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

Abstract

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 Na-20 has been measured to be 6498.4 +/- 0.2(stat) +/- 0.4(syst) keV by using the superallowed 0(+) -> 0(+) beta decay of Mg-20 to access it and an array of high-purity germanium detectors to detect its gamma-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.