Second T = 3/2 state in $^9$B and the isobaric multiplet mass equation.

Recent high-precision mass measurements and shell-model calculations [M. Brodeur et al., Phys. Rev. Lett. 108, 212501 (2012)] have challenged a longstanding explanation for the requirement of a cubic isobaric multiplet mass equation for the lowest A = 9 isospin quartet. The conclusions relied upon the choice of the excitation energy for the second T = 3/2 state in B-9, which had two conflicting measurements prior to this work. We remeasured the energy of the state using the Be-9(He-3, t) reaction and significantly disagree with the most recent measurement. Our result supports the contention that continuum coupling in the most proton-rich member of the quartet is not the predominant reason for the large cubic term required for A = 9 nuclei.