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

TL;DR

This paper reports a new measurement of the second T=3/2 state in $^9$B, providing insights into the isobaric multiplet mass equation and challenging previous explanations for its cubic nature.

Contribution

It presents a remeasurement of the $^9$B state energy, clarifying the role of continuum coupling in the isobaric multiplet mass equation for A=9 nuclei.

Findings

New $^9$B state energy measurement disagrees with previous data.

Supports that continuum coupling is not the main cause of the cubic term.

Challenges previous explanations for 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 $^9{\rm Be}(^3{\rm He},t)$ 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.