Isospin-forbidden β-decay of 28Mg

The branching ratio for the isospin-forbidden Fermi β-decay of 28Mg to the 0.972 MeV 0+, T=1 level in 28Al has been measured. This decay occurs through an admixture of the 0+, T=2 analog state at 5.992 MeV into the anti-analog level at 0.972 MeV. A charge-dependent matrix element of 17.0sub(-5.8)sup(+4.3) keV is deduced from the observed (0.21+-0.12)% branch. Comparisons are made with matrix elements deduced in other nuclei. An analysis based on a simple shell model with fourfold degenerate orbitals indicates the importance of the two-body Coulomb interaction in isospin mixing in nuclei with more than one valence proton. (Auth.)