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 Mg20,21, 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 Mg21 is in good agreement but that the mass of Mg20 deviates by 3σ. These measurements reduce the uncertainties in the masses of Mg20,21 by 15 and 22 times, respectively, resulting in a significant departure from the expected behavior of the isobaric multiplet mass equation in both the A=20 and A=21 multiplets. This presents a challenge to shell model calculations using either the isospin nonconserving universal sd USDA and USDB Hamiltonians or isospin nonconserving interactions based on chiral two- and three-nucleon forces. © 2014 American Physical Society.

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Author Jens Dilling Physics