstract

The $\ensuremath{\gamma}$-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 ($p, {p}^{\ensuremath{'}}\ensuremath{\gamma}$) 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 $E1$ transition probabilities, $R=\frac{B(E1:{{3}_{1}}^{\ensuremath{-}}\ensuremath{\rightarrow}{{2}_{2}}^{+})}{B(E1:{{3}_{1}}^{\ensuremath{-}}\ensuremath{\rightarrow}{{2}_{1}}^{+})}$, has been observed. The experimental value $R\ensuremath{\le}0.5$ for $^{48}\mathrm{Ti}$ is much lower than the shell-model estimate ($R=\ensuremath{\infty}$) due to Soga, which assumed a pure ${({f}_{\frac{7}{2}})}^{8}$ configuration and characterized the ${2}_{1}^{+}$, ${2}_{2}^{+}$, and ${3}_{1}^{\ensuremath{-}}$ states according to their isospin properties. This result seems to require a substantial isovector component in the ${3}_{1}^{\ensuremath{-}}$ states of $^{48}\mathrm{Ti}$. Comparisons are also made with Leper's estimate for the $B(E1:3\ensuremath{\rightarrow}2)$ value based on a vibrational model, as well as with the predictions of Soga, using perturbation theory to estimate isovector components in the ${2}_{1}^{+}$ and ${2}_{2}^{+}$ states.