Precise half-life measurement of the superallowed β+ emitter 26Si

We measured the half-life of the superallowed 0(+) -> 0(+) beta(+) emitter Si-26 to be 2245.3(7) ms. We used pure sources of Si-26 and employed a high-efficiency gas counter, whichwas sensitive to positrons from both this nuclide and its daughter Al-26(m). The data were analyzed as a linked parent-daughter decay. To contribute meaningfully to any test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the f t value of a superallowed transition must be determined to a precision of 0.1% or better. With a precision of 0.03%, the present result is more than sufficient to be compatible with that requirement. Only the branching ratio now remains to be measured precisely before a +/- 0.1% f t value can be obtained for the superallowed transition from Si-26.