Precise measurement of branching ratios in the β decay of Ca 38

We present the full description of a measurement of the branching ratios for the beta decay of Ca-38. This decay includes five allowed 0(+) -> 1(+) branches and a superallowed 0(+) -> 0(+) one. With our new result for the latter, we determine its f t value to be 3062.3 +/- 6.8 s, a result whose precision (0.2%) is comparable to the precision of the 13 well-known 0(+) -> 0(+) transitions used up until now for the determination of V-ud, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix. The Ca-38 superallowed transition thus becomes the first addition to this set of transitions in nearly a decade and the first for which a precise mirror comparison is possible, thus enabling an improved test of the isospin-symmetry-breaking corrections required for the extraction of V-ud.