Isobaric multiplet mass equation in the $A=31$ $T=3/2$ quartets

Abstract

The observed mass excesses of analog nuclear states with the same mass number $A$ and isospin $T$ can be used to test the isobaric multiplet mass equation (IMME), which has, in most cases, been validated to a high degree of precision. A recent measurement [Kankainen et al., Phys. Rev. C 93 041304(R) (2016)] of the ground-state mass of ${}^{31}$Cl led to a substantial breakdown of the IMME for the lowest $A=31,T=3/2$ quartet. The second-lowest $A=31,T=3/2$ quartet is not complete, due to uncertainties associated with the identity of the ${}^{31}$S member state. Using a fast ${}^{31}$Cl beam implanted into a plastic scintillator and a high-purity Ge $\gamma$-ray detection array, $\gamma$ rays from the ${}^{31}$Cl$(\beta \gamma )$${}^{31}$S sequence were measured. Shell-model calculations using USDB and the recently-developed USDE interactions were performed for comparison. Isospin mixing between the ${}^{31}$S isobaric analog state (IAS) at 6279.0(6) keV and a nearby state at 6390.2(7) keV was observed. The second $T=3/2$ state in ${}^{31}$S was observed at $E_x=7050.0(8)$ keV. Isospin mixing in ${}^{31}$S does not by itself explain the IMME breakdown in the lowest quartet, but it likely points to similar isospin mixing in the mirror nucleus ${}^{31}$P, which would result in a perturbation of the ${}^{31}$P IAS energy. USDB and USDE calculations both predict candidate ${}^{31}$P states responsible for the mixing in the energy region slightly above $E_x=6400$ keV. The second quartet has been completed thanks to the identification of the second ${}^{31}$S $T=3/2$ state, and the IMME is validated in this quartet.

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Cited by in corpus (3)

• The Masses of the $T_z=-3/2$ Nuclei ${}^{27}$P and ${}^{29}$S
• Precision mass measurements of magnesium isotopes and implications on the validity of the Isobaric Mass Multiplet Equation
• Beyond Wigner's isobaric multiplet mass equation: Effect of charge-symmetry-breaking interaction and Coulomb polarization