Beta Decay of theAl26Ground State

The decay scheme of the ground state of ${}_{}{}^{26}{}_{}{}^{}\mathrm{Al}$ has been reexamined by measurement of its $\gamma$-ray spectrum with Ge(Li) detectors. The $\gamma$-ray spectrum shows very strong positron annihilation radiation, a strong peak at 1808.65 ± 0.07 keV, a weak peak at 1129.67 ± 0.10 keV, and a very weak peak at 2938 keV. These $\gamma$-ray energies correspond to transitions from the first and second excited states of ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$, which have been determined more accurately in this experiment. The ${}_{}{}^{26}{}_{}{}^{}\mathrm{Al}$ branching ratios, calculated from the $\gamma$-ray relative intensities, are (82.1 ± 2.5)% positron emission to the 1809-keV level, (15.2 ± 2.5)% electron capture to that same level, and (2.7 ± 0.2)% electron capture to the 2938-keV level of ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$. The 2938-keV state decays (10.2 ± 2.0)% directly to the ground state and (89.8 ± 2.0)% by cascade. These branching ratios combine with previous results to give a half-life of (7.16 ± 0.32) × ${10}^5$ yr for the ground state of ${}_{}{}^{26}{}_{}{}^{}\mathrm{Al}$.