High-Precision Mass Measurements of 52Ni and 51Co Reveal Breakdown of the Isobaric Multiplet Mass Equation in the f p Shell
We performed high-precision mass measurements of the proton-rich nuclei $^{52}$Ni and $^{51}$Co with the LEBIT Penning trap at the Facility for Rare Isotope Beams (FRIB). For $^{52}$Ni, a mass excess of $-22474.8(2.2)$~keV was determined, which is consistent with a recent storage-ring measurement at the Cooler-Storage Ring (CSRe) but has a factor 37 improved precision. For $^{51}$Co, we obtained a mass excess of $-27375.1(5.7)$~keV, agreeing with a recent CSRe result, while reducing the uncertainty by a factor~2. Combining our mass value for $^{52}$Ni with the known two-proton decay energy of $^{54}$Zn, we determined the mass excess of $^{54}$Zn to be $-6463(42)$~keV. These new mass values reveal a substantial breakdown of the isobaric mass multiplet equation for $A=52$ and $A=54$, and provide stringent benchmarks for isospin-symmetry-breaking effects in the proton-rich $fp$-shell, favoring theoretical descriptions that omit the Coulomb-exchange term.