Q value of the superallowed decay of Mg-22 and the calibration of the Na-21(p,gamma) experiment
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The masses of the radioactive nuclei Mg-22 and Na-22 have been measured with the Canadian Penning trap on-line mass spectrometer to a precision of 3x10(-8) and 1x10(-8), respectively. A Q(EC) value of 4124.39(73) keV for the superallowed beta decay of Mg-22 is obtained from the difference of these two masses. With this precise Q value, the Ft value for this decay is determined with improved precision and is found to be consistent with the existing precision data set of superallowed Fermi emitters. This provides an important test of the isospin symmetry-breaking corrections. If the mass of Mg-22 determined here is used in the calibration of a recent Na-21(p,gamma)Mg-22 measurement, part of the discrepancy observed in that measurement is removed.