β+-delayed-proton decay of 21Mg

Identified protons with energies from 900 keV to 6.3 MeV have been observed following the positron decay of ²¹Mg. The half-life of ²¹Mg was measured to be 123.1 ± 3.3 msec, which, combined with previous results, gives a best value of 122.5 ± 2.8 msec. The energies and intensities of the proton groups have been used to determine precise excited state energies in ²¹Na above 5 MeV and the strength of the preceding β⁺ transition feeding each level. These data, and the assumption of isospin purity for the lowest T=3/2 level in ²¹Na, then yield absolute ft values for the beta decay. The ²¹Mg transition rates compared with the negatron decay rates in its mirror, ²¹F, give $\frac{{\left(ft\right)}^{+}}{{\left(ft\right)}^{-}}=1.10\pm 0.08$. All significant particle decays of the lowest T=3/2 level have been observed, including an upper limit on energetically possible, but unobserved, α decay. The measured transition strengths were compared to recent shell model calculations for β decay in the s-d shell.