Completion of the Mass-20 Isospin Quintet by Employing a Helium-Jet-Fed On-Line Mass Separator

Abstract

Observation of $\ensuremath{\beta}$-delayed protons from the decay of the mass-separated ${T}_{z}=\ensuremath{-}2$ nuclide $^{20}\mathrm{Mg}$ (${t}_{\frac{1}{2}}\ensuremath{\sim}95$ ms) establishes the mass excess of the lowest $T=2({0}^{+})$ state in $^{20}\mathrm{Na}$ (13.42 \ifmmode\pm\else\textpm\fi{} 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.