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

Background: 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 , Phys. Rev. C 93, 041304(R) (2016)2469-998510.1103/PhysRevC.93.041304] of the ground-state mass of Cl31 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 S31 member state. Purpose: Our goal is to populate the two lowest T=3/2 states in S31 and use the data to investigate the influence of isospin mixing on tests of the IMME in the two lowest A=31,T=3/2 quartets. Methods: Using a fast Cl31 beam implanted into a plastic scintillator and a high-purity Ge γ-ray detection array, γ rays from the Cl31(βγ)S31 sequence were measured. Shell-model calculations using USDB and the recently-developed USDE interactions were performed for comparison. Results: Isospin mixing between the S31 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 S31 was observed at Ex=7050.0(8) keV. Calculations using both USDB and USDE predict a triplet of isospin-mixed states, including the lowest T=3/2 state in P31, mirroring the observed mixing in S31, and two isospin-mixed triplets including the second-lowest T=3/2 states in both S31 and P31. Conclusions: Isospin mixing in S31 does not by itself explain the IMME breakdown in the lowest quartet, but it likely points to similar isospin mixing in the mirror nucleus P31, which would result in a perturbation of the P31 IAS energy. USDB and USDE calculations both predict candidate P31 states responsible for the mixing in the energy region slightly above Ex=6400 keV. The second quartet has been completed thanks to the identification of the second S31 T=3/2 state, and the IMME is validated in this quartet.