Multipole mixing ratios in 77Se from γγ directional correlations following 56-h 77Br decay

The decay properties of the levels in ⁷⁷Se were investigated following the decay of 56-h ⁷⁷Br. Singles γ-ray energy and intensity measurements were obtained with a 42-cm³ high-resolution Ge(Li) γ-ray detector in an anti-Compton arrangement employing a large NaI(T1) annular detector. Extensive γγ-coincidence and directional-correlation measurements were obtained with two large Ge(Li) detectors. An improved decay scheme was constructed. From γγ correlation measurements, deduced logft values, and internal-conversion coefficients, the $J^{\pi }$ values in parentheses for the following levels in keV were confirmed: g.s. (1/2⁻), 161.83 (7/2⁺), 238.98 (3/2⁻), 249.65 (5/2⁻), 439.42 (5/2⁻), 520.59 (3/2⁻), 817.78 (1/2⁻), and 1005.08 (3/2⁻); and further $J^{\pi }$ assignments in parentheses for the following levels are proposed: 300.78 (5/2⁺), 580.90 (5/2⁺), 679.7 (5/2⁺), 824.35 (3/2⁻,5/2⁻), 911.40 (3/2⁺), 1186.55 (3/2⁻), and 1230.62 (3/2⁺,5/2⁻). With the 249.77- and 439.47-keV transitions assumed to be pure E2, the mixing ratio δ(M2/E1)=-0.09±0.05 was obtained for the 87.59-keV transition, while the δ(E2/M1) mixing ratios obtained were: 200.40 keV, 0.00 ± 0.09; 238.98 keV, 0.13 ± 0.04; 270.83 keV, ${0.30}_{-0.06}^{+0.05}$; 281.65 keV, ${-0.12}_{-0.04}^{+0.03}$; 297.23 keV, 0.17 ± 0.03 or -2.7 ± 0.2; 303.76 keV, ${-0.44}_{-0.16}^{+0.05}$ or ${0.09}_{-0.04}^{+0.05}$; 384.99 keV, ${-0.23}_{-0.05}^{+{0.07}^{{}^{\prime }}}$; 484.57 keV, ${-0.27}_{-0.03}^{+0.04}$; 520.69 keV, ${0.17}_{-0.07}^{+0.06}$; 574.64 keV, 0.23 ± 0.06 for 3/2⁻→5/2⁻ or ${-0.31}_{-0.08}^{+0.07}$ for 5/2⁻→5/2⁻; 578.91 keV, ${0.16}_{-0.04}^{+0.03}$ or ${-2.6}_{-0.3}^{+0.2}$; and 755.35 keV, ${0.31}_{-0.08}^{+0.09}$. 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 ⁷⁷Se is discussed in terms of recent theoretical models for nuclear structure.[RADIOACTIVITY ⁷⁷Br, ⁷⁵As(α,2n) at 22 MeV, measured $E_{\gamma }$, $I_{\gamma }$, γγ, γγ(θ); deduced ⁷⁷Se levels, logft, J, π, δ(E2/M1), B(E2), B(E1), and B(M1) values.]