Elucidation of the anomalous A = 9 isospin quartet behaviour

Abstract

Recent high-precision mass measurements of ${}^9$Li and ${}^9$Be, performed with the TITAN Penning trap at the TRIUMF ISAC facility, are analyzed in light of state-of-the-art shell model calculations. We find an explanation for the anomalous Isobaric Mass Multiplet Equation (IMME) behaviour for the two $A$ = 9 quartets. The presence of a cubic $d$ = 6.3(17) keV term for the $J^{\pi }$ = 3/2${}^{-}$ quartet and the vanishing cubic term for the excited $J^{\pi }$ = 1/2${}^{-}$ multiplet depend upon the presence of a nearby $T$ = 1/2 state in ${}^9$B and ${}^9$Be that induces isospin mixing. This is contrary to previous hypotheses involving purely Coulomb and charge-dependent effects. $T$ = 1/2 states have been observed near the calculated energy, above the $T$ = 3/2 state. However an experimental confirmation of their $J^{\pi }$ is needed.

Comments: 5 pages, 2 figures