Breakdown of the Isobaric Multiplet Mass Equation for the A=20 and 21 Multiplets

Using the Penning trap mass spectrometer TITAN, we performed the first direct mass measurements of (20,21)Mg, isotopes that are the most proton-rich members of the A = 20 and A = 21 isospin multiplets. These measurements were possible through the use of a unique ion-guide laser ion source, a development that suppressed isobaric contamination by 6 orders of magnitude. Compared to the latest atomic mass evaluation, we find that the mass of (21)Mg …