Energy Levels ofBr81Populated by the Decay ofSe81mandSe81

Abstract

The $\gamma$ rays following the decay of ${\mathrm{Se}}^{81m}$ ($T_{\frac{1}{2}}=57\mathrm{}$ min) and ${\mathrm{Se}}^{81}$ ($T_{\frac{1}{2}}=18\mathrm{}$ min) were investigated using a Ge(Li) detector, a $\gamma -\gamma$ coincidence apparatus, and a high-pressure gas proportional counter. ${\mathrm{Se}}^{81m}$ and ${\mathrm{Se}}^{81}$ were produced by neutron bombardment of 94.4% enriched ${\mathrm{Se}}^{80}$ and natural selenium. Nine $\gamma$ rays have been identified as belonging to the decay of ${\mathrm{Se}}^{81m}$ and ${\mathrm{Se}}^{81}$ by using the Ge(Li) detector. The energies (in keV) and relative intensities (in brackets) of these $\gamma$ rays are as follows. ${\mathrm{Se}}^{81m}$: 103.3±0.2 (100) and 1146±2 (3.4±0.6). ${\mathrm{Se}}^{81}$: 276.3±0.4 (100), 290.2±0.4 (79.6±5.6), 538.7±0.7 (6.2±0.7), 552.7±0.7 (12.8±1.4), 566.1±0.7 (28.9±2.6), 650.4±0.9 (2.6±0.4), and 829.2±0.8 (36.6±2.6). A $\gamma$ ray of energy 180±2 keV (relative intensity=0.94±0.19) has also been attributed to ${\mathrm{Se}}^{81}$ through $\gamma -\gamma$ coincidence measurements. An energy-level scheme of ${\mathrm{Br}}^{81}$ populated by the decay of ${\mathrm{Se}}^{81m}$ and ${\mathrm{Se}}^{81}$ has been proposed by introducing levels at 276, 566, 650, 815, 829, and 1146 keV. The 1146-keV level is fed by a direct $\beta$ decay of ${\mathrm{Se}}^{81m}$ with a transition intensity of (0.34±0.13)%. The $\beta$-decay transition intensities of ${\mathrm{Se}}^{81}$ to the 0-, 566-, 650-, 815-, and 829-keV levels of ${\mathrm{Br}}^{81}$ were computed to be (98.8±0.33)%, (0.76±0.21)%, (0.014±0.006)%, (0.042±0.013)%, and (0.36±0.10)%, respectively.