Gallant A T, Brodeur M, Andreoiu C, Bader A, Chaudhuri A, Chowdhury U, Grossheim A, Klawitter R, Kwiatkowski A A, Leach K G, Lennarz A, Macdonald T D, Schultz B E, Lassen J, Heggen H, Raeder S, Teigelhöfer A, Brown B A, Magilligan A, Holt J D, Menéndez J, Simonis J, Schwenk A, Dilling J
TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 Canada.
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA.
Phys Rev Lett. 2014 Aug 22;113(8):082501. doi: 10.1103/PhysRevLett.113.082501. Epub 2014 Aug 19.
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 is in good agreement but that the mass of (20)Mg deviates by 3 σ. These measurements reduce the uncertainties in the masses of (20,21)Mg 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.
