Precise ft‐value Measurement for the Superallowed 0+ → 0+ β Decay of 22Mg

Very accurate measurements of the half‐life, 3.8755(12) s, and the branching ratio, 0.5315(12), are reported for the superallowed 0+ → 0+ β‐decay of 22Mg. The precision in the branching ratio was achieved by the efficiency calibration of an HPGe detector to 0.15% precision for γ‐ray energies between 50 and 1400 keV. The ft‐value extracted from the measurement, corrected for small calculated radiative effects is Ft=3071(9) s. With this measurement, we open up a new series of superallowed emitters with Tz = −1 to precision measurements. This series will permit direct tests of the calculated nuclear‐structure‐dependent correction terms essential to the determination of the vector coupling constant, GV, and of Vud, the up‐down quark‐mixing element of the Cabibbo‐Kobayashi‐Moskawa (CKM) matrix. As the top row of the CKM matrix currently fails the unitarity test by more than two standard deviations, measurements to improve the precision with which Vud is known are of paramount importance.