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

Abstract

Recent high-precision mass measurements and shell model calculations~[Phys. Rev. Lett. {\bf 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 ${}^9$B, which had two conflicting measurements prior to this work. We remeasured the energy of the state using the ${}^9\mathrm{Be}{(}^3\mathrm{He},t)$ 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.