The disintegration scheme of the long-lived ground-state isomer of ${\mathrm{Al}}^{26}$ has been determined. It is based on the following observations.

The maximum positron energy as determined by absorption measurements is 1.16±0.05 Mev; this agrees with the expected 1.17 Mev. The spectrum appears to be simple.

The scintillation gamma spectrum shows intense positron annihilation radiation, a strong peak at 1.83±0.03 Mev, and weak peaks at 1.12±0.03 Mev and 2.96±0.05 Mev, corresponding to transitions from the known ${\mathrm{Mg}}^{26}$ states at 1.82 and 2.97 Mev. A peak at 0.68 Mev is from the addition of two annihilation photons, one being backscattered from the source and surroundings; the gamma of ∼0.7-Mev energy reported by others is not present. The peak at 2.97 Mev is shown to result from 2.97-Mev photons as well as from addition of 1.82- and 1.15-Mev photons.

The relative intensities of the annihilation and gamma radiations indicate that ${\mathrm{Al}}^{26}$ undergoes (84.6±1.8)% positron emission to the 1.82-Mev state of ${\mathrm{Mg}}^{26}$; (11.4±1.9)% electron capture to the same state; (3.7±0.3)% electron capture to the 2.97-Mev state followed by emission of 1.15- and 1.82-Mev gamma-rays; and (0.30±0.03)% electron capture to the same state followed by 2.97-Mev radiation. Other energetically possible transitions are apparently negligible in intensity.

Auger electrons and x-rays were observed in a proportional counter spectrometer. Analysis of the spectra yielded $K$-shell fluorescence yields of 0.008±0.003 for magnesium and 0.008±0.003 for aluminum.

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