ISOSPIN-FORBIDDEN BETA-DECAY OF MG-28

We have measured the branching ratio for the isospin-forbidden Fermi β-decay of 28Mg to the 0.972 MeV 0+, T = 1 level in 28Al. 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 17.0-5.8+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. © 1978.