An unexpected breakdown of the isobaric multiplet mass equation in the A=20, T=2 quintet was recently reported, presenting a challenge to modern theories of nuclear structure. In the present work, the excitation energy of the lowest T=2 state in 20Na has been measured to be 6498.4±0.2stat±0.4syst keV by using the superallowed 0+→0+ beta decay of 20Mg to access it and an array of high-purity germanium detectors to detect its γ-ray deexcitation. This value differs by 27 keV (1.9 standard deviations) from the recommended value of 6525±14 keV and is a factor of 28 more precise. The isobaric multiplet mass equation is shown to be revalidated when the new value is adopted.
@article{arxiv.1511.01809,
title = {Revalidation of the isobaric multiplet mass equation for the $A=20$ quintet},
author = {B. E. Glassman and D. Pérez-Loureiro and C. Wrede and J. Allen and D. W. Bardayan and M. B. Bennett and B. A. Brown and K. A. Chipps and M. Febbraro and C. Fry and M. R. Hall and O. Hall and S. N. Liddick and P. O'Malley and W. Ong and S. D. Pain and S. B. Schwartz and P. Shidling and H. Sims and P. Thompson and H. Zhang},
journal= {arXiv preprint arXiv:1511.01809},
year = {2015}
}