Superallowed 0+ → 0+ β decay of T =2 20Mg: QEC value and βγ branching
Abstract
Background: Superallowed 0+ → 0+ β decays of isospin T=2 nuclides can be used to test theoretical isospin symmetry breaking corrections applied to extract the CKM matrix element Vud from T = 0,1 decays by measuring precise ft values and also to search for scalar currents using the β- angular correlation. Key ingredients include the QEC value and branching of the superallowed transition and the half life of the parent. Purpose: To determine a precise experimental QEC value for the superallowed 0+ → 0+ β decay of T=2 20Mg and the intensity of 20Mg β-delayed γ rays through the isobaric analog state in 20Na. Method: A beam of 20Mg was produced using the in-flight method and implanted into a plastic scintillator surrounded by an array of high-purity germanium detectors used to detect β-delayed γ rays. The high-resolution γ-ray spectrum was analyzed to measure the γ-ray energies and intensities. Results: The intensity of 20Mg β-delayed γ rays through the isobaric analog state in 20Na was measured to be (1.60 0.04stat 0.15syst 0.15theo) × 10-4, where the uncertainties are statistical, systematic, and theoretical, respectively. The QEC value for the superallowed transition was determined to be 4128.7 2.2 keV based on the measured excitation energy of 6498.4 0.2stat 0.4syst keV and literature values for the ground-state masses of 20Na and 20Mg. Conclusions: The β-delayed γ-decay branch and QEC value are now sufficiently precise to match or exceed the sensitivity required for current low-energy tests of the standard model.
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