Superallowed β-Decay Branching Ratio Measurement of 26Si

Abstract

As part of the continued effort to test the unitarity of the Cabbibo-Kobayashi-Maskawa matrix, the branching ratio for the Tvz = -1 superallowed 0^+ → 0^+ positron-emitter ^26Si was measured precisely for the first time. Since the QvEC value, 4840.86(10) keV, and half-life, 2245.3(7) ms, are known precisely, the branching ratio is all that is required to obtain a precise ft value. This completes the second pair of mirror superallowed transitions, ^26Si → ^26mAl and 26mAl → ^26Mg. A previous measurement of the mirror transitions, ^38Ca → ^38mK and ^38mK → ^38Ar, showed that the ratio of mirror ft-values is very sensitive to the model used to calculate the small isospin symmetry-breaking corrections required to extract Vvud. In calculating this correction, both Woods-Saxon and Hartree-Fock radial wave functions have been used, with the experimental results from the first pair favoring Woods-Saxon. The result for the A = 26 mirror pair demonstrates that this conclusion can be generalized.