Beta-delayed protons have been observed from the decays of the mass separated $T_z=-2\mathrm{}$ nuclides ${}_{}{}^{20}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Si}$, and ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ca}$. From these proton spectra the mass excesses of the lowest $T=2\mathrm{}$ states in the $T_z=-1\mathrm{}$ nuclei ${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Al}$, and ${}_{}{}^{36}{}_{}{}^{}\mathrm{K}$ are determined to be 13420±50 keV (${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$), 5903±9 keV (${}_{}{}^{24}{}_{}{}^{}\mathrm{Al}$), and - 13168±22 keV (${}_{}{}^{36}{}_{}{}^{}\mathrm{K}$). The complete $A=20,24, \mathrm{and} 36$ isospin quintets have all their members bound against isospin allowed particle-decay modes, providing a stringent test of the isobaric multiplet mass equation. Good agreement is observed for all these quintets using only the quadratic form of this equation.

RADIOACTIVITY ${}_{}{}^{20}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ca}$ (mass separated); measured $\beta$-delayed protons; deduced $T_{\frac{1}{2}}$ and proton branching; derived mass excesses of the lowest $0^{+}$, $T=2\mathrm{}$ states in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Al}$, and ${}_{}{}^{36}{}_{}{}^{}\mathrm{K}$; deduced coefficients of the isobaric multiplet mass equation.

• Received 22 July 1980

DOI:https://doi.org/10.1103/PhysRevC.23.879