Ground-state mass of the odd-odd N = Z nuclide 70Br

We have analyzed the systematics of b and c coefficients of the quadratic isobaric multiplet mass equation (IMME) using the newly measured masses of the upper fp-shell nuclei in the experimental cooler storage ring (CSRe) in Lanzhou. The c coefficient for the A = 70, T = 1 isospin multiplet is found to be negative, which is severely deviating from overall systematics and in conflict with theoretical expectations of the isospin symmetry. This anomaly is analyzed and attributed to the presently adopted mass of 70Br in the latest Atomic Mass Evaluation (AME2020). Under the assumption of a smooth variation of Coulomb energy differences with changing mass number A, the ground-state mass excess of 70Br is deduced to be -51934(16) keV, which is 508(22) keV more bound than the adopted value. Using this mass value, the calculated c coefficient of IMME at A = 70 fits well into the systematics, and the obtained corrected Ft value for the 0+ -> 0+ superallowed beta decay of 70Br is consistent with the world-average value.