QEC Values of the superallowed β Emitters 50Mn and 54Co

Using a new fast cleaning procedure to prepare isomerically pure ion samples, we have measured the beta-decay Q(EC) values of the superallowed beta emitters Mn-50 and Co-54 to be 7634.48(7) and 8244.54(10) keV, respectively, results which differ significantly from the previously accepted values. The corrected Ft values derived from our results strongly support new isospin-symmetry-breaking corrections that lead to a higher value of the up-down quark mixing element V-ud and improved confirmation of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.