Ground-state mass of the odd-odd N=Z nuclide Br70

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 $^{70}\mathrm{Br}$ 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 $^{70}\mathrm{Br}$ is deduced to be $\ensuremath{-}51934(16)$ 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 ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ superallowed $\ensuremath{\beta}$ decay of $^{70}\mathrm{Br}$ is consistent with the world-average value.