Revalidation of the isobaric multiplet mass equation for the A=20 quintet

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 Na20 has been measured to be 6498.4±0.2stat±0.4syst keV by using the superallowed 0+→0+ beta decay of Mg20 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.