Precise β branching-ratio measurement for the 0+ → 0+ superallowed decay of 34Ar

We have measured the branching ratio for the superallowed 0(+) -> 0(+) beta transition from Ar-34 to be 0.9448(8), and determined its f t value to be 3058.1(28) s, a result with +/- 0.09% precision, which is a factor of 3 improvement over the previous result based on current world data. The f t-value ratio for the mirror pair of superallowed transitions Ar-34 -> Cl-34 and Cl-34 -> S-34 becomes the most precise yet measured and, in a sensitive test of the method used to calculate the isospin-symmetry-breaking correction, delta(C), it agrees well with the ratio as calculated with Woods-Saxon radial wave functions. This confirms the method used in the most recent survey of superallowed decays to extract Vud, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix. In addition, our branching-ratio results for the four observed Gamow-Teller branches to 1(+) states in Cl-34 are shown to agree well with shell-model calculations based on the same effective interactions that were used in the determination of delta(C).