By employing the reactions ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}(p,{}_{}{}^6{}_{}{}^{}\mathrm{He}){}_{}{}^{19}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}(p,{}_{}{}^6{}_{}{}^{}\mathrm{He}){}_{}{}^{23}{}_{}{}^{}\mathrm{Al}$ [$\frac{d\sigma }{d\Omega }\sim 100$ nb/sr], the mass excesses of ${}_{}{}^{19}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{23}{}_{}{}^{}\mathrm{Al}$ are found to be 12.974 ± 0.070 and 6.766 ± 0.080 MeV, respectively [${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ = 0]. Since ${}_{}{}^{19}{}_{}{}^{}\mathrm{Na}$ is determined to be proton unstable, ${}_{}{}^{23}{}_{}{}^{}\mathrm{Al}$ should be the lightest, nucleon-stable member of the $A=4n+3\mathrm{}$, $T_Z=-\frac{3}{2}$ mass series.

• Received 3 February 1969

DOI:https://doi.org/10.1103/PhysRevLett.22.612