Nuclear Orientation Study of the Decay ofNp239Polarized in ZrFe2: Parity Mixing inPu239and Nuclear Structure ofPu239and Fission ProductsXe131,132,133

Investigations of the angular distribution of $\gamma$ radiation from ${}_{}{}^{239}{}_{}{}^{}\mathrm{Np}$ polarized at low temperatures in Zr${\mathrm{Fe}}_2$ have suggested small forward-backward asymmetries for several of the hindered $E1\mathrm{}$ and $M1\mathrm{}$ $\gamma$ transitions of ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$. From the anisotropies of the $\gamma$-ray angular distributions, the following multipole mixing ratios were deduced: $\frac{E2\mathrm{}}{M1\mathrm{}}:\delta \mathrm{}\left(210\right)=-0.02\mathrm{}\pm 0.02$, $\delta \mathrm{}\left(228\right)=-0.07\mathrm{}\pm 0.03$, $\delta \mathrm{}\left(254\right)=-0.12\mathrm{}\pm 0.06$, $\delta \mathrm{}\left(278\right)=+0.16\mathrm{}\pm 0.03$; $\frac{M2\mathrm{}}{E1\mathrm{}}:\delta \mathrm{}\left(106\right)=+0.05\mathrm{}\pm 0.03$, $\delta \mathrm{}\left(316\right)=-0.10\mathrm{}\pm 0.12$, $\delta \mathrm{}\left(334\right)=+0.02\mathrm{}\pm 0.03$. The small $\frac{E2\mathrm{}}{M1\mathrm{}}$ mixing ratios of the $\Delta K=2\mathrm{}$ transitions are explainable in terms of Coriolis mixing. The 512-keV level of ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ has been identified as the ${\frac{7}{2}}^{+}\mathrm{}\left[624\right]\mathrm{}$ Nilsson state, which assignment yields the $\frac{E2\mathrm{}}{M1\mathrm{}}$ mixing ratio $\delta \mathrm{}\left(182\right)=-0.12\mathrm{}\pm 0.08$. The hyperfine splitting of ${}_{}{}^{239}{}_{}{}^{}\mathrm{Np}$ in (${\mathrm{U}}_{0.005}$ ${\mathrm{Zr}}_{0.995}$)${\mathrm{Fe}}_2$ has been determined to be 60 ± 5 mK. Difficulties encountered in the selection of a suitable ferromagnetic environment for ${}_{}{}^{239}{}_{}{}^{}\mathrm{Np}$ are discussed. Spin assignments and $\gamma$-ray multipole mixing ratios in ${}_{}{}^{131,132,133}{}_{}{}^{}\mathrm{Xe}$ were deduced from angular distributions of $\gamma$ rays following the decays of fission products ${}_{}{}^{131,132,133}{}_{}{}^{}\mathrm{I}$.