Multipole mixing ratios inSe77fromγγdirectional correlations following 56-hBr77decay

Abstract

The decay properties of the levels in $^{77}\mathrm{Se}$ were investigated following the decay of 56-h $^{77}\mathrm{Br}$. Singles $\ensuremath{\gamma}$-ray energy and intensity measurements were obtained with a 42-${\mathrm{cm}}^{3}$ high-resolution Ge(Li) $\ensuremath{\gamma}$-ray detector in an anti-Compton arrangement employing a large NaI(T1) annular detector. Extensive $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence and directional-correlation measurements were obtained with two large Ge(Li) detectors. An improved decay scheme was constructed. From $\ensuremath{\gamma}\ensuremath{\gamma}$ correlation measurements, deduced $log\mathrm{ft}$ values, and internal-conversion coefficients, the ${J}^{\ensuremath{\pi}}$ values in parentheses for the following levels in keV were confirmed: g.s. (${\mathrm{\textonehalf{}}}^{\ensuremath{-}}$), $161.83 ({\frac{7}{2}}^{+})$, $238.98 ({\frac{3}{2}}^{\ensuremath{-}})$, $249.65 ({\frac{5}{2}}^{\ensuremath{-}})$, $439.42 ({\frac{5}{2}}^{\ensuremath{-}})$, $520.59 ({\frac{3}{2}}^{\ensuremath{-}})$, 817.78 (${\mathrm{\textonehalf{}}}^{\ensuremath{-}}$), and $1005.08 ({\frac{3}{2}}^{\ensuremath{-}})$; and further ${J}^{\ensuremath{\pi}}$ assignments in parentheses for the following levels are proposed: $300.78 ({\frac{5}{2}}^{+})$, $580.90 ({\frac{5}{2}}^{+})$, $679.7 ({\frac{5}{2}}^{+})$, $824.35 ({\frac{3}{2}}^{\ensuremath{-}},{\frac{5}{2}}^{\ensuremath{-}})$, $911.40 ({\frac{3}{2}}^{+})$, $1186.55 ({\frac{3}{2}}^{\ensuremath{-}})$, and 1230.62 (${\frac{3}{2}}^{+},{\frac{5}{2}}^{\ensuremath{-}}$). With the 249.77- and 439.47-keV transitions assumed to be pure $E2$, the mixing ratio $\ensuremath{\delta}(\frac{M2}{E1})=\ensuremath{-}0.09\ifmmode\pm\else\textpm\fi{}0.05$ was obtained for the 87.59-keV transition, while the $\ensuremath{\delta}(\frac{E2}{M1})$ mixing ratios obtained were: 200.40 keV, 0.00 \ifmmode\pm\else\textpm\fi{} 0.09; 238.98 keV, 0.13 \ifmmode\pm\else\textpm\fi{} 0.04; 270.83 keV, ${0.30}_{\ensuremath{-}0.06}^{+0.05}$; 281.65 keV, ${\ensuremath{-}0.12}_{\ensuremath{-}0.04}^{+0.03}$; 297.23 keV, 0.17 \ifmmode\pm\else\textpm\fi{} 0.03 or -2.7 \ifmmode\pm\else\textpm\fi{} 0.2; 303.76 keV, ${\ensuremath{-}0.44}_{\ensuremath{-}0.16}^{+0.05}$ or ${0.09}_{\ensuremath{-}0.04}^{+0.05}$; 384.99 keV, ${\ensuremath{-}0.23}_{\ensuremath{-}0.05}^{+{0.07}^{\ensuremath{'}}}$; 484.57 keV, ${\ensuremath{-}0.27}_{\ensuremath{-}0.03}^{+0.04}$; 520.69 keV, ${0.17}_{\ensuremath{-}0.07}^{+0.06}$; 574.64 keV, 0.23 \ifmmode\pm\else\textpm\fi{} 0.06 for ${\frac{3}{2}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\frac{5}{2}}^{\ensuremath{-}}$ or ${\ensuremath{-}0.31}_{\ensuremath{-}0.08}^{+0.07}$ for ${\frac{5}{2}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\frac{5}{2}}^{\ensuremath{-}}$; 578.91 keV, ${0.16}_{\ensuremath{-}0.04}^{+0.03}$ or ${\ensuremath{-}2.6}_{\ensuremath{-}0.3}^{+0.2}$; and 755.35 keV, ${0.31}_{\ensuremath{-}0.08}^{+0.09}$. From these multipole mixing ratios, present branching ratios and previously measured lifetimes improved and new values of reduced transition probabilities were calculated for the transitions at 87.59, 200.40, 238.98, 249.77, 270.83, 277.47, 281.65, 439.47, and 520.69 keV. The results of this study are compared with previous work and the structure of the lower levels in $^{77}\mathrm{Se}$ is discussed in terms of recent theoretical models for nuclear structure.[RADIOACTIVITY $^{77}\mathrm{Br}$, $^{75}\mathrm{As}(\ensuremath{\alpha},2n)$ at 22 MeV, measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, $\ensuremath{\gamma}\ensuremath{\gamma}$, $\ensuremath{\gamma}\ensuremath{\gamma}(\ensuremath{\theta})$; deduced $^{77}\mathrm{Se}$ levels, $log\mathrm{ft}$, $J$, $\ensuremath{\pi}$, $\ensuremath{\delta}(\frac{E2}{M1})$, $B(E2)$, $B(E1)$, and $B(M1)$ values.]