Complete Isobaric Quintet

By experimental observation of the lowest $T=2\mathrm{}$ states in ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$, an isobaric quintet has been completed for the first time. The $T=2\mathrm{}$ state in ${}_{}{}^8{}_{}{}^{}\mathrm{B}$, populated via the reaction ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$(${}_{}{}^3{}_{}{}^{}\mathrm{He}$, ${}_{}{}^6{}_{}{}^{}\mathrm{He}$)${}_{}{}^8{}_{}{}^{}\mathrm{B}$, lies at 10.619±0.009 MeV excitation, and its analog in ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$, found in ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}(p, {}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^8{}_{}{}^{}\mathrm{Li}$, lies at 10.8222±0.0055 MeV. The excitation of the previously known $T=2\mathrm{}$ state in ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ was measured by ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}(p, t){}_{}{}^8{}_{}{}^{}\mathrm{Be}$ to be 27.4922±0.0027 MeV. A significant departure from the isobaric-multiplet mass equation is indicated.