Isobaric multiplet mass equation in theA=31,T=3/2quartets Academic Article uri icon

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  • 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.


  • Bennett, MB
  • Wrede, C
  • Brown, BA
  • Liddick, SN
  • Pérez-Loureiro, D
  • Bardayan, DW
  • Chen, Alan
  • Chipps, KA
  • Fry, C
  • Glassman, BE
  • Langer, C
  • Larson, NR
  • McNeice, EI
  • Meisel, Z
  • Ong, W
  • O'Malley, PD
  • Pain, SD
  • Prokop, CJ
  • Schwartz, SB
  • Suchyta, S
  • Thompson, P
  • Walters, M
  • Xu, X

publication date

  • June 13, 2016