Observation of $\beta$-delayed protons from the decay of the mass-separated $T_z=-2\mathrm{}$ nuclide ${}_{}{}^{20}{}_{}{}^{}\mathrm{Mg}$ ($t_{\frac{1}{2}}\sim 95$ ms) establishes the mass excess of the lowest $T=2\mathrm{}\left(0^{+}\right)$ state in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$ (13.42 ± 0.05 MeV), thereby completing the mass-20 isospin quintet. All members of this multiplet are bound to isospin-allowed particle-decay modes, providing the first test of the isobaric multiplet mass equation for such a quintet. Excellent agreement is observed using only the quadratic form of this equation.

• Received 31 October 1978

DOI:https://doi.org/10.1103/PhysRevLett.42.43